title: Carbon isotope effects on the fructose-1,6-bisphosphate aldolase reaction, origin for non-statistical 13C distributions in carbohydrates.
abstract: The kinetic and equilibrium isotope effects on the fructose-1, 6-bisphosphate aldolase reaction have been determined using the rabbit muscle enzyme. The natural 13C abundance for both atoms participating in the bond splitting were measured in position C-1 of dihydroxyacetone phosphate and glyceraldehyde 3-P after irreversible conversion to glycerol-3-P and 3-phosphoglycerate, respectively, and chemical degradation. The carbon isotope effects were determined comparing the 13C content of the corresponding positions after partial and complete turnover, and after complete equilibration of the reactants. 13(Vmax/Km) on C-3 was 1.016 +/- 0.007 and 0.997 +/- 0.009 on position C-4, and the equilibrium isotope effects K12/K13 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 comes to equilibrium before the rate determining release of glyceraldehyde 3-P from the ternary complex. The equilibrium isotope effect is seen as the reason for an earlier-found relative 13C enrichment in position C-3 and C-4 of glucose and for varying enrichments in 13C of carbohydrates from different compartments of cells. The kinetic isotope effect is suggested to cause 13C discriminations in the C-3 pool in context with the hexose formation in competition with other dihydroxyacetone phosphate turnover reactions.
authors.affiliation: Lehrstuhl für Allgemeine Chemie und Biochemie, TU-München, Vöttingerstrabetae 40, 85350 Freising, Germany. gerd@ibm1.chem.agrar.tu-muenchen.de
authors.forename: G
authors.initials: G
authors.lastname: Gleixner
authors.forename: H L
authors.initials: HL
authors.lastname: Schmidt
mesh_terms: D000818:Animals; D002247:Carbon Isotopes; D004099:Dihydroxyacetone Phosphate / Q000378:metabolism; D005634:Fructose-Bisphosphate Aldolase / Q000378:metabolism*; D005947:Glucose / Q000378:metabolism*; D005990:Glycerol / Q000378:metabolism; D007700:Kinetics; D008956:Models, Chemical; D009132:Muscles / Q000201:enzymology; D011817:Rabbits; D013816:Thermodynamics
chemical_list: D002247:Carbon Isotopes; D004099:Dihydroxyacetone Phosphate; D005634:Fructose-Bisphosphate Aldolase; D005947:Glucose; D005990:Glycerol