Growth promotion by keto and hydroxy acids and its relation to vitamin B6.

Many of the amino acids required in the diet of animals or microorganisms are non-essential if the corresponding keto acids are supplied (cf. (l-7)). How these keto acids are transformed to amino acids in viva is not generally known, but the change is assumed to occur either by reductive amination or by transamination. The transaminating enzymes so far studied all require pyridoxal phosphate as a coenzyme (8, 9), whereas the known enzymes that carry out reductive amination 01 keto acids or the reverse reaction are either pyridinoproteins or flavoproteins and vitamin Bs does not appear to be involved. If it could be shown that vitamin Bg was specifically required for utilization of keto acids by growing organisms, strong presumptive evidence for their utilization via transamination reactions, and thus for the general significance of various transamination reactions in viva, would be provided. Such experiments are described below. The test system used was based on the observations that many lactic acid bacteria grow well in the absence of vitamin Bs if a complete assortment of amino acids, including n-alanine, is provided, and synthesize little or no vitamin Bo under these conditions (10, 11). In such a medium substitution of a keto or hydroxy acid for an essential amino acid would result in growth failure if vitamin Be is required for its transformation to the corresponding amino acid; upon addition of vitamin Bs, growth should again become possible. Broquist and Snell (5) demonstrated in this way that vitamin B, was necessary for utilization of imidazolepyruvic acid in place of histidine by Streptococcus faecalis; on these grounds they postulated that histidine participates in transamination in this organism. The present experiments demonstrate that vitamin Bs is required quite generally for utilization of the keto and hydroxy acids that substitute for the essential amino acids in these bacteria. The organisms employed are widely used for the determination of amino acids, and