Products Based on Decarboxylation and Amine Oxidation

Decarboxylation of amino acids is a typical feature of the bacterial decomposition of proteins. Both phenylethylamine and tyramine were isolated from putrid meat by Barger and Walpole (30), who considered it extremely probable that they were derived from phenylalanine and tyrosine, respectively. No cell-free preparation of phenylalanine decarboxylase appears to have been reported, but decarboxylation by a crude Streptococcus faecalis preparation provides a valuable method of phenylalanine assay (887). Bacterial tyrosine decarboxylase has been studied in detail (495), especially by Gale and co-workers (summarized in 284). It requires pyridoxal phosphate as coenzyme (26, 326, 327) and, unlike mammalian tyrosine decarboxylase, also attacks dihydroxyphenylalanine. Decarboxylation normally only occurs in acid media and is considered primarily to be a protective mechanism tending to restore the pH to neu- 

Diagram 16. Interrelationships of some side-chain metabolic reactions used by microorganisms. Ar can be phenyl, p-hydroxyphenyl (as in tyrosine), indol-3-yl-(as in tryptophan), 3,4-dihydroxyphenyl, etc. Names given above are in general for substances derived from phenylalanine. 

The amine resulting from decarboxylase action can be oxidatively de-aminated to give the aldehyde or, more usually, the acid by further oxidation  

This reaction has been primarily studied in higher organisms (review 77) but probably occurs equally in microorganisms and also in plants, where it is probably intimately connected with biosynthesis of alkaloids q.v.). 

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