D-amino acid dehydrogenase: The enzyme of the first step of D-histidine and D-methionine racemization inSalmonella typhimurium*

Abstract

Mutants ofSalmonella typhimurium deficient in D-amino acid dehydrogenase were isolated in histidine auxotrophs able to utilize D-histidine(his-dhuA) 1. The mutants have lost the ability to utilize D-histidine and D-methionine due to mutations in the locusdadA mapped in co-transducible vicinity of the genehemA. ThedadA mutants were unable to deaminate D-histidine, D-methionine, D-alanine and several other D-amino acids to the respective keto products. Indad + strains the enzyme activity was the highest in toluenized cells. In crude sonieates it was 5 to 10 times less. Reduction of artificial electron accepters in the presence of D-amino acids behaved similarly. Keto product formation was strongly inhi-bited by cyanide. It has been concluded thereof that the deaminating enzyme is a D-amino acid dehydrogenase, the activity of which depends on structural integrity of a cell component or on a structure-bound electron accepter. The enzyme activity was inducible by adding L-or D-alanine to growth media. The induction was the highest in media with poor carbon sources. A temperature-sensitivedadA mutant was isolated. I t mapped indadA and had thermolabile D-amino acid dehydrogenase. This has indicated thatdadA is structural gene for the D-amino acid dehydrogenase.