S-methylation of diethyldithiocarbamic acid in rat liver microsomes

1. Human hepatic thiol methyltransferase (TMT) is a microsomal enzyme known to catalyse the in vitro S-methylation of diethyldithiocarbamic acid (DDC) to form diethyldithiocarbamic acid methyl ester (methyl DDC). In vivo data are needed to investigate further the biotransformation of DDC to methyl DDC. Thus, we have characterized the in vitro conversion of DDC to methyl DDC using rat liver microsomes with the ultimate goal of establishing an animal model. 2. Formation of methyl DDC in rat liver microsomes was confirmed by hplc analysis. 3. Rat liver microsomes catalysed methylation of DDC with low and high Km's of 5 +/- 6 and 260 +/- 80 microM respectively and with corresponding Vmax's of 0.09 +/- 0.05 and 0.59 +/- 0.04 nmol/min/mg protein. 4. Rat liver TMT activity was maximally inhibited by 57 +/- 6% by 1000 microM 2,3-dichloro-alpha-methylbenzylamine (DCMB), whereas human TMT was completely inhibited. The concentration of half maximal inhibition of rat TMT for DCMB was 2 microM. 5. Incomplete inhibition of rat TMT activity by DCMB suggests a possible alternative pathway of methylation.