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



but as yet no hydroxypyruvate kinase or oxidase has been 

 reported. However, [2-^*C]- and [3-^*C]hydroxypyruvates are 

 in fact quite well incorporated into liver glycogen, with less 

 randomization of radioactivity in glucose carbons than that 

 reported for pyruvate (Dickens, 1958a; Dickens and William- 

 son, 1959). The incorporation of hydroxypyruvate into 

 muscle glycogen is now being studied, and may provide some 

 evidence as to the possible existence of such a pathway. It 

 is not yet excluded that the incorporation observed in liver 

 may depend on TK acting on hydroxypyruvate and "formal- 

 dehyde" (Dickens and Williamson, 1958). 



Krebs and Romberg (1957) reject, on the basis of the 

 earlier experiments of Krebs (1954), the view that in liver 

 G6P- and 6-PG-dehydrogenases could provide enough TPNH 

 for the "malic" enzyme in the above by-pass (cf. Fig. 2). 

 However, the experiments quoted were made with dilute 

 (1 : 30 pigeon liver) homogenates, in which the enzymes 

 themselves and the concentration of total TPN (apparently 

 none was added) must have been very low compared with 

 those in the intact tissue. Their finding that more malate 

 was formed under conditions where ATP was being actively 

 synthesized (aerobic; no DNP added) could perhaps be 

 explained if the possibility of a "direct" type of PEP forma- 

 tion, based on a high ATP level under oxidative conditions, 

 cannot be excluded in these experiments even in the case of 

 liver tissue. This would avoid the necessity for the ingenious 

 explanation advanced by Krebs and Kornberg (1957), that 

 a high ATP level can cause a reduction of TPN+ to TPNH by 

 a kind of reverse oxidative phosphorylation reaction of the 

 following type : 



Reduced flavoprotein + ATP^- + TPN+ + Ufi -> flavo- 

 protein + ADP^- + TPNH + 2H+ + HPO42-. 



As was pointed out above, such a reaction even in the usual 

 direction of oxidative phosphorylation has not yet been clearly 

 experimentally demonstrated for TPN, although the idea 

 itself is undoubtedly of great theoretical interest. 



