The following points highlight the three groups of nitrogen containing secondary plant products. The products are: 1. Alkaloids 2. Cyanogenic Glycosides and Glucosinolates and 3. Non-Protein Amino Acids.
1. Alkaloids:
Alkaloids are an extremely heterogeneous group of so called secondary metabolites containing one or more nitrogen atoms, usually in a heterocyclic ring. However, all compounds with heterocylic ring and containing nitrogen are not alkaloids e.g., purines, thiamine etc.
Literary meaning of alkaloid is ‘alkali like’ and as their name implies, they are all basic in reaction.
Most of the alkaloids are colourless, crystalline, non-volatile solids but some of them such as coniine and nicotine are liquids at ordinary temperatures. Berberine is yellow in colour.
They are usually bitter in taste, insoluble in water (or slightly soluble) but soluble in most of the organic solvents.
Alkaloids are usually optically active being laevorotatory. Some of them like coniine are dextrorotatory, while a few such as papapverine are optically inactive.
Many of the alkaloids exhibit important pharmacological properties.
Distribution in Plants:
More than 3000 alkaloids have been isolated from plants. Alkaloidal plants are scattered almost in every group of plants, except probably the algae. They are especially common in families of angiosperms e.g., Magnoliaceae, Solanaceae, Papaveraceae, Leguminosae, Ranunculaceae, Rubiacae, Apocyanaceae etc. The alkaloidal plant species may contain one to a large number of alkaloids. For example, more than twenty different alkaloids have been isolated from opium poppy including morphine, codeine, thebaine etc.
Examples of some of the more commonly known alkaloids in plants are: morphine from opium poppy (Papaver somniferum), nicotine from tobacco (Nicotiana tabacum) quinine from cinchona (Cinchona officinalis or C. pubescens), atropine from nightshade (Atropa belladonna), colchicine from meadow saffron (Colchicum autumnale) strychnine and brucine from Strychnos nux vomica, cocaine from Coca (Erythroxylon Sp.) and cannabidiol from hemp (marijuana) i.e., Cannabis sativa.
The alkaloids are usually known to accumulate in:
(i) Young actively growing parts of plants,
(ii) Epidermal and hypodermal cells,
(iii) Bundle sheaths and
(iv) Latex vessels.
The alkaloids in a particular plant species are often confined to a certain organ such as root, leaves, bark etc. Often, the alkaloids are synthesised in a particular plant organ but accumulate in another. For example in tobacco, nicotine is synthesized in roots but is trans located to and stored in leaves.
Classification:
The alkaloids may be divided into 3 categories:
(a) Protoalkaloids,
(b) True alkaloids, and
(c) Pseudo alkaloids
Protoalkaloids and true alkaloids are directly derived from amino acids while pseudo alkaloids are not directly derived from amino acids e.g., terpenoid containing alkaloids.
(a) Protoalkaloids:
These alkaloids do not contain heterocyclic rings and are amines e.g., hordenine and ephedrine.
(b) True alkaloids:
These alkaloids contain heterocyclic rings and on the basis of the ring system present in their molecules are further classified into many groups:
(i) Pyridine alkaloids e.g., Nicotine.
(ii) Pyrrolidine alkaloids e.g., Stachydrine.
(iii) Piperidine alkaloids e.g., Coniine
(iv) Tropane alkaloids e.g., Atropine.
(v) Quinoline alkaloids e.g., Quinine.
(vi) Isoquinoline alkaloids e.g., Papaverine, Narcotine and Berberine.
(vii) Quinolizidine alkaloids e.g, Lupinine.
(viii) Indole alkaloids e.g., Reserpine, Ergatomine.
(ix) Pyrrolizidine alkaloids e.g., Heliotridine.
(x) Imidazol alkaloids e.g., Pilocarpine.
(c) Pseudo alkaloids:
These alkaloids may be subdivided into three categories:
(i) Terpenoid containing alkaloids (sterol alkaloids). These alkaloids occur as glycosides. For example the aglycone (i.e., non-carbohydrates part of glycoside) in tomatin is tomatidine while in solanine the aglycone is solanidine.
(ii) Phenanthrene alkaloids e.g., Morphine, codeine and thebaine.
(iii) Tropolone alkaloids e.g., Colchicine.
The structures of some of these alkaloids are given in Fig. 24.10.
Physiological role of alkaloids in plants:
In-spite of the widespread distribution of alkaloids in plants their physiological role in plants is yet unknown.
It has been suggested by different workers that:
(i) Alkaloids may provide protection against predators;
(ii) They may act as nitrogen reserve, but this has not been established;
(iii) They may act as growth regulators, especially as germination inhibitors;
(iv) They may help to maintain ionic balance due to their chelating power.
(Sir Robert, Robinson, Nobel Laureate of 1947 in Chemistry has done extensive investigations on plant products of biological importance especially the alkaloids.
2. Cyanogenic Glycosides and Glucosinolates:
These groups of nitrogen containing secondary metabolites in plants emit volatile poisons or toxins when the plants are crushed. The poisons or toxins so released are feeding deterrents to many insects and other herbivores.
Cyanogenic Glycosides:
Cyanogenic glycosides are widely distributed in plants especially legumes, grasses and members of the family Rosaceae. Amygdalin is commonly known cyanogenic glycoside which occurs in Cotoneaster and many species of Prunus. Some other examples of these substances are Linamarin from Phaseolus lunatus, Lotaustralin from Lotus tenuis, Dhurrin from sorghum and Heterodendria from African Acacia.
Cyanogenic glycosides are derived from various amino acids and correspond to the following general formula:
Where R1 is usually an aliphatic or aromatic group and R2 in most cases is H. The sugar residue is almost always D-Glucose joined by an O-β-glucosyl linkage. In amygdalin, the sugar is a disaccharide β-Gentiabiose. Chemical structures of a few cyanogenic glycosides are given in Fig. 24.11).
Cyanogenic glycosides release poisonous hydrogen cyanide (HCN) when plants containing them are crushed and they come in contact with enzymes glycosidase and hydroxynitrile lyase released from other parts of the plants. Thus, cyanogenic glycosides play a defensive role in plants.
Glucosinolates (Mustard Oil Glycosides):
These compounds such as benzylglucosinolate (Fig. 24.12) contain nitrogen and sulphur and are found mainly in plants of the family Cruciferae. When such plants are crushed and they come in contact with enzyme thioglucosidase released from other parts of the plants, they give rise to pungent volatile toxins such as isothiocyanates and nitriles which provide strong deterrent to feeding infects and other herbivores.
3. Non-Protein Amino Acids:
Apart from those 20 amino acids which constitute proteins in plants there is a large groups of over 200 different amino acids which occur free in plant cells and are not incorporated into proteins. These free amino acids are called as non-protein amino acids. Their main function appears to be protective against herbivores. A good number of different kinds of these amino acids are found in plants of the family Leguminosae.
Many non-protein amino acids closely resemble in their structure to proteins amino acids. For example, canavanine closely resembles in structure with arginine and azetidine-2-carboxylic acid is a close analog of proline (Fig. 24.13).
Non-protein amino acids may block the synthesis or uptake of protein amino acids or they may wrongly be incorporated into proteins which become non-functional. Various categories of non-protein amino acids with examples and major plant sources are given in Table 24.1.
