ALKALOIDS | Properties and Determination

Abstract

Plants produce a wide variety of secondary metabolites, many of which function as allelochemicals or signal compounds to attract pollinating insects and seed- or fruit-dispersing animals. Their main function is chemical defense against herbivores, microorganisms, or competing plants. Over 22 000 nitrogen-containing secondary metabolites have been described so far in plants. Alkaloids contribute over 21 000 compounds, followed by amines, nonprotein amino acids, cyanogenic glycosides, and glucosinolates. The nitrogen-containing secondary compounds have in common that they derive from amino acids, which donate the carbon skeleton and/or the nitrogen. Many N-containing natural compounds function as defense compounds, which affect molecular targets in animal cells, such as receptors of neurotransmitters or ion channels. To fulfill this function, the molecules need to be synthesized in a stereochemically ‘correct’ configuration; that is, they must fit like a key in a lock. For a correct stereochemistry, highly specific enzymes must be involved in the biosynthesis of alkaloids. Because alkaloids and other nitrogen-containing allelochemicals mainly serve as defense compounds, they need to be present in an adequate concentration and at the right place and time. This demands that alkaloid metabolism must be highly coordinated, which can be seen from the observation that alkaloid formation often depends on the developmental stage of a plant. Alkaloid contents and composition may vary in an annual or even diurnal cycle. Allelochemical effects are dose-dependent. To be effective, plants need to store sufficient amounts of their defense compounds, usually at a strategic site (often epidermal tissues or plant parts important for reproduction and/or survival, such as bark, flowers, seeds, or fruits), and to prevent any intoxication of its own metabolism by storing the allelochemicals in vacuoles, latex, or resin ducts or dead tissues. As a common theme, we can observe that plants that produce seeds rich in energy supplies (carbohydrates, lipids, and proteins) concomitantly accumulate potent chemical defense compounds, often alkaloids, nonprotein amino acids, cyanogenic glycosides, glucosinolates, protease inhibitors, lectins, or other toxalbumins. Their presence in seeds can be mutually exclusive, that is, legume seeds store either alkaloids (e.g., quinolizidines and pyrrolizidines) or nonprotein amino acids but not both at the same time. During germination, the breakdown of nutrient reserves is a general procedure and usually includes the nitrogenous defense compounds. They serve a double purpose, that is, that of N-storage and that of protection. They are thus degradable and toxic N-storage compound.