Essential arginyl residues in thymidylate synthetase.

Thymidylate synthetase from amethopterin-resistant $\text{Lactobacillus}$$\text{casei}$ is rapidly and completely inactivated by 2,3-butanedione in borate buffer, a reagent that is highly selective for the modification of arginyl residues. The reversible inactivation follows pseudo-first order kinetics and is enhanced by borate buffer. dUMP and dTMP afford significant protection against inactivation while (±)-5,10-methylenetetrahydrofolate and 7,8-dihydrofolate provide little protection. Unlike native enzyme, butanedione-modified thymidylate synthetase is incapable of interacting with 5-fluoro-2′-deoxyuridylate and 5,10-(+)-methylenetetrahydrofolate to form stable ternary complex. The results suggest that arginyl residues participate in the functional binding of dUMP.