264 Essays in Biochemistry 



With the F-6-P pool (III) resulting from reaction 1, reactions 3 

 and 4a each yield a molecule of tetrose of the composition G-3, 4 > 3, 

 5 > 2, 6 > 1. Reaction 5 would then give rise to a molecule of the 

 composition G-2, 3 = 4, 2 = 5, 1 = 6. Alternatively, the production 

 of two molecules of the first kind of tetrose in reaction 3 can be followed 

 by reactions 4b and 5, again yielding a molecule of the second kind 

 of tetrose. In either case the ratio of G-3 to G-2 in carbon 1 of the 

 pooled tetrose would be 2. 



The F-6-P pool (V) from reaction 2 would yield through the same 

 process a pair of tetrose phosphates identical with the above in carbon 

 atoms 2 to 4. In carbon atom 1, however, G-3 > 4 would replace 

 G-3 and G-2 > 5 would replace G-2. 



It is clear that the isotope distribution observed in atoms 4, 5, 6 

 of SA is consistent with their origin from carbon atoms 2, 3, 4 re- 

 spectively of tetrose phosphate formed as described above. In one- 

 third of the tetrose molecules these atoms are derived from triose 

 phosphate, which would account for a G-l, 2, 3/G-6, 5, 4 ratio of 

 1/6 in S-6, 5, 4. From the higher ratio observed ( 1/2.5 1, it can be 

 calculated that about two-thirds of the pooled F-6-P was derived from 

 the intact chain of glucose and one-third had incorporated triose phos- 

 phate, possibly by direct exchange (reaction 1) or by hydrolysis of 

 equilibrated FDP (reaction 2).* 



Similarly, the isotope distribution observed in S-3 (G-3, 4/G-2 in a 

 ratio of 2.5/1) is consistent with the origin of S-3 from carbon atom 1 

 of tetrose phosphate. In the latter atom a ratio of 2/1 would be 

 expected, as noted above, for the G-3/G-2 contribution derived from 

 "unexchanged" F-6-P (I). The effect of reaction 1 on the F-6-P pool 

 would not influence the isotopic composition of this atom of tetrose, 

 whereas reaction 2 would cause small incorporations of G-4 and G-5. 

 Theoretically the importance of reaction 2 could be evaluated from a 

 precise determination of these incorporations, but such data are not 

 available.! 



* Let a = fraction of G-l in position 6 of pooled F-6-P (III or V). Assume 

 fraction of G-l in position 3 of pooled triose phosphate = 0.5. Assume also that 

 S-3, 4, 5, 6 is derived from tetrose molecules of which two-thirds obtain then 

 bottom carbon atoms from F-6-P and one-third from triose. Then: 



((1-1) 0.25 (0.5 X 1) + 2n 



Fraction of G-l in S-6 = - = = 0.29 = — — 



(G-l) + (G-6) 0.85 3 



a = 0.185 



Fraction of F-6-P pool derived by exchange (reactions 1 or 2, Fig. 3) is 2a, or 0.37. 



t As a further consequence of reactions 3 to 5 (Fig. 4), the F-6-P regenerated 



in reaction 5 (which would amount to one-third of the glucose entering these 



