Substrate and product inhibition in the xanthine oxidase catalyzed oxidation of acetaldehyde

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The xanthine oxidase catalyzed oxidation of acetaldehyde was studied in phosphate buffers at 25.0°. The reactions were carried out in the presence of ferricytochrome c which, upon reduction, results in an increase in optical density at 550 nm. It was observed that the enzymatic oxidation of acetaldehyde is sensitive to some form of substrate inhibition. Since acetaldehyde in aqueous solution exists in equilibrium with its hydrate, experiments were carried out to determine the nature of the actual inhibitory species, viz., the aldehyde or its conjugate hydrate. Substrate inhibition was studied at various stages of the aldehyde-hydrate equilibration. It was observed that both the aldehyde and hydrate inhibit the enzymatic reaction, the latter being the more potent inhibitor (Ki = 1.6 X 10-3 M). Inhibition of the enzymatic process by acetic acid, a common contaminant of acetaldehyde and indeed the product of enzymatic oxidation, was also investigated. The acetate ion appears to function as a noncompetitive inhibitor (Ki = 7.6 X 10-2 M).


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Copyright © 1974, American Chemical Society