Diadenosine polyphosphates inhibit the action of ribonucleotide reductase on ADP

Using permeabilized cells prepared according to Lammers, M. and Follmann, H. (Arch. Biochem. Biophys. 244, 430-438, 1986), the ribonucleotide reductase (EC 1.17.4.1) of yeast was assayed in situ. In our experimental conditions, 40-200 pmoles of ADP were reduced in 10 min per 5 x 10(7) cells. Concentrations of 0.01 mM of diadenosine tri, tetra, penta and hexaphosphates elicited inhibitions of 37, 40, 63 and 58%, respectively. The enzyme was almost completely inhibited in the presence of 0.05 mM diadenosine pentaphosphate. As some of these dinucleotides are present in yeast cells at these concentrations, the reported inhibition may have physiological meaning.