Summary
Hepatic glucose output is decreased by hyperglycaemia through an unknown mechanism. We hypothesize that free radicals generated by hyperglycaemic pseudohypoxia might cause glucose output to decrease by inhibiting glucose 6-phosphatase — a key enzyme of gluconeogenesis. To prove this a model experiment was performed on a microsome fraction of rat liver. One of the characteristic features of pseudohypoxia due to hyperglycaemia is an increase in the ratio of NADH/NAD +, so in the present study the changes in NADH — induced glucose 6-phosphatase activity were investigated as related to the release of inorganic phosphate (Pi) derived from glucose 6-phosphate. After incubation for 50 min, Pi release was significantly reduced by NADH (4.026 ± 0.189 vs 2.696 ± 0.429 µmol · 1-1 · mg protein-1, control vs NADH samples, p < 0.01). The decrease in the activity of glucose 6-phosphatase generated by NADH was prevented by using desferrioxamine, an irreversible ferric chelator, butylated hydroxytoluene and Trolox, two agents which inhibit lipid peroxidation, and reduced glutathione, a non-specific radical scavanger. Superoxide dismutase, catalase and the hydroxyl radical scavenger dimethyl sulphoxide proved to be ineffective. When the above investigations were carried out in the presence of a ferric-EDTA complex the inhibition of glucose 6-phosphatase was found to be inducible by hydrogen peroxide and/or hydroxyl free radicals. These investigations seem to indicate that pseudohypoxia due to hyperglycaemia can inhibit the activity of glucose 6-phosphatase both by lipid peroxidation and by inducing hydrogen peroxide and/or hydroxyl free radicals and thus it may play a part in the glucose-induced decrease of hepatic glucose output.
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Abbreviations
- BHT:
-
Butylated hydroxytoluene
- DMSO:
-
dimethyl sulphoxide
- G6Pase:
-
glucose 6-phosphatase
- GSH:
-
glutathione (reduced form)
- HGO:
-
hepatic glucose output
- Pi:
-
inorganic phosphate
- SOD:
-
Superoxide dismutase.
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Wittmann, I., Mazák, I., Wagner, L. et al. Possible role of free radicals generated by pseudohypoxia in the regulation of hepatic glucose output. An in vitro model using rat liver microsomal glucose 6-phosphatase. Diabetologia 40, 1251–1254 (1997). https://doi.org/10.1007/s001250050817
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DOI: https://doi.org/10.1007/s001250050817