Classification And Properties

Alkaloids are naturally occurring substances with a particularly wide range of structures and pharmacologic activities. They may be conveniently divided into three main categories: the true alkaloids, the protoalkaloids, and the pseudoalkaloids.

The true alkaloids have the following characteristics: they show a wide range of physiological activity, are usually basic, normally contain nitrogen in a heterocyclic ring, are biosynthesized from amino acids, are of limited taxo-nomic distribution, and occur in the plant as the salt of an organic acid. Exceptions are colchicine, aristolichic acid, and the quaternary alkaloids. The protoalkaloids, also known as the biological amines, include mescaline and Af,7V-dimethyltryptamine. They are simple amines synthesized from amino acids in which the nitrogen is not in a heterocyclic ring. The pseudoalkaloids, those not derived from amino acids, include two major series of compounds: the steroidal and terpenoid alkaloids (eg, cones-sine) and the purines (eg, caffeine). Most alkaloids occur in the An-giosperms, the flowering plants, but they are also found in animals, insects, marine organisms, microorganisms, and the lower plants. The only common characteristic of alkaloid names is that they end in "ine" (except camptothecin).

Alkaloids are preferably grouped by their biosynthetic origin from amino acids (eg, ornithine, lysine, phenylalanine, tryptophan, histidine, and anthranilic acid) rather than their heterocyclic nucleus. The pseudoalkaloids, which are not derived from amino acids and clearly cannot be classified this way, are best organized in terms of their parent terpenoid class, eg, diterpenoid and steroidal alkaloids, or as purines or ansamacrolides.

Physical Properties

Most alkaloids are colorless crystalline solids with a defined melting point or decomposition range (eg, vindoline and morphine). Some alkaloids are amorphous gums and some are liquids (eg, nicotine and conine) and some are colored (eg, berberine is yellow and betanidine is red).

The free base of the alkaloid is normally soluble in an organic solvent; however, the quaternary bases are only water soluble, and some of the pseudo- and protoalkaloids show substantial solubility in water. The salts of most alkaloids are soluble in water.

The solubility of alkaloids and their salts is of considerable significance in the pharmaceutical industry, both in the extraction of the alkaloid from plant or fungus and in the formulation of the final pharmaceutical preparation. Solubility is also of considerable significance in the clinical distribution of an alkaloidal drug.

Chemical Properties

Most alkaloids are basic, which makes them extremely susceptible to decomposition, particularly by heat and light.

Ornithine-Derived Alkaloids

Ornithine-derived alkaloids include the tropanes (atropine, Z-hyoscyamine, /-scopolamine, and cocaine), the Senecio alkaloids, and nicotine.

Tropane. Tropane alkaloids are derived from plants in the Solanaceae, Erythroxylaceae, and Convolvulaceae families. These alkaloids comprise two parts: an organic acid and an alcohol (normally a tropan-3a-ol). The phar macologically active members of this group include atropine, the optically inactive form of /-hyoscyamine, which is isolated from deadly nightshade (Atropa belladonna)-, l-hyoscyamine and /-scopolamine, which are found in the leaves of Duboisia metel L., D. meteloides L., and D. fastuosa var. alba.

The tropane alkaloids are parasympathetic inhibitors. For example, atropine acts through antagonism of muscarinic receptors, the receptors responsible for the slowing of the heart, constriction of the eye pupil, vasodilation, and stimulation of secretions. Atropine prevents secretions (eg, sweat, saliva, tears, and pancreas) and dilates the pupil. Atropine is used to reduce pain of renal and intestinal cholic and other gastrointestinal tract disorders, to prolong mydriasis when necessary, and as an antidote to poisoning by cholinesterase inhibitors. Small doses produce respiratory and myocardial stimulation and decrease nasal secretion, and the drug has little local anesthetic action. Hyoscyamine and scopolamine have mydriatic effects. They are also used in combination as sedatives, in anti-motion-sickness drugs, and in antiperspirant preparations.

Cocaine. Cocaine is a potent central nervous system (CNS) stimulant and adrenergic blocking agent. It is extracted from South American cocoa leaves or prepared by converting ester alkaloids to exgonine, followed by meth-ylation and benzoylation. It is too toxic to be used as an anesthetic by injection, but the hydrochloride is used as a topical anesthetic. It has served as a model for a tremendous synthetic effort to produce an anesthetic of increased stability and reduced toxicity.

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