SOME FACTORS AFFECTING THE PHOSPHORYLATION OF THYMIDINE BY TRANSPLANTABLE RAT HEPATOMAS.

Abstract

The phosphorylation of thymidine and thymidylate and some factors affecting the in vitro reactions have been investigated in a series of transplantable hepatomas of different growth rates. The tumors (Novikoff, McCoy MDAB, Dunning, Novikoff ascites, and Morris 5123, in order of decreasing growth rate) all were capable of phosphorylating thymidine, and a correlation between growth rate and rate of thymidine utilization was observed. All hepatomas were responsive to feedback inhibition by TTP4 and this effect was potentiated by low levels of Mg++. Some properties of the kinase systems in the different hepatomas were examined. Thymidine kinase required only endogenous levels of Mg++ while thymidylate kinase required added Mg++. Both activities were abolished by excess ethylenediaminetetraacetate. Kinase activity was moderately stable to storage in the frozen state. TMP kinase was generally undetectable in the Dunning hepatoma. TMP phosphatase was inhibited by adenosine triphosphate and activated by Mg++, but the action of this phosphatase does not explain the lack of TTP formation by Dunning hepatoma supernatant fraction. The activity of the kinases varied with the growth stage of the tumor and was generally low at early and late times in the tumor growth cycle. These changes were interpreted in terms of the number of actively proliferating tumor cells present.