Function of the vitamin B6 group; mechanism of transamination.

The r&e of vitamin Bg in amino acid metabolism has been well established through enzyme studies (l-3) and by the substitution for various amino acids in growth studies (4-6). The coenzyme form, synthetic pyridoxal phosphate (7), functions as the coenzyme for amino acid decarboxylases (S-10), transaminases (2, 3), tryptophan formation (11)) and tryptophan breakdown by tryptophanase (12). The reactions involved in the amino acid replacement of vitamin Bg have not been determined. The mechanism by which pyridoxal phosphate functions is still in question. Snell (13), however, found that pyridoxal and pyridoxamine were readily and reversibly interconverted by in vitro transamination and suggested on this basis that biological transamination might be mediated by the transfer of the amino group via the aldehyde and amino forms of vitamin BG to the keto acid. The demonstration of pyridoxal phosphate as t,he coenzyme of transamination quite naturally suggested the existence of pyridoxamine phosphate and its possible r81e in those systems for which pyridoxal phosphate is the coenzyme. Umbreit, O’Kane, and Gunsnlus (14) found activation of the apotransaminase of dried bacteria by “pyridoxamine phosphate” which had been prepared by the method of Snell for the interconversion of pyridoxal and pyridoxamine (13). The pyridoxamine phosphate was judged to be free of pyridoxal phosphate by its lack of coenzyme activity for tyrosine apodecarboxylase and by the spectrum. Later Ames, Sarma, and Elvehjem (15) found pyridoxamine phosphate, prepared as above to stimulate the transaminase in liver homogenates from vitamin Be-deficient animals. These data could be interpreted as substantiating the hypothesis of pyridoxamine phosphate as an intermediate in transamination, although the alternative possibility of the transformation of the pyridoxamine phosphate to pyridoxal phosphate by a separate enzyme was not excluded. The latter type