THE REACTION OF XANTHINE-OXIDASE WITH ALDEHYDIC PRODUCTS OF LIPID-PEROXIDATION

A quantitative structure-activity relationship for the reaction of xanthine oxidase with a homologous series of alpha,beta-unsaturated aldehydes, which are known to be products of lipid peroxidation, was investigated. Aldehydes in the series 2-butenal through 2-nonenal and 4-hydroxy-2-nonenal, displayed differential reactivity toward xanthine oxidase as measured by production of the superoxide radical anion. Kinetic parameters for the rate of superoxide production and substrate affinity were determined via the superoxide dismutase-sensitive reduction of cytochrome c. Trends in kinetic parameters as a function of carbon number for the series of trans-2-enals was consistent with a dependence on substrate hydrophobicity. Log k(w)', a hydrophobicity constant widely employed as a model for the octanol/water partition coefficient, was determined by reversed phase liquid chromatography for the alpha,beta-unsaturated aldehydes in this study. Linear relationships for the correlation of substrate binding (pK(m)) and efficiency of superoxide production (log k(cat)/K(m)) with substrate hydrophobicity (log k(w)') were found. The mode of inhibition of xanthine oxidation by 2-butenal is shown to be noncompetitive, suggesting distinct binding sites for purine and aldehydic substrates. It is suggested that the reaction of xanthine oxidase with unsaturated aldehydes could be an important route of amplification of oxidative damage in cells.