Protection of endogenous β-carotene in LDL oxidized by oxygen free radicals in the presence of supraphysiological concentrations of melatonin

Abstract

This study was designed to evaluate the effect of high concentrations of melatonin on the peroxidation of human low density lipoproteins (LDLs) initiated by O₂^•- and ethanol-derived peroxyl radicals (RO₂^•) from water gamma radiolysis in the presence of ethanol. LDL (3 g/l; total LDL concentration) was oxidized in the absence of melatonin or in its presence at three concentrations (50 × 10^-6, 100 × 10^-6 or 250 × 10^-6 mol/l) in ethanol. Radiolytic yields (i.e. number of mole consumed or produced per Joule) of the markers of lipid peroxidation were determined (i.e. decrease in the endogenous antioxidants α-tocopherol and β-carotene, formation of conjugated dienes and of thiobarbituric acid-reactive substances [TBARS]). Melatonin decreased the yields of lipid peroxidation products and delayed the onset of the propagation phase for conjugated dienes and TBARS in a concentration-dependent manner. Nevertheless, melatonin did not protect endogenous α-tocopherol against peroxyl-induced oxidation (probably due to a lower scavenging capacity than that of α-tocopherol towards peroxyl radicals), but delayed the consumption of LDL endogenous β-carotene and decreased its rate of disappearance. The effect of melatonin seemed to be the highest for a melatonin concentration of 250 × 10^-6 mol/l.