In sorbitol dehydrogenase only one cysteine residue, Cys-43, is reactive in both anionic buffer (phosphate) and zinc-liganding buffer (imidazole) upon carboxymethylation. This is in contrast to the situation in the structurally related liver alcohol dehydrogenase, with either of two alternative Cys residues being reactive, and is compatible with differences in zinc-binding and active site relationships between these two metalloenzymes. Unrelated aldehyde dehydrogenase, upon carboxamidomethylation, shows a third pattern, now less well defined but confirming the presence of a thiol function of Cys-302 close to the active site.
