Abstract

Diethylstilbestrol and 17-ethynyl-β-estradiol irreversibly inhibited thymidine-5'-monophosphokinase (EC 2.7.4.9) which had been purified from L5178Y mouse leukemia cells. Preparations of TMP-kinase from mouse spleen, calf thymus, and Escherichia coli were not inhibited by these compounds. A high concentration of thymidylate protected TMP-kinase from inactivation by diethylstilbestrol but could not reactivate the inhibited enzyme. The inhibition of TMP-kinase by the synthetic estrogens was prevented by several sulfhydryl compounds of which dithiothreitol was most effective. The addition of dithiothreitol to the inhibited enzyme did not restore activity. Diethylstilbestrol-¹⁴C did not specifically associate with the enzyme protein in a partially purified TMP-kinase and the addition of the protective agents, dithiothreitol and thymidylate, did not alter the degree of binding. Treatment of the inhibition kinetics by the method of Ackermann and Potter showed that diethylstilbestrol did not titrate enzyme activity.

Diethylstilbestrol caused no inhibition of thymidylate metabolism in L5178Y cells incubated in vitro nor of the development of ascitic tumor in the mouse.