Stereochemistry of the hydrogen transfer to NAD catalyzed by (S)alanine dehydrogenase from Bacillus subtilis.

The stereochemistry of the hydrogen transfer to NAD catalyzed by (S)alanine dehydrogenase [ (S)alanine: NAD oxidoreductase (EC 1.4.1.1) ] from B. subtilis was investigated. The label at C-2 of (S) [2,3--3H] alanine was enzymatically transferred to NAD, and the [4--3H]NADH produced isolated and the stereochemistry at C-4 investigated. It was found that the label was exclusively located at the (R) position which indicates that (S)alanine dehydrogenase is an A-type enzyme. This result was confirmed in an alternate way by reducing enzymatically [4--3H]NAD with non labeled (S)alanine and (S)alanine dehydrogenase and investigating the stereochemistry of the ]4--3H]NADH produced. As expected, the label was now exclusively located at the (S) position. This proves that (S)alanine dehydrogenase isolated from B. subtilis should be classified as an A-enzyme with regard to the stereochemistry of the hydrogen transfer to NAD.