Serum and interstitial fluid apolipoprotein E levels in the healthy and in hyperlipoproteinemia type III as studied by radioimmunoassay

Abstract

In order to study the relationships between serum lipoprotein lipid concentrations and the concentrations of apo E in serum and interstitial fluid, we have developed a specific, sensitive and rapid radioimmunoassay for this apolipoprotein. Studies of the interstitial fluid lipoproteins and of the gradient between the lipoprotein concentrations in interstitial fluid and serum may add to our understanding of the development of atherosclerosis and xanthomatosis. Serum, interstitial fluid, lipoproteins or standards were incubated with 125I-labelled apo E and rabbit antiserum against apo E for 90–120 min at room temperature. The immune complexes were harvested with the use of formalin-treated staphylococci. The displacement curves produced by standard and samples of serum, interstitial fluid and isolated lipoproteins were linear in logit-log plots and had identical slopes. Delipidation did not change the results and the recovery of added apo E to a serum sample was 96 ± 5% ( n = 5). Apo E was found in all major lipoprotein classes and the concentrations of apo E in serum and in interstitial fluid were 36 ± 19 mg/l and 8 ± 4 mg/l, respectively, in normals ( n = 21) and 305 ± 125 mg/ml and 20 ± 9 mg/l, respectively, in patients with HLP type III ( n = 11). Highly significant positive correlations were found in HLP type III between the interstitial fluid level of apo E and the corresponding concentrations of cholesterol and triglyceride. Interstitial fluid apo E concentrations were significantly correlated to apo E but not to the lipid levels in serum, indicating that only some subclasses of the serum lipoproteins are transported to the interstitial compartment.