Abstract
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 μM indicated an inhibition pattern which fitted an equation of the general form ν 0 = ν max · [SJ/( K m + a[S] + b[S] 2/ K i). Univalent electron flux to oxygen was favored at substrate concentrations above 50 μM. This augmentation of univalent flux percentage that appeared at a high substrate concentration was greater for hypoxanthine than 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.
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