Reversible inactivation of horse liver aldehyde dehydrogenase by 2-hydroxyethyl disulfide

Incubation of horse liver aldehyde dehydrogenase (aldehyde:NAD oxidoreductase, EC 1.2.1.3) with 2-hydroxyethyl disulfide formed mixed-disulfides between protein sulfhydryl groups and beta-mercaptoethanol. Reduction of aldehyde dehydrogenase activity may be associated with formation of one, or at most two, mixed-disulfides per dehydrogenase subunit. Characteristically in the case of a mixed-disulfide, inactivation was was reversed by addition of thiols. Other disulfides also inactivated aldehyde dehydrogenase. The pseudo first-order rate constants for the forward and reverse reactions (aldehyde dehydrogenase + 2-hydroxyethyl disulfide in equilibrium or formed from modified aldehyde dehydrogenase + beta-mercaptoethanol) were 0.70 and 2 liter mole-1 sec-1, respectively. The equilibrium constant was approximately 0.4. After extended incubation under conditions expected to result in complete modification of aldehyde dehydrogenase, 30% of the initial catalytic activity remained. This suggests that 2-hydroxyethyl disulfide-treated aldehyde dehydrogenase retains catalytic activity and that the sulfhydryl group modified by 2-hydroxyethyl disulfide is not essential for aldehyde dehydrogenase activity.