The chemistry of the isomeric cytidylic acids; enzymic hydrolysis to cytidine.

It has been shown in previous publications that neither of the cytidylic acid isomers isolated from hydrolysates of yeast ribonucleic acid is oxidized by sodium periodate under conditions in which an unsubstituted Q: ,@-glycol grouping is readily oxidized (1, 2). These results indicate that each compound is phosphorylated in either the 2’ or the 3’ position and eliminate cytidine-5’-phosphate as a possibility for the structure of either isomer. The remaining possibilities for isomerism involve either a different location of the phosphate group on the same nucleoside unit (Fig. 1, a and b) or, alternately, the presence of an LY or /l configuration of the ribose residue phosphorylated in the same or in different positions (Fig. 1, b and c). In the case of the isomeric adenylic acids obtained from ribonucleic acid hydrolysates, conflicting evidence has been presented as to the reason for the isomerism. Carter obtained evidence on enzymic dephosphorylation that both isomers formed adenosine (3). In a later presentation, however, Doherty (4) found that both compounds were 3’-phosphates and that the isomerism was explained by the presence of an o( configuration (Fig. 1, c) in one isomer and of the p configuration (Fig. 1, b) in the other. It is apparent from the structures shown that dephosphorylation in one case (Fig. 1, a and b) would lead to the formation of the same nucleoslde and in the other (Fig. 1, b and c) would yield different nucleosides. Studies of the utilization of the isomeric cytidylic acids for growth of the pyrimidine-deficient Neurospora were made before and after dephosphorylation to provide evidence regarding the identity of the nucleosides concerned. Contrary to previously published reports which showed utilization of the pyrimidine nucleotides after autoclaving (5), it was now found that the phosphorylated compounds were not utilized by the deficient Neurospora mutant and that the previous results were due to dephosphorylation during autoclaving. After dephosphorylation with prostatic phosphatase, the two cytidylic acid isomers gave a product with the same growth-promoting activity for the pyrimidine-deficient Neurospora mutant 1298, in-