Dynamics of the oxidation of low density lipoprotein induced by free radicals

Although the oxidation of low density lipoprotein (LDL) has been studied extensively, its dynamics have received much less attention. The present study was carried out aiming to elucidate the rates and products of the oxidation of LDL induced by free radicals generated in three different ways by hydrophilic and lipophilic azo radical initiators and copper. The oxidations were followed by measuring the formation of lipid oxidation products, the uptake of oxygen, consumption of vitamin E, and the fragmentation and acquisition of negative charge by apolipoprotein B within a single oxidation reaction. LDL was oxidized by a free radical chain mechanism independent of the manner of initiating free radical generation to give phosphatidylcholine hydroperoxide (PCOOH) and cholesteryl ester hydroperoxide (CEOOH) as major primary products. When the radicals were generated within the LDL compartment, CEOOH was formed almost exclusively and quantitatively at the initial stage. The kinetic chain length for the oxidation of cholesteryl ester was larger than that for phosphatidylcholine in the presence and absence of vitamin E even when the radicals were generated initially in the aqueous phase. PCOOH and CEOOH accumulated with increasing extent of oxidation, but then decreased at the later stage of oxidation. The decrease in hydroperoxides was more significant in copper-induced oxidations. The thiobarbituric acid reactive substances also increased with time, but they accounted for less than 10% of total oxygen uptake. Oxidative modification of apo B was also observed. The modification of apo B, that is, fragmentation and increase in negative charge correlated well with the oxidation of lipids independent of the manner of chain initiation. It was also suggested that some radicals formed in the aqueous phase attacked apo B directly. These results suggest the importance of chain propagation in the oxidative modification of LDL and also the lipophilic, chain-breaking antioxidants within LDL in its inhibition.