Oxidative Pathways of Carbohydrate Metabolism 165 



Data on enzymes concerned with G6P metabolism 



Among the factors controlling the relative activities of the 

 glycolytic and pentose P pathways are the enzyme activities 

 both of these pathways and of competing ones, Michaelis 

 constants, substrate concentrations and the presence or 

 absence of inhibitors. Table I shows the activities in liver 

 tissue and some of the characteristics of the two dehydro- 

 genases of the pentose P pathways and of competing enzymes. 

 Although the maximal activities, measured in homogenates 

 or soluble fractions of homogenates under optimal conditions, 

 do not necessarily reflect what is happening in the cell, the 

 relatively low activities of G6P- and 6-PG-dehydrogenases 

 indicate that only a small proportion of the total G6P meta- 

 bolism proceeds via the pentose P pathway. However, since 

 the Michaelis constants for both substrate and TPN are very 

 low, it is possible that this pathway would compete more 

 efficiently for G6P when the substrate concentration is 

 limiting. The effect of lactonase on the dehydrogenation 

 product of G6P will be to promote this reaction in the forward 

 direction. 



Interesting relationships have been demonstrated between 

 the rate of formation of certain intermediates in the pentose 

 P pathway and the activity of enzymes of alternative routes. 

 Thus, 6-PG has been shown to have a marked inhibitory 

 action on phosphoglucose isomerase when present in the same 

 molecular concentration as G6P or F6P (Parr, 1956). In 

 addition, the rate of glycogen synthesis is modified by accumu- 

 lation of R5P since this inhibits phosphoglucomatase activity 

 (Segal and Foley, 1958). A more direct effect is the inhibition 

 of brain hexokinase activity by G6P, the product of the 

 reaction (Weil-Malherbe and Bone, 1951; Crane and Sols, 

 1953). It is possible that the high concentrations of G6P in 

 some tissues (Weil-Malherbe, 1955) may regulate to some 

 extent the activity of this enzyme. 



