We have studied two enzymes of a newly described family of dehydrogenases with high sequence homology, 1,2-propanediol oxidoreductase of Escherichia coli and alcohol dehydrogenase II of Zymomonas mobilis. These enzymes perform their metabolic role under anaerobic conditions; in the presence of oxygen, they show a very similar inactivation pattern by a metal-catalyzed oxidation system. Titration of histidine residues with diethyl pyrocarbonate showed one histidine residue less in the oxidized enzymes. Comparison of subtilisin peptide maps of active and inactivated enzymes showed a difference in one histidine-containing peptide, the sequence of which is YNTPH277GVAN for propanediol oxidoreductase and YNLPH277GV for alcohol dehydrogenase II. This histidine residue lies 10 residues away from a proposed metal-binding site, H263XXXH267, necessary to explain a site-specific free radical mechanism. The three histidine residues here described are strictly conserved in all enzymes of this family. In this report we propose that histidine 277 is a target for oxidation by a metal-catalyzed oxidation system and that this modification leads to the irreversible inactivation of both enzymes.