Apolipoprotein B-bound lipids as a marker for evaluation of low density lipoprotein oxidation in vivo

It has been generally accepted that oxidized low density lipoprotein (LDL) plays an important role in atherogenesis. However, oxidized LDL was not detected in patients' blood and the extent of LDL oxidation in vivo is unknown. We have suggested that LDL oxidation may lead to a formation of covalent links between lipids and apolipoprotein B. LDL were oxidized by copper ions, 2,2'-azobis-(2-aminopropane hydrochloride), sodium hypochlorite or by incubation with macrophages. Oxidized LDL were delipidated by repeated extraction with organic solvents. After mild alkaline hydrolysis protein-bound sterols were identified colorimetrically and by high-performance liquid chromatography. Protein-bound phospholipid residues were detected by nuclear magnetic resonance and colorimetric determination of phosphate. Using radiolabeled lipids it was also shown that free and esterified cholesterol, phospholipids, as well as triglyceride and free fatty acid residues can form covalent bonds with apolipoprotein B. The ability of lipids to bind to apolipoprotein B correlates with the degree of unsaturation of their fatty acids and depends on the nature of polar head of phospholipids. When LDL were oxidized with copper ions, the content of protein-bound lipids increased gradually up to 24 h of incubation, while the levels of conjugated dienes, hydroperoxides and thiobarbituric acid-reactive substances changed in varying manners. It has been demonstrated that the content of protein-bound sterols in multiple-modified desialylated LDL of patients with coronary atherosclerosis is higher than that in native LDL. Our results suggest that the level of protein-bound lipids may be a marker of LDL oxidation and can be used to evaluate the association of lipoprotein oxidation and atherogenesis.