PHOSPHORYLATION OF CARBOHYDRATES 187 



given at the bottom of the table. There can be no doubt that lactic 

 acid is formed here by a phosphorylating mechanism. This is not to 

 imply that such a mechanism is the only one that has been invented 

 by nature for the degradation of carbohydrate, but so far as animal 

 tissues are concerned it would seem that the burden of proof is on 

 those who claim that a non-phosphorylating glycolysis exists. 



Table 8.— Glycolysis in rat brain extract 



(0.2 cc. of extract (equivalent to 40 mg. of brain) were made up to 2 cc. with addition 

 of Mg"*""*", phosphate-bicarbonate buffer, and catalytic amounts of adenosinetriphos- 

 phate and cozymase. Incubated 90 minutes at 38° C. From J. Biol. Chem., IJtl, 

 245, 1941.) 



* 2 hexose + 2H3P04= 2 lactic acid + 1 hexose diphosphate. 



t 2 hexose monophosphate = 2 lactic acid+1 hexose diphosphate. 



% 1 hexose diphosphate = 2 lactic acid + 2H3P04. 



Table 9 summarizes the essential reactions of the phosphate cycle. 

 One important feature of this scheme is that the concentration of 

 inorganic phosphate has a marked influence on the rate of enzymatic 

 reactions. This is obvious in the case of the first reaction, since 

 whether glycogen will be broken down or synthesized depends en- 

 tirely on the relative concentrations of inorganic phosphate and 

 glucose-1-phosphate. It may also be pointed out that the oxidation of 

 triosephosphate and of pyruvate cannot occur in the absence of 

 inorganic phosphate and that its concentration therefore has a 

 marked effect on the rate of oxidation. Another feature of the scheme 

 which needs to be emphasized is that the rate of oxidation is also 

 dependent on the availability of a phosphate acceptor, because of the 

 fact that oxidation and phosphate transfer are coupled reactions. It 

 is for this reason that the addition of adenylic acid is often found 

 to have a marked stimulating effect on carbohydrate oxidation in an 

 enzyme system. Finally, other phosphate acceptors, particularly 

 creatine, and the dephosphorylating enzymes (phosphatase, adenyl- 



