COOH 

 HCOH 

 HOCH 



HCOH 



I 

 HCOH 



I 

 CH2OPO3H2 



6-Phospho- 

 gluconate 



BIOSYNTHESIS OF PENTOSES 

 COOH 



HCOH 

 C=0 



HCOH 



I 

 HCOH 



I 

 CH2OPO3H2, 



3-Keto-6-phospho 

 gluconate 



— C02 



CH2OH 

 C=0 



HCOH 



1 

 HCOH 



CH2OPO3H2 



Ribulose-5- 



phosphate 



251 



HC=0 

 HCOH 



HCOH 



I 

 HCOH 



CH2OPO3H2 

 Ril)Ose-5- 

 phosphate 



Fig. 2. Formation of pentose phosphates from 6-phosphogluconate according to 

 Horecker et al.^^ 



librium is reached when 70 to 80 % of the pentose phosphate is in the form 

 of ribose-5-phosphate, this interconversion being catalyzed by a pentose 

 phosphate isomerase. Similar results were obtained with a partially purified 

 liver 6-phosphogluconate dehydrogenase preparation.^^ 



The occurrence of ribulose-5-phosphate as a precursor of ribose-5-phos- 

 phate suggested that 6-phosphogluconate might be oxidized at the 3- 

 position to give the hypothetical intermediate 3-keto-6-phosphogluconate, 

 which on decarboxylation would yield ribulose-5-phosphate according to 

 Fig. 2.'^ No arabinose-5-phosphate was formed with this purified dehy- 

 drogenase preparation, indicating that this is not an intermediate in the 

 conversion of 6-phosphogluconate to ribose-5-phosphate. 



Axelrod and Jang^^ have recently obtained similar results with a par- 

 tially purified spinach leaf preparation, using C^^-labeled 6-phosphogluco- 

 nate. 



3. Metabolism of Ribulose-5-Phosphate 



a. Formation of Triose Phosphate 



Triose phosphate has been identified as one of the products of pentose 

 phosphate cleavage by enzymes of red cells,^" bacteria and yeast,^^ liver,^^ '^^ 



'8 J. PI SeegmillerandB.L. Horecker,/. B/oLC/iem. 194, 261 (1952). 

 '9 B. Axelrod, R. S. Bandurski, C. M. Greiner, and R. Jang, J. Biol. Chem. 202, 619 

 (1953). 



20 Z. Dische, Naturwissenschaften 26, 252 (1938). 



21 E. Racker, m "Phosphorus Metabolism" (McElroy and Glass, eds.), Vol. 1, p. 

 147. Johns Hopkins Press, Baltimore, 1951; H. Z. Sable, Federation Proc. 10, 241 

 (1951). 



" B. L. Horecker and P. Z. Smyrniotis, J. Am. Chem. Soc. 74, 2123 (1952). 

 "G. E. Clock, Biochem. J. 52, 575 (1952). 



