| Abstract: | Thymidylate synthetase (EC 2.1.1.B.) from blast cells of patients with acute myelocytic leukemia has been purified more than 1470-fold by affinity column chromatography. Methotrexate was the affinity ligand. dUMP was found to be a necessary additive for retention of the enzyme by the affinity column. Disc electrophoresis and sucrose density gradient centrifugation revealed a single enzyme species with a molecular weight of 76,000. The enzyme exhibits a temperature-dependent conformational change with activation energies of 5.3 +/- 0.4 and 17.3 +/- 1.9 kcal/mol, respectively, above and below a transitional temperature of 35 degrees. This conformational change is reflected in the binding affinity of dUMP but not of 5,10-methylenetetrahydrofolate. The enzyme displays a broad pH maximum in the range of pH 7.4 to 8.8. The Michaelis constants for dUMP and (+/--L-5,10-methylenetetrahydrofolate are 1.8 +/- 0.2 and 31 +/- 8.3 micrometer, respectively. Initial velocity and product inhibition studies reveal the enzymic mechanism to be ordered sequential. dUMP binds before 5,10-methylenetetrahydrofolate and dihydrofolate is released before TMP. 5-Fluoro-2'-deoxy-5'-uridylate (FdUMP) behaves as in irreversible inhibitor with a Ki of 1.68 +/- 0.45 nM. The enzyme has a turnover number of 6 min-1 per FdUMP binding site. Methotrexate inhibits noncompetitively with respect to dUMP and binds tighter to the enzyme in the presence of dUMP. Methotrexate antagonizes inactivation of the enzyme by FdUMP. |
