BIOSYNTHESIS OF PENTOSES 271 



Thymidine + phosphate -^ deoxyribose-1-P + thymine 



i 

 deoxyribose-5-P 



i 

 glyceraldehyde-3-P + acetaldehyde 



Fig. 10. Suggested pathway of thymidine degradation in E. coli (after Lampen'"''). 



For a discussion of the evidence indicating the existence of more complex methods 

 of pentose degradation by bacteria, the reader is referred to the review by Lampen.'" 

 Routes of ethanol formation are discussed by DeMoss.^' 



c. Fermentation of Deoxyrihose 



Very little information is available concerning the fermentation of deoxyribose. 

 Balance studies carried out in Lampen's laboratory" •''* on the fermentation of 

 thymidine by a strain of E. coli, indicated the conversion of 1 mole of deoxyribose to 

 1 mole each of formate, acetate, and ethanol. Since Racker^^ had already demon- 

 strated the reversible breakdown of deoxyribose-5-phosphate to glyceraldehyde-3- 

 phosphate and acetaldehyde in extracts of E. cob', the scheme shown in Fig. 10 was 

 suggested as the probable pathway of thymidine fermentation by this organism. In- 

 tact cells of E. coli cause no degradation of free deoxyribose. 



2. In Mammals 



In contrast to many bacteria, it is doubtful whether mammalian tissues can utilize 

 free pentoses. D-ribose is not metabolized by brain cortex slices or erj'throcytes,^ 

 nor D-xylose or D-ribose by rat diaphragm. i'^ An increased R.Q. has, however, been 

 reported in guinea pigs after ingestion of o-xjiose."* It is possible that animal tissues 

 can only metabolize phosphorylated pentoses. So far, however, only one animal 

 pentokinase, catalyzing the phosphorylation of D-xylose by ATP in the presence of 

 rat intestinal mucosa homogenates, has been reported."' Extracts of kidney cortex 

 cannot phosphorylate L-xylose, D-ribose, or L-arabinose."^ 



The tolerance of man and most mammals for pentoses is low and when ingested as 

 such they are largely eliminated unchanged in the urine and faeces."' In addition to 



"5 C. E. Hoffmann and L. A. Manson, Federation Proc. 10, 198, (1951) ; J. O. Lampen, 

 in "Phosphorus Metabolism" (McElroy and Glass, eds.), Vol. 1, p. 160. Johns 

 Hopkins Press, Baltimore, 1951. 



"6N. Northdurft, Pflugers Arch. ges. Physiol. 238, 567 (1937). 



1" M. P. Hele, Nahire 166, 786 (1950). 



"8 G. E. Youngberg, Arch. Biochem. 4, 137 (1944). 



"9 M. Rangier, P. M. de Traverse, and M. Bonvallet, Bxdl. soc. chim. hiol. 30, 583 

 (1948). 



