262 GERTRUDE E. GLOCK 



Significant incorporation of labeled P into nucleic acids of E. coli grow- 

 ing in the presence of fructose-6-P*^ has been reported by Roberts and 

 Wolff e.^^ Since far less effect was obtained with glucose-G-F^^ and none with 

 fructose- 1,6-P^2 they suggested that fructose-6-phosphate does not pass 

 through glucose-6-phosphate as an intermediate but is oxidized by a similar 

 independent process giving rise to pentose phosphate and nucleic acid. 

 This interesting observation requires confirmation. Incorporation of iso- 

 tope into nucleic acid was also found on feeding gluconate uniformly labeled 

 with C^'* to rats,^" but no degradation was carried out to determine to what 

 extent the pentose was labeled. 



Bernstein^^ has studied the in vivo synthesis of ribose in the chicken after 

 feeding acetate- 1-C^'*. The patterns of incorporation of the isotope into 

 glucose (glycogen) and PNA ribose differed to such an extent that the 

 direct conversion of hexose to ribose was excluded as a major pathway for 

 ribose synthesis. The results could, however, be explained by a condensa- 

 tion of C2 and C3 units. 



It is obvious that far more experimental data are required before it will 

 be possible to evaluate precisely the part played by the direct oxidative 

 pathway in the synthesis of PNA. Such investigations are, however, re- 

 stricted by the impermeability of most cells to phosphorylated sugars. 



V. Other Methods of Biosynthesis of Pentose Phosphates 



1. Condensation of C2 and C3 Units 



a. Synthesis of Xyloketose-1 -phosphate 



The aldolase-catalyzed condensation of dihydroxyacetone phosphate and 

 glycolaldehyde to form ketopentose-1 -phosphate was originally reported 

 by Lohmann.'^^ j^ h^g subsequently been prepared by Racker^^ with crys- 

 talline muscle aldolase and generally assumed to be xyloketose-1-phosphate. 

 The same ketopentose-1 -phosphate was presumably also synthesized from 

 glycolaldehyde and triose phosphate by extracts of Micrococcus pyogenes J'^ 

 The final characterization from the rate of acid hydrolysis^' and by identi- 

 fication of the sugar as D-xylulose'^^ is, however, relatively recent. This 

 pentose phosphate is of doubtful metabolic significance. Since only trans- 

 linkages are formed from aldolase-catalyzed condensations, the direct 

 formation of ribose-5-phosphate by this mechanism is precluded. 



«9 I. Z. Roberts and E. L. Wolffe, Arch. Biochem. and Biophys. 33, 165 (1951). 

 ■"> M. R. Stetten and DeW. Stetten, Jr., J. Biol. Chem. 187, 241 (1950). 

 '1 I. A. Bernstein, J. Am. Chem. Soc. 73, 5003 (1951). 

 " K. Lohmann, Angew. Chem. 49, 327 (1936). 

 73 E. Racker, Federation Proc. 7, 180 (1948). 



'^ J. Marmur and F. Schlenk, Arch. Biochem. and Biophys. 31, 154 (1951). 

 75 L. Hough and J. K. N. Jones, J. Chem. Soc. 1952, 4047; R. S. Forrest, L. Hough, 

 and J. K. N. Jones, Chemistry & Industry 1093 (1951). 



