The ribonucleoside diphosphate reductase (E.C.1.17.4.1) was partially purified from Ehrlich ascites carcinoma and regenerating rat liver. The specific activity of the two enzyme preparations did not vary with regard to the reduction of UDP, ADP, CDP and GDP. Similarly the regulation of the enzyme system by deoxyribonucleoside triphosphate (dNTP) is almost identical. Through the application of deoxyribonucleosides (10(-3) or 2 x 10(-3)M) and measurement of the dNTP content it was found in Ehrlich and Yoshida ascites tumours that these control mechanisms are transmissible to whole cells. dATP inhibits the reduction of all four nucleoside diphosphates. dTTP stimulates the reduction of GDP, dCTP that of UDP and dGTP that of ADP.
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