Effect of pH on Low-Density Lipoprotein Oxidation by O₂^·−/HO₂^· Free Radicals Produced by Gamma Radiolysis

This study aimed to analyze quantitatively the initiation and the consequences of low-density lipoprotein (LDL) peroxidation by O₂^·−/HO₂^· free radicals produced by gamma radiolysis. The action of increasing radiation doses on aqueous LDL solutions has been monitored simultaneously by several parameters: a decrease in endogenous vitamin E, the formation of thiobarbituric acid-reactive substances (TBARS) and conjugated dienes, the appearance of a differential fluorescence (excitation wavelength = 360 nm), and an increase of the relative electrophoretic mobility. Initial radiation yields (decrease in vitamin E, formation of TBARS) have been determined at pH 7 and pH 5.7 as a function of LDL concentration (from 0.75 to 9 g liter^-1). From the comparison of these yields with those of O₂^·− radicals produced by water radiolysis, we have deduced reaction mechanisms for LDL peroxidation initiated by O₂^·−/HO₂^· free radicals.