Substrate specificity of human ribonucleotide reductase from Molt-4F cells

Nucleoside triphosphates were examined as the activator for various nucleoside diphosphate reductions catalyzed by a highly purified ribonucleotide reductase obtained from Molt-4F cultured human cells. It was found that cytidine 5'-diphosphate and uridine diphosphate reductions are activated by adenosine 5'-triphosphate with apparent Ka's of 0.63 +/- 0.03 (S.E.) and 1.25 +/- 0.10 mM, respectively. Guanosine 5' diphosphate reduction is activated by deoxythymidine 5'-triphosphate with an apparent Ka of 1.25 +/- 0.11 microM, and adenosine 5'-diphosphate reduction is activated by guanosine 5'-triphosphate or deoxyguanosine 5'-triphosphate with an apparent Ka of 1.1 +/- 0.09 or 1.1 +/- 0.08 mM, respectively. In the presence of saturating amounts of their best activating nucleoside triphosphates, the Km's of various nucleotide diphosphates for this purified enzyme were studied. Double reciprocal plots of velocity against substrate concentration were found to be linear for all four substrates in the concentration range tested and yielded apparent Km's of 7 +/- 0.3 microM for cytidine 5'-diphosphate, 80 +/- 6.5 microM for adenosine 5'-diphosphate, 33 +/- 3.1 microM for guanosine 5'-diphosphate, 50 +/- 2.0 microM for uridine 5'-diphosphate. The reduction of one ribonucleoside diphosphate could be inhibited by other ribonucleoside diphosphates in a noncompetitive manner.