The pH dependence of xanthine oxidase catalysis in basic solution

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Abstract
The steady-state kinetics of the oxidation of the following 6 heteroaromatic substrates by xanthine oxidase were investigated over the range pH 9.0-11.1 at 25.degree. C, ionic strength 0.1: 1-methylquinolinium, 6-methoxy-1-methylquinolinium, 1-methylnicotinamide, 3-acetyl-1-methylpyridinium and 1-(4-methoxyphenyl)pyridinium cations and 1-methylnicotinate zwitterion. For the 1st 4 of these species, kc and Km were evaluated as a function of pH while only kc/Km was accessible in the latter 2 cases. Where available, kc is pH independent, whereas plots of log (kc/Km) vs. pH are linear with slopes in the range 0.54-1.17. The rats of enzymic oxidation of the 1-methylquinolinium cation and its 2-deuterio derivative were investigated and kinetic isotope effects were calculated at pH 9.8 and 10.6: kcH/kcD = 1.7 and KmH/KmD = 0.4 at each pH. Detailed comparisons of the oxidation of heteroaromatic cations and xanthine-derived substrates indicate that similar rate-determining steps control the enzymic oxidations of these 2 classes of substrate.

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