Activities of Thymidine Kinase and Thymine Deoxyribonucleotide Phosphatase during Growth of Cells in Tissue Culture.

Accumulating evidence indicates that the initiation of deoxyribonucleic acid synthesis is intimately associated with the appearance of elevated levels of enzymes involved in the synthesis of thymidine triphosphate. Thus, enhanced levels of such enzymes as thymidine kinase (l-11), thymidylate kinase (5-7, 12I4), thymidine diphosphate kinase (6,7), thymidylate synthetase (10, 15, 16), and deoxycytidylate deaminase (10, 15-18)’ have been observed in cells and tissues undergoing rapid cellular proliferation, and in cultured mammalian cells after infection with virus. The fact that the activities of these enzymes gradually decrease to their resting levels upon cessation of deoxyribonucleic acid synthesis provides further support for the hypothesis that the enzymes are involved in processes related to cell division. Since the presence of thymidine triphosphate is required for deoxyribonucleic acid synthesis (19, 20), it has been suggested (6-10, 15, 16, 21) that these enzymes, by regulating the rate of formation of thymidine triphosphate, may play an important role in the control of deoxyribonucleic acid synthesis, and hence of cell division. In a previous communication (22) we have reported the presence in human liver cell homogenates of enzymes that are capable of dephosphorylating the mono-, di-, and triphosphates of thymidine. Deoxyribonucleotide phosphatase activity has previously been shown in extracts of different mammalian tissues (10, 12, 15, 16, 21-30), and it has been suggested (16, 23, 24) that a correlation may exist between the cellular concentration of these catabolic enzymes and the mitotic index. The present work was undertaken to study in more detail the ability of mammalian tissue culture cells to phosphorylate thymidine, and to dephosphorylate thymidine nucleotides, in relation to their growth rate. It was found that the levels of thymidine kinase, and of the phosphatases responsible for the breakdown of the monoand diphosphates of thymidine, showed considerable variations during different phases of growth. The activity of the phosphatase catalyzing the cleavage of thymidine triphosphate was not similarily affected.