BIOSYNTHESIS OF PENTOSES 267 



ing, preparation of ribose-1 -phosphate has been effected by the use of this 

 crude system. ^^ Klenow and Larsen^'' have recently found that crystalUne 

 phosphoglucomutase catalyzes the conversion of ribose-1 -phosphate to 

 ribose-5-phosphate and that glucose- 1,6-diphosphate acts as a coenzyme. 

 Evidence is also presented to support the view that phosphoribomutase is 

 probably identical with phosphoglucomutase, and, by analogy with the 

 findings of Leloir^* in connection with the conversion of mannose-1-phos- 

 phate to mannose-6-phosphate, it is suggested that the following reaction 

 occurs: 



ribose-1 -P -)- glucose-1 ,6-diP ;^ riboge-l,5-diP -|- glucose-6-P 



This view is supported by the isolation of ribose-1, 5-diphosphate from the 

 reaction products obtained on incubating glucose-l,6-diphosphate and 

 ribose-1-P^- with muscle phosphoglucomutase, and the demonstration that 

 it can serve as a coenzyme for both phosphoribomutase and phospho- 

 glucomutase. 



b. Phosphodeoxyribomutase 



Lampen and his co-workers^® have shown that the degradation of deoxy- 

 ribosenucleosides by liver and thymus extracts and by intact cells of E. 

 coll is accompanied by the formation of deoxyribose-1-phosphate. This is 

 transformed irreversibly into deoxyribose-5-phosphate by "phosphodeoxy- 

 ribomutase." 



hypoxanthine deoxyriboside -{- phosphate — > deoxyribose-1-P -|- hypoxanthine 



deoxj'ribose-5-P 



Moderate concentrations of phosphate, sulfate, and arsenate inhibit this 

 mutase. 



3. Pentose Isomerases 



Two adaptive pentose isomerases have been reported, one in E. coli^^ 

 and the other in Lactobacillus pentosus,^'' which catalyze the interconversion 

 of D-arabinose and D-ribulose, and D-xylose and D-xylulose, respectively. 

 In both cases, approximately 15 % of the keto-sugar is present at equi- 

 librium. 



^* H. Klenow and B. Larsen, Abstr. 2nd Intern. Congr. Biochem., Paris p. 235 (1952); 



Arch. Biochem. and Biophijs. 37, 488 (1952). 

 '* L. F. Leloir, in "Phosphorus Metabolism" (McElroy and Glass, eds.), Vol. 1, 



p. 67. Johns Hopkins Press, Baltimore, 1951. 

 " L. A. Manson and J. O. Lampen, /. Biol. Chem. 191, 95 (1951) ; C. E. Hoffmann and 



J. O. Lampen, J. Biol. Chem. 198, 885 (1952). 

 "J. O. Lampen in "Phosphorus Metabolism" (McElroy and Glass, eds.), Vol. 2, 



p. 363. Johns Hopkins Press, Baltimore, 1952. 



