INTERMEDIATE CARBOHYDRATE METABOLISM 7 



is not reduced. On the contrary, such an oxidation of dihydro- 

 cozymase shifts the equihbrium in the opposite direction, so that 

 lactic acid, if present, would be oxidized by cozymase to pyruvic acid, 

 whereas in the stationary state of sugar oxidation pyruvic acid would 

 be continuously formed by way of phosphoglyceric acid, without a 

 compensating reduction. 



Therefore only pyruvic acid, and not lactic acid, is formed in the 

 stationary state of oxidation. This interpretation at the same time 

 gives a clue to the oxidation quotient, the numerical relationship 

 between the oxygen consumed and the lactic acid that is prevented 

 from being formed: if one atom of oxygen is required to oxidize the 

 two hydrogen atoms of dihydrocozymase, then this atom prevents 

 one niolecule of pyruvic acid from being reduced to lactic acid or in 

 yeast fermentation to alcohol. Therefore six atoms of oxygen (corre- 

 sponding to the complete oxidation of one molecule of lactic acid) 

 can prevent six molecules of lactic acid from being formed, and we 

 obtain the normal oxidation quotient of 6. Of course this refers only 

 to the principle. The cozymase reoxidized by oxidative catalysts 

 must dehydrogenate other intermediary stages besides triosephos- 

 phate, because every oxidative step in the breakdown of sugar acts 

 in the same way, preventing the formation of one molecule of lactic 

 acid per one atom of oxygen taken up. 



And this is only one side of the picture. If the breakdown of sugar 

 in oxygen and in nitrogen proceeded with the same speed to the 

 stage of pyruvic acid, and the only difference consisted in the fate 

 of pyruvic acid to be reduced or further oxidized, then the oxidation 

 would not prevent, as it actually does, by this so-called "Pasteur 

 effect," the greater part of sugar from disappearing. But here the 

 concept of metabolic cycles has its place. Actually every oxidative 

 step is coupled with the phosphorylation of the adenylic system, and 

 by this means a corresponding phase of anaerobic breakdown is 

 reversed, so that for every oxygen atom consumed one three-carbon 

 molecule can return to its initial stage as sugar or glycogen. This 

 state of affairs is very neatly shown by the recent experiments of 

 Cori, Kalckar, and co-workers (16) with dialyzed extracts of kidney 

 and heart, and by experiments of Behtzer and Tzibakowa (17) with 

 washed pigeon muscle. Cori and his group found that in the pres- 

 ence of the complete glycolytic coenzyme system the organ extracts 

 oxidize glucose and phosphorylate an excess of it, so that for every 

 hexose molecule burned to carbon dioxide, ten molecules of phos- 

 phate are taken up to form five molecules hexosediphosphate; and 



