State and accessibility of zinic in yeast alcohol dehydrogenase.

1. Yeast alcohol dehydrogenase (EC 1.1.1.1) is inhibited in the presence of 1,10-phenanthroline. 2. A conformational change in the enzyme's structure is induced by 1,10-phenanthroline, and is abolished in the presence of NADH. 1,10-Phenanthroline binds to the enzyme competitively with respect to NADH, with a stoicheiometry of 2 mol of 1,10-phenanthroline/144000g of enzyme. 3. 1,10-Phenanthroline induces a time-dependent dissociation of Zn2+ from the enzyme, which is in correlation with its inhibitions. 4. Spectrophotometric measurement indicates that the dissociation of half (2 zinc atoms/tetramer) of the total zinc content of the enzyme correlates with the full inhibition of its activity. Measurement of the tightly bound Zn2+ by atomic absorption photometry confirms this. 5. A proposition is advanced that the tetrameric molecule of yeast alcohol dehydrogenase possesses an inherent asymmetry, with four monomeric subunits being arranged in two mutually symmetrical pairs.