Substrate inhibition of xanthine oxidase and its influence on superoxide radical production.
The influence of substrate inhibition on xanthine oxidase-intramolecular electron transport was studied by steady-state kinetic analysis. Experiments with hypoxanthine and xanthine up to 900 microM indicated an inhibition pattern which fitted an equation of the general form nu 0 = nu max . [S]/(Km + a[S] + b[S]2/Ki). Univalent electron flux to oxygen was favored at substrate concentrations above 50 microM. This augmentation of univalent flux percentage that appeared at a high substrate concentration was greater for hypoxanthine that xanthine and at pH 8.3 than at 9.5. Our results support a mechanism of inhibition in which a substrate-reduced enzyme, non-productive Michaelis complex was formed. It is possible that this non-productive complex favored the univalent pathway of enzyme reoxidation (superoxide production) by increasing the midpoint redox potential of the molybdenum active site.
Rubbo
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H
Department of Biochemistry, Faculty of Medicine, University of the Republic, Montevideo, Uruguay.
Radi
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R
Prodanov
E
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D000818:Animals; D002417:Cattle; D004579:Electron Transport; D005609:Free Radicals; D006736:Horses; D019271:Hypoxanthine; D007042:Hypoxanthines / Q000494:pharmacology; D007700:Kinetics; D013379:Substrate Specificity; D013481:Superoxides / Q000378:metabolism*; D014527:Uric Acid / Q000378:metabolism; D019820:Xanthine; D014969:Xanthine Oxidase / Q000037:antagonists & inhibitors* / Q000378:metabolism; D014970:Xanthines / Q000494:pharmacology
D005609:Free Radicals; D007042:Hypoxanthines; D014970:Xanthines; D013481:Superoxides; D019820:Xanthine; D014527:Uric Acid; D019271:Hypoxanthine; D014969:Xanthine Oxidase