Inhibition of copper-mediated low density lipoprotein peroxidation by quinoline and indolinone nitroxide radicals.

Quinoline and indolinone nitroxide radicals are known to be efficient scavengers of oxygen-centred (rate constants (k) between 10(3) and 10(5)/M/sec) and carbon-centred radicals (almost diffusion-controlled rate). In this study, the relative effects of these compounds in protecting low density lipoprotein (LDL) from oxidation induced by copper have been investigated. The extent of lipid peroxidation was assessed by monitoring the increased conjugated diene formation, the altered surface charge of the apolipoprotein B and the generation of aldehydic breakdown products of oxidized LDL. All the nitroxides inhibited LDL peroxidation in a concentration-dependent manner. The corresponding hydroxylamines of the nitroxides were also studied and were shown to inhibit lipid peroxidation to almost the same extent as the parent nitroxide. The data indicates that this class of nitroxide radicals (and their reduced hydroxylamine forms) are effective lipophilic antioxidants with the quinoline nitroxide being more efficient than the indolinone nitroxides.