Properties of sorbitol dehydrogenase and characterization of a reactive cysteine residue reveal unexpected similarities to alcohol dehydrogenases.

Sorbitol dehydrogenase was characterized as a homogeneous protein on affinity chromatography and ion-exchange chromatography. Tests of stability, sensitivity to inhibitors, and protection by coenzyme suggest that the enzyme has essential cysteine, metal, and probably histidine. The native enzyme has a molecular weight around 140 000 and a subunit around 35 000--40 000, suggesting a tetrameric quaternary structure. Subunits are highly similar if not identical as judged by characterization of one unique 45-residue sequence containing a single reactive cysteine residue. Properties resemble those of mammalian and yeast alcohol dehydrogenases, and the sequence determined for the region around the reactive cysteine residue is homologous to that around one of the zinc-liganding cysteine residues at the active site of horse liver alcohol dehydrogenase. Sorbitol dehydrogenase thus reveals an unexpected relationship to alcohol dehydrogenases, from which ancestral connections and functional mechanisms in this group of enzymes may be further elucidated.