52 INTERMEDIARY METABOLISM AND GROWTH I 



5. Deoxyribose synthesis 

 [a) Deoxyribose aldolase 



The enzyme, deoxyribose aldolase, which catalyzes reaction i, has been ob- 

 served in E. coli, C. diphtheriae, S.faecalis, liver and thymus (Racker, 1951, 1952). 



i) Acetaldehyde + glyceraldehyde-phosphate ' — >• deoxyribose phosphate 



This reaction constitutes a possible mechanism for deoxyribose synthesis from 

 two and three carbon compounds. Consistent with this mechanism is the obser- 

 vation that when E. coli cells, or E. coli cells infected with bacteriophage were grown 

 with glucose- 1 -'■*C as the sole carbon source, they converted glucose- i-'^^G to 

 deoxyribose of the DNA of the bacteria or the phage (Lanning and Cohen, 1955). 

 In these experiments, the deoxyribose of is. coli had 20-26% and the deoxyribose 

 of phage 41-59% of the molar specific activity of the original glucose. It was esti- 

 mated that deoxyribose phosphate synthesized by the combination of acetaldehyde 

 and glyceraldehyde should have had an activity of 76 to 100% of that of the ref- 

 erence glucose. It is therefore apparent that the functioning biosynthetic mechan- 

 ism was not correctly represented by the enzymatic condensation alone. 



{b) Ribose as a deoxyribose precursor 



A second possible source of deoxyribose is ribose or a derivative of ribose. In 

 growing E. coli cells, approximately 37% of the glucose which is metabolized is 

 degraded via the phosphogluconate pathway. The ribose which is required for 

 ribonucleic acid synthesis is derived principally from this pathway. However, the 

 participation of the transaldolase-transketolase pathway for ribose synthesis is 

 also indicated since the ribose of RNA contained about 25% of the molar specific 

 activity of the glucose- i-'^^C, in experiments in which the latter substance was 

 the sole carbon source (Lanning and Cohen, 1954, 1955)- In phage infected cells, 

 ribonucleic acid synthesis is reduced and the activities of the bacteria are directed 

 to the synthesis of viral constituents. It is conceivable that an increase in the trans- 

 aldolase-transketolase pathway of pentose synthesis is one of the alterations in 

 E. coli metabolism incident to the synthesis of viral deoxyribose. Thus, the ob- 

 served ratio of the deoxyribose to glucose radioactivity in growing E. coli cells and 

 in phage infected E. coli cells which were quoted in the last section are as plausible 

 on the basis of a C3 plus C, condensation involving the transaldolase-transketo- 

 lase system as on the basis of the deoxyribose aldolase reaction. There is evidence 

 to suggest that a ribose to deoxyribose conversion can occur at the nucleoside or 

 nucleotide level. Rose and Schweigert (1953) administered uniformly labelled 

 cytidine-^'*C to rats and isolated the cytidine and deoxycytidine from the nucleic 

 acids of the rat tissues. It was observed that the molar ratio of the radioactivity of 

 the pyrimidine alone to that of the entire nucleoside was approximately one in 

 both cases, indicating that the original nucleoside had been incorporated into 

 the nucleic acids without a rupture of the sugar-pyrimidine linkage. Similar 

 experiments have been performed with labelled cytidylic and adenylic acids (Roll 

 et al., 1956b). On the other hand, with uridylic acid or guanylic acid, some cleav- 

 age of the glycosidic linkage occurs during deoxynucleoside synthesis (Roll et al., 

 1956a). 



