162 



F. Dickens, G. E. Clock and P. McLean 



The 12 TPNH so formed could carboxylate 12 moles of 

 pyruvate : 



Pyruvate- + TPNH + CO2 ^ malate^- + TPN+ 



thus yielding, by the "by-pass", 12 PEP and so eventually 

 6 moles of glucose: ratio of glucose resynthesized/glucose 

 oxidized = 6/1. 



The possibility of this coupling of a TPNH-dependent 



MB 



t-0-5 



"F6Pl-^ 



2PEP — ^ 

 F6P+ HPO4" 



AG'-6/moIe F6P 



6 Pentose P -fH20 

 ^5F6P + HPO4 



AG'-2/mole F6P 



ITP 



DPNH, 



IDP 



t 



PEP 



ILACTATEI 



Qv. AG -3-5/mole 

 approx. 



Fig. 4. Coupling of HMP oxidative pathway with carboxylation of 

 pyruvate by means of TPNH. 



The numbers on arrows indicate approximate value of AG' for the 

 reactions shown (cf. Fig. 2). 



reductive carboxylation with reversal of glycolytic reactions 

 requires the presence of G6P- and 6-PG-dehydrogenases as well 

 as TPN in the tissue concerned. This is the case in liver, a 

 tissue in which randomization of pyruvate carbon atoms in 

 the glucose of the glycogen synthesized accords well with an 

 indirect route of pyruvate incorporation, in which carboxyla- 

 tion to give a stage involving a symmetrical dicarboxylic acid 



