Quantification of human apolipoprotein E in plasma and lipoprotein subfractions by a non-competitive enzyme immunoassay.

A non-competitive enzyme linked immunosorbent assay (ELISA) has been developed to quantitate apolipoprotein E (Apo E) concentrations in serum and in isolated lipoproteins. Microtiter plates coated with affinity-purified antibodies to Apo E were used and the Apo E bound to the plates was estimated with peroxidase-labelled antibodies to Apo E. The average concentration of Apo E in the serum from normolipidemic subjects (n = 132) was 54 +/- 19 mg/l. The within and between assay coefficients of variation were 4.65 and 7.08%, respectively. The standard curves for Apo E in serum, in VLDL and in HDL were parallel. There was a good correlation (r = 0.81) between estimation of Apo E by our assay and that by electroimmunoassay. Assay sensitivity (1 ng of Apo E) was sufficient to enable a study of the distribution of Apo E in plasma lipoproteins separated by density gradient ultracentrifugal fractionation.