S-METHYL N,N-DIETHYLTHIOCARBAMATE SULFONE, A POTENTIAL METABOLITE OF DISULFIRAM AND POTENT INHIBITOR OF LOW K-M MITOCHONDRIAL ALDEHYDE DEHYDROGENASE

Disulfiram inhibits hepatic aldehyde dehydrogenase (ALDH) causing an a ccumulation of acetaldehyde after ethanol ingestion. it is thought tha t disulfiram is too short-lived in vivo to directly inhibit ALDH, but instead is biotransformed to reactive metabolites that inhibit the enz yme. S-Methyl N,N-diethylthiocarbamate (MeDTC) sulfoxide has been iden tified in the blood of animals given disulfiram and is a potent inhibi tor of ALDH (Hart and Faiman, Biochem Pharmacol 46: 2285-2290, 1993). MeDTC sulfone is a logical metabolite of MeDTC sulfoxide. Therefore, w e investigated the effects of MeDTC sulfone on the activity of rat hep atic low K-m mitochondrial ALDH, the major enzyme in the metabolism of acetaldehyde. MeDTC sulfone inhibited the low K,mitochondrial ALDH in vitro with an IC50 of 0.42 +/- 0.04 mu M (mean +/- SD, N = 5) compare d with disulfram, which had an IC50 of 7.5 +/- 1.2 mu M under the same conditions, The inhibition of ALDH by MeDTC sulfone was time dependen t. The decline in ALDH activity followed pseudo first-order kinetics w ith an apparent half-life of 2.1 min at 0.6 mu M MeDTC sulfone. Inhibi tion of ALDH by MeDTC sulfone was apparently irrversible; dilution of the inhibited enzyme did not restore lost activity. The substrate (ace taldehyde, 80 mu M) and cofactor (NAD, 0.5 mM) together completely pro tected ALDH from inhibition by MeDTC sulfone; substrate alone partiall y protected the enzyme. Addition of either thiol-containing compound g lutathione (GSH) or dithiothreitol (DTT) to MeDTC sulfone before incub ation with the enzyme increased the IC50 of MeDTC sulfone by 7- to 14- fold. Neither GSH nor DTT could restore lost ALDH activity after expos ure of the enzyme to MeDTC sulfone. Results of these studies indicate that MeDTC sulfone, a potential metabolite of disulfram, is a potent, irreversible inhibitor of low K-m mitochondrial ALDH.