Secondary Plant Metabolites (Active Constituents)

The secondary metabolites act as a protective agents. They constitute as the waste product of metabolism (detoxification). They are biosynthesized to aid the producers' survival. Thus they are energy producer and physiologically active. Secondary metabolites are divided into several classes of various organic compounds.

I- Carbohydrates and related compounds Compounds composed of carbon, hydrogen and oxygen as polyhydroxy aldehydes or ketone alcohols: sucrose, lactose, starch, gums, mucilage and pectin. 1- Gums: They are translucent, amorphous substances that are frequently produced in higher plants as a protective after injury. Gums are heterogeneous in composition. Upon hydrolysis arabinose, galactose, glucose, mannose and various uronic acids are produced. The uronic acids may form salts with calcium, magnesium and other cations. Examples of gums are gum Acacia and gum Tragacanth.

2- Mucilages : 1- They are polysaccharide complexes formed from sugar and uronic acid units. 2- They are insoluble in alcohol but forming a viscous non-adhesive colloidal solution with water. 3- They are stained red with Ruthenium Red reagent (pectose type present in Senna, buchu), and with solution of Corallin soda (callose type found in Squill). Others are stained by methylene blue (neutral type of Fenugreek).

3- Pectin: It is formed of partially methoxylated polygalactouronic acids. Pectin is a coarse or fine powder, yellowish white in colour, almost odourless and has a mucilaginous taste. Soluble in 20 parts of water and the solution is viscous, colloidal and acidic to litmus. One part of pectin heated in nine parts of water forms a stiff gel. Pectin is used as a suspending agent and is an ingredient in many antidiarrheal formulations.

II- Glycosides Definition: Glycosides are non-reducing organic compounds that yield on hydrolysis with acids, alkalis or enzymes: 1- A sugar (or glycone, formed of one or more sugar units). 2- A non-sugar moiety (also called aglycone or genin). Glycosides are widely distributed in nature. They usually accompanied with enzymes used in either their synthesis or hydrolysis.

Glycosides are considered as acetals or sugar ethers. Glycosides are generally soluble in water and hydroalcoholic solvents due to the hydrophilic nature of the sugar. The higher the sugar content the less soluble in organic solvent. The aglycones are soluble in organic solvents.

Classification of glycosides A- Phenolic glycosides: They produce phenolic aglycone when subjected to hydrolytic processes. The phenolic aglycone may be: 1- Simple phenolic glycosides: e.g. Arbutin present in Uva ursi leaf

2- Flavonoid glycosides: They occur in plant in both free aglycone and /or as glycosides. This group is regarded as C6-C3-C6 compounds in which each C6 moiety is a benzene ring e.g. Diosmin glycoside of Buchu and hespiridin glycoside of Citrus species. Chemical test: flavonoides + KOH → canary yellow colour

3- Anthraquinone glycosides: They are pharmacologically active constituents of several laxatives and purgatives of plant origin e.g. Senna, Aloe, Cascara, Frangula, Rhubarb.

1- Borntrager's test for anthraquinone glycosides: Boil the powder with dil. HCl, filter, cool, shake with organic solvent, separate organic layer, shake with NH4OH, the aqueous layer becomes rose pink or cherry red. 2- Modified Borntrager's test: in case of dianthrone e.g. sennosoides: a- Boil the powder with alc. KOH, filter. b- Add dil. HCl to the filtrate and extract with ether. c- Oxidize with H2O2. d- Add NH4OH to the ethereal extract and shake, a rose red colour is produced in the aqueous layer.

4- Anthocyanin glycosides: they are structurally related to flavonoids and coloured. They constitute the pigments of the coloured petals of the flowers. Their colours are affected by the pH of the cell sap and ranges from red in acidic medium to blue in alkaline medium. e.g. Red Rose petals, karkadeh and corn silk.

B- Cyanophore glycosides (Cyanogenic) They yield HCN as one of the products of hydrolysis. Examples are; amygdalin in bitter almond, prunasin in wild cherry and linamarin in linseed. Chemical test: by Guignard test: turns the yellow colour of sodium picrate paper into orange.

C- Thioglycosides (thiocyanate or sulphated glycosides) S-glycosides, S atom is present conjugated to glucose and a second S atom present in a sulfonated oxime grouping. Singrin from black mustard seeds giving allyl isothiocyanate (mustard irritant gas) upon hydrolysis.

D- Steroidal cardioactive glycosides These drugs are used in medicine as mainly to increase the tone and contractility of the cardiac muscle. Also as diuretic due to the increased renal circulation. Thet are present in leaves of Digitalis, seeds of Strophanthus and the bulb of Squill.

Structure activity relationship (SAR) Steroidal nucleus with alcoholic OH group at C3 at which sugar is attached. Attachment of unsaturated lactone ring at C17. the lactone ring is present either as a 5- membered ring (Cardinolides) e.g. lannatosides in Digitalis or as 6- membered ring (bufadienolides) e.g. Scillarin in Squill. Deoxy sugar, e.g. digitoxose.

Detection of cardiac glycosides Several chemical tests could be used for identification of cardiac glycosides Colour reactions due to aglycone moiety: A- Reactions due to (-CH2-) group of the lactone ring: These are characteristic for the 5-mambered lactone ring of cardenolides: 1- Kedde's test: card. + Kedde's reagent (3,5-dinitrobenzoic acid + NaOH → violet colour. 2- Baljet's test: card. +Baljet's reagent (picric acid +NaOH) → orange or red.

B- Reactions due to the steroidal nucleus: Liebermann's test: card. in glacial acetic acid + H2SO4 → red, violet, blue to green. Reactions due to the presence of 2-deoxy sugar unit in the sugar moiety: Keller-Killiani's test: card. in glacial acetic acid containing traces of FeCl3, conc. H2SO4 on the wall of the test tube → blue ring is formed in between the two layers.

E- Saponins e.g. saponins of Quillaia bark This group of glycosides is characterized by the following: it foams in aqueous solution forming froth. It causes haemolysis of red blood cells. Drugs containing saponins are usually sternutatory and irritating the mucous membrane of the eyes and the nose. They are highly toxic when injected into blood stream. They are used in cleaning industrial equipments and fabrics, also as a powerful emulsifier of certain resins, fats and fixed oils.

III- Tannins They are non-crystalline compounds that with water form colloidal solutions possessing an acid reaction and a sharp astringent taste. They precipitate gelatin and alkaloids. They precipitate protein and can combine with protein of animal hide (Gold beater's test), rendering them resistant to proteolytic enzymes and so prevent their putrefaction.

Uses of drugs containing tannins: Astringent in GIT and skin (haemostatic). In treatment of burns. Tanning of leather. Antidote of alkaloidal and heavy metals poisoning. Preparation of paints and ink. Types of true tannins: True tannins: have molecular weight 1000-5000. there are two main types of true tannins. Pseudotannins: compounds of lower molecular weight than true tannins, they don't respond to the gold beater's skin test.

Hydrolysable (Pyrogallol) Condensed (Catechol) Hydrolysis Hydrolysable Non-Hydrolysable Dry distillation Gives pyrogallol Gives catechol FeCl3 Blue-black colour Green colour Br2 water No ppt. Ppt Examples Gall, Clove, Rhubarb Cinnamon, Tea, Henna.

IV- Alkaloids Alkaloids, which mean alkali-like, are basic nitrogenous compounds of biological origin, they are physiologically active. They occur in the form of salts with organic acids, or in combination with specific acids e.g. opium alkaloids occur with meconic acid . generally alkaloidal bases are insoluble in water but soluble in organic solvents. While alkaloidal salts are soluble in water sparingly soluble in organic solvents.

Their physiological actions vary widely being Analgesic and Narcotic e Their physiological actions vary widely being Analgesic and Narcotic e.g. morphine and codeine Central stimulant e.g. caffeine and strychnine Mydriatic e.g. atropine Myotic e.g. pilocarpine Antiasthmatic e.g. ephedrine Antihypertensive e.g. reserpine smooth muscle relaxant e.g. atropine and papaverine skeletal muscle relaxants e.g. d-tubocurarine.

Detection of alkaloids Alkaloidal precipitants: Most alkaloids are precipitated from neutral or acidic solution by a number of reagents which contain certain heavy metals e.g. Mayer's reagent (potassium mercuric iodide solution) gives creamy white ppt. with most alkaloids except caffeine. Alkaloidal colour reagents: The most common Dragendorff's reagent gives orange colour.

V- Volatile oils Volatile or essential oils are volatile in steam. They are secreted in oil cells, in secretion canals or cavities or in glandular hairs. They are generally mixtures of hydrocarbons and oxygenated compounds derived from these hydrocarbons. These oxygenated constituents mainly determine the odour and taste of volatile oils.

Volatile oils are used for flavoring e.g. oil of Lemon, in perfumery e.g. oil of Rose as spices e.g. Pepper, Clove , Cardamom and for their therapeutic action as antiseptic e.g. Thyme, antispasmodic e.g. Melissa, Menthe, and carminative e.g. Chamomile.

VI- Resins, Gum-resins and similar substances Resins are more or less solid, amorphous substances of complex chemical nature, on heating they soften and finally melt. They are insoluble in water but dissolve in alcohol, chloroform and ether. Resins are associated with volatile oil, so called oleoresin. With gum and called gum resin. With oil and gum and called oleo-gum resin. Resins give brown colour with iodine reagent. VII- Others: Enzymes, vitamins, hormones, antibiotics, natural insecticides, etc.........