Plant compounds, amino acids

A considerable variety of nitrogen-containing compounds, other than proteins and nucleic acids, occur in plants and animals. In plants, free amino acids are usually present those in greatest amount include glutamic acid, aspartic acid, alanine, serine, glycine and proline. Other compounds are nitrogenous lipids, amines, amides, purines, pyrimidines, nitrates and alkaloids. In addition, most members of the vitamin B complex contain nitrogen in their structure. 

GLSs are amino acid derived secondary plant metabolites which contain a sulphate and a thioglucose (HaUder and Du 1997). These compounds are mainly found in the seeds, although they may also be present in other tissues of the plants. Various amino acids such as tyrosine, phenylalanine and tryptophan, are precursor components and are converted to GLSs by decarboxylation. Table 4.4 hsts the GLSs, their degradation products and precursors. 

Early investigators grouped alkaloids according to the plant families in which they are found, the stmctural types based on their carbon framework, or their principal heterocycHc nuclei. However, as it became clear that the alkaloids, as secondary metaboUtes (30—32), were derived from compounds of primary metabohsm (eg, amino acids or carbohydrates), biogenetic hypotheses evolved to link the more elaborate skeletons of alkaloids with their simpler proposed pregenitors (33). These hypotheses continue to serve as valuable organizational tools (7,34,35). 

Proteins. Proteins (qv) supply amino acids (qv), palatabiHty enhancement, and, when present in more than requited amounts, energy as the proteins are degraded and nitrogen compounds excreted. Dogs and cats can consume and meet amino acid requirements in the form of pure amino acids with complete success. However, animal tissue cannot differentiate between pure, plant, or animal sources of those amino acids, and those amino acids can be obtained much more economically from either plant or animal proteins. 

Naturally Occurring Compounds. Many derivatives of iadole are found ia plants and animals where they are derived from the amino acid tryptophan. Several of these have important biological function or activity. Serotonin [50-67-9] (12) functions as a neurotransmitter and vasoconstrictor (35). Melatonin [73-31-4] (13) production is controlled daily by the circadian cycle and its physiological level iafluences, and seasonal rhythms ia humans and other species (36). Indole-3-acetic acid [87-51-4] (14) is a plant growth stimulant used ia several horticultural appHcations (37). 

Amino Acids. Early observations on the liberation of amino acids by plant roots were reviewed by Loehwing (94), Rademacher (121), and Borner (12). Free amino acids have been isolated from soil fractions (119), and the excretion of a variety of ninhydrin-positive compounds by plant roots has been demonstrated under controlled conditions by Katznelson et al. (18), Rovira (121), and Pearson and Parkinson (115). 

Stewart, G.R. Larher, F. (1981). The accumulation of amino acids and related compounds in relation to environmental stress. In Biochemistry of Plants, Vol. V, ed. B.J. Miflin, pp. 609-35. London Academie Press. 

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