Ribonucleotide reductase, a possible agent in deoxyribonucleotide pool asymmetries induced by hypoxia

While investigating the basis for marked natural asymmetries in deoxyribonucleoside triphosphate (dNTP) pools in mammalian cells, we observed that culturing V79 hamster lung cells in a 2% oxygen atmosphere causes 2-3-fold expansions of the dATP, dGTP, and dTTP pools, whereas dCTP declines by a comparable amount. Others have made similar observations and have proposed that, because O(2) is required for formation of the catalytically essential oxygen-bridged iron center in ribonucleotide reductase, dCTP depletion at low oxygen tension results from direct or indirect effects upon ribonucleotide reductase. We have tested the hypothesis that oxygen limitation affects ribonucleotide specificity using recombinant mouse ribonucleotide reductase and an assay that permits simultaneous monitoring of the reduction of all four nucleotide substrates. Preincubation and assay of the enzyme in an anaerobic chamber caused only partial activity loss. Accordingly, we treated the enzyme with hydroxyurea, followed by removal of the hydroxyurea and exposure to atmospheres of varying oxygen content. The activity was totally depleted by hydroxyurea treatment and nearly fully regained by exposure to air. By the criterion of activities regained at different oxygen tensions, we found CDP reduction not to be specifically sensitive to oxygen depletion; however, GDP reduction was specifically sensitive. The basis for the differential response to reactivation by O(2) is not known, but it evidently does not involve varying rates of reactivation of different allosteric forms of the enzyme or altered response to allosteric effectors at reduced oxygen tension.