266 GERTRUDE E. CLOCK 



ribose ester and 7 % an ester of either L-arabinose or glucose. This method 

 of formation of pentose phosphate does not appear to have been investi- 

 gated further. 



Horecker and Smyrniotis^^ have suggested that the transfer of a dihy- 

 droxyacetone group by "transaldolase" from one phosphorylated sugar to 

 another, which they demonstrated in connection with the formation of 

 fructose-6-phosphate from sedoheptulose-7-phosphate (see Sect. II.3.6.), 

 is probably not confined to this particular reaction. It may be a general 

 method of synthesis of 2-keto-sugar phosphates including 2-ketopentose 

 phosphates. 



Although not completely relevant in connection with the synthesis of 

 pentose phosphates, the formation of ketopentoses by oxidation of penti- 

 tols should be mentioned. This has been demonstrated by Hudson and his 

 co-workers,^^ who found that Acetobader suboxydans produced good yields 

 of D-xylulose from D-arabitol. 



VI. Interconversion of Pentose Phosphates and Pentoses 



1. Phosphopentose Isomerase 



The presence of a specific phosphoribose isomerase catalyzing the inter- 

 conversion of ribulose-5-phosphate and ribose-5-phosphate was first demon- 

 strated in a purified yeast enzyme preparation by Horecker and Smyrni- 

 otis.'^'^^ At equilibrium, approximately 80% of the ribose ester is present. 

 The ubiquitous distribution of these two pentose phosphates suggests that 

 this also applies to the isomerase. Purified phosphoribose isomerase from 

 alfalfa leaves was found to be inhibited by p-chloromercuribenzoate." 



2. Phosphopentomutases 



a. Phosphor ibomutase 



The existence of an enzyme catalyzing the formation of ribose-5-phos- 

 phate from ribose- 1 -phosphate was suggested by Schlenk.^" Kalckar^^ 

 showed that the transformation of inosine into hypoxanthine by crude liver 

 nucleoside phosphorylase in the presence of inorganic phosphate was 

 accompanied by the formation of ribose- 1 -phosphate. This was converted 

 into an acid-stable ester, presumably ribose-5-phosphate, by the presence 

 of phosphoribomutase in the liver extract. Later work confirmed this^^'^^ 

 and, on account of the rapid surface denaturation of this mutase on foam- 



89 R. M. Hann, E. B. Tilden, and C. S. Hudson, J. Am. Chem. Soc. 60, 1201 (1938). 



90 F. Schelnk, Advances in Enzymol. 9, 473 (1946). 

 " H. M. Kalckar, /. Biol. Chem. 167, 477 (1947). 



92 J. Wajzer and F. Baron, Bull. soc. chim. biol. 31, 750 (1949). 



93 A. Abrams and H. Klenow, Federation Proc. 10, 153 (1951); Arch. Biochem. and 

 Biophijs. 34, 285 (1951). 



