Cephalosporin-induced hypoprothrombinemia: Possible role for thiol methylation of 1-methyltetrazole-5-thiol and 2-methyl-1,3,4-thiadiazole-5-thiol

Heterocyclic thiol metabolites of cephalosporin antibiotics may play an important role in the pathophysiology of hypoprothrombinemia and hemorrhage in patients treated with these drugs. A heterocyclic thiol metabolite of moxalactam, 1-methyltetrazole-5-thiol (MTT), inhibits the gamma carboxylation of glutamic acid that is required for the formation of active clotting factors. One possible pathway for the biotransformation of thiol compounds such as MTT is S-methylation catalyzed by either thiopurine methyltransferase (TPMT), a soluble enzyme, or by thiol methyltransferase, a microsomal enzyme. Therefore, MTT and 2-methyl-1,3,4-thiadiazole-5-thiol (MTD), a thiol 'leaving group' structurally related to MTT that is present in cefazolin, were tested as possible substrates for S-methylation catalyzed by purified human kidney TPMT or by human liver microsomes, a source of thiol methyltransferase. MTT and MTD were methylated by both human kidney TPMT and human liver microsomes. The products of these reactions were shown by high-performance liquid chromatography to be S-methyl MTT and S-methyl MTD. Apparent K(m) constants for the methylation of MTT and MTD by TPMT were 0.26 and 0.068 mM, respectively. Apparent K(m) constants for the methylation of MTT and MTD by human liver microsomes were 0.60 and 0.20 mM, respectively. Maximal velocity (V(max)) values for the S-methylation of MTD catalyzed by TPMT and by human liver microsomes were 3.58- and 678-fold greater than were those for the thiol methylation of MTT. Finally, S-methyl derivatives of MTT and MTD were one to two orders of magnitude less potent as inhibitors of the in vitro gamma carboxylation of glutamic acid than were MTT and MTD themselves. These results are compatible with the conclusion that S-methylation may play a role in the biotransformation of MTT and MTD in vivo. Individual variations in S-methylation might be one factor responsible for individual differences in susceptibility to cephalosporin-induced hypoprothrombinemia.