156 



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



phosphate pathway. At this stage, G6P can undergo a two- 

 stage TPN-Unked oxidation, the first oxidation yielding 

 6-phosphogluconolactone (Cori and Lipmann, 1952): 



G6P2- + TPN+ -> 6-PG lactone^- + TPNH + H+ 



whilst the slow hydrolysis of this lactone is then accelerated 

 by a widely distributed but little studied lactonase (see 



I GLYCOGEN I <-^fGTPl 



ATP 



GLUCOSE 



^^ H 



ME ^^^, 





TPN 

 -1 ^TPNH 



ATPx |F6Pk ^ I6PGLACT0NE | 



-4i \(AG;, XH^O -5 



IDP J ' "' 'vT^'v RECYCLING 



^^ / \6\ (AG'ca.O) 



\l?^(iO/—^ \?'^I? -^^ ^^}^ 



OAA^ C; iC02j^ y lPYRUVATEl ^^ lLACTATEl 



DPNiK L^TPNH 



/ 1?H 



ICITRATE.etcl ^^^ IMALATEI —fv^ iFUMARATE.etcl 



Fig. 2. Approximate free-energy changes involved in glycolysis and the 



hexose monophosphate oxidative pathways. 

 Values of AG' represent free energy changes at pH 7 and are rounded to 

 nearest 0-5 kg. cal./mole (Krebs and Romberg, 1957; see also text). 

 "Citrate, etc" and "Fumarate, etc" indicate points of linking with Krebs 



cycle reactions. 



Fig. 2); the same enzyme appears to hydrolyse S-glucono- 

 lactone QLipmann and Brodie, 1955): 



6-PG lactone^- + H2O -> G-PG^- + H+. 



It is unfortunate that accurate free energy values for these 

 reactions are not available. The similar reaction with partially 



