Kinetic analysis of copper-induced peroxidation of HDL, autoaccelerated and tocopherol-mediated peroxidation

Comparison of the kinetic profiles of copper-induced peroxidation of HDL an d LDL at different copper concentrations reveals that under all the studied experimental conditions HDL is mon susceptible to oxidation than LDL. The mechanism responsible for HDL oxidation is a complex function of the copper /HDL ratio and of the tocopherol content of the HDL. At high copper concent rations, the kinetic profiles were similar to those observed for LDL oxidat ion, namely, relatively rapid accumulation of oxidation products, via an au toaccelerated, noninhibited mechanism, was preceded by an initial "lag phas e." Under these conditions, the maximal peroxidation rate (V-max) of HDL an d LDL depended similarly on the molar ratio of bound copper/lipoprotein. An alysis of this dependency in terms of the binding characteristics of copper to lipoprotein, yielded similar dissociation constant (K = 10(-6) M) but d ifferent maximal binding capacities for the two Lipoproteins (8 Cu+2/HDL as compared to 17 Cu+2/LDL). Given the size difference between HDL and LDL, t hese results imply that the maximal surface density of bound copper is at l east 2-fold higher for HDL than for LDL. This difference may be responsible for the higher susceptibility of HDL to copper-induced oxidation in the pr esence of high copper concentrations. At relatively low copper concentratio ns, the kinetic profile of HDL oxidation was biphasic, similar to but more pronounced than the biphasic kinetics observed for the oxidation of LDL lip ids at the same concentration of copper. Our results are consistent with th e hypothesis that the first phase of rapid oxidation occurs via a tocophero l-mediated-peroxidation (TMP) mechanism. Accordingly, enrichment of HDL wit h tocopherol resulted in enhanced accumulation of hydroperoxides during the first phase of copper-induced oxidation. Notably, the maximal accumulation during the first phase decreased upon increasing the ratio of bound copper /HDL. This behavior can be predicted theoretically for peroxidation via a T MP mechanism, in opposition to autoaccelerated peroxidation. The possible p athophysiological significance of these findings is discussed. (C) 2000 Els evier Science Inc.