Dense low density lipoprotein subspecies with diminished oxidative resistance predominate in combined hyperlipidemia.

Patients presenting combined hyperlipidemia (CHL) display an elevated risk of coronary heart disease. The atherogenic lipoprotein particles implicated in this disorder remain ill defined. We determined the qualitative and quantitative characteristics of the density distribution of low density lipoprotein (LDL) particle subspecies in nine subjects defined phenotypically as presenting CHL, and under strict dietary control. Seven CHL patients possessed familial antecedents of premature coronary heart disease; none were E2E2 homozygotes. Five LDL subspecies were isolated by density gradient ultracentrifugation in the density range 1.019-1.063 g/ml. In all patients, the LDL profile was skewed towards the dense subspecies (LDL-4, d 1.039-1.050 g/ml and LDL-5, d 1.050-1.063 g/ml), representing 47% of total LDL mass; by contrast, these subspecies accounted for only 30% of LDL mass in five normolipidemic subjects (P < 0.01). In addition, plasma LDL mass concentrations were some twofold higher in CHL patients as compared to normolipidemic subjects. The % mass of LDL-4 was positively correlated with plasma triglyceride and apoB levels. LDL-2 and LDL-3 in CHL patients were triglyceride-enriched (11.9 and 7.2%, respectively) as compared to the corresponding subspecies in normolipidemic subjects (6.6 and 3.7%, respectively; P < 0.05 in each case). LDL particle size decreased with increase in density in both groups; however, significant differences were found between corresponding LDL subspecies (LDL-1, -3, -4, and -5) in CHL patients and normolipidemic subjects, a finding suggestive of dissimilar molecular organization, despite correspondence in hydrated density. The copper-induced oxidative modification of LDL subspecies was assessed by determination of conjugated diene formation. In both groups, LDL-5 was distinct in exhibiting a marked diminution in oxidative resistance as indicated by a significant reduction (P < 0.01) in mean lag time. The oxidative susceptibility of LDL subspecies in both groups was independent of vitamin E content when expressed as the ratio vitamin E/LDL mass, although dense LDL in CHL patients tended to be deficient in this antioxidant. The diminished oxidative resistance of dense LDL subspecies could not be accounted for by enrichment in polyunsaturated fatty acids in either group. These studies suggest that in consequence of their elevated circulating concentration and diminished oxidative resistance, dense LDL subspecies represent putative atherogenic subspecies in combined hyperlipidemia.