Effect of Divalent Cations on Bovine Pancreatic Ribonucrease*

(RNase A) activity was reinvestigated with well-characterized substrates such as benzyl cytidine 3'-phosphate (C-3'-p-Bz), cytidine 2', 3'-cyclic phosphate (C-cyclic-p) at pH 7.0, ft 0.1 and 37°C. At concentrations of 10" * and 10~4 M, Cu++ ion, and to a lesser extent Zn++ and Hg++ ions had an inhibitory effect. Other ions such as Mg++, Ca++ and Mn++ about which there have been reports of diverse results were found to have little effect. 2. The observed inhibition of RNase A activity at a low concen-tration of Cu++ and Zn++ at the initial stage of hydrolysis of C-3'-p-Bz cannot satisfactorily be explained by the formation of a ternary complex [E-Me++-cytidine 3'-phosphate (C-3'-p)] suggested by Ross ct al. A possible mechanism of the inhibition in the early stage of the reaction seems to be due to the formation of [E-Me++] complex. In the later stage of the reaction when the product, C-3'-p, accumulates to some extent, the formation of a ternary complex [E-Me++-C-3'-p] might be