The aim of this study was to analyze quantitatively the oxidative modification of low-density lipoproteins (LDLs) induced by OH· free radicals produced by gamma radiolysis, in the absence or in the presence of oxygen (action of OH· free radicals, or simultaneous action of OH· and${\rm O}_{2}^{\overline{\cdot}}$ free radicals, respectively). The effects of increasing radiation doses on aqueous LDL solutions have been monitored by several parameters: a decrease in endogenous vitamin E, the formation of thiobarbituric acidreactive substances (TBARS) and conjugated dienes, the appearance of a differential fluorescence (excitation wavelength = 360 nm), and an increase in the relative electrophoretic mobility. Initial radiation yields (decrease in vitamin E, formation of TBARS) have been determined at pH 7 as a function of LDL concentration (from 0.75 to 9 g liter-1). From the comparison of these yields with those of OH· and${\rm O}_{2}^{\overline{\cdot}}$ free radicals produced by water radiolysis, we have deduced reaction mechanisms for the initiation of oxidation of LDLs by OH· and${\rm OH}\cdot /{\rm O}_{2}^{\overline{\cdot}}$ free radicals.

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