Allosteric regulation of calf thymus ribonucleotide reductase

Ribonucleotide reductase was purified 3400-fold from calf thymus. The enzyme preparation was essentially free of kinases and phosphatases and therefore allowed a conclusive study of the allosteric regulation of a eukaryotic ribonucleotide reductase to be made for the first time. Comparable maximal activities were obtained for the reduction of all four ribonucleotide substrates in the presence of their optimal stimulatory effectors. These and other results strongly argue for the existence of only one ribonucleotide reductase in mammalian cells. No reduction was observed in the absence of effector. The reduction of CDP and UDP both required ATP, with no stimulatory effect of any other nucleoside triphosphate. The only activator of GDP reduction was dTTP and the only activator of ADP reduction was dGTP. Reduction of the purine ribonucleotides was further stimulated by ATP but only in combination with dTTP or dGTP. The reduction of all four ribonucleotides was strongly inhibited by dATP, the inhibition being partly released by ATP. The data can be integrated into a scheme which links ribonucleotide reduction to DNA synthesis.