Phosphoglucoisomerase-catalyzed interconversion of hexose phosphates; diastereotopic specificity, isotopic discrimination and intramolecular hydrogen transfer.
When D-[1-3H]fructose 6-phosphate generated from D-[2-3H]glucose 6-phosphate is converted, in a monodirectional manner to D-glucose 6-phosphate and then 6-phospho-D-gluconate, about 42% of the radioactivity is transferred from the C1 of the ketohexose ester to the C2 of the aldohexose phosphate, whereas the remaining 58% are produced as 3H2O. The velocity of the reaction catalyzed by phosphoglucoisomerase represents, in the case of the tritiated substrate, only 43% of that recorded with D-[U-14C]fructose 6-phosphate, such an isotopic discrimination being attributable mainly to a difference in maximal velocity rather than affinity. The phenomena of both intramolecular hydrogen transfer and isotopic discrimination were less pronounced than when D-[2-3H]glucose 6-phosphate is converted, in a monodirectional manner, to D-fructose 6-phosphate and then D-fructose 1,6-bisphosphate. In contrast, when either D-[1-3H]glucose 6-phosphate or D-[1-3H]fructose 6-phosphate prepared from D-[1-3H]glucose were tested, no 3H2O was formed, all radioactivity being recovered, respectively, in tritiated D-fructose 1,6-bisphosphate or NADP3H. Nevertheless, phosphoglucoisomerase was also found to discriminate between D-[U-14C]glucose 6-phosphate and D-[1-3H]glucose 6-phosphate or between D-[U-14C]fructose 6-phosphate and D-[1-3H]fructose 6-phosphate prepared from D-[1-3H]glucose. The reaction velocity with the tritiated esters averaged 78-83% of those recorded with the 14C-labelled esters. Such an isotopic discrimination was again attributable mainly to a difference in maximal velocity rather than affinity. These findings indicate that the mode of preparation of D-[1-3H]fructose cannot be ignored in considering the fate of this tritiated hexose, as ruled by the intrinsic properties, and especially the diastereotopic specificity of phosphoglucoisomerase.
Liemans
V
V
Laboratory of Experimental Medicine, Brussels Free University, Belgium.
Malaisse-Lagae
F
F
Willem
R
R
Malaisse
W J
WJ
D002250:Carbon Radioisotopes; D005636:Fructosephosphates / Q000378:metabolism*; D005956:Glucose-6-Phosphate Isomerase / Q000378:metabolism*; D005958:Glucosephosphates / Q000378:metabolism*; D006859:Hydrogen; D007700:Kinetics; D013237:Stereoisomerism; D014316:Tritium; D015003:Yeasts / Q000201:enzymology
D002250:Carbon Radioisotopes; D005636:Fructosephosphates; D005958:Glucosephosphates; D014316:Tritium; D006859:Hydrogen; D005956:Glucose-6-Phosphate Isomerase