Physiological function and kinetic mechanism of human liver alcohol dehydrogenase as 5 beta-cholestane-3 alpha,7 alpha,12 alpha,26-tetrol dehydrogenase.

Abstract

To test whether human liver alcohol dehydrogenase (alcohol:NAD+ oxidoreductase, EC 1.1.1.1) plays a role in the metabolism of cholesterol in man or not, a major isozyme of human liver alcohol dehydrogenase, beta 2 beta 2, was purified from homozygous atypical human livers, measuring both liver acetaldehyde reductase activity and that of 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestan-26-al reductase (5 beta-cholestane-3 alpha,7 alpha,12 alpha,26-tetraol:NAD+ 26-oxidoreductase, EC 1.1.1.161) in the course of purification, and it was found that both enzyme activities were accompanied with each other at any step. Both enzyme activities of the highly purified isozyme, beta 2 beta 2, were inhibited by a chelating agent, 1,10-phenanthroline, for Zn which resides in the active site of the enzyme, the pKiapp value of which was 4.1 for both activities. They were also inhibited by a known competitive inhibitor, isobutyramide, in the same fashion and the Ki values calculated from both activities were the same (2.0 mM). From these results it was suggested that both enzyme activities are catalyzed by the same active site of the same enzyme protein. Kinetic studies of 5 beta-cholestane-3 alpha,7 alpha,12 alpha,26-tetrol dehydrogenase have shown that neither Theorell-Chance mechanism nor simple Ordered Bi Bi mechanism holds in the reaction; instead a mechanism which is asymmetric in both directions is operative.