Phosphorylation of cytidine monophosphate in animal tissues.

Enzymes catalyzing the phosphorylation of adenosine monophosphate by adenosine triphosphate have been found in many tissues, and the adenylate kinases from liver and muscle have been extensively purified and studied (l-3). Yeast extracts also phosphorylated uridine monophosphate and guanosine monophosphate (4), and similar reactions were shown to be present in other tissues (5-7). Recently, Strominger, Heppel, and Maxwell found several nucleoside monophosphate kinases in calf liver (8). The present communication reports the occurrence of an enzyme in muscle extracts which catalyzes the phosphorylation of cytidine monophosphate to cytidine diphosphate by adenosine triphosphate. In order to provide more information concerning the properties of this cytidylate kinase, the enzyme was purified 120-fold from the muscle extracts. The properties of the purified enzyme, including the Michaelis constants of the substrates and the specificity for different nucleotides, are described. The equilibrium constant of the reaction was determined. This enzyme in the presence of an adenosine triphosphate-generating system containing partially purified pyruvic kinase and catalytic amounts of adenosine triphosphate may be employed for the quantitative conversion of cytidine monophosphate to cytidine triphosphate. The relative distribution of cytidylate kinase activity in different mammalian tissues was also investigated.