Oxidative Pathways of Carbohydrate Metabolism 159 



Transketolase (TK) requires thiamine pyrophosphate and 

 magnesium ions. Since all of these reactions are quite freely 

 reversible, the individual values of AG' must be quite small, 

 as is indicated in Fig. 2 by " AG' ca. O". 



According to Axelrod and Jang (1954), AG' of reaction (1) 

 is —0-7 kg. cal. (37°, pH 7). There is still considerable doubt 

 about the exact equilibrium positions of these reactions (1)- 

 (5) (see Dickens, 19586; Tabachnick et al., 1958) but the 

 equilibrium constants all appear to lie within the range 

 0-8-3, i.e. AG' is in the region of zero to unity. 



By this method F6P is resynthesized from pentose phos- 

 phates, including the d-Xu5P mentioned above as being pro- 

 duced from hexose in the glucuronate cycle, and also from the 

 D-Ru5P arising oxidatively from the TPN-linked dehydro- 

 genases acting as G6P (stage a, above). For each turn of the 

 cycle (Figs. 2 and 3), one terminal carbon of hexose is removed 

 as COg and 2 TPN+ are reduced, one pentose being formed. 

 After six such turns the equivalent of one hexose mole has 

 been completely oxidized to COg and water; the 6 pentose 

 moles formed could then yield, by the sequence of reactions 

 (l)-(5) above, 5 molecules of hexose — one of them being 

 thought to be produced via the action of triose phosphate 

 isomerase, aldolase and fructose diphosphatase, on the GA3P 

 appearing in equation (6) above. During this process no less 

 than 12 moles of TPNH should be produced per mole of 

 hexose disappearing. 



Reoxidation of reduced TPN 



Whereas the formation of three moles of ATP from ADP is 

 coupled with the oxidation of one mole of DPNH by the 

 flavoprotein-cytochrome oxidative chain of reactions (Lehn- 

 inger, 1955), a similar proof for the oxidative phosphorylations 

 accompanying TPNH oxidation is lacking at present (Kaplan 

 et al, 1956; Ball and Cooper, 1957). 



Joshi, Newburgh and Cheldelin (1957) who used a mito- 

 chondrial fraction of heart, supplemented by a soluble fraction, 



