Antioxidant content in low density lipoprotein and lipoprotein oxidation in vivo and in vitro

Human blood contains naturally occurring multiple-modified low density lipoprotein (nomLDL) capable of inducing the accumulation of cholesteryl esters in the cells of human aortic intima. NomLDL is desialylated particles of small size with an increased electronegative charge which can be separated from native low density lipoprotein (LDL) by lectin chromatography. The purpose of this study was to determine the content of antioxidants in native and nomLDL obtained from healthy subjects and from patients with coronary heart disease as well as to elucidate a possible relationship between the level of antioxidants and the degree of in vivo and in vitro LDL oxidizability. The apoB-bound cholesterol level in native and nomLDL of healthy subjects was 0.25 +/- 0.08 and 0.28 +/- 0.05 mol/mol apoB, respectively. The level of apoB-bound cholesterol in native LDL of coronary atherosclerosis patients showed no significant difference from that in healthy subjects' native lipoprotein. At the same time, the level of apoB-bound cholesterol in patients' nomLDL was 7-fold higher than in native LDL. The average duration of the lag phase of native LDL oxidation did not show a significant difference between the lipoprotein of healthy subjects and coronary atherosclerosis patients. The lag phase of nomLDL obtained from healthy subjects and patients was significantly shorter (3- and 6-fold, respectively) than for their native LDL. The latter finding points to their increased susceptibility to in vitro oxidation. Oxidizability of total LDL preparations correlated positively with their nomLDL content. The content of all the antioxidants studied (coenzyme-Q(10), alpha- and gamma-tocopherols, p-carotene and lycopene) in nomLDL was 1.5- to 2-fold lower than in native LDL. The level of apoB-bound cholesterol in nomLDL, correlated positively with the ubiquinone-10 content and showed negative correlation with ubiquinol-10 and p-carotene levels. On the other hand, the content of apoB-bound cholesterol in native LDL correlated positively with the ubiquinol-10 level. Susceptibility of nomLDL to in vitro oxidation exhibited negative correlation with cu-tocopherol acid p-carotene levels and a positive correlation with the ubiquinone-10 content. On the contrary, oxidizability of native LDL correlated positively with the ubiquinone-10 level. Conclusions: (a) elevated apoB-bound cholesterol level in nomLDL of coronary atherosclerosis patients indicates that peroxidation of lipids occurs in vivo; (b) in vivo lipoperoxidation in nomLDL is corroborated by increased proportion of oxidized form of coenzyme-Q(10); (c) content of lipid-soluble antioxidants in nomLDL is lower than in native lipoprotein; (d) nomLDL has a higher susceptibility to in vitro oxidation than native LDL; (e) it is necessary to use isolated subfractions of native LDL and nomLDL, but not total lipoprotein preparations, to study the mechanisms of lipid peroxidation.