Possible regulation of ribonucleoside diphosphate reductase

Two inhibitors of ribonucleoside diphosphate reductase (RR) (EC 1.17.4.1) in vitro were isolated from normal rat liver: they were a nondialyzable, heat-labile, high-molecular-weight ribonucleoside diphosphate reductase inhibitor (HRRI, and a dialyzable, heat-stable, low-molecular-weight ribonucleoside diphosphate reductase inhibitor (LRRI). The activities of both inhibitors varied inversely with the cell growth rate. HRRI from the cytosol fraction of rat liver was partially purified by ammonium sulfate fractionation (0 - 50%), and gel filtration on a Sepharose 6B column. It was eluted in the void volume from this column, together with ATP-hydrolyzing activity. The HRRI fraction also contained CDP kinase and CDPase activities, suggesting that HRRI is a complex of several enzymes that reduce the concentrations of the substrate of RR, CDP, and of the allosteric activator, ATP. LRRI was extracted from the cytosol of rat liver with ethanol (80% final concentration) and purified further by washing with organic solvent, and be chromatographies of Amberlite IR-45 and Dowex 50. Finally, it was identified as glucose, which was phosphorylated to glucose 6-phosphate by hexokinase present tin the RR enzyme solution ( 0 - 35% ammonium sulfate fraction of AH-130 cytosol), thus causing ATP depletion. Thus, neither inhibitor reacted directly with the RR enzyme, but both may regulate the enzyme activity in vivo by reducing the intracellular levels of substrates or cofactors.