The mechanism of action of disulfiram involves inhibition of hepatic aldehyde dehydrogenase (ALDH). Although disulfiram inhibits ALDH in vitro, it is believed that the drug is too short-lived in vivo to inhibit the enzyme directly. The ultimate inhibitor is thought to be a metabolite of disulfiram. In this study, we examined the effects of S -methyl- N,N -diethylthiocarbamate (MeDTC) sulfoxide and S -methyl- N,N -diethylthiocarbamate sulfone (confirmed and proposed metabolites of disulfiram, respectively) on rat liver mitochondrial low K m ALDH. MeDTC sulfoxide and MeDTC sulfone, in 10-min incubations with detergent-solubilized mitochondria, inhibited ALDH activity with an IC 50 (mean ± SD) of ± and ± μM, respectively, compared with ± μM for the parent drug disulfiram. Inhibition by MeDTC sulfone and MeDTC sulfoxide, both at μM, was time-dependent, following apparent pseudo-first-order kinetics with a t 1/2 of inactivation of and min, respectively. Dilution of ALDH inhibited by either sulfoxide or sulfone did not restore activity, an indication of irreversible inhibition. Addition of glutathione (50 to 1000 μM) to ALDH before the inhibitors did not alter the inhibition by MeDTC sulfoxide. In contrast, the inhibition by MeDTC sulfone was decreased > 10-fold (IC 50 = μM) by 50 μM of glutathione and almost completely abolished by 500 μM of glutathione. The cofactor NAD, in a concentration-dependent manner, protected ALDH from inhibition by MeDTC sulfoxide and MeDTC sulfone. In incubations with intact mitochondria, the potency of the two compounds was reversed (IC 50 of ± and ± μM for the MeDTC sulfone and sulfoxide, respectively). Our results suggest that MeDTC sulfone is highly reactive with normal cellular constituents (., glutathione), which may protect ALDH from inhibition, unless this inhibitor is formed very near the target enzyme. In contrast, MeDTC sulfoxide is a better candidate for the ultimate active metabolite of disulfiram, because it is more likely to be sufficiently stable to diffuse from a distant site of formation, such as the endoplasmic reticulum, penetrate the mitochondria, and react with ALDH located in the mitochondrial matrix.