Unit 2: Secondary Metabolites

Semester 5

BP503T

Secondary Metabolites – Detailed Study

This is the most content-heavy unit of Pharmacognosy 2 — a systematic, monograph-style study of all major classes of secondary metabolites. For each crude drug, you must know the biological source, family, part used, chemical constituents, chemical tests, therapeutic uses, and commercial applications. The drugs cover Alkaloids (the most important pharmacologically active class), Phenylpropanoids & Flavonoids, Steroids & Cardiac Glycosides, Volatile Oils, Tannins, Resins, Glycosides, and Iridoids.

Syllabus & Topics

1Alkaloids – Vinca (Catharanthus roseus, Apocynaceae): Contains Vincristine (Oncovin) and Vinblastine (Velban) – powerful antineoplastic alkaloids that bind tubulin and arrest mitosis. Used in leukemia, Hodgkin’s lymphoma. Extremely low yield (500 kg dry leaves → 1 g vincristine).
2Alkaloids – Rauwolfia (Rauwolfia serpentina, Apocynaceae): Contains Reserpine – depletes catecholamines from sympathetic nerve terminals → antihypertensive + tranquilizer. Also contains Ajmaline (antiarrhythmic), Ajmalicine. ‘Sarpagandha’ in Ayurveda for insanity and snake bite.
3Alkaloids – Belladonna (Atropa belladonna, Solanaceae): Contains Atropine (racemic hyoscyamine) and Hyoscine (scopolamine). Tropane alkaloids – competitive muscarinic antagonists. Uses: mydriatic (eye dilation), antispasmodic, pre-anesthetic medication. All parts poisonous. ‘Beautiful lady’ – Italian women used drops to dilate pupils.
4Alkaloids – Opium (Papaver somniferum, Papaveraceae): Air-dried latex from unripe capsules. Contains 25+ alkaloids: Morphine (10-15%, narcotic analgesic → gold standard for severe pain), Codeine (antitussive), Papaverine (smooth muscle relaxant), Thebaine (convulsant, precursor for semi-synthetic opioids like Oxycodone). Schedule II controlled substance.
5Phenylpropanoids & Flavonoids – Lignans: Dimeric phenylpropanoids. Podophyllotoxin (from Podophyllum peltatum) → semi-synthetic Etoposide and Teniposide (anticancer). Sesame lignans (Sesamin – antioxidant).
6Phenylpropanoids & Flavonoids – Tea (Camellia sinensis, Theaceae): Contains Catechins (Epigallocatechin gallate/EGCG – powerful antioxidant, anticancer), Caffeine (CNS stimulant), Theanine (relaxant amino acid). Green tea (unfermented) has highest catechin content.
7Phenylpropanoids & Flavonoids – Ruta (Ruta graveolens, Rutaceae): Contains Rutin (Vitamin P – capillary-strengthening flavonoid glycoside → reduces capillary fragility and permeability). Hydrolysis yields Quercetin (aglycone) + Rutinose (sugar). Used in venous insufficiency.
8Steroids & Cardiac Glycosides – Liquorice (Glycyrrhiza glabra, Leguminosae): Root contains Glycyrrhizin (triterpenoid saponin – 50x sweeter than sugar, expectorant, anti-inflammatory, anti-ulcer by ↑prostaglandin synthesis). ADR: Pseudoaldosteronism (Na⁺ retention, K⁺ loss, edema, hypertension) at high doses.
9Steroids & Cardiac Glycosides – Dioscorea (Dioscorea species, Dioscoreaceae): Tubers contain Diosgenin – steroidal sapogenin. Most important raw material for semi-synthesis of ALL steroidal drugs (corticosteroids, sex hormones, oral contraceptives). Marker degradation converts diosgenin → progesterone.
10Steroids & Cardiac Glycosides – Digitalis (Digitalis purpurea/lanata, Scrophulariaceae): Leaves contain Cardiac glycosides – Digitoxin (D. purpurea, most lipophilic, longest acting), Digoxin (D. lanata, most widely used). Inhibit Na⁺/K⁺-ATPase → positive inotropic effect. Narrow therapeutic index → digitalis toxicity. Biological standardization (cat/pigeon assay) essential.
11Volatile Oils – Mentha (Mentha piperita, Lamiaceae): Contains Menthol (major – cooling, carminative, counter-irritant), Menthone. Steam distilled from aerial parts. Used in cough drops, analgesic balms, toothpastes. Indian (Mentha arvensis) is world’s major menthol source.
12Volatile Oils – Clove (Syzygium aromaticum, Myrtaceae): Dried flower buds. Contains Eugenol (85-90% – powerful antiseptic, analgesic). Used as dental analgesic (clove oil for toothache), flavoring agent, and source of vanillin synthesis.
13Volatile Oils – Cinnamon (Cinnamomum zeylanicum, Lauraceae): Dried inner bark. Contains Cinnamaldehyde (65-80% – responsible for flavor/aroma), Eugenol. Carminative, anti-diabetic activity, widely used spice. Cassia (C. cassia) is the cheaper alternative.
14Volatile Oils – Fennel & Coriander: Fennel (Foeniculum vulgare) – Anethole (carminative, galactagogue). Coriander (Coriandrum sativum) – Linalool (carminative, flavoring).
15Tannins – Catechu & Pterocarpus: Catechu (Acacia catechu, Leguminosae) – heartwood extract contains Catechin and Epicatechin (condensed tannins). Astringent, used in pan masala. Pterocarpus (Pterocarpus marsupium) – heartwood contains Pterostilbene – anti-diabetic activity (Vijaysar in Ayurveda).
16Resins – Benzoin, Guggul, Ginger, Asafoetida, Myrrh, Colophony: Benzoin (Styrax benzoin – balsamic resin, expectorant, antiseptic). Guggul (Commiphora wightii – Guggulsterones lower cholesterol). Ginger (Zingiber officinale – Gingerols, Shogaols – antiemetic, carminative). Asafoetida (Ferula asafoetida – oleo-gum resin, antiflatulent). Myrrh (Commiphora molmol – antiseptic, astringent). Colophony (Pinus – residue after turpentine distillation, used in varnishes, plasters).
17Glycosides – Senna, Aloes, Bitter Almond: Senna (Cassia angustifolia – Sennosides A & B – anthraquinone laxatives, act on large intestine). Aloes (Aloe barbadensis – Aloin/Barbaloin – purgative; Aloe vera gel – wound healing, moisturizer). Bitter Almond (Prunus amygdalus var. amara – Amygdalin → HCN + Benzaldehyde on hydrolysis, flavoring agent).
18Iridoids & Other Terpenoids: Gentian (Gentiana lutea – Gentiopicroside, bitter stomachic). Artemisia (Artemisia annua – Artemisinin, sesquiterpene lactone endoperoxide, most important antimalarial drug today). Taxus (Taxus brevifolia – Taxol/Paclitaxel, diterpene, anti-cancer – stabilizes microtubules). Carotenoids (C₄₀ terpenoids – β-Carotene = Provitamin A, Lycopene – antioxidant).

Learning Objectives

Crude Drug Monographs: For each drug, state the biological source, family, part used, chemical constituents, chemical tests, and therapeutic uses.

Chemical Classification: Classify the crude drugs by their chemical nature (alkaloid, glycoside, terpenoid, tannin, resin) and biosynthetic origin.

Identify Key Constituents: Name the major active phytoconstituent of each crude drug and its pharmacological significance.

Commercial Importance: Explain the industrial/commercial applications of Diosgenin, Artemisinin, and Vincristine.

Chemical Tests: Perform and interpret the specific chemical identification tests for each class of secondary metabolite.

Exam Prep Questions

Q1. Why Is Dioscorea (Diosgenin) So Commercially Important?

Diosgenin obtained from Dioscorea is a steroidal sapogenin widely used as the starting material for the semi-synthesis of many steroidal drugs. Through the Marker degradation, diosgenin can be converted into Progesterone, which then serves as a precursor for corticosteroids such as Prednisolone and Dexamethasone, as well as sex hormones like Testosterone and Estradiol. Because it provides a convenient natural starting structure for steroid synthesis, diosgenin is one of the most commercially valuable phytochemicals in pharmaceutical manufacturing.

Q2. What Makes Artemisinin Unique Among Antimalarial Drugs?

Artemisinin is a sesquiterpene lactone isolated from Artemisia annua. Its structure contains a rare endoperoxide bridge (–O–O–), which is essential for its antimalarial action. Inside malaria parasites such as Plasmodium falciparum, the endoperoxide reacts with iron in heme to generate highly reactive free radicals that damage parasite proteins and membranes. Artemisinin acts rapidly and remains effective against chloroquine-resistant malaria. The discovery of this drug by Tu Youyou led to the Nobel Prize in Physiology or Medicine 2015.

Q3. What Is the Difference Between Condensed and Hydrolysable Tannins?

Hydrolysable tannins are esters of gallic or ellagic acid with glucose and can be easily hydrolyzed by acids, enzymes, or boiling water to yield sugars and phenolic acids. Condensed tannins, also known as proanthocyanidins, are polymers of flavonoid units such as Catechin and cannot be hydrolyzed. When heated with acids they condense further to produce reddish-brown compounds called phlobaphenes. Condensed tannins are generally more common in nature.

Q4. Why Is Vincristine So Expensive?

Vincristine is obtained from the plant Catharanthus roseus. The drug occurs in extremely low concentrations in the plant; approximately 500 kg of dried leaves are required to obtain only about 1 gram of pure vincristine. This very low yield, combined with complex extraction and purification procedures, contributes to its high cost. Despite this, vincristine is an essential chemotherapeutic agent used particularly in treating Acute Lymphoblastic Leukemia.

Q5. How Do Sennosides Work as a Laxative?

Sennoside A and Sennoside B are anthraquinone glycosides found in Senna. They are not absorbed in the small intestine and pass unchanged into the large intestine, where bacterial enzymes convert them into the active metabolite Rhein anthrone. This metabolite stimulates the Auerbach’s plexus of the colon, increasing intestinal peristalsis and reducing water and electrolyte reabsorption. The result is softer stool and a bowel movement typically within 6–12 hours.