Ribonucleotide Reductase in Immature Testes of Salmon

When cell-free extracts obtained by ultracentrifugation of homogenates of immature salmon testes were incubated with radioactive ribonucleoside di- or triphosphates, the corresponding deoxyribonucleosides were isolated from the reaction mixtures. The specific activities of the crude enzyme extracts were between 15.8 and 54 picomoles of deoxyribonucleoside formed per milligram of protein per hour at 25 C. All attempts to purify the enzyme, which lost activity quite rapidly at 0 C, were unsuccessful. Acid hydrolysis of guanosine deoxyribosides of constant specific radioactivity formed by enzymic reduction of tritiated guanosine triphosphate (GTP) or guanosine diphosphate (GDP) yielded radioactive guanosine of similar specific radioactivity. The crude nature of the preparation, which possessed phosphoribokinase and phosphatase activities, made it impossible to decide whether nucleotide reduction occurred at the mono-, di-, or triphosphate level. Evidence was obtained that allosteric effectors are required for ribonucleotide reduction, as with mammalian and bacterial ribonucleoside diphosphate reductase. The enzyme required dithiothreitol and iron, was strongly stimulated by ethylenediaminetetraacetate (EDTA) and was inhibited by dADP.