Ribonucleotide reduction in intact human diploid fibroblasts

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Abstract
There have been very few studies on ribonucleotide reductase activity in human tissue. In this report we describe a rapid and convenient procedure for determining purine and pyrimidine ribonucleotide reduction in normal human diploid fibroblasts and use the method to examine some general properties of the activity in these cells. ADP and CDP reductase was characterized for its response to the positive effectors, ATP and dGTP, the negative effector dATP, and the reducing agent dithiothreitol. Apparent Km values for ADP and CDP were determined to be 0.1 mM and 0.04 mM respectively. The antitumor agent hydroxyurea inhibited both purine and pyrimidine reductase in a noncompetitive fashion, giving Ki values of 0.40 mM and 0.41 mM for ADP and CDP respectively. These Ki estimates are about four to five times higher than those reported for some permanent cell lines. An examination of the cytotoxic effects of hydroxyurea indicated a close correlation between the concentration of drug which inhibited enzyme activity and decreased colony‐forming ability. Clearly the ability to investigate ribonucleotide reduction in low numbers of normal human diploid cells will be useful for genetic and biochemical studies.