THE SIMPLEST HISTORICAL MODEL THAT EXPLAINS DEHYDROGENASE STEREOSPECIFICITY*

Stereospecificities are reported for seven dehydro-genases from Acholeplaama laidlawii, an organism from an evolutionarily distinct branch of life which has not previously been studied from a stereochemical point of view. Three of the activities examined (alcohol dehydrogenase, lactate dehydrogenase, and alanine de-hydrogenase) catalyze the transfer of the pro-R (A) hydrogen from NADH. Four other activities (3-hy-droxy-3-methylglutaryl-CoA reductase, glyceralde-hyde-3-phosphate dehydrogenase, glucose-6-phos-phate dehydrogenase, and NADH oxidase) catalyze the transfer of the pro-S (B) hydrogen from NAD(P)H. The stereospecificity of hydroxymethylglutaryl-CoA re-ductase is notable because it is the opposite of that of hydroxymethylglutaryl-CoA reductases from yeas