Phosphoglucoisomerase-catalyzed interconversion of hexose phosphates; comparison with phosphomannoisomerase.
The isotopic discrimination, diastereotopic specificity and intramolecular hydrogen transfer characterizing the reaction catalyzed by phosphomannoisomerase are examined. During the monodirectional conversion of D-[2-3H]mannose 6-phosphate to D-fructose 6-phosphate and D-fructose 1,6-bisphosphate, the reaction velocity is one order of magnitude lower than with D-[U-14C]mannose 6-phosphate and little tritium (less than 6%) is transferred intramolecularly. Inorganic phosphate decreases the reaction velocity but favours the intramolecular transfer of tritium. Likewise, when D-[1-3H]fructose 6-phosphate prepared from D-[1-3H]glucose is exposed solely to phosphomannoisomerase, the generation of tritiated metabolites is virtually restricted to 3H2O and occurs at a much lower rate than the production of D-[U-14C]mannose 6-phosphate from D-[U-14C]fructose 6-phosphate. However, no 3H2O is formed when D-[1-3H]fructose 6-phosphate generated from D-[2-3H]glucose is exposed to phosphomannoisomerase, indicating that the diastereotopic specificity of the latter enzyme represents a mirror image of that of phosphoglucoisomerase. Advantage is taken of such a contrasting enzymic behaviour to assess the back-and-forth flow through the reaction catalyzed by phosphomannoisomerase in intact cells exposed to D-[1-3H]glucose, D-[5-3H]glucose or D-[6-3H]glucose. Relative to the rate of glycolysis, this back-and-forth flow amounted to approx. 4% in human erythrocytes and rat parotid cells, 9% in tumoral cells of the RINm5F line and 47% in rat pancreatic islets.
Malaisse-Lagae
F
F
Laboratory of Experimental Medicine, Brussels Free University, Belgium.
Liemans
V
V
Yaylali
B
B
Sener
A
A
Malaisse
W J
WJ
D000818:Animals; D002238:Carbohydrate Epimerases / Q000378:metabolism*; D002478:Cells, Cultured; D005636:Fructosephosphates / Q000378:metabolism; D005947:Glucose / Q000378:metabolism; D005956:Glucose-6-Phosphate Isomerase / Q000378:metabolism*; D006600:Hexosephosphates / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D008359:Mannose-6-Phosphate Isomerase / Q000378:metabolism*; D008360:Mannosephosphates / Q000378:metabolism; D051381:Rats; D014316:Tritium; D015003:Yeasts / Q000201:enzymology
D005636:Fructosephosphates; D006600:Hexosephosphates; D008360:Mannosephosphates; D014316:Tritium; D002238:Carbohydrate Epimerases; D008359:Mannose-6-Phosphate Isomerase; D005956:Glucose-6-Phosphate Isomerase; D005947:Glucose