Synthesis and Interconversion of Simple Esters of Ribomononucleotides

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.