IV. BIOCHEMICAL SYSTEMS 501 



Cohen and Scott""' '•''^ confirmed the formation of ribose-5-phosphate in 

 these reactions. 



Horecker and Smyrniotis"'- have purified the enzyme from yeast which 

 catalyzes this reaction and have been able to characterize the reaction. 



6-Phosphogluconate + TPN ^ TPNHo + CO. + ribulose-5-phosphate 



u 



ribose-5-phosphate 



The significance of these reactions may be considerably greater than 

 just as a mechanism for ribose synthesis. They may consitute an important 

 alternate pathway for the oxidation of glucose, and as a source of other 

 biological compounds. All the steps in the further metabolism of these 

 pentoses have not yet been elucidated. However, Horecker and Smyrniotis"^ 

 have demonstrated the formation of a 7-carbon sugar, sedoheptulose, which 

 is apparently formed by the condensation of ribulose phosphate and a 2- 

 carbon fragment. In addition it should be noted that these reactions are 

 reversible,"^ thus constituting a mechanism for the fixation of carbon di- 

 oxide, and may be of importance in photosynthesis. 



5. In Lipid Metabolism 



The dependence of fat metabolism on DPN-TPN-linked reactions has 

 been studied less extensively than with carbohydrates. It is clear that DPN 

 is required for the synthesis and degradation of glycerol. Dihydroxyacetone 

 phosphate, derived from the splitting of fructose- 1 ,6-diphosphate, can be 

 reversibly converted to L-a-glycerophosphate by an enzyme which requires 

 DPNHo as a hydrogen donor.^'*^' "^ 



O H O H 



II I II I 



HO— P— O— C— C— C— OH + DPNH2 ^ 



I I I 



O H H 



H H 



O H O H 



II III 



DPN + HO— P— O— C— C— C— OH 



I 111 



O H H H 



H 



'30 S. S. Cohen and D. B. M. Scott, Science 111, 543 (1950). 



»3i D. B. M. Scott and S. S. Cohen, J. Biol. Chem. 188, 509 (1951). 



"2 B. L. Horecker and P. Z. Smyrniotis, J. Biol. Chem. 193, 371, 383 (1951). 



'" B. L. Horecker and P. Z. Smyrniotis, Federation Proc. 11, 232 (1952). 



'3' B. L. Horecker and P. Z. Smyrniotis, J. Biol. Chem. 196, 135 (1952). 



'35 E. Baer and H. O. L. Fisher, /. Biol. Chem. 128, 463 (1939). 



