The pH dependence of xanthine oxidase catalysis in basic solution

Author:

Bunting John W.,Laderoute Keith R.,Norris Donald J.

Abstract

The steady-state kinetics of the oxidation of the following six heteroaromatic substrates by xanthine oxidase have been investigated over the range pH 9.0–11.1 at 25 °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 first four of these species, kc and Km were evaluated as a function of pH while only kc/Km was accessible in the latter two 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 rates 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 two classes of substrate.

Publisher

Canadian Science Publishing

Subject

General Medicine

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