IV 



PENTOSE FORMATION 



53 



Experiments have also been carried out with E. coli B cells (Rose and Schweigert, 

 1953). In this organism, free cytosine and cytidine are essentially equivalent as 

 sources of RNA pyrimidines. It was observed that extensive cleavage of the gly- 

 cosidic linkage occurred during the incorporation of uniformly labelled cytidine 

 into both the RNA-pyrimidine nucleosides and the DNA-pyrimidine nucleosides. 

 The ratio of radioactivity of pyrimidine to sugar was essentially the same for the 

 RNA-cytidine and uridine and the DNA-deoxycytidine and thymidine although 

 in each case considerably greater than that for the cytidine substrate. Vitamine 

 Bj2 may be involved in the de novo synthesis of deoxyribose (Downing and Schweig- 

 ert, 1956). 



[c) Phospho gluconic dehydrogenase pathway 



Pseudomonas saccharophila degrades glucose to pyruvate and D-glyceraldehyde- 

 3-phosphate by the metabolic pathway shown in Fig. 18 (McGee and Doudoroff, 



1) Glucose 



ATP 



Glucose — 6— phosphate 



TPN' 



6-Phosphogluconate 



2) COOH 



I 

 HCOH 



HOCH 



1 

 HCOH 



HCOH 



■H,0 



H2COPO3H2 



2 — Keto — 3 — deoxy- 

 6 — phosphogluconic 



H2COPO3H2 

 6 — P— Gluconic 

 Fig. 18. Metabolism of 6-phosphogluconate by Pseudomonas saccharophila 



Deoxyribose— 5— phosphate 



COOH CHO 



C = -f- CHOH 



I I 



CHj CH20P03H2 



Pyruvic 



Glyceraldehyde 

 phosphate 



1954). 2-Keto-3-deoxy-6-phosphogluconate has been identified as an intermediate 

 in the reaction. The latter compound is extremely interesting in that a decar- 

 boxylation of this compound would result in the formation of deoxyribose-5- 

 phosphate. However, evidence for such a decarboxylation is still lacking. This 

 intermediate is also of interest in that it is produced from 6-phosphogiuconate 

 by a dehydration reaction, a reaction similar to one which may take place during 

 the conversion of cytidine to deoxycytidine. 



B. The Biosynthesis of Amino Acids 



I. Distribution of free amino acids in tissues 



Animal tissues, plants, and many microorganisms contain intracellular pools of 

 free amino acids (Kit and Awapara, 1953, 1954; Kit, 1953; Awapara et al., 1950; 

 Roberts and Tanaka, 1956; Schurr et al., 1950; Solomon et al., 1951 ; Spiegelman 

 et al., 1955). The concentrations of the free amino acids differ from tissue to tissue 



Literature p. 124 



