The Ribonuclease ofPhaseolus AureusRoxb: III. — Effectors and Specificity

Abstract

Several divalent metal ions (Hg++, Cu++, Zn++ are powerful inhibitors of the Phaseolus aureus RNase activity. Other divalent metal ions inhibit less; none of those investigated activate the enzyme. Anionic polymers do not inhibit in conditions where they are very effective against pancreatic RNase. This is the case for heparin, sulphated pectins and poly-p, p-dioxydiphenyldimethyl metaphosphate. Polyphloroglucin phosphate is a rather potent inhibitor, although its action is less drastic than in the case of pancreatic RNase. The plant enzyme is less inhibited by its own degradation products than pancreatic RNase. DNA also shows inhibitory properties against the plant enzyme. The specificity of Phaseolus aureus RNase resembles that of other plant RNases. It works by a preliminary transphosphorylation so that cyclic nucleotides are formed. This happens at the level of both purine-3′- and pyrimidine-3′-phosphate linkages. The liberation of cyclic nucleotides proceeds in the order : guanylic, adenylic, uridylic and cytidylic acid. The enzyme is furthermore able to decyclize cyclic 2′-3′ purine nucleotides with formation of 3′-nucleotides only, at least in the case of cyclic adenylic acid. Actually one of the three chromatographic RNase fractions (fraction B) showed also a very small decyclizing ability against cyclic pyrimidine nucleotides.