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Recent studies have considerably clarified the mechanism ofhydrolysis of ribonucleic acids (RNA) by pancreatic ribonuclease and by spleen phosphodiesterase. Digestion ofRNA with ribonuclease results in the splitting of all pyrimidine-pyrimidine internucleotidic links and of those pyrimidine-purine internucleotidic links which involve C(3w) of the pyrimidine nucleotide (Schmidt et al. 1951; Markham & Smith, 1952; Merrifield & Woolley, 1952; Volkin & Cohn, 1953). The first stage in the degradation involves the formation of pyrimidine nucleogide2':3' phosphates (cyclic nucleotides) and the simultaneous splitting of the internucleotidic phosphoester bond distal to C(31) ofthe latter. Thus it is possible to isolate from early ribonuclease-digests of RNA pyrimidine cyclic nucleotides and small polynucleotides comprised of purine nucleotide(s) terminating in a pyrimidine cyclic nucleotide (Markham & Smith, 1952). Further treatment with ribonuclease hydrolyses these pyrimidine cyclic nucleotides giving pyrimidine nucleoside-3' phosphates only (Brown, Dekker & Todd, 1952; Markham & Smith, 1952). Ribonuclease also splits simple esters of pyrimidine nucleoside-3' phosphates but not of pyrimidine nucleoside-2' phosphates to give the nucleoside-3' phosphate and an alcohol, the corresponding cyclic nucleotide appearing as an intermediate in the reaction (Brown & Todd, 1953). Purine nucleotide esters and purine cyclic nucleotides are not attacked by the enzyme (Markham & Smith, 1951a; Brown, Dekker & Todd, 1952; Brown & Todd, 1953). Thus, in these reactions, ribonuclease can be regarded as a phosphodiesterase specific for esters of pyrimidine nucleoside-3' phosphates. A phosphodiesterase fraction from spleen hydrolyses dinucleotides and simple esters ofnucleoside-3' phosphates but not of nucleoside-2' phosphates to yield in each case a nucleoside-3' phosphate (Heppel, Markham & Hilmoe, 1953; Brown, Heppel & Hilmoe, 1954). This enzyme fraction, however, * Present address: National Institutes of Health, Bethesda, Md., U.S.A.

biochemical journal. cellular aspects

Nucleotide exchange reactions catalysed by ribonuclease and spleen phosphodiesterase. 1. Synthesis and interconversion of simple esters of ribomononucleotides