324 F. SCHLEXK 



Klein^ suggests separate enzymes in tissues for the two cytosine nucleo- 

 sides. No cytidylic acid deaminase free from phosphatase has been ob- 

 tained. Deamination of cytosine by a different enzyme from some micro- 

 organisms has been observed; it does not act on cytidine or cytidyHc acid, 

 but can deaminate 5-methylcytosine to thymine. ^^ As yet nothing is known 

 about the deamination of 5-methylcytosine deoxyriboside, nor is there any 

 information about reversal of these deaminations. 



/. Decarboxylation of Orotic Acid and Its Homologues 



The activity of orotic acid as a precursor of pyrimidines is apparent 

 from the results of numerous experiments on the incorporation of the 

 labeled acid and the appearance of the isotope in the nucleic acid pyrimi- 

 dines.^^ All attempts to decarboxylate orotic acid in vitro have failed, how- 

 ever, and it appears that removal of the carboxyl group takes place on the 

 nucleoside or nucleotide level. This assumption is supported by the recent 

 isolation of orotidine^^ and by the observations of Hurlbert and Potter.®*'^* 

 They found that injection of orotic acid-6-C^^ into rats leads to accumula- 

 tion of labeled uridine-5'-phosphate and diphosphate in the liver. In vitro 

 experiments should soon clarify the steps of orotic acid utilization. 



g. Formation of 5 -Methyldeoxy cytidine and Thymidine 



It is not known whether 5-methyldeoxycytidine and thymidine are 

 formed by methylation of deoxy cytidine and deoxyuridine ; thymidine could 

 also result from deamination of 5-methyldeoxycytidine. The methyl group 

 may be introduced into the bases or even precursors of the latter. There 

 are no experiments with isolated enzyme systems. In vivo studies have 

 shown that the methyl group originates from one-carbon units such as for- 

 mate.*® ^^ Folic acid appears to play a part in the transfer of the single car- 

 bon unit. Shive believe that a direct synthesis of a conjugated form of thv- 

 mine is possible.*^ 



III. Biosynthesis of Nucleotides 



Information on the biosynthesis of nucleosides is far from complete; our 

 knowledge of nucleotide formation is scanty at best. A tempting speculation 

 is the assumption of enzymic phosphorylation in position 3 or 5 of the 

 pentose of nucleosides. On the other hand, observations have been recorded 

 which are contrary to this hypothesis. According to these recent reports, 



" E. Chargaff and J. Kream, J. Biol. Chem. 175, 993 (1948). 



S'' R. B. Hurlbert and V. R. Potter, /. Biol. Chem. 195, 267 (1952). 



" R. B. Hurlbert and V. R. Potter, J. Biol. Chem. 209, 1 (1954). 



6« D. Elwyn and D. B. Sprinson, /. Am. Chem. Soc. 72, 3317 (1950). 



" J. R. Totter, E. Volkin, and C. E. Carter, J. Am. Chem. Soc. 73, 1521 (1951). 



«8 W. Shive, Federation Proc. 12, 639 (1953). 



