The pH dependence of xanthine oxidase catalysis in basic solution.
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 degrees 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 o 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 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.
D003903:Deuterium; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009536:Niacinamide / Q000031:analogs & derivatives / Q000378:metabolism; D009539:Nicotinic Acids / Q000378:metabolism; D011726:Pyridinium Compounds / Q000378:metabolism; D011806:Quinolinium Compounds / Q000378:metabolism; D014969:Xanthine Oxidase / Q000378:metabolism*
D009539:Nicotinic Acids; D011726:Pyridinium Compounds; D011806:Quinolinium Compounds; C026557:1-methylquinolinium; C026558:6-methoxy-1-methylquinolinium iodide; D009536:Niacinamide; C026560:1-(4-methoxyphenyl)pyridinium; C026559:3-acetyl-1-methylpyridinium; C008473:methyl nicotinate; D003903:Deuterium; D014969:Xanthine Oxidase; C024058:N(1)-methylnicotinamide