IV PENTOSE FORMATION 5I 



basis of these inferences, Kit has estimated that twice as much exogenous glucose 

 is utilized for pentose formation via the direct oxidative pathway by lymphomas 

 as by normal lymphatic tissues (Kit, 1956). 



4. Evaluation of the transketolase-transaldolase 

 pathway and pentose formation 



The significance of the transketolase-transaldolase pathway of pentose formation 

 has been investigated by Bernstein (1953, 1956). Labelled acetate or formate was 

 administered to chicks and labelled bicarbonate to rats which had been fasted for 

 48 h. Tissue glycogen and the ribose of ribonucleic acids were isolated and de- 

 graded. The results may be illustrated by the experiments with rats. Labelled 

 bicarbonate is converted to carbons three and four of the glucose of glycogen by 

 the rat but there is very little isotope to be found in the other glucose carbons. It 

 was therefore anticipated that pentose carbons two and three should have been 

 labelled, if the pentose had been formed from glucose via the direct oxidative 

 pathway of glucose metabohsm. It was found, however, that ribose carbons three 

 and four contained by far the highest specific activity and that the specific activity 

 of carbon three of ribose was twice as great as that of carbon four. This pattern 

 of labelling was therefore indicative of the formation of ribose by a "C3 and C2" 

 condensation rather than of ribose formation by the direct oxidative pathway. 

 However, AgranofF has shown that the ratio of ^'^C derived from glucose- i-'^^C 

 compared with glucose-G-'^^C is reduced in the livers of rats fasted for 72 h. (Agranoff 

 et aL, 1954) while as indicated previously, Glock and McLean, 1956) reported 

 diminution in the dehydrogenases of the direct oxidative pathway in liver tissue 

 of fasting rats. It is therefore possible that the nutritional status of the animals in 

 part accounts for the results of Bernstein. 



In E. coli B, ribose is synthesized predominantly by the loss of hexose carbon- 1 (Lanning 

 and Cohen, 1954; Bernstein, 1956b). The distribution of radioactivity from labelled 

 glucose in the ribityl moiety of riboflavin has been measured in the growing mold, Ashbya 

 gossypii (Plaut and Broberg, 1956). The following results were obtained: 



When glucose-2-i-'C was the substrate, there was twice as much radioactivity in carbons 

 (2-4') of the ribityl molecule as in carbon i' and very little label in carbon 5'. These 

 results are consistent with the possibility that pentose is derived both by the direct oxidative 

 pathway and by the transaldolase-transketolase pathway. If both processes were operating 

 to an equal extent, one might expect with glucose-2-i'»C as substrate, that the molar 

 specific activity of carbons 2' to 4' portion of the ribityl molecule would be about 11/2 

 times as great as that of carbon i'; this is fairly consistent with the results obtained. 



Literature p. 124 



