STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 179 



active in degradation than in synthesis. However, even if given deoxyribose- 

 5-phosphate, a mutase for conversion to deoxyribose-1-phosphate is not 

 present in all organisms, thereby obviating a phosphorylase route to nucleo- 

 sides for some organisms. In addition, the pyrophosphorylation of deoxyribose- 

 5-phosphate has been sought without success, making it very dubious that 

 deoxyribose-5-phosphate is on a more direct route to deoxyribotide synthesis. 

 Isotope studies of deoxyribose synthesis have not revealed a single instance 

 wherein deoxyribose could arise by this aldolase mechanism. On the other 

 hand, existing isotope studies in intact organisms have indicated the direct 

 reductive conversion of ribosyl derivatives to deoxyribosyl derivatives 

 (Brown, 1956; Loeb and Cohen, 1957; Reichard, 1957). 



Isotopic competition experiments have indicated that the pyrimidine 

 deoxyribosides are not intermediates m the formation of DNA pyrimidines 

 in virus-infected E. coli (Cohen et al., 1957). Although significant 

 amounts of pyrimidine deoxyribosides have been isolated from a number of 

 normal and tumor tissues (Schneider, 1955), it was not felt that these com- 

 ponents rather than the nucleotides were important intermediates in DNA 

 synthesis in regenerating liver (Hecht and Potter, 1956c). Thus, it has been 

 thought that the conversion occurs most actively at the nucleotide level. 

 However, in a recent report, evidence has been presented for an enzymatic 

 conversion of a ribonucleoside to deoxyriboside in bacterial extracts 

 (Grossman and Hawkins, 1957), 



b. Synthesis of Purine Ribotides. The outstandmg work of Buchanan, 

 Greenberg and their collaborators has resulted in an almost complete dis- 

 section of the routes of purine biosynthesis. Unlike pathways of carbo- 

 hydrate metabolism, for which variants are known at almost every step, 

 this pathway appears essentially the same in all organisms tested. As indi- 

 cated earlier, 1-pyrophosphoryl ribose-5-phosphate is converted by reaction 

 with glutamine to the 5-phosphoribosylamine and this compound becomes 

 the foundation on which the purine superstructure is built. The reaction 

 sequence is presented in formula (XXVIII) at top of j). 180. 



It can be seen that inosinic acid is the branch point for the formation of 

 both adenylic acid and guanylic acid. The conversion of inosinic acid to 

 adenylic acid is effected by the intermediary formation of adenylosuccinic 

 acid as a result of condensation of aspartic acid with inosinic acid and de- 

 acylation of adenylosuccinic acid. This deacylase is apparently the same as 

 the deacylase involved m the cleavage of the earlier succinylo-derivative 

 formed in the reaction chain (MiUer et al., 1957; Gots and Gollub, 1957). The 

 formation of guanylic acid from inosinic acid requires an initial dehydrogena- 

 tion with DPN to xanthylic acid (Lagerkvist, 1955; Magasanik et al., 1957) 

 which is then aminated by glutamine in the presence of ATP to guanylic 

 acid. 



