Diurnal heterogeneity in structure and function of low density lipoproteins of normolipidemic males

Structural changes in low density lipoproteins (LDL) have been shown to alter their metabolism and atherogenic potential. We investigated the diurnal changes in size and composition of LDL in seven healthy, non-obese, normolipidemic male volunteers consuming a standard diet (14.5% protein, 31.9% fat, 53.6% carbohydrate and 383 mg cholesterol/day) and continuing their daily routine. The food was divided into three meals and three snacks, and blood samples were obtained at 7 AM (after 12 h fasting), noon, 8 PM, midnight and 3 AM. LDL were isolated by both sequential and density gradient uluacentrifugation (d = 1.019-1.050 g/ml), and analyzed for lipids, apolipoproteins, size, and affinity to LDL receptors. Diurnal LDL preparations differ from fasting LDL in both chemical and physical parameters. The former get richer in triglyceride (TG/cholesterol weight ratio 0.23 vs. 0.16), larger in diameter (21.2 ± 0.2 vs. 22.4 ± 0.1 nm), and enriched in a more buoyant fraction (74.0 ± 4.6 vs. 41.9 ± 3.8% of LDL cholesterol in d = 1.019-1.035 g/ml). These structural changes in LDL were associated with enhanced affinity to LDL receptors in both human skin fibroblasts and HepG2 cells, as demonstrated by competition experiments with fasting human ¹²⁵I-LDL. The observed diurnal heterogeneity in both the structure and the function of LDL may be attributed to the absorptive state as it did not occur during prolonged fasting. These diurnal changes may be important for better understanding LDL metabolism in vivo and for the elucidation of the atherogenic process.