Liver Alcohol Dehydrogenase. IV. Kinetics in the Presence of Zinc Binding Agents.

Abstract

Measurements of the kinetics of both the oxidation of alcohol by DPN and the reduction of aldehyde by DPNH have been made in the presence of a variety of zinc binding agents. These substances show a varied pattern of behavior, which is summarized as follows. Binding of inhibitors to the enzyme parallels the binding of the third bidentate ligand to free zinc, indicating the zinc to be bound in the enzyme octahedrally at more than two of its coordination positions. Bindentate ligands compete with DPN and DPNH, thus indicating that at least one of their normal binding sites are at the zinc. In addition ternary complexes are formed between enzyme-substrate-inhibitor which show markedly increased affinity of inhibitor and substrate for the enzyme. Furthermore, monodentate ligands compete with substrate in forming ternary complexes with enzyme-coenzyme complexes. Both these results are taken to indicate that the substrates may also be bound to the zinc.