DIETHYLDITHIOCARBAMATE S-METHYLATION - EVIDENCE FOR CATALYSIS BY HUMAN LIVER THIOL METHYLTRANSFERASE AND THIOPURINE METHYLTRANSFERASE

Disulfiram is used in the treatment of alcoholism to inhibit the enzym e aldehyde dehydrogenase. Disulfiram is rapidly reduced in vivo to for m diethyldithiocarbamate (DDC), and DDC can undergo methyl conjugation to form S-methyl-DDC. Human tissues contain two separate genetically regulated enzymes that can catalyze thiol S-methylation. Thiol methylt ransferase (TMT) is a microsomal enzyme that preferentially catalyzes the S-methylation of aliphatic sulfhydryl compounds, whereas thiopurin e methyltransferase (TPMT) is a cytoplasmic enzyme that preferentially catalyzes the S-methylation of aromatic and heterocyclic sulfhydryl c ompounds. Our experiments were performed to determine whether human li ver microsomal and/or cytosolic preparations could catalyze the S-meth ylation of DDC, and, if so, to determine whether TMT or TPMT might be the enzymes involved. We found that both human liver microsomes and cy tosol could catalyze DDC S-methylation. The microsomal activity displa yed biphasic substrate kinetics, with apparent K(m) values for DDC of 7.9 and 1500 muM for the high- and low-affinity activities, respective ly. The high-affinity activity had an apparent K(m) value for S-adenos yl-L-methionine, the methyl donor for the reaction, of 5.8 muM. The th ermal inactivation profile and response to methyltransferase inhibitor s of the high-affinity microsomal DDC S-methyltransferase activity wer e similar to those of human liver microsomal TMT. In addition, TMT act ivity and the activity catalyzing the S-methylation of DDC were highly correlated in 19 individual liver samples (r(s) = 0.956; P <.0001). H epatic cytosolic DDC S-methyltransferase activity had an apparent K(m) value for DDC of 95 muM. The cytosolic enzyme which catalyzed DDC S-m ethylation and TPMT activity had similar thermal inactivation profiles , similar patterns of response to methyltransferase inhibitors and the two activities coeluted during ion exchange chromatography. Furthermo re, the activities of TPMT and cytosolic DDC S-methyltransferase were highly correlated in 20 individual liver samples (r(s) = 0.963; P <.00 01). These results were compatible with the conclusion that both TMT a nd TPMT could catalyze the S-methylation of DDC in the human liver. Be cause the activities of both TMT and TPMT are controlled by inheritanc e, our observations raise the possibility of pharmacogenetic variation in the biotransformation and therapeutic effect of DDC in humans.