2-Mercaptoethanol as a substrate for liver alcohol dehydrogenase.

An activity was identified in a phosphate buffer extract of calf liver acetone powder which utilized 2-mercaptoethanol and NAD⁺ as substrates and formed NADH as one product. The activity responsible for catalyzing this reaction is associated with calf liver alcohol dehydrogenase based on copurification, similarity in pH optima, and similarity in response to chelating agents and other inactivating agents. Crystalline horse liver alcohol dehydrogenase also catalyzes the formation of NADH from NAD⁺ using 2-mercaptoethanol as the substrate. Although the K_m for mercaptoethanol is much lower than that for ethanol, 30 μm as compared to 0.625 mm, the maximum velocity with mercaptoethanol as the substrate is only 7% of that when ethanol is the substrate. Because of this difference in maximum velocity, 2-mercaptoethanol is an apparent competitive inhibitor with respect to ethanol with crystalline horse liver alcohol dehydrogenase, consistent with ethanol and 2-mercaptoethanol binding at the same site. The apparent K_i for 2-mercaptoethanol is 14 μm. 2-Butanethiol is a competitive inhibitor with respect to both 2-mercaptoethanol and ethanol with horse and beef liver alcohol dehydrogenases.