The specificities and configurations of ternary complexes of yeast and liver alcohol dehydrogenases.

1. Some aspects of the substrate specificities of liver and yeast alcohol dehydrogenases have been investigated with pentan-3-ol, heptan-4-ol, (+)-butan-2-ol, (+/-)-butan-2-ol, (+/-)-hexan-3-ol and (+/-)-octan-2-ol as potential substrates. The liver enzyme is active with all substrates tested, including both isomers of each optically active alcohol. In contrast, the yeast enzyme is completely inactive towards those secondary alcohols where both alkyl groups are larger than methyl and active with only the (+)-isomers of butan-2-ol and octan-2-ol. 2. The absence of stereospecificity of liver alcohol dehydrogenase towards optically active secondary alcohols and its broad specificity towards secondary alcohols in general are explained in terms of an alkyl-binding site that will react with a variety of alkyl groups and the ability of the enzyme to accommodate a fairly large unbound alkyl group in an active enzyme-NAD(+)-secondary alcohol ternary complex. The absolute optical specificity of the yeast enzyme towards n-alkylmethyl carbinols and its unreactivity towards pentan-3-ol, hexan-3-ol and heptan-4-ol are explained by its inability to accept alkyl groups larger than methyl in the unbound position in a viable ternary complex. 3. Comparison of the known configurations of the n-alkylmethyl carbinols and [1-(2)H]ethanol and [1-(3)H]geraniol, which have been used in stereospecificity studies with these enzymes by other workers, provides strong evidence for which alkyl group of the substrate is bound to the enzyme in the oxidation of n-alkylmethyl carbinols. The conclusions reached are, for butan-2-ol oxidation with the liver enzyme, confirmed by deductions from kinetic data obtained with (+)-butan-2-ol and a sample of butan-2-ol containing 66% of (-)-butan-2-ol. 4. Initial-rate parameters for the oxidations of (+)-butan-2-ol, 66% (-)-butan-2-ol and pentan-3-ol by NAD with liver alcohol dehydrogenase are presented. The data are completely consistent with a general mechanism of catalysis previously proposed for this enzyme.