CARBOHYDRATE METABOLIC PATHWAYS 83 



Horecker and Mehler (66) showed that Bernstein's results 

 were in agreement with expectations from the transaldolase- 

 transketolase sequence operating in reverse. 



2. In higher plants, a close relative of the cycle, ribulose 

 1,5-diphosphate, acts as a primary acceptor of COo (67, 68); 

 the subsequent reactions leading to polysaccharide synthesis 

 appear to employ the reverse, or reductive pentose cycle 

 (69): 



3 Pentose-P + 3 ATP -^ 3 Ru-di-P + 3 ADP 



3 Ru-di-P + 3C02 + 3H2O -> 6 P-glycerate 



6 P-glycerate + 6 ATP -> 6 di-P glycerate + 6 ADP 



6 di-P glycerate + 6 TPNH + 6 H+ 



^ 6 GLA-P + 6 TPN+ + 6 PO/" 

 2 GLA-P ;=± 2 DHA-P 

 2 GLA-P + 2 DHA-P ^ 2 F-1, 6-di P 

 2 Fl,6-di P + 2H2O -^ 2 F-6-P + 2 Pi 



F-6-P + GLA-P ^ Xu-5-P + E-4-P 



F-6-P + E-4-P ^ SH-7-P + GLA-P 



SH-7-P + GLA-P ^ 2 pentose-P 



Sum: 3 CO2 + 9 ATP + 5 H2O + 6 TPNH + 6 H+ 



-^ GLA-P + 9 ADP + 6 TPN+ + 8 PO/~ 



3. A third experiment which points to a synthetic role 

 for the pentose cycle is the finding by Jolley et al. (29) 

 that this complex of enzymes is much more active in the 

 mammalian foetus than in the adult, and is most active 

 in the very young embryo (about one-fourth of the total 

 glucose oxidation appears to proceed by phosphogluco- 

 nate cleavage). There is a gradation toward adult character- 

 istics which is reached about three-fourths of the way 

 through the gestation period. From this point onward the 

 metabolic "traffic" in the heart of the foetus becomes vir- 



