S-Methyl N,N-Diethylthiocarbamate Sulfone, a Potential Metabolite of Disulfiram and Potent Inhibitor of Low Km Mitochondrial Aldehyde Dehydrogenase

Article abstract of DOI:10.1016/0006-2952(94)00504-F

Disulfiram inhibits hepatic aldehyde dehydrogenase (ALDH) causing an accumulation of acetaldehyde after ethanol ingestion. It is thought that disulfiram is too short-lived in vivo to directly inhibit ALDH, but instead is biotransformed to reactive metabolites that inhibit the enzyme. S-Methyl N,N-diethylthiocarbamate (MeDTC) sulfoxide has been identified in the blood of animals given disulfiram and is a potent inhibitor of ALDH (Hart and Faiman, Biochem Pharmacol 46: 2285-2290, 1993). MeDTC sulfone is a logical metabolite of MeDTC sulfoxide. Therefore, we investigated the effects of MeDTC sulfone on the activity of rat hepatic low K_m mitochondrial ALDH, the major enzyme in the metabolism of acetaldehyde. MeDTC sulfone inhibited the low K_m mitochondrial ALDH in vitro with an IC₅₀ of 0.42 +/- 0.04 μM (mean +/- SD, N = 5) compared with disulfiram, which had an IC₅₀ of 7.5 +/- 1.2 μM under the same conditions. The inhibition of ALDH by MeDTC sulfone was time dependent. The decline in ALDH activity followed pseudo first-order kinetics with an apparent half-life of 2.1 min at 0.6 μM MeDTC sulfone. Inhibition of ALDH by MeDTC sulfone was apparently irreversible; dilution of the inhibited enzyme did not restore lost activity. The substrate (acetaldehyde, 80 μM) and cofactor (NAD, 0.5 mM) together completely protected ALDH from inhibition by MeDTC sulfone; substrate alone partially protected the enzyme. Addition of either thiol-containing compound glutathione (GSH) or dithiothreitol (DTT) to MeDTC sulfone before incubation with the enzyme increased the IC₅₀ of MeDTC sulfone by 7- to 14-fold. Neither GSH nor DTT could restore lost ALDH activity after exposure of the enzyme to MeDTC sulfone. Results of these studies indicate that MeDTC sulfone, a potential metabolite of disulfiram, is a potent, irreversible inhibitor of low K_m mitochondrial ALDH.