한약 - secondary metabolite 역사탐구, 임상응용까지 ...

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음식 --> 약이 되기까지

herbal medicine --> 진짜 약이 되기까지

secondary metabolite는 leaky gut 환자에게는 

anti-nutrient가 될 수 있다

https://cafe.daum.net/panicbird/K5v7/438

Re: 한약의 대변신.. herb의 anti-nutrient 제거, efficacy, potency 증진 방법 탐구!!

한약재(식물성 생약)의 성분을 체계적으로 분석하는 최선의 방법은

Kossel(1891)이 처음 개념화한

‘secondary metabolites(2차 대사산물)’로 분류하는 것.

한약재의 주요 활성 성분(알칼로이드, 테르페노이드, 페놀성 화합물, 플라보노이드 등)은

대부분 primary metabolism(1차 대사)과 구별되는 2차 대사산물로,

식물의 방어·적응·생태 상호작용에 관여하며 약리 효과의 핵심입니다.

1950년대부터 방사성 동위원소(tracer) 기술 도입으로

생합성 연구가 본격화되었고,

이후 화학적·분자생물학적·유전체학적 접근으로 폭발적으로 발전했습니다.

방사성 동위원소 (Radioactive Isotope, Tracer) 

1. 기본 개념

방사성 동위원소란,
원소의 원자핵이 불안정하여 스스로 붕괴하면서 방사선(α선, β선, γ선)을 방출하는 동위원소.

동위원소(isotope)는 같은 원소지만 중성자 수가 다른 원자들입니다. 예를 들어:

• 탄소의 안정 동위원소: ¹²C (98.9%), ¹³C (1.1%)
• 방사성 동위원소: ¹⁴C (탄소-14, β선 방출, 반감기 5,730년)

생합성 연구에서 가장 많이 사용된 방사성 동위원소:

• ¹⁴C (탄소-14)
• ³H (트리튬, 수소-3)
• ³²P, ³⁵S, ¹⁵N (안정 동위원소지만 tracer로도 사용) 등

2. Tracer (추적자) 기술이란?

방사성 동위원소를 표지(labeling)하여 특정 분자에 넣은 뒤,
그 분자가 생물체 안에서 어떻게 이동하고 변형되는지를 방사선 검출기로 추적하는 방법.

→ 마치 GPS 추적기를 분자에 붙여서 생화학 반응의 경로와 속도, 중간체를 밝히는 기술.

3. 1950년대 생합성 연구에서의 혁명적 역할 1950년대 이전까지는 생합성 경로를 밝히는 것이 매우 어려웠습니다.

방사성 tracer 기술 도입으로 상황이 완전히 바뀌었습니다.

주요 장점:

• 극미량으로도 검출 가능 (매우 민감)
• 정상적인 생리 조건에서 실험 가능
• 반응 순서와 속도를 시간대별로 추적 가능
• 중간 대사산물을 동정(identify)할 수 있음

대표적인 성공 사례 (1950~1960년대):

• Calvin cycle (광합성 탄소 고정 경로): Melvin Calvin이 ¹⁴C-labeled CO₂를 이용해 광합성 중간체를 밝힘 (1961년 노벨 화학상)
• Cholesterol 생합성 경로: Konrad Bloch 등이 ¹⁴C-acetate로 메발론산 경로 발견
• 알칼로이드, 테르펜, 페놀 화합물 등 식물 이차 대사물질 생합성 연구
• 페닐알라닌 → 페닐프로파노이드 경로 (리그닌, 플라보노이드 등)
• 메발론산 경로 vs. MEP 경로 (테르펜 생합성)

4. 실제 실험 방식 (간단 예시)

1. 식물이나 세포에 ¹⁴C로 표지된 포도당 또는 아세트산을 공급
2. 일정 시간 후 식물을 추출
3. 방사성 TLC, autoradiography, HPLC 등으로 방사능이 어디에 들어갔는지 확인
4. 시간에 따라 방사능이 어떻게 이동하는지 관찰 → 생합성 경로 재구성

5. 이후 발전

• 1970~80년대: 안정 동위원소 (¹³C, ¹⁵N, ²H) + NMR, 질량분석기로 비방사성 tracer 기술 발전
• 1990년대 이후: 분자생물학 (유전자 클로닝), 유전체학, CRISPR 등으로 효소 유전자 → 기능 → 경로 규명이 폭발적으로 진행

요약

1950년대 방사성 동위원소 tracer 기술은
생합성 연구의 '눈'을 열어준 혁명적인 방법이었습니다.

분자가 어떻게 만들어지고, 어디로 가는지를 직접 볼 수 있게 해주면서,
지금 우리가 알고 있는 대부분의 1차·2차 대사 경로의 기초를 마련

아래는

1950년대 이후 식물 2차 대사산물 연구의 주요 고인용 논문·리뷰(인용지수 높은 순으로 선별, 10개 이상)를 중심으로

간단히 정리.

대부분 Google Scholar·PMC 등에서 확인된 고인용(수백~수천 회) 논문으로, 역사적 개관·생합성 경로·환경 영향·in vitro 생산 등 한약재 분석에 직접적으로 활용되는 내용입니다. 

1. Fraenkel GS (1959). “The raison d’être of secondary plant substances.” Science. (인용 ~1,940회) 1950년대 대표 seminal paper. 2차 대사산물을 단순 ‘폐기물’이 아닌 곤충·초식동물 방어 물질로 재정의. 한약재의 생태적·방어적 역할을 이해하는 기초.

핵심 내용 (한 줄 요약)

자세한 설명

• 1959년에 발표된 고전적인 논문으로, 식물 이차 대사물질의 존재 이유(raison d’être)를 처음으로 체계적으로 설명한 대표적 연구입니다.
• Fraenkel은 식물이 만들어내는 알칼로이드, 테르펜, 페놀류 등의 이차 대사물질을 “odd chemicals(이상한 화학물질)”이라고 표현하며, 이들이 주로 방어(anti-insect) 목적으로 진화했다고 주장했습니다.
• 그러나 진화 과정에서 일부 화합물은 곤충에게 먹이 신호로 작용하게 되어, 식물-곤충 간의 공진화(co-evolution) 관계를 형성했다고 설명합니다.
• 즉, 보호 → 유인이라는 이중적인 역할을 강조한 최초의 중요한 논문

1. Bourgaud F et al. (2001). “Production of plant secondary metabolites: a historical perspective.” Plant Science. (인용 ~2,030회) 1960년대 이후 세포배양·생합성 연구 역사 정리. 한약재 대량 생산·분석 기술(plant cell culture)의 기초를 제시.

핵심 내용 (한 줄 요약)

자세한 설명

• 2001년에 발표된 리뷰 논문으로, 식물 이차 대사물질 생산 연구의 역사적 흐름을 체계적으로 정리한 대표적인 논문입니다.
• 주요 포인트:
  • 1960년대 후반에 식물 세포·조직 배양 기술이 처음 도입됨.
  • 이 기술은 이차 대사물질의 생합성 연구뿐만 아니라 대량 생산을 위한 실용적 방법으로 주목받음.
  • 이후 30여 년간(2001년까지) 배양 기술이 어떻게 발전해 왔는지, 성공 사례와 한계를 역사적으로 분석.
  • 식물 세포 배양이 산업적으로 중요한 고부가 이차 대사물질(알칼로이드, 플라보노이드, 테르펜 등)을 생산하는 대안이 될 수 있음을 강조.

“1960년대 말 식물 세포 배양 기술 도입

→ 이차 대사물질 연구와 생산의 새로운 시대 개막 (Bourgaud, 2001)

1. Wink M (2003). “Evolution of secondary metabolites from an ecological and molecular phylogenetic perspective.” Phytochemistry. (인용 ~2,066회) 진화·계통발생학적 관점에서 2차 대사산물 다양성 설명. 한약재 간 화학적 유사성·차이를 이해하는 데 유용.

핵심 내용 (한 줄 요약)

자세한 설명

• 2003년에 발표된 리뷰 논문으로, 식물 이차 대사물질의 진화적 기원과 의미를 생태학(ecological)과 분자계통학(molecular phylogenetic) 관점에서 종합적으로 정리한 대표적인 고전입니다.
• 주요 포인트:
  • 이차 대사물질은 단순한 “부산물”이 아니라, 자연선택을 통해 진화한 적응 전략이다.
  • 곤충·병원균 방어, 자외선 차단, 알레로파시(타 식물 억제), 꽃·열매 색상·향기 유인 등 다양한 생태학적 기능을 수행.
  • 분자계통학 데이터를 이용해 서로 다른 식물군에서 동일한 이차 대사물질이 수렴진화(convergent evolution)를 일으킨 사례를 분석.
  • 생합성 유전자들의 계통 발생(phylogeny)을 통해 이차 대사물질의 진화적 역사를 추적.

의의

• 1959년 Fraenkel 논문이 “존재 이유”를 처음 제기했다면,
• 2003년 Wink 논문은 진화생물학적·분자적 증거를 바탕으로 현대적으로 재정립한 중요한 업그레이드 버전입니다.
• 현재 번역 중인 리뷰 논문에서 “환경 스트레스와 방어 기전” 부분의 이론적 배경으로 자주 인용되는 핵심 문헌입니다.

한 줄 기억하기: “이차 대사물질은 진화 과정에서 자연선택을 받은 적응적 산물이다 — Wink (2003)”

1. Hartmann T (2007). “From waste products to ecochemicals: fifty years research of plant secondary metabolism.” Phytochemistry. (인용 ~1,182회) 1950년대 방사성 tracer 기술부터 2000년대 초까지 정확히 50년을 리뷰. ‘폐기물’에서 ‘생태화학물질(ecochemicals)’로의 인식 변화, 메커니즘 연구 역사 정리. 1950년대 연구의 필수 참조.

핵심 내용 (한 줄 요약)

자세한 설명

• 2007년에 발표된 리뷰 논문으로, 식물 이차 대사물질 연구의 최근 50년(1957~2007) 역사를 체계적으로 정리한 논문입니다.
• 주요 포인트:
  • 19세기 후반 ~ 1950년대: 주로 화학자들에 의해 새로운 식물 화합물의 분리(isolation)와 구조 결정(structure elucidation)에 집중.
  • 1950년대 이후: 방사성 동위원소 tracer 기술 도입으로 생합성 경로 연구가 본격화.
  • 1970~1990년대: 생태학적 역할(방어, 상호작용), 생합성 효소, 유전자 연구로 확대.
  • 2000년대: 분자생물학, 유전체학, 대사공학, 식물 세포배양 기술 등을 통해 산업적 생산까지 이어짐.
  • 연구 패러다임이 “화합물 찾기” → “왜 만드는가? 어떻게 만드는가? 어떻게 활용할까?”로 전환되었음을 강조.

이 리뷰는:

• 역사적으로 중요한 육상·해양 천연물과 그 민간 사용 사례
• Dereplication(중복 물질 빠르게 식별) 기술
• 천연물 화학이 어떻게 신약 개발로 이어졌는지
• 첨단 분광학 기법(하이픈네이티드 기술)의 역할
• 대사체학(metabolomics) + dereplication을 활용한 미래 방향

을 다룹니다.

주요 내용 구조

1. 천연물의 역사
   • 메소포타미아 점토판(2600 BC), 에베르스 파피루스(이집트, 2900 BC), 중국 본초서(Shennong 등), Dioscorides·Theophrastus(그리스), 아랍 의학(Avicenna) 등에서 천연물이 약으로 사용된 기록.
2. 민간요법(Folklore) 사례
   • 식물: Salvia(출산 보조), Alhagi maurorum(manna, 다양한 질환), Atropa belladonna(독성 주의) 등
   • 균류·지의류: Piptoporus betulinus(지혈·소독), Usnea(비듬·상처)
   • 해양: 붉은 해조류(Chondrus crispus 등, 감기·암 치료 민간요법)
3. 1차 vs 2차 대사산물
   • 1차: 생존 필수 (당, 아미노산 등)
   • 2차: 방어·적응용 (알칼로이드, 테르페노이드, 페놀 등) → 대부분의 약효 성분
4. 신약 발견에서의 역할
   • 천연물은 구조적 다양성이 뛰어나 조합화학(combinatorial chemistry)을 보완
   • 많은 블록버스터 약물이 천연물 또는 그 유도체(예: 아스피린, 페니실린, 택솔 등)
   • 현대: 고해상도 NMR, LC-MS 등 첨단 분석으로 발견 가속화
5. 미래 방향
   • Metabolomics profiling + dereplication으로 효율적 스크리닝
   • 미개척 생물다양성(세계 생물의 10% 미만 탐색) 활용

의의

이 논문은 천연물 연구의 역사적·과학적 가치를 체계적으로 정리한 입문서 역할

1. Kabera JN et al. (2014). “Plant Secondary Metabolites: Biosynthesis, Classification, Function and Pharmacological Properties.” Journal of Pharmacy and Pharmacology. (인용 ~924회) 생합성 경로(shikimate, meval‎onate, MEP 등), 분류(테르페노이드·알칼로이드·페놀류), 약리학적 특성 체계적으로 정리. 한약재 성분 분석의 교과서적 리뷰.

핵심 내용 (한 줄 요약)

자세한 설명

• 2014년에 발표된 리뷰 논문으로, 식물 이차 대사물질에 대한 전반적인 개요를 제공하는 실용적인 논문입니다.
• 주요 다루는 내용:
  • 생합성(Biosynthesis): 주요 경로(시킴산 경로, 메발론산 경로, 지방산 경로 등) 설명
  • 분류(Classification): 테르펜, 페놀, 알칼로이드, 플라보노이드 등 화학 구조에 따른 체계적 분류
  • 기능(Function): 식물 내 방어, 스트레스 반응, 생태학적 역할
  • 약리학적 특성(Pharmacological Properties): 항산화, 항암, 항염증, 항균 등 의약·산업적 활용 가능성 및 최근 연구 동향
• 총 16페이지 분량으로, 비교적 간결하면서도 실용적인 내용을 담고 있습니다.

1. Hussein RA, El-Anssary AA (2018). “Plants Secondary Metabolites: The Key Drivers of the Pharmacological Actions.” (인용 ~879회) 2차 대사산물이 약리 작용(항산화·항염·항암 등)의 핵심임을 강조. 전통의학(한약재)과 현대 약학 연결.

https://www.intechopen.com/chapters/61866

Plants Secondary Metabolites: The Key Drivers of the Pharmacological Actions of Medicinal Plants

식물 2차 대사산물(Secondary Metabolites)은

식물이 성장·발달·번식에 직접적으로 필수적이지 않은 유기 화합물입니다.

대신

환경 적응, 방어, 상호작용을 위해 만들어집니다.

1. Primary Metabolites vs Secondary Metabolites

구분                     Primary (1차)                                              Secondary (2차)

역할	성장, 에너지, 번식	방어, 스트레스 대응, 생태계 상호작용
예시	당, 아미노산, 지질, 핵산	알칼로이드, 테르페노이드, 페놀화합물 등
필수성	생존에 필수	없어도 성장 가능하지만, 경쟁력↑

2. 주요 종류와 기능

• 알칼로이드 (Alkaloids): 카페인, 모르핀, 니코틴 등 → 독성·약리 효과 강함
• 테르페노이드 (Terpenoids): 에센셜 오일, 카로티노이드 → 향기, 색소, 항염증
• 페놀화합물 (Phenolics): 플라보노이드, 탄닌 → 항산화, UV 보호, 병원균 방어
• 기타: 글리코시드, 사포닌 등

이 물질들은 약용식물의 약효를 주로 담당합니다. 한의학·서양의학에서 사용하는 대부분의 천연약물 성분이 바로 2차 대사산물입니다.

왜 중요한가?

• 식물 스스로를 보호: 초식동물·병원균·자외선·가뭄 등으로부터 방어
• 인간에게 유익: 약, 향료, 색소, 농약 대체 물질로 활용
• 최근 연구 트렌드: 항암, 항바이러스, 항노화 등 신약 개발의 주요 타겟

이 논문은

약용식물의 약리작용을 이해하는 데 좋은 입문 자료

1. Jan R et al. (2021). “Plant Secondary Metabolite Biosynthesis and Transcriptional Regulation in Response to Biotic and Abiotic Stress Factors.” Agronomy. (인용 ~885회) 스트레스(생물·비생물)에 따른 생합성·전사 조절 메커니즘 상세 설명. 한약재 재배·성분 증강 전략에 활용.

식물 2차 대사산물(SMs)은

식물의 생존, 생태계 상호작용, 병원균·초식동물 방어, 산화 스트레스 완화에 핵심 역할을 합니다.

식물은 움직일 수 없기 때문에 (sessile nature)

다양한 생물적(biotic)·비생물적(abiotic) 스트레스에 대응해 SMs를 축적합니다.

이 리뷰는:

• 개별 환경 요인(빛, 온도, 가뭄, 염분, 토양 영양 등)이 SMs 축적에 미치는 영향
• Shikimate pathway를 중심으로 한 생합성 메커니즘
• 스트레스 조건에서 SMs 생합성을 조절하는 전사인자(TFs) (WRKY, MYB, AP2/ERF, bZIP, bHLH, NAC 등)와 관련 유전자
• 생물·비생물 elicitor(유도물질)를 이용한 배양 시스템에서의 SMs 증산 전략

을 중점적으로 다룹니다.

주요 내용 구조

1. 서론: Primary vs Secondary Metabolites 차이, 스트레스 대응에서의 SMs 역할
2. 생합성 경로: Shikimate pathway (aromatic amino acids: phenylalanine, tyrosine, tryptophan) → 페놀화합물, 테르페노이드, 알칼로이드, 질소·황 함유 화합물 등
3. SMs 개요: 페놀류, 테르페노이드, 알칼로이드 등 주요 그룹
4. 비생물 스트레스(Abiotic) 반응: 빛, 온도, 가뭄, 염분, 중금속 등에 따른 SMs 변화
5. 생물 스트레스(Biotic) 반응: 병원균, 초식동물 공격 시 phytoalexin 등 방어 물질 증가
6. 전사 조절: 다양한 TFs가 스트레스 신호를 받아 SMs 관련 유전자 발현 조절
7. Elicitor 적용: 인공 배양에서 SMs 생산 증대 기술

1. Reshi ZA et al. (2023). “From Nature to Lab: A Review of Secondary Metabolite Biosynthetic Pathways, Environmental Influences, and In Vitro Approaches.” Metabolites. (사용자 제공 이미지 논문) (인용 ~245회, 최근 고인용) 생합성 경로·환경 영향(스트레스)·in vitro(세포배양·elicitor) 접근법을 종합 리뷰. 한약재 성분 체계적 분석·생산에 가장 직접적으로 적용 가능한 최신 리뷰.

이 리뷰는 약용식물의 2차 대사산물(SMs) 생합성 경로를 자연 상태에서 실험실(체외 배양)까지 체계적으로 정리합니다. 특히 다양한 환경 스트레스(생물적·비생물적)가 SMs 생산에 미치는 영향과, 이를 활용해 생산성을 높이는 in vitro 기술을 중점적으로 다룹니다.

주요 내용 구조

1. 2차 대사산물의 생합성 경로
   • 주요 그룹: 페놀화합물(Shikimate pathway), 테르페노이드(MEP/MVA pathway), 알칼로이드, 질소·황 함유 화합물 등
   • 핵심 전구체와 효소, 유전자 조절 메커니즘 상세 설명
2. 환경 영향 (Environmental Influences)
   • 비생물 스트레스: 가뭄, 염분, 고온/저온, 중금속, UV, 빛 강도 등
   • 생물 스트레스: 병원균, 초식동물
   • 스트레스가 SMs 축적을 어떻게 유도하는지 (방어 기작으로서의 역할 강조)
3. In Vitro Approaches (실험실 생산 기술)
   • 세포/조직 배양, elicitor(유도물질: jasmonate, salicylic acid 등) 처리
   • bioreactor, hairy root culture, metabolic engineering
   • 자연 상태보다 안정적·대량 생산 가능하게 하는 전략
4. 약용식물 중심 실용적 관점
   • SMs의 약리학적 가치(항산화, 항염, 항암 등)
   • 기후변화 시대에 스트레스 대응형 재배 및 바이오테크놀로지 적용

1. Dixon RA (2024). “A century of studying plant secondary metabolism—From ‘what?’ to ‘where, how, and why?’” Plant Physiology. (인용 ~99회, 최근 but authoritative) 100년 연구 타임라인(1950년대 PAL 효소 발견, 1970년대 elicitor·cell culture, 1980~90년대 유전자 클로닝, 2000년대 omics·genome mining까지) 제시. ‘무엇(what)’에서 ‘어떻게·왜(how & why)’로의 발전 과정 요약.

https://academic.oup.com/plphys/article/195/1/48/7499152

지난 100년 동안

식물 2차 대사산물 연구는

“무엇이 존재하는가(what)?” (화학 구조 규명)에서

“어디에 존재하는가(where)?”,

“어떻게 만들어지는가(how)?”,

“왜 만들어지는가(why)?” 로 발전해 왔습니다.

초기 추출물 분석부터

현대 ‘오믹스(omics)’ 기술까지,

식물 2차 대사산물의 구조·기능·국소화·생합성 경로가 밝혀졌으며,

이는 의학·농업 바이오테크놀로지에 큰 기여를 했습니다.

그러나

여전히 방대한 화학 다양성과 생물학적 역할에 대한 미스터리가 남아 있습니다.

이 리뷰는

역사적 발전을 정리하고, 미래 연구 방향과 신기술을 제안합니다.

주요 내용 구조

1. 역사적 개요 (100년 타임라인)
   • 초기: 식물 추출물의 화학 성분 동정 (예: 알칼로이드, 플라보노이드, 리그닌 등)
   • 중기: 생합성 경로 규명 (효소, 유전자)
   • 최근: 오믹스(유전체·전사체·대사체), 이미징 기술, 합성생물학 등으로 “where/how/why” 탐구
2. 주요 발전 영역
   • What? → 구조 동정 (선택된 대사산물의 화학 구조)
   • Where? → 세포·조직 내 국소화 (MS imaging, FRET 센서, 형광 프로브 등)
   • How? → 생합성 경로와 조절 (유전자, 효소, 전사인자)
   • Why? → 생물학적 기능 (발달, 환경 스트레스 대응, 생태계 상호작용, 인간 의학적 가치)
3. 신기술 하이라이트
   • 단일세포 전사체학, MALDI-MS 이미징, 형광 센서, 합성생물학, CRISPR 등
   • 예: 리그닌·플라보노이드·알칼로이드 경로의 역사적 타임라인

https://www.intechopen.com/chapters/61866

Plants Secondary Metabolites: The Key Drivers of the Pharmacological Actions of Medicinal Plants

Abstract

The vast and versatile pharmacological effects of medicinal plants are basically dependent on their phytochemical constituents. Generally, the phytochemical constituents of plants fall into two categories based on their role in basic metabolic processes, namely primary and secondary metabolites. Primary plant metabolites are involved in basic life functions; therefore, they are more or less similar in all living cells. On the other hand, secondary plant metabolites are products of subsidiary pathways as the shikimic acid pathway. In the course of studying, the medicinal effect of herbals is oriented towards the secondary plant metabolites. Secondary plant metabolites played an important role in alleviating several aliments in the traditional medicine and folk uses. In modern medicine, they provided lead compounds for the production of medications for treating various diseases from migraine up to cancer. Secondary plant metabolites are classified according to their chemical structures into various classes. In this chapter, we will be presenting various classes of secondary plant metabolites, their distribution in different plant families and their important medicinal uses.

Keywords

• secondary plant metabolites
• phenolics
• alkaloids
• saponins
• terpenes

Author InformationShow +

1. Introduction

Plant chemistry is the basis of the therapeutic uses of herbs. A good knowledge of the chemical composition of plants leads to a better understanding of its possible medicinal value. Modern chemistry has described the role of primary plant metabolites in basic life functions such as cell division and growth, respiration, storage and reproduction. They include the components of processes such as glycolysis, the Krebs or citric acid cycle, photosynthesis and associated pathways. Primary metabolites include small molecules such as sugars, amino acids, tricarboxylic acids, or Krebs cycle intermediates, proteins, nucleic acids and polysaccharides. Eventually, the primary metabolites are similar in all living cells [1].

Secondary plant metabolites are numerous chemical compounds produced by the plant cell through metabolic pathways derived from the primary metabolic pathways. The concept of secondary metabolite was first defined by Albrecht Kossel, Nobel Prize winner for physiology or medicine in 1910 [2]. Thirty years later, Czapek described them as end-products [3]. According to him, these products are derived from nitrogen metabolism by what he called ‘secondary modifications’ such as deamination. In the middle of the twentieth century, advances of analytical techniques such as chromatography allowed the recovery of more and more of these molecules, and this was the basis for the establishment of the discipline of phytochemistry.

Secondary metabolites have shown to possess various biological effects, which provide the scientific base for the use of herbs in the traditional medicine in many ancient communities. They have been described as antibiotic, antifungal and antiviral and therefore are able to protect plants from pathogens. Besides, they constitute important UV absorbing compounds, thus preventing serious leaf damage from the light. It was noticed that some herbs as forage grasses such as clover or alfalfa can express estrogenic properties and interact with fertility of animals [4].

Secondary plant metabolites are classified according to their chemical structures into several classes. In this chapter, the nature of secondary plant metabolites will be discussed as a foundation for a review of the main categories of constituents considered to be of therapeutic importance. Each section includes an overview of a class of the secondary plant metabolites regarding structure, botanical distribution and generalizations about pharmacology, followed by examples of representative molecules. The classes of secondary plant metabolites include:

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Advertisement2. Phenolics

Phenolics probably constitute the largest group of plant secondary metabolites. They share the presence of one or more phenol groups (Figure 1) as a common characteristic and range from simple structures with one aromatic ring to highly complex polymeric substances. They are widespread in plants where they contribute significantly to the color, taste and flavor of many herbs, foods and drinks. Some phenolics are valued pharmacologically for their anti-inflammatory activities such as quercetin or antihepatotoxic properties such as silybin. Others exert phytoestrogenic activity as genistein and daidzein, while others are insecticidal as naringenin [5]. Many of the phenolic molecules are also effective antioxidants and free radical scavengers, especially flavonoids. Phenolics can be classified according to their structure or biosynthetic origin. According to their structures, phenolics can be classified into:

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Figure 1.

Phenol.

2.1. Simple phenolics

Phenolic acids are ubiquitous among plants; although free phenols are rare, gallic acid is relatively widespread and is the parent compound of the gallotannins (Figure 2). Gallic acid is well known for its astringent properties but has demonstrated many other activities in vitro, including antibacterial, antiviral, antifungal, anti-inflammatory, antitumor, antianaphylactic, antimutagenic, choleretic and bronchodilatory actions. It also inhibits insulin degradation and promotes smooth muscle relaxation [6]. The phenolic compounds in this group vary according to their functional group, which may be hydroxyl, aldehydic, or carboxylic group; these include eugenol (a phenolic phenylpropane), vanillin (a phenolic aldehyde) and salicylic, ferulic and caffeic acids (phenolic acids). Hydroquinone is also among the most widely distributed of the simple phenols, occurring in a number of plants as the glycoside arbutin. Glycoside formation is common, and the widely distributed glycoside coniferin and other derivatives of phenolic cinnamic alcohols are precursors of lignin [7, 8].

Figure 2.

Gallic acid.

The pharmacological properties of these widely found constituents are probably best demonstrated by the urinary tract antimicrobial arbutin [9] and the anti-inflammatory salicylates [10]. A property shared by all phenols is antimicrobial activity. In fact, phenol itself was the first antiseptic used in surgery [11].

The pharmacological activities of many plants are attributed to simple phenolics among which the antimicrobial and diuretic activities of Arctostaphylos uva-ursi were attributed to its phenolic content [12]. Capsicum spp. showed circulatory stimulant, rubefacient and analgesic activities due to the presence of capsaicinoids, which are simple phenolic compounds [13]. Moreover, the cholagogue activity of Cynara scolymus, the anthelmintic activity of Dryopteris filix-mas, the anti-inflammatory analgesic activity of Filipendula ulmaria as well as the anticatarrhal and diuretic activities of Solidago virgaurea are all attributed to the action of simple phenolics [8]. Figure 3 illustrates some examples of simple phenolics.

Figure 3.

Examples of simple phenolics.

2.2. Tannins

Tannins are polyphenols which have the ability to precipitate protein. These compounds have been used for decades to convert raw animal hides into leather. In this process, tannin molecules crosslink the protein and make it more resistant to bacterial and fungal attack. Today, however, many substances considered to be tannins by virtue of their structure and biosynthetic origin have limited, if any, ability to make leather [14]. There are two major types of tannins: hydrolyzable tannins and condensed tannins. Hydrolyzable tannins are formed from several molecules of phenolic acids such as gallic and hexahydroxydiphenic acids, which are united by ester linkages to a central glucose molecule. Two principal types of hydrolysable tannins are gallotannins and ellagitannins, which are, respectively, composed of gallic acid and ellagic acid units. Ellagitannins found in plants of medicinal interest and for which structures have been elucidated include geraniin (isolated from Geranium robertianum (Herb Robert) and Geranium maculatum (American cranesbill) [15]) and tellimagrandins 1 and 2 [16] (isolated from Quercus alba (Oak bark), Punica granatum (pomegranate) and Filipendula ulmaria (Meadowsweet)) [7].

Condensed tannins, or proanthocyanidins, are compounds whose structures are based on oligomeric flavonoid precursors and vary in the type of linkages between flavonoid units; hydroxylation patterns; stereochemistry of carbons 2, 3 and 4 of the pyran ring and the presence of additional substituents. Some drugs (e.g., Camellia sinensis (tea), Hamamelis virginiana leaves and bark) contain both hydrolyzable and condensed tannins [17].

Tannin-containing drugs act as antidiarrhoeals and have been employed as antidotes in poisoning by heavy metals and alkaloids. Epigallocatechin-3-gallate, the active principal in tea, has been shown to be antiangiogenic in mice. Vaccinium oxycoccos (cranberry) juice has long been used as urinary antiseptic [18], which was scientifically proven in a randomized, double-blind, placebo-controlled trial that has been carried out on 153 elderly women [19]. Figure 4 illustrates some examples of hydrolysable tannins.

Figure 4.

Examples of hydrolysable tannins.

2.3. Coumarins

Coumarins are derivatives of benzo-α-pyrone, the lactone of O-hydroxycinnamic acid, coumarin. Some 1000 natural coumarins have been isolated. Coumarin itself has been found in about 150 species belonging to over 30 different families. The richest sources of coumarin are sweet clover or melilot (Melilotus spp.), Dipteryx odorata (tonka bean) and Galium odoratum (sweet woodruff) [8]. Aesculetin, umbelliferone and scopoletin are common coumarins present in plants both in the free state and as glycosides. Plants rich in coumarins include Atropa belladonna, Datura stramonium (Solanaceae), Daphne mezereum (Thymeliaceae), Ruta graveolens (Umbelliferae) and certain Aesculus hippocastanum (Horse-chestnut) (Hippocastanaceae) and certain Rosaceae [7]. Anti-inflammatory, anticoagulant, anticancer and anti-Alzheimer’s activities are the most important biological activities reported for coumarins [20]. Examples of coumarins are shown in Figure 5.

Figure 5.

Examples of coumarins.

2.4. Flavonoids

Flavonoids are the largest group of naturally occurring phenols. More than 2000 of these compounds are now known, with nearly 500 occurring in the free state [7]. The structural skeleton of flavonoids includes a chroman ring bearing an aromatic ring in position 2, 3 or 4. Flavonoids may be divided into various classes according to the oxidation level of the central ring (ring C). The most common of these are anthocyanins, flavones and flavonols. The flavones and their close relations are often yellow (Latin flavus, yellow). They are widely distributed in nature but are more common in the higher plants and in young tissues, where they occur in the cell sap. They are abundant in the Polygonaceae, Rutaceae, Leguminosae, Umbelliferae and Compositae. Recent researches have demonstrated the medicinal action of drugs containing flavonoids such as Glycyrrhiza glabra (liquorice root), Chamaemelum nobile (Roman chamomile) and Ginkgo biloba (gingko). A number of flavonoid-containing herbs have now been included in the British Pharmacopeia, examples are Betula pendula (Birch Leaf), Calendula officinalis Flower, Sambucus nigra (ElderFlower), Equisetum ramosissimum (Horsetail), Tilia cordata (Lime Flower), Leonurus cardiaca (Motherwort) and Passiflora edulis (passion flower). The group is known for its anti-inflammatory and antiallergic effects, for antithrombotic and vasoprotective properties, for inhibition of tumor promotion and as a protective for the gastric mucosa [21, 22]. Examples of flavonoids are shown in Figure 6.

Figure 6.

Examples of flavonoids.

2.5. Chromones and xanthones

These compounds are structural derivatives of benzo-γ-pyrone, and although not of great pharmaceutical importance, a few compounds are worthy of mention; eugenin is found in the clove plant and khellin from mustard seeds [7]. More complex are the furanochromones, the active constituents of the fruits of Ammi visnaga. Xanthones are found mainly in the Gentianaceae and Guttiferae, otherwise scattered sporadically throughout the plant kingdom as in the Moraceae and Polygalaceae. Polygala nyikensis is used by the highlanders of Malawi and bordering countries to treat various skin problems of fungal origin. The root of the plant was recently shown to exert its antifungal activity owing to the presence of xanthones [23].

2.6. Stilbenes

Stilbenes are a relatively small, but widely distributed, group of plant secondary metabolites found mostly as heartwood constituents in a heterogeneous assembly of plant species. They are especially important in the heartwood of trees of the genera Pinus (Pinaceae), Eucalyptus(Myrtaceae) and Madura (Moraceae) [1]. The para-hydroxylated compound, resveratrol, is the most widespread stilbene in nature. Resveratrol possesses estrogen-like activity and occurs in Picea, Pinus, the Fabaceae, Myrtaceae and the Vitaceae [24].

2.7. Lignans

Lignans are dimeric compounds formed essentially by the union of two molecules of a phenylpropene derivative reported from the members of Asteraceae (e.g., Achillea lingulata [25]), Pinaceae (e.g., Cedrus deodara [26]) and Rutaceae (e.g., Fagara heitzii) [27]. Four major subtypes occur: dibenzylbutane derivatives, dibenzylbutryolactones (lignanolides or derivatives of butanolide), monoepoxy lignans or derivatives of tetrahydrofuran and bisepoxylignans or derivatives of 3,7-dioxabicyclo(3.3.0)-octane. Many of these compounds showed antimicrobial and antifungal activities [1], while others showed cytotoxic activities such as wikstromal, matairesinol and dibenzyl butyrolactol from Cedrus deodara [26].

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3. Alkaloids

Alkaloids are organic compounds with at least one nitrogen atom in a heterocyclic ring. Their definition is problematic, as they do not represent a homogeneous group of compounds from any standpoint, whether chemical, biochemical, or physiological. Except for the fact that they are all nitrogen-containing compounds, no general definition fits all alkaloids. Alkaloids can be divided according to their basic chemical structure into different types. The following are basic types of alkaloids: acridones, aromatics, carbolines, ephedras, ergots, imidazoles, indoles, bisindoles, indolizidines, manzamines, oxindoles, quinolines, quinozolines, phenylisoquinolines, phenylethylamines, piperidines, purines, pyrrolidines, pyrrolizidines, pyrroloindoles, pyridines and simple tetrahydroisoquinolines [28].

Although plants containing alkaloids have been used by man for at least 3000 years as medicines, teas and potions, the compounds responsible for activity were not isolated and characterized until the nineteenth century [1]. Alkaloids are not common in lower plants. Lysergic acid derivatives and sulfur-containing alkaloids, e.g., the gliotoxins, are detected in fungi. Concerning the pteridophytes and gymnosperms alkaloids reported for their medicinal uses include the lycopodium, ephedra and Taxus alkaloids. Alkaloids are unevenly distributed among the angiosperms. The following are the orders reported to be rich in alkaloids: Centrospermae (Chenopodiaceae), Magnoliales (Lauraceae, Magnoliaceae), Ranunculales (Berberidaceae, Menispermaceae, Ranunculaceae), Papaverales (Papaveraceae, Fumariaceae), Rosales (Leguminosae, subfamily Papilionaceae), Rutales (Rutaceae), Gentiales (Apocynaceae, Loganiaceae, Rubiaceae), Tubiflorae (Boraginaceae, Convolvulaceae, Solanaceae) and Campanulales (Campanulaceae, sub-family Lobelioideae; Compositae, subfamily Senecioneae). However, there is no report for the presence of alkaloids in Salicales, Fagales, Cucurbitales and Oleales dicot orders till the present time [7].

Alkaloids demonstrate a diverse array of pharmacological actions including analgesia, local anesthesia, cardiac stimulation, respiratory stimulation and relaxation, vasoconstriction, muscle relaxation and toxicity, as well as antineoplastic, hypertensive and hypotensive properties. The activity of alkaloids against herbivores, toxicity in vertebrates, cytotoxic activity, the molecular targets of alkaloids, mutagenic or carcinogenic activity, antibacterial, antifungal, antiviral and allelopathic properties have been reported in literature. Many alkaloids are sufficiently toxic to animals to cause death if eaten. Several (e.g., nicotine and anabasine) are used as insecticides [1, 8].

Examples of some alkaloids:

3.1. Nicotine

Nicotine is found in the tobacco plant (Nicotiana tabacum) and other Nicotiana species; it has tranquilizing properties and is the addictive component of tobacco. It is also extremely toxic, causing respiratory paralysis at high doses (Figure 7). Nicotine is a ganglion cholinergic-receptor agonist with complex pharmacological actions, including effects mediated by binding to receptors in the autonomic ganglia, the adrenal medulla, the neuromuscular junction and the brain [29].

Figure 7.

Examples of alkaloids.

3.2. Caffeine

Caffeine occurs in a number of botanically unrelated species, including coffee (Coffea spp.), tea (Camellia sinensis), mate (Ilex paraguariensis), guarana (Paullinia cupana) and kola (Cola acuminata) (Figure 7). Caffeine is bound to chlorogenic acid in raw coffee beans. The roasting process liberates the caffeine and other compounds that contribute to the aroma of coffee. Caffeine is a diuretic and has stimulant effects on the respiratory, cardiovascular and central nervous systems [30].

3.3. Vinblastine

Vinblastine is isolated from Catharanthus roseus G. (Figure 7) and has been used to treat diabetes and high blood pressure and as disinfectant. Nevertheless, Vinblastine is so important for being cancer fighters. It is used along with the other vinca alkaloids vinorelbine, vincristine and vindesine, which are in clinical use in the United States and Europe [31].

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4. Saponins

Saponins are compounds that possess a polycyclic aglycone moiety with either a steroid (steroidal saponins) or triterpenoid (triterpenoidal saponins) attached to a carbohydrate unit (a monosaccharide or oligosaccharide chain) (examples illustrated in Figures 8 and 9). These sugar units are composed variously of pentoses, hexoses, or uronic acids. This hydrophobic-hydrophilic asymmetry means that these compounds have the ability to lower surface tension and are soap-like. They form foam in aqueous solutions and cause hemolysis of blood erythrocytes in vitro. The aglycone portion of the saponin molecule is called the genin or sapogenin. Saponins are widespread among plants, having been reported from more than 500 plants from at least 90 different families; these substances have been isolated from all parts of plants: leaves, stems, roots bulbs, flowers and fruits, although they tend to be concentrated in the roots of many species such as Digitalis purpurea (foxglove), Dioscorea villosa (wild yam), Eleutherococcus senticosus (Siberian ginseng), Gentiana lutea (gentian), Glycyrrhiza spp. (licorice) and Panax ginseng (Korean ginseng) [32].

Figure 8.

Example of triterpenoidal saponin.

Figure 9.

Example of steroidal saponin.

Saponins have demonstrated numerous pharmacological properties. Some saponins have antitumor, piscicidal, molluscicidal, spermicidal, sedative, expectorant and analgesic properties. Glycyrrhizin from glycyrrhizae radix (from Glycyrrhiza glabra, Fabaceae) is useful as expectorant and antitussive agent. It is also used to treat chronic hepatitis and cirrhosis. Some saponins have anti-inflammatory properties as the saponins from Bupleurum falcatum (Apiaceae). Phytolacca americana roots are reputed to possess anti-inflammatory properties in Korean medicine. Similar properties have been demonstrated for a number of other saponins, for example aescin, from horse chestnut (Aesculus hippocastanum), has been shown to be 600 times more effective than rutin in reducing rat paw edema [33].

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5. Terpenes

Terpenes are the largest and most diverse group of plant secondary compounds. The name “terpene” is derived from the word “turpentine,” which in turn comes from the old French ter(e)bintb, meaning “resin.” They are all derived chemically from 5-carbon isoprene units assembled in different ways [8]. Terpenes are classified according to the number of isoprene units in the molecule; a prefix in the name indicates the number of terpene units as follows.

5.1. Hemiterpenes

They consist of a single isoprene unit. Isoprene itself is considered the only hemiterpene, but oxygen-containing derivatives such as angelic acid isolated from Angelica archangelica and isovaleric acid from Vaccinium myrtillus are hemiterpenoids [1].

5.2. Monoterpenes

They consist of two isoprene units and have the molecular formula C10H16 (see Figure 10). They are important components of plant essential oils or volatile oils. Monoterpenes tend to occur in members of certain plant families, such as Lamiaceae, Pinaceae, Rutaceae and Apiaceae, from which many essential oils are commercially produced. Some of these compounds, such as geraniol, are almost ubiquitous and can be found in small amounts in the volatile secretions of most plants. Monoterpenes are further classified into unsaturated hydrocarbons (e.g., limonene), alcohols (e.g., linalool), alcohol esters (e.g., linalyl acetate), aldehydes (e.g., citronellal) and ketones (e.g., Carvone). Monoterpenes and other volatile terpenes have a number of widespread medicinal uses. Compounds such as camphor and menthol are used as counterirritants analgesics and anti-itching agents. Many monoterpenes have been used as anthelmintics. A series of monoterpene glycosides appear to have vasodilation effect on coronary vessels and the femoral vascular bed [16].

Figure 10.

Examples of monoterpenes.

5.3. Sesquiterpenes

They consist of three isoprene units and have the molecular formula C15H24 (see Figure 11). Based on biogenetic origin, there are more than 200 different structural types of sesquiterpenes, and several thousand such compounds are known. These compounds can be conveniently classified into three main groups according to structure: acyclic (e.g., farnesol), monocyclic (e.g., bisabolol) and bicyclic (e.g., caryophyllene). A number of sesquiterpene lactones show antibacterial, antifungal and antiprotozoan activities. Sesquiterpenes from Vernonia colorata inhibit Entamoeba histolytica at concentrations comparable to metronidazole, an antiamoebic drug. Helenalin and a series of related compounds are responsible for the cardiotonic properties of Arnica montana flowers. Atractylodis rhizoma, from Atractylodis macrocephala (Asteraceae), is clinically used as diuretic, analgesic and anti-inflammatory. The activity is related to the presence of active compounds including eudesma-4(14)-7(1 l)-dien-8-one and atractylenolide I. Several related medicinal plants are also used for the same purposes due to the presence of sesquiterpenes [1, 34].

Figure 11.

Examples of sesquiterpenes.

5.4. Diterpenes

They are composed of four isoprene units and have the molecular formula C20H32 (see Figure 12). Diterpenes are classified into acyclic and macrocyclic compounds. Moreover, macrocyclic diterpenes are classified according to the number of ring systems present. Diterpenes may be 6-membered ringed structures or they may have fused 5- and 7-membered ringed structures. In addition, many diterpenes have additional ring systems. These occur as side substitutions as esters or epoxides [8]. Diterpenoids constitute the active constituents of a number of medicinal plants. Vitamin K1, an antihemorrhagic compound, first discovered in plants in 1929, is a diterpene. Vitamin A, a diterpenoid, is referred to, together with the related compounds, as “carotenes.” The bitter principles of Jateorhiza palmata (calumba root) belong to furanoditerpenes. Teucrium chamaedrys (wall germander) and T. scorodonia (wood sage) family Labiatae, both produce diterpenes of the neoclerodane type. They are used in herbal medicine as diaphoretics and antirheumatics [35]. Like all groups of terpenes, diterpenes have demonstrated a range of pharmacological properties including: analgesic, antibacterial, antifungal, anti-inflammatory, antineoplastic and antiprotozoal activities [8]. Some diterpenes from Kalmia latifolia (Ericaceae) have antifeedant properties with respect to the gypsy moth. The gibberellins, first obtained from fungi of the genus Gibberella but also found in higher plants, are diterpenoid acids, which have a marked effect on growth of seedlings [7].

Figure 12.

Examples of diterpenes.

5.5. Sesterterpenes

Terpenes having 25 carbons and five isoprene units are rare relative to the other sizes (the sester- prefix means half to three, i.e. two and a half). An example of a sesterterpenoid is geranyl farnesol isolated from seed oils of Camellia sasanqua (sasanqua) and Camellia japonica (camellia), family Theaceae [36]. Geranyl farnesol showed cytotoxic activity in mouse leukemic M1 cells [37].

5.6. Triterpenes

They consist of six isoprene units and have the molecular formula C30H48 (see Figure 13). The linear triterpene squalene, the major constituent of shark liver oil, is derived from the reductive coupling of two molecules of farnesyl pyrophosphate. Triterpenes constitute a significant portion of the lipid substances of all plants; more than 4000 triterpenoids have been isolated. These compounds are precursors to steroids in both plants and animals. Both triterpenes and steroids occur free, as glycosides or in other combined forms. The structures of triterpenes and steroids may be subdivided into about 40 major types [1]. β-Boswellic acids (ursane-type triterpene) and α-boswellic acids (oleanane-type triterpene) that are isolated from the oleo-gum-resin of Boswellia carterii are known for their anti-inflammatory and anti-rheumatic activities [38].

Figure 13.

Example of triterpene.

One group of compounds showing a range of interesting biological activity is the quassinoids isolated from Quassia amara. These are degradation and rearrangement products of triterpenes. Quassia is used as a bitter tonic, as an insecticide and as an enema for the expulsion of thread worms.

Terpenes also include sesquarterpenes (seven isoprene units, C35H56), tetraterpenes (eight isoprene units, C40H64) as well as polyterpenes and norisoprenoids (long chains of many isoprene units.

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6. Lipids

Lipids comprise a group of naturally occurring molecules that include fixed oils, waxes, essential oils, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), phospholipids and others. Lipids serve various biological actions as major structural components of all biological membranes and as energy reservoirs and fuel for cellular activities in addition to being vitamins and hormones [39, 40]. Although lipids are considered primary plant metabolites, recent studies revealed pharmacological activities to members of this class of phytochemicals.

6.1. Fixed oils

Fixed oils constitute of high molecular aliphatic long-chain fatty acids, such as palmitic, stearic and oleic acids, esterified with glycerol. Fixed oils contain a relatively higher percentage of liquid glycerides (polyunsaturated) such as glycerin oleate, while fats are rich in solid glycerides such as glycerin stearate. [39]. Flax and linseed and its oil are obtained from Linum usitatissimum, family Linaceae. Polyunsaturated fatty acids in some fixed oils cause reduced excretion of lipid peroxidation products and hence are potent antioxidants and anti-inflammatory. They are used as prophylactic to decrease the risk of atherosclerosis and cardiovascular disease [41].

6.2. Waxes

Waxes are lipoidal matter constituting mainly from long aliphatic chains that may contain one or more functional groups. They may contain hydroxyl groups as in the case of primary and secondary long-chain alcohols that are frequently present in the form of esters. Others contain unsaturated bonds, aromatic systems, amide, ketonic, aldehydic or carboxylic functional groups. On the other hand, synthetic waxes constitute of long-chain hydrocarbons (alkanes or paraffins) that lack functional groups. They are similar to the fixed oils and fats since they are esters of fatty acids, but with the difference that the alcohol is not glycerin. The seeds of Simmondsia chinensis yield the liquid wax, jojoba wax, which consists of straight chain esters of fatty acids and alcohols [42]. Jojoba wax has anti-inflammatory, anti-aging and wound healing activities, and hence it can be utilized in several skin conditions. Jojoba wax has also been used in topical medications to enhance drug absorption. In addition, it is used in skin care products and in cosmetics such as sunscreens and moisturizers [43].

6.3. Essential oils

Essential oils are volatile aromatic complex mixtures of relatively low molecular weight compounds. Although they may contain up to 60 components, yet they are characterized by the presence of two or three major components at fairly high concentrations (20–70%) compared to other components present in trace amounts. For example, Origanum compactum essential oil contains carvacrol (30%) and thymol (27%) as major components. Linalol is the major component of Coriandrum sativum essential oil reaching up to 68%. Other examples are Artemisia herba-alba essential oil which contains α- and β-thuyone (57%) and camphor (24%) as major constituent, Cinnamomum camphora essential oil with 1,8-cineole (50%) as major constituent and finally Mentha piperita essential oil with menthol (59%) and menthone (19%) being the major constituent. Generally, these major components determine the biological properties of the essential oils [44]. They have many and important medical uses such as antiseptic, antimicrobial, analgesic, sedative, anti-inflammatory, spasmolytic and locally anesthesic remedies. They are also used as fragrances in embalmment and in food preservation [45].

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7. Carbohydrates

Carbohydrates are universally present in living beings on our planet. As the first product of photosynthesis, carbohydrates are the starting point for all phytochemicals and also, by extension, for all animal biochemicals. More carbohydrates occur in nature than any other type of natural compound. The most abundant single organic substance on Earth is cellulose, a polymer of glucose, which is the main structural material of plants. Although carbohydrates are primary metabolites, they are incorporated in plenty of secondary metabolites through glycosidation linkages. Polymers of simple sugars and uronic acids produce mucilages and gums [46].

Carbohydrates consist of carbon, hydrogen and oxygen with the last two elements usually present in the same proportions as in water. They are classified into four chemical groups: monosaccharides, disaccharides, oligosaccharides and polysaccharides. Monosaccharides contain from three to nine carbon atoms, although those with five and six carbon atoms (pentoses, C5H10O5, and hexoses, C6H12O6) are accumulated in plants in greatest quantity. Condensation of monosaccharides results in the other types according to the number of saccharide units involved. In addition to the important biological and structural function of carbohydrates in plants, some members show medicinal effects such as mucilage. Mucilage, viscous sticky material produced by almost all plants and some microorganisms, plays a protective role in thickening membranes in plants. It also serves in storage of water and food and in seed germination. Chemically it constitutes of a polar glycoprotein and an exopolysaccharide. Mucilage is used medicinally as demulcent. Cactus (and other succulents) and Linum usitatissimum (flax seeds) are the major sources of mucilage. The extract of the mucilaginous root of the marshmallow plant (Althaea officinalis); used traditionally to make marshmallows, were used as cough suppressant due to its demulcent effect. Ulmus rubra (the slippery elm) inner bark, is also used as a demulcent due to its mucilaginous content. Mucilage acts primarily as a local demulcent or emollient when it comes in direct contact with mucous membrane surfaces or skin. Here, they produce a coating of “slime” that soothes and protects exposed or irritated surfaces of the gastrointestinal tract. They are used extensively in the management of inflammatory digestive disorders, especially when there is ulceration. Their relative indigestibility and hydrophilic properties have important influences on bowel behavior [47].

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8. Conclusion

According to the abovementioned data, there are several classes of secondary plant metabolites that are responsible for the biological activities of herbal medicines. Eventually, secondary plant metabolites exert their action on molecular targets that differ from one case to the other. These targets may be enzymes, mediators, transcription factors or even nucleic acids. The use of herbal medicines should be based on comprehensive phytochemical studies for the determination of the chemical constituents of the herbs involved. Hence the knowledge of the resultant pharmacological and toxicological effects can be deduced, as well as the possible synergistic or antagonistic effects due to the use of multiple component herbal formulae. For this reason, the isolation and structural elucidation of secondary plant metabolites, though ancient, is still a huge and fast growing approach, and the techniques used for separation and analysis are advancing continuously.

References

1. 1.Seigler DS. Plant Secondary Metabolism. New York: Springer Science: Business Media; 1995. ISBN: 978-1-4613-7228-8; ISBN: 978-1-4615-4913-0 (eBook). DOI: 10.1007/978-1-4615-4913-0
2. 2.Jones ME, Kossel A. A biographical sketch. Yale Journal of Biology and Medicine. 1953;26(1):80-97
3. 3.Bourgaud F, Gravot A, Milesi S, Gontier E. Production of plant secondary metabolites: A historical perspective. Plant Science. 2001;161:839-851
4. 4.Bennets HW, Underwood EJ, Shier FL. A specific breeding problem of sheep in subterranean clover pastures in Western Australia. Australian Veterinary Journal. 1946;22(1):2-12
5. 5.Goławska S, Sprawka I, Łukasik I, Goławski A. Are naringenin and quercetin useful chemicals in pest-management strategies? Journal of Pest Science. 2014;87(1):173-180
6. 6.Harborne JB, Baxter H. Phytochemical Dictionary: A Handbook of Bioactive Compounds from Plants. London; Washington, DC: Taylor & Francis; 1993
7. 7.Evans WC. Pharmacognosy. 16th ed. Edinburgh London New York Philadelphia St Louis Sydney Toronto; 2009
8. 8.Hoffmann D. Medical Herbalism : The Science and Practice of Herbal Medicine. Healing Arts Press One Park Street, Rochester, Vermont; 2003. ISBN: 978-089281749-8
9. 9.Zbigniew S, Beata Ż, Kamil J, Roman F, Barbara K, Andrzej D. Antimicrobial and antiradical activity of extracts obtained from leaves of three species of the genus Pyrus. Microbial Drug Resistance. 2014;20(4):337-343. DOI: 10.1089/mdr.2013.0155
10. 10.Amann R, Peskar BA. Anti-inflammatory effects of aspirin and sodium salicylate. European Journal of Pharmacology. 2002;447(1):1-9
11. 11.Pelczar MJ, Chan ECS, Krieg NR. Control of microorganisms, the control of microorganisms by physical agents. In: Microbiology. New York: McGraw-Hill International; 1988. pp. 469-509
12. 12.Eric Y. Botanical medicines for the urinary tract. World Journal of Urology. 2002;20(5):285-293

Metabolites

. 2023 Jul 28;13(8):895. doi: 10.3390/metabo13080895

From Nature to Lab: A Review of Secondary Metabolite Biosynthetic Pathways, Environmental Influences, and In Vitro Approaches

Zubair Altaf Reshi 1, Waquar Ahmad 1, Alexander S Lukatkin 2,*, Saad Bin Javed 1,*

Editor: Wolfgang Eisenreich

• Author information
• Article notes
• Copyright and License information

PMCID: PMC10456650  PMID: 37623839

Abstract

Secondary metabolites are gaining an increasing importance in various industries, such as pharmaceuticals, dyes, and food, as is the need for reliable and efficient methods of procuring these compounds. To develop sustainable and cost-effective approaches, a comprehensive understanding of the biosynthetic pathways and the factors influencing secondary metabolite production is essential. These compounds are a unique type of natural product which recognizes the oxidative damage caused by stresses, thereby activating the defence mechanism in plants. Various methods have been developed to enhance the production of secondary metabolites in plants. The elicitor-induced in vitro culture technique is considered an efficient tool for studying and improving the production of secondary metabolites in plants. In the present review, we have documented various biosynthetic pathways and the role of secondary metabolites under diverse environmental stresses. Furthermore, a practical strategy for obtaining consistent and abundant secondary metabolite production via various elicitation agents used in culturing techniques is also mentioned. By elucidating the intricate interplay of regulatory factors, this review paves the way for future advancements in sustainable and efficient production methods for high-value secondary metabolites.

이차 대사물질은

제약, 염료, 식품 등 다양한 산업에서 점점 더 중요한 역할을 하고 있으며,

이러한 화합물을 안정적이고 효율적으로 확보하는 방법에 대한 필요성도 증가하고 있다.

지속 가능하고 비용 효과적인 접근 방식을 개발하기 위해서는

생합성 경로와 이차 대사물질 생산에 영향을 미치는 요인에 대한

포괄적인 이해가 필수적이다.

이 화합물들은

식물이 스트레스로 인한 산화 손상을 인식하여

식물 내 방어 기전을 활성화하는 독특한 천연물이다.

식물에서

이차 대사물질 생산을 향상시키기 위한 다양한 방법이 개발되었다.

elicitor-induced in vitro 배양 기술은

식물 내 이차 대사물질의 연구 및 생산 향상을 위한 효율적인 도구로 여겨진다.

본 리뷰에서는

다양한 생합성 경로와 다양한 환경 스트레스 하에서 이차 대사물질의 역할을 정리하였다.

또한,

배양 기술에서 사용되는 다양한 elicitation agent를 통해

일관되고 풍부한 이차 대사물질 생산을 얻기 위한 실용적인 전략도 언급하였다.

조절 인자들의 복잡한 상호작용을 밝힘으로써,

본 리뷰는 고부가가치 이차 대사물질의 지속 가능하고 효율적인 생산 방법에 대한 미래 발전의 길을 열어준다.

Keywords: plant tissue culture, environmental stress, defence action, industrial use, sustainable production

1. Introduction

Plant metabolites, both primary and secondary, play crucial roles in the growth and survival of plant species [1]. Primary metabolites, such as lipids, proteins, carbohydrates, amino acids, and vitamins, directly contribute to essential cellular processes like cell division, respiration, and photosynthesis, crucial for plant growth and development [2]. In contrast, secondary metabolites have multifunctional roles, primarily involved in defence and interactions with the environment [3]. Additionally, they contribute to plant colour, specific fragrances, flavours, and responses to various stresses. The concept of plant secondary metabolites in plant biology was introduced by Kossel et al. [4]. Secondary metabolites are highly reactive, and their accumulation is influenced by both biotic and abiotic stress conditions, which can have detrimental effects on physiological and morphological characteristics like leaf number, leaf area, plant height, and productivity [1].

Secondary metabolites play a crucial role in helping plants cope with different stress conditions. The response is initiated through the activation of plant defence mechanisms, triggered by the recognition of foreign agents through sensors and receptors in plants [5]. Moreover, plant survival and productivity rely on the expression‎ of defensive transcriptional factors [3]. Detection of threat signals and increased production of secondary metabolites through elicitation contribute to the downstream expression‎ of transcriptional factors [3]. For example, in response to abiotic stress, the expression‎ of β-lycopene cyclase in Bixa orellana and phytoene synthase in Daucus carota is elevated, leading to the accumulation of carotenoids [5,6]. The production of secondary metabolites is highly dependent on the developmental stages and physiological conditions of the plant. In various interactions, such as mutualism observed in root nodules of legumes, or antagonistic relationships, such as pathogenicity and herbivory, secondary metabolites play a pivotal role by exerting irreversible effects [7].

Teoh [8] classified plant secondary metabolites into various groups based on functional groups and chemical structure. These groups include terpenes (including volatile compounds, sterols, and carotenoids), polysaccharides, phenolic compounds, phytoalexins (sulfur-containing compounds), alkaloids (nitrogen-containing compounds), flavonoids, and hydrocarbons. Almost all of these metabolites contribute significantly to defence against stressful situations. Plant hormones, such as abscisic acid (ABA), jasmonates (JA), polyamines, and salicylic acid (SA), are also involved in responding to environmental stresses, and their accumulation often results from various biotic and abiotic stresses and the response to elicitors and other signalling molecules [9].

The low-molecular-weight secondary metabolites have garnered significant interest among researchers due to their dramatic implications for pharmaceutical, nutritional, and industrial purposes. Recent advancements in the research of secondary metabolites have focused on finding a reliable source for production and extractions of important secondary metabolites for industrial use [10].

In vitro culture-based elicitation mechanisms are considered advantageous for the production of secondary metabolites as they offer independence from environmental conditions and reduced the risk of microbial contamination. In vitro micropropagation techniques are efficient for mass-producing secondary metabolites applicable to various industrial and pharmaceutical companies. Other natural products derived from plants, including steroids, codeine, morphine, pilocarpine, digitoxin, and quinine, are used in various pharmaceutical products [8].

Environmental and developmental factors influence the synthesis and accumulation of secondary metabolites in plants greatly. Therefore, this review aims to provide a comprehensive summary of how various environmental conditions influence the synthesis and accumulation of secondary metabolites. The qualitative and quantitative aspects of the environment can serve as tools to improve the accumulation of secondary metabolites in plants by modifying their growing conditions as well as the use of in vitro culture techniques for sustainable production and extraction of these secondary metabolites for industrial use.

1. 서론

식물 대사물질(일차 및 이차)은

식물 종의 성장과 생존에 중요한 역할을 한다 [1].

지질, 단백질, 탄수화물, 아미노산, 비타민과 같은 일차 대사물질은

세포 분열, 호흡, 광합성과 같은 필수 세포 과정에 직접 기여하여

식물의 성장과 발달에 핵심적이다 [2].

반면

이차 대사물질은

주로 방어와 환경과의 상호작용에 관여하는 다기능적 역할을 한다 [3].

또한

식물의 색상, 특정 향기, 풍미,

그리고 다양한 스트레스에 대한 반응에 기여한다.

식물 생물학에서

식물 이차 대사물질의 개념은 Kossel et al.에 의해 소개되었다 [4].

이차 대사물질은

반응성이 높으며, 생물적·비생물적 스트레스 조건에 의해 축적이 영향을 받는데,

이는 잎 수, 잎 면적, 식물 높이, 생산성 등의 생리적·형태적 특성에 해로운 영향을 줄 수 있다 [1].

이차 대사물질은

식물이 다양한 스트레스 조건에 대처하는 데 중요한 역할을 한다.

반응은

식물의 센서와 수용체를 통해 외부 물질을 인식함으로써

식물 방어 기전의 활성화를 통해 시작된다 [5].

또한 식물의 생존과 생산성은

방어 전사 인자의 발현에 의존한다 [3].

위협 신호의 탐지와 elicitation을 통한

이차 대사물질의 증가된 생산은

하류 전사 인자의 발현에 기여한다 [3].

예를 들어,

비생물적 스트레스에 대한 반응으로

Bixa orellana의 β-lycopene cyclase와 Daucus carota의 phytoene synthase 발현이 증가하여

카로티노이드의 축적이 일어난다 [5,6].

이차 대사물질의 생산은

식물의 발달 단계와 생리적 조건에 크게 의존한다.

콩과 식물 뿌리혹에서 관찰되는 상리공생(mutualism)과 같은 상호작용,

또는 병원성(pathogenicity)과 초식(herbivory)과 같은 길항 관계에서

이차 대사물질은 돌이킬 수 없는 효과를 발휘함으로써 핵심적인 역할을 한다 [7].

Teoh [8]은

작용기와 화학 구조에 따라 식물 이차 대사물질을 다양한 군으로 분류하였다.

이 군에는

테르펜(휘발성 화합물, 스테롤, 카로티노이드 포함),

다당류,

페놀 화합물,

피토알렉신(황 함유 화합물),

알칼로이드(질소 함유 화합물),

플라보노이드,

탄화수소 등이 포함된다.

이들 거의 모두가 스트레스 상황에 대한 방어에 크게 기여한다. 아브시스산(ABA), 자스몬산염(JA), 폴리아민, 살리실산(SA)과 같은 식물 호르몬도 환경 스트레스에 반응하는 데 관여하며, 이들의 축적은 다양한 생물적·비생물적 스트레스와 elicitor 및 기타 신호 분자에 대한 반응의 결과인 경우가 많다 [9].

저분자량 이차 대사물질은 제약, 영양, 산업 목적으로의 극적인 함의 때문에 연구자들 사이에서 큰 관심을 받고 있다. 최근 이차 대사물질 연구의 발전은 산업적으로 중요한 이차 대사물질의 안정적인 공급원과 추출 방법을 찾는 데 초점을 맞추고 있다 [10].

In vitro 배양 기반 elicitation 기전은 환경 조건으로부터의 독립성과 미생물 오염 위험 감소로 인해 이차 대사물질 생산에 유리한 것으로 여겨진다. In vitro 미세증식 기술은 다양한 산업 및 제약 회사에 적용할 수 있는 이차 대사물질을 대량 생산하는 데 효율적이다. 스테로이드, 코데인, 모르핀, 필로카르핀, 디기톡신, 퀴닌 등 식물 유래의 다른 천연물들도 다양한 제약 제품에 사용된다 [8].

환경 및 발달 요인은 식물에서 이차 대사물질의 합성과 축적에 큰 영향을 미친다. 따라서 본 리뷰는 다양한 환경 조건이 이차 대사물질의 합성과 축적에 어떻게 영향을 미치는지에 대한 포괄적인 요약을 제공하는 것을 목적으로 한다. 환경의 질적·양적 측면은 식물의 재배 조건을 수정하고, 지속 가능한 생산 및 추출을 위한 in vitro 배양 기술을 활용함으로써 식물 내 이차 대사물질 축적을 향상시키는 도구로 작용할 수 있다.

2. Biosynthesis of Secondary Metabolites in Plants

Secondary metabolites in plants are categorized into distinct chemical groups based on their biosynthetic pathways: phenolic compounds, terpenes and steroids, and nitrogenous compounds. These chemical structures determine the function and stress adaptation of secondary metabolites. Various stresses such as drought, pathogenesis, herbicides, salinity, and heavy metals promote the accumulation of secondary metabolites [11].

Plants develop numerous adaptive strategies to overcome harsh conditions by upregulating the synthesis and accumulation of secondary metabolites. The production levels of these metabolites are greatly influenced by factors such as growing temperature and environmental constraints [12]. Under suitable conditions, more than 100,000 secondary metabolites are synthesized through various metabolic pathways. The interrelation between the synthesis of primary and secondary metabolites is fundamental for most plants.

Primary and secondary metabolites are distinct in their distribution, chemical structure, and functional roles in plants. Figure 1 illustrates the production of secondary metabolites and their interconnections with primary metabolism within the plant cell. The alternative mechanisms involved in the biosynthesis of secondary metabolites lead to common products, such as phenols, flavonoids, and terpenes (Figure 1). However, the critical precursors for secondary metabolites are primary metabolites. The shikimic acid pathway and Krebs cycle produce essential precursors required for the production of phenolic metabolites [13]. The aromatic molecules synthesized by the shikimic acid pathway play featured roles in electron transport, antioxidants, wound response, structural agents, and defence systems [14]. The aromatic amino acids L-tryptophan (L-Trp), L-tyrosine (L-Tyr), and L-phenylalanine (L-Phe) serve as precursors for secondary metabolite synthesis produced by the shikimate pathway [15]. Chorismate is the final product in the seven-step pathway of shikimic acid, and it serves as the starting material for the biosynthesis of secondary metabolites. Chorismate mutase, aminodeoxychorismate synthase, and isochorismate synthase play regulatory roles in higher plants for chorismate, which is also the precursor of folate, phenylalanine, phylloquinone, and tryptophan [16]. In fungi, the AROM complex undergoes catalysis by enzymes, such as 3-dehydroquinate dehydratase (DHD) and shikimate dehydrogenase (SDH), which facilitate the third and fourth reactions within the shikimate pathway. In plants, SDH and DHD act bifunctionally, while they have single functions in Escherichia coli.

2. 식물에서 이차 대사물질의 생합성

식물의 이차 대사물질은

생합성 경로에 따라

페놀 화합물, 테르펜 및 스테로이드, 질소 함유 화합물과 같은 Distinct한 화학 군으로 분류된다.

이러한 화학 구조가

이차 대사물질의 기능과 스트레스 적응을 결정한다.

가뭄, 병원균 감염, 제초제, 염분, 중금속과 같은 다양한 스트레스가 이차 대사물질의 축적을 촉진한다 [11].

식물은 가혹한 조건을 극복하기 위해 이차 대사물질의 합성과 축적을 상향 조절하는 수많은 적응 전략을 발달시켰다. 이러한 대사물질의 생산 수준은 재배 온도와 환경 제약과 같은 요인에 크게 영향을 받는다 [12]. 적합한 조건에서 100,000개 이상의 이차 대사물질이 다양한 대사 경로를 통해 합성된다. 일차 대사물질과 이차 대사물질의 합성 사이의 상호관계는 대부분의 식물에게 근본적이다.

일차 및 이차 대사물질은 식물 내 분포, 화학 구조, 기능적 역할에서 구별된다. 

Figure 1은 식물 세포 내에서 이차 대사물질의 생산과 일차 대사와의 상호연결성을 보여준다. 이차 대사물질 생합성에 관여하는 대체 기전들은 페놀, 플라보노이드, 테르펜과 같은 공통 생성물을 유도한다(Figure 1). 그러나 이차 대사물질의 핵심 전구체는 일차 대사물질이다. 시킴산 경로와 크렙스 회로는 페놀 대사물질 생산에 필요한 필수 전구체를 생성한다 [13]. 시킴산 경로에 의해 합성된 방향족 분자들은 전자 전달, 항산화제, 상처 반응, 구조 물질, 방어 시스템에서 중요한 역할을 한다 [14]. 방향족 아미노산 L-트립토판(L-Trp), L-티로신(L-Tyr), L-페닐알라닌(L-Phe)은 시킴산 경로에 의해 생산되어 이차 대사물질 합성의 전구체로 작용한다 [15]. 코리스메이트는 시킴산 7단계 경로의 최종 산물이며, 이차 대사물질 생합성의 출발 물질로 작용한다. 코리스메이트 뮤타아제, 아미노데옥시코리스메이트 합성효소, 이소코리스메이트 합성효소는 고등 식물에서 코리스메이트에 대한 조절 역할을 하며, 이는 또한 엽산, 페닐알라닌, 필로퀴논, 트립토판의 전구체이기도 하다 [16]. 곰팡이에서는 AROM 복합체가 3-데하이드로퀴네이트 데하이드라타아제(DHD)와 시킴산 데하이드로게나아제(SDH)와 같은 효소에 의해 촉매되어 시킴산 경로 내 3번째와 4번째 반응을 촉진한다. 식물에서는 SDH와 DHD가 이중 기능을 하는 반면, Escherichia coli에서는 단일 기능을 한다.

Figure 1.

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Schematic illustration of biosynthetic pathways for secondary metabolite production. The figure demonstrates the intricate biosynthesis of secondary metabolites and their interconnections with primary metabolism within plants. Plant cells employ diverse mechanisms through major pathways including meval‎onic acid (MVA) and the 2-C-methylerythritol 4-phosphate (MEP) and shikmate pathway to synthesize terpenes, phenols, flavonoids, and alkaloids. Abbreviations: phosphoenolpyruvate (PEP); 1-deoxy-d-xylulose 5-phosphate (DXP); 4-hydroxy-3-methylbut-2-enyl diphosphate (HMBPP); isopentenyl pyrophosphate (IPP); dimethylallyl pyrophosphate (DMAPP); geranyl pyrophosphate (GPP); 2-C-methyl-l-erythritol 4-phosphate (MEP); 1-deoxy-D-xylulose 5-phosphate (DXP); acetyl coenzyme A (Acetyl-CoA); β-hydroxy β-methylglutaryl-CoA (HMG-CoA); geranylgeranyl diphosphate (GGPP); erythrose 4-phosphate (E4P); and pentose phosphate pathway (PPP).

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