Assay of ribonucleotide reduction in nucleotide‐permeable hamster cells

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Abstract
Ribonucleotide reduction was measured in Chinese hamster ovary cells made permeable to nucleotides by treatment with the detergent Tween‐80. When compared to the respective ribonucleotide reductase activity in partially purified cell extracts, CDP and GDP reductase activities in permeabilized cells responded in a similar fashion to dithiothreitol, pH, MgCl2, FeCl3, substrate concentration and the presence of positive or negative allosteric effectors. At low protein concentrations both CDP and GDP reduction with whole cells increased linearly with cell number and was greater than the activity in corresponding cell extracts. Permeabilized cells were used to measure the level of CDP and GDP reductase in a hamster cell line resistant to the cytotoxic effects of hydroxyurea. The hydroxyurea‐resistant cell line contained four to ten times more CDP and GDP reductase activity compared to parental or revertant cell lines. The permeabilized cell assay was also used to measure CDP and GDP reductase activities in Chinese hamster ovary cells synchronized by isoleucine starvation. CDP reductase activity was low in G1 arrested cells but increased 10‐fold by 16 hours after the readdition of isoleucine to the growth medium. GDP reductase, which is present at much higher levels, is similarly induced after isoleucine addition, but only by 2‐fold. The maximum activity of both CDP and GDP reductase occurred from 14 to 16 hours after isoleucine addition, which corresponded to the period of maximum DNA synthesis.