The Effect of X-Irradiation on Thymidine Kinase Activity in Synchronous Populations of HeLa Cells

Abstract

The periodic synthesis of thymidine kinase during the cell cycle has been studied in synchronous populations of HeLa cells. There is a sharp decrease in enzyme activity after cell division followed by a rise in thymidine kinase levels which reaches a maximum several hours after the peak of DNA synthetic activity. Cells treated with inhibitors of protein or RNA synthesis early in the cycle have low levels of thymidine kinase activity. Cells exposed to irradiation, however, have a pattern of prolonged elevated levels of thymidine kinase activity. This altered pattern of enzyme activity resulting from x-irradiation can be mimicked by treatment of the cells with inhibitors of DNA synthesis or of cell division. The data indicate that radiation of a dose of 2000 rads does not interfere with the synthesis of the enzyme and that the prolonged elevation of thymidine kinase activity in x-irradiated cells reflects interference with the progression of the cell population to a phase of the cell cycle that is detriment...