Characteristics of ten charge-differing subfractions isolated from human native low-density lipoproteins (LDL). No evidence of peroxidative modifications

Abstract

Native plasma low-density lipoproteins (LDL) were fractionated into ten subfractions with increasingly negative charges (LDL-1, the least electronegative, to LDL-10) using an anion-exchange column coupled to a fast protein-liquid chromatography system. Prior to fractionation, contaminating Lp(a) and apo A-I-containing lipoproteins were removed from LDL preparations by immunoaffinity chromatography. No significant difference in thiobarbituric acid-reactive substances, vitamin E or free aminogroup content was found among subfractions, and no peptide with a higher molecular weight than apo B was observed on SDS-PAGE. We observed a gradual increase in cholesterol esters and a concomitant decrease in triglycerides from LDL-1 to LDL-7, and a reverse tendency from LDL-8 to LDL-10 ( P < 0.01). Free cholesterol increased linearly from LDL-1 to LDL-10 ( P < 0.01). LDL-1 to -3 had a homogeneous density profile, while other more electronegative subfractions showed a bimodal distribution with a second, minor peak of slightly higher density. A gradual increase in apolipoprotein C-III content related to LDL electronegativity was observed ( P < 0.01). Apolipoprotein E content was also increased in the last two subfractions ( P < 0.01). LDL subfractions displayed a similar binding fate on human fibroblasts, with the exception of the most electronegative subfractions [LDL-(9 ± 10)], which bound more actively to apo9 B/E receptors ( P < 0.05). This study shows that charge heterogeneity of native LDL is not related to lipid peroxidation or derivatization of free aminogroups of apolipoprotein B. In contrast, the enrichment of LDL in apolipoproteins other than apo B may explain, in part, the difference in their particle charge.