The inactivation of thymidylate synthase by diethyl pyrocarbonate

Thymidylate synthase (5,10-methylenetetrahydrofolate: dUMP C-methyltransferase, EC 2.1.1.45) from Lactobacillus casei was inactivated by treatment with diethyl pyrocarbonate. The inactivation was apparently due to the modification of a large proportion of the enzyme's histidine residues. Neither the substrate dUMP nor the product dTMP prevented inactivation by diethyl pyrocarbonate. The inactivated enzyme was not reactivated by treatment with hydroxylamine. These results indicate that histidine residues are involved in the maintenance of enzyme structure.