DNA synthesis in tumor-bearing rats: purification of liver thymidine kinase stimulating factor from Yoshida sarcoma

Yoshida sarcoma cells contain a factor that stimulates thymidine kinase activity in the liver of mice in vivo; intraperitoneal injection of an extract from Yoshida sarcoma into normal mice stimulated their liver thymidine kinase activity 2- to 3-fold, whereas injection of a crude extract of normal rat liver did not stimulate the activity at all. A factor that stimulates the de novo synthesis of thymidine kinase in the liver was partially purified from Yoshida sarcoma by ammonium sulfate fractionation, DEAE-cellulose column chromatography and gel filtration. It appeared to be thermolabile and sensitive to trypsin treatment. These results suggested that it was a high-molecular weight protein. Intraperitoneal injection of this factor into 67% hepatectomized rats stimulated thymidine kinase activity 2- to 3-fold. Increase of liver thymidine kinase activity after injection of the factor into mice was blocked by simultaneous injection of actinomycin-D. These results suggest that this factor stimulates de novo synthesis of thymidine kinase in the liver.