Heterogeneity in the rapidly exchanging metals of horse liver alcohol dehydrogenase.

Substitution of the two rapidly exchanging zinc atoms of liver alcohol dehydrogenase by cobalt is biphasic; replacement by the first cobalt occurs at a rate ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 15 minutes}$) approximately ten times faster than substitution by the second cobalt atom. The hybrid enzyme containing one gram atom of cobalt has a characteristic visible absorption spectrum which is not perturbed by NADH or 1,10-phenanthroline. The fluorescence of NADH or ε-NAD bound to the hybrid is not quenched. These data indicate a previously unrecognized heterogeneity in the rapidly exchanging zinc atoms; one of the exchange labile zinc atoms is located at a structural metal binding site rather than an active site.