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End-Product Inhibition of Thymidine Kinase Activity in Normal and Leukemic Human Leukocytes^*

( Department of Pharmacology, Baylor University College of Medicine, Houston, Texas)

Thymidine kinase activity in sonicated soluble preparations obtained from normal and leukemic human leukocytes (both chronic myelogenous and chronic lymphocytic leukemia) was inhibited by the 5'-mono-, di-, and triphosphates of 2'-deoxythymidine (TMP, TDP, and TTP, respectively), by 2'-deoxycytidine-5'-monophosphate (d-CMP), and by 2'-deoxycytidine-5'-triphosphate (d-CTP). The inhibitions by TTP and d-CTP were of the competitive type (with thymidine). The ‘apparent’ K_m for thymidine and the K_i for TTP and d-CTP were 3.6 x 10^-5, 1.5 x 10^-6, and 7.6 x 10^-6M, respectively. Comparable results were obtained with preparations obtained from normal or leukemic leukocytes. Thymidine kinase in these leukocyte preparations was gradually inactivated at 50° C. The degree of end-product inhibition by d-CTP or TTP was not affected by this heat treatment of the leukocyte preparations.

^* This work was supported in part by an institutional grant from the American Cancer Society, ACS In 27E Project 8, and by a grant from the National Cancer Institute (CA 06571), United States Public Health Service.

Received 12/28/63.

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