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Purification and properties of an endoribonuclease existing as a complex with inhibitor in rat liver cytosol.

An endoribonuclease existing as a complex with inhibitor in the cytosol of rat liver has been purified about 128,000-fold after inactivation of the inhibitor with CdCl2. The enzyme had a molecular weight of 16,000 and produced 3'-CMP via 2',3'-cyclic phosphate of cytidine from poly(C). The breakdown of poly(U) by the enzyme was less than 5% of poly(C) breakdown. Poly(A) was not hydrolyzed by the enzyme. The enzyme had a pH optimum of 7.5-8, was heat-stable and had a Km of 952 micrograms yeast RNA and a Km of 198 micrograms poly(C) per ml. The maximal velocities for yeast RNA and poly(C) degradation were 3,970 A260/min/mg protein and 1,890 A260/min/mg protein, respectively. The enzyme was slightly stimulated by polyamines or monovalent and divalent cations except Mn2+, but was inhibited by nucleoside triphosphate, poly(G) and rat liver RNase inhibitor. Inhibition of the enzyme by rat liver RNase inhibitor was not prevented by monovalent and divalent cations or polyamines, although inhibition by poly(G) was prevented by these ions.