EVALUATION OF METABOLIC PATHWAYS 59 



experiment merely equates fructose regenerated in the pen- 

 tose cycle with the glucose administered as substrate. The 

 equation used previously is indeed too simple; data obtained 

 through its use are somewhat in error, as follows: 



When an organism degrades glucose by both the gly- 

 colysis-Krebs cycle and the pentose cycle routes, some triose 

 phosphate will be formed by either pathway. Glucose 

 carbons 1 and 6 will appear in triose in equal amounts by 

 glycolysis, but the triose from the pentose cycle will con- 

 tain only carbons 4, 5, and 6 (carbon 1 will have been con- 

 verted to COo). The total triose pool will therefore be 

 enriched with carbon 6, and carbon 6 yields in CO2 will 

 be too high. 



This means that corrections should be provided which 

 enlarge the differences between Gi and G^, specifically by 

 reducing Gq-, in other words, the reported contribution of 

 phosphogluconate cleavage, as calculated by equation 1, 

 is spuriously low. Such refinements have been made (12, 14), 

 and it is believed that the newer equations will give a more 

 nearly correct evaluation of simultaneously participating 

 pathways. Briefly, where glycolysis and phosphogluconate 

 cleavage occur together, Wang (12, 14) has developed the 

 following expression: 



G, = ^- - ^^- - ^-^-^ (2) 



Gt — Grj" 



where G^ = total radioactivity of each administered substrate 

 (always taken as 100%, or unity) 

 Gt' = fraction of the labeled substrate engaged in ana- 

 bolism 

 Gi, Ge = per cent total yields of C^^02 from systems me- 

 tabolizing equal amounts of glucose labeled in 

 these positions 



