ALLOSTERIC REGULATION OF CALF THYMUS RIBONUCLEOSIDE DIPHOSPHATE REDUCTASE

A highly purified ribonucleotide reductase from calf thymus catalyzed the reduction of all four ribonucleoside diphosphates at almost identical rates. Substrate specificity is regulated by allosteric effects. The activities toward CDP and GDP were purified in parallel and the two nucleotides competed for the same catalytic site. Taken together the data show that the same enzyme also in mammalian cells can reduce all four ribonucleotides. In the absence of positive effectors, the enzyme was inactive with any ribonucleoside diphosphate. Reduction of CDP and UDP was stimulated by ATP, reduction of GDP by dTTP, and reduction of ADP by dGTP. Reduction of the purine ribonucleotides was further stimulated by ATP combined with dTTP or dGTP. dATP served as a general inhibitor whose negative effects could be reversed by ATP. Inhibition was also caused by dTTP or dGTP which, in stimulating reduction of a single substrate, inhibited reduction of the other three substrates at the same time. The general pattern of regulation is similar to that observed for the Escherichia coli enzyme, but the effector requirements were more distinct with the mammalian enzyme. Our results fully explain the variations in pools of deoxy-ribonucleoside triphosphates observed earlier for cells in tissue cultre upon exposure to certain inhibitors of DNA synthesis. They may also explain the mechanism behind two immunodeficiency diseases associated with an inherited deficiency in the enzymes adenosine deaminase or purine nucleoside phosphorylase. © 1979, American Chemical Society. All rights reserved.