THE EFFECT OF CORTISOL ADMINISTRATION ON THE ACTIVITIES OF URIDINE AND DEOXYURIDINE PHOSPHORYLASES OF NORMAL AND REGENERATING RAT LIVER

The phosphorolysis of uridine and deoxyuridine by homogenates of normal and regenerating liver of rats was assayed by a modification of the method of Canellakis (J. Biol. Chem. 227, 701 (1957)). By this assay the Kmvalues for uridine and deoxyuridine were 1.035 × 10−3 M and 2.95 × 10−4 M, respectively. The activities at an arsenate concentration of 0.114 M were 87 to 95% of the activities at an equivalent phosphate concentration.Forty-eight hours after partial hepatectomy the activity of uridine phosphorylase of regenerating liver was 1.1 times that of normal liver, and the activity of deoxyuridine phosphorylase of regenerating liver was 0.75 times that of normal liver. After a single injection of 9 mg of cortisol sodium succinate ester per 100 g of body weight there was a 1.8-fold increase in the activity of uridine phosphorylase but no significant increase in the activity of deoxyuridine phosphorylase of regenerating liver. After a single injection of 9 mg of cortisol per 100 g of body weight into normal rats there was a 1.6-fold increase in the activity of uridine phosphorylase and a 1.4-fold increase in the activity of deoxyuridine phosphorylase of normal liver. Thus, the phosphorolysis of uridine and deoxyuridine increases at different rates in liver after partial hepatectomy, or after cortisol, and this finding lends support to the view of others that the activities are due to two enzymes. The effect of cortisol, whether direct or indirect, in increasing the activities of the two enzymes in normal or regenerating liver would aid in the regulation of concentrations of intermediates of nucleic acid metabolism.