Detailed studies of apolipoprotein E (apoE)-containing lipoproteins in abetalipoproteinemia have been performed in an attempt to resolve the apparent paradox of a suppressed low density lipoprotein (LDL) receptor pathway in the absence of apoB-containing lipoproteins. It was hypothesized that apoE-containing high density lipoproteins (HDL) in abetalipoproteinemia might functionally substitute for LDL in regulation of cholesterol metabolism in these patients.

The mean (±standard deviation) plasma concentration of apoE in nine patients with abetalipoproteinemia was 44.8±8.2 μg/ml, slightly higher than the corresponding value for a group of 50 normal volunteers, 36.3±11 μg/ml. Fractionation of plasma lipoproteins by agarose column chromatography or by ultracentrifugation indicated that in abetalipoproteinemia, plasma apoE was restricted to a subfraction of HDL. This was in contrast to the results obtained with plasma from 30 normal volunteers, in whom apoE was distributed between very low density lipoproteins (VLDL) and HDL. Consequently, the mean apoE content of HDL in abetalipoproteinemia (44.8 μg/ml) was more than twice that found in the normal volunteers (20.3 μg/ml).

ApoE-rich and apoE-poor subfractions of HDL₂ were isolated by heparin-agarose affinity chromatography. ApoE comprised a mean of 81% of the protein mass of the apoE-rich subfraction. Compared with the apoE-poor subfraction, the apoE-rich HDL₂ was of larger mean particle diameter (141±7 vs. 115±15 Å) and had a higher ratio of total cholesterol/protein (1.01±0.11 vs. 0.63±0.14).

Plasma and HDL fractions from three patients were studied with respect to their ability to compete with ¹²⁵I-LDL in specific binding to receptors on cultured human fibroblasts. The binding activity of plasma from patients (per milligram of protein) was about half that of plasma from normal volunteers. All binding activity in the patients' plasma was found to reside in the HDL fraction. The binding activity of the patients' HDL (on a total protein basis) was intermediate between that of normal HDL and normal LDL. However, the large differences in binding between patients' HDL and normal HDL entirely disappeared when data were expressed in terms of the apoE content of these lipoproteins. This suggested that the binding activity was restricted to that subfraction of HDL particles that contain apoE. These apoE-rich HDL particles had calculated binding potencies per milligram of protein 10-25 times that of normal LDL. Direct binding studies using ¹²⁵I-apoE-rich HDL₂ and ¹²⁵I-apoE-poor HDL₂, confirmed the suggestion that binding is restricted to the subfraction of HDL particles containing apoE. The apoE-rich HDL₂ were found to be very potent inhibitors of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase activity in cultured fibroblasts, providing direct evidence of the ability of these lipoproteins to regulate cholesterol metabolism.

On the basis of binding potencies of apoE-rich HDL, apoE concentrations, and the composition of apoE-rich HDL, it could be calculated that apoE-rich HDL in abetalipoproteinemia have a capacity to deliver cholesterol to tissues via the LDL receptor pathway equivalent to an LDL concentration of 50-150 mg/dl of cholesterol. Thus, these apoE-rich lipoproteins are capable of producing the suppression of cholesterol synthesis and LDL receptor activity previously observed in abetalipoproteinemia.