Uridine phosphorylase and uridine kinase activities of the Ehrlich ascites carcinoma

Uridine phosphorylase and uridine kinase in cell-free extracts of the Ehrlich ascites carcinoma were assayed by a radioactive-tracer technique. Under the assay conditions uridine phosphorylase had an optimum pH of 8.0 and an apparent Km value for uridine of 2.5 × 10−3 M. Uridine kinase had a pH optimum of 6.6 and an apparent Michaelis constant for uridine of 2.4 × 10−3 M.The growth of the Ehrlich ascites carcinoma as indicated by the increase in the total packed cell volume was determined concurrently with the specific activities of uridine phosphorylase and uridine kinase, in order to ascertain the relationship of the specific activities of these enzymes to the progression of tumor growth. The specific activities of both enzymes varied during tumor growth, reaching maxima on the 11th day following the inoculation of 5.2 × 106 tumor cells into each mouse.