STUDIES ON NUCLEAR EXORIBONUCLEASES .3. ISOLATION AND PROPERTIES OF ENZYME FROM NORMAL AND MALIGNANT TISSUES OF MOUSE

An exoribonuclease, which degrades singlestranded ribonucleic acid to nucleoside 5′-monophosphates, has been isolated from the nuclei of mouse liver, kidney, embryo, mammary tumor, and Ehrlich ascites tumor. The enzyme is inactivated by heating at 50° and treatment with 3.2 m urea. The enzyme is specific for polyribonucleotides; it will not hydrolyze deoxyribonucleic acid, pTpT, or thymidine 5′-pnitrophenylphosphate. Studies on the mechanism of attack on polyribonucleotides show that: (1) the enzyme (which degrades from the 3′-OH end) catalyzes attack on the 5′-phosphorus atom of the termina nucleotide, with resultant P-O splitting to yield a nucleoside 5′-phosphate; (2) a single polynucleotide molecule stays bound to the enzyme until that polynucleotide molecule is degraded to completion, in a manner similar to the mechanism for degradation of polynucleotides by bacterial polynucleotide phosphorylase and ribonuclease II; and (3) the enzyme is strongly inhibited by the presence of terminal 3′-phosphate or 2′,3′-cyclic phosphate groups on a potential substrate molecule. © 1969, American Chemical Society. All rights reserved.