S-methyl N,N-diethylthiocarbamate sulfone, a potential metabolite of disulfiram and potent inhibitor of low Km mitochondrial aldehyde dehydrogenase

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 Km mitochondrial ALDH, the major enzyme in the metabolism of acetaldehyde. MeDTC sulfone inhibited the low Km mitochondrial ALDH in vitro with an IC50 of 0.42 +/- 0.04 microM (mean +/- SD, N = 5) compared with disulfiram, which had an IC50 of 7.5 +/- 1.2 microM 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 microM 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 microM) 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 Km mitochondrial ALDH.

Simultaneous structure-activity determination of disulfiram photolysis products by on-line continuous-flow liquid secondary ion mass spectrometry and enzyme inhibition assay

Disulfiram (DSF) is used in the treatment of recovering alcoholics and exerts its effect by inhibiting the enzyme aldehyde dehydrogenase (ALDH). We analyzed a mixture of products derived photochemically from DSF with on-line microbore HPLC-continuous-flow liquid secondary ion mass spectrometry (HPLC-CF-LSI-MS). By utilizing the post-HPLC column split of solvent flow, a small proportion (ca. 5%) was sent directly into the mass spectrometer, and the remainder was collected. Simultaneous MS analysis and enzyme inhibition studies on ALDH were then possible. Furthermore, using HPLC-CF-LSI-MS-MS, we were able to structurally characterize an interesting sulfine compound that inhibited ALDH.