CTP: CMP phosphotransferase activity in L1210 cells.

Substrate converting enzymes interfering with the measurement of ribonucleotide reductase were assessed in cell-free extracts prepared from L1210 cells. Data show the presence of a myokinase-type enzyme activity (CTP:CMP phosphotransferase) which catalyzes the reaction: 2CDP in equilibrium CMP + CTP. This enzyme is not removed by passage of cell extracts over ATP-agarose columns. Monitoring of nucleoside diphosphate substrate level is, therefore, mandatory for obtaining accurate measurements of CDP reductase activity in crude cell extracts.