The Role of Zinc in Alcohol Dehydrogenase IV. THE KIh’ETICS OF THE INSTANTANEOUS INHIBITION OF HORSE LIVER ALCOHOL DEHYDROGENASE BY 1 , lo-PHENANTHROLINE*

The alcohol dehydrogenase crystallized from horse liver contains 2 atoms of zinc per molecule and the activity of this enzyme is inhibited by metal-binding agents (1, 2). The inhibition of liver alcohol dehydrogenase by one of these agents, 1, IO-phenanthroline, differs from that which is observed when the alcohol dehydrogenase isolated from yeast is exposed to 1, lo-phenanthroline (3). The liver enzyme is inhibited by 1, lo-phenanthroline only instantaneously and reversibly (2) ; under identical experimental conditions, the yeast enzyme, although showing a similar instantaneous type of inhibition, in addition exhibits a slow, irreversible inhibition (4). Direct experimental evidence (5) indicates that l,lO-phenanthroline acts at a zinc atom of these enzymes. These differences in the 1, IO-phenanthroline inhibitions may reflect differences in the mechanism of reaction, in the structure of the active loci of these enzymes, or in both. The present communication examines the kinetics of the inhibition of liver alcohol dehydrogenase by 1, lo-phenanthroline, and the effects of coenzyme and substrates thereon. Mechanisms of action of this enzyme have been postulated based on the experimental findings.