Apolipoprotein and size heterogeneity in human umbilical cord blood low density lipoproteins

Neonatal umbilical cord blood plasma low density lipoproteins (LDL, d = 1.019-1.063 g/ml) were subfractionated by density gradient ultracentrifugation into seven fractions (from 1.024 to 1.062 g/ml); the bulk of the LDL mass was in a density region of 1.034-1.042 g/ml. Apolipoprotein B by 10% SDS-polyacrylamide gel electrophoresis varied inversely with density, with only trace amounts present in the most dense fraction. The distribution of apolipoprotein B molecular weight forms was assessed by both 3% SDS-polyacrylamide gel electrophoresis and relative aminoacyl mass determination. Lower molecular weight forms of apolipoprotein B (B74 and B26) increased relative to apolipoprotein B100 with increasing density, ranging from undetectable in fraction 1 to apolipoproteins B26 and B74 comprising 30% of the total mass of apolipoprotein B in fraction 6. No apolipoprotein B48 was detectable in the LDL. Apolipoprotein E as determined by both SDS-polyacrylamide gel electrophoresis and radioimmunoassay increased with density with a maximum (14% of the protein) in the most dense fraction, fraction 7. Apolipoprotein A-I by SDS-polyacrylamide gel electrophoresis increased with increasing density and was the major apolipoprotein in fraction 7. Electron microscopic analysis revealed spherical particles whose diameters decreased with increasing density, ranging from 28.6 nm in the top fraction (fraction 1) to 15.6 nm in the bottom fraction (fraction 7). Gradient gel electrophoresis revealed that most of the fractions contained several different sized particles. The bottom fraction (fraction 7), enriched in apolipoproteins E and A-I, had a unique, poorly defined peak at 14.6 nm on gradient gel electrophoresis and showed a tendency to pack hexagonally upon electron microscopy. The unusual composition and apolipoprotein distribution in neonatal LDL fractions suggests that the LDL in the neonate are metabolically very diverse.