CARBON-ISOTOPE EFFECTS ON THE FRUCTOSE-1,6-BISPHOSPHATE ALDOLASE REACTION, ORIGIN FOR NONSTATISTICAL C-13 DISTRIBUTIONS IN CARBOHYDRATES

The kinetic and equilibrium isotope effects on the fructose-1,6-bispho sphate aldolase reaction have been determined using the rabbit muscle enzyme, The natural C-13 abundance for both atoms participating in the bond splitting were measured in position C-1 of dihydroxyacetone phos phate and glyceraldehyde 3-P after irreversible conversion to glycerol -3-P and 3-phosphoglycerate, respectively, and chemical degradation, T he carbon isotope effects were determined comparing the C-13 content o f the corresponding positions after partial and complete turnover, and after complete equilibration of the reactants. (13)(V-max/K-m) on C-3 was 1.016 +/- 0.007 and 0.997 +/- 0.009 on position C-4, and the equi librium isotope effects K-12/K-13 on these positions were 1.0036 +/- 0 .0002 and 1.0049 +/- 0.0001. The observed kinetic isotope effect on C- 3 is discussed to originate from the formation of the enamine, which c omes to equilibrium before the rate determining release of glyceraldeh yde 3-P from the ternary complex. The equilibrium isotope effect is se en as the reason for an earlier-found relative C-13 enrichment in posi tion C-3 and C-4 of glucose and for varying enrichments in C-13 of car bohydrates from different compartments of cells. The kinetic isotope e ffect is suggested to cause C-13 discriminations in the C-3 pool in co ntext with the hexose formation in competition with other dihydroxyace tone phosphate turnover reactions.