Apolipoprotein A-II content of human plasma high density lipoproteins measured by radioimmunoassay

Abstract

A double antibody radioimmunoassay for human ApoA-II is reported. ApoA-II isolated from human plasma high density lipoprotein (HDL) by column chromatography migrated as a single band on polyacrylamide disc gel electrophoresis, had the appropriate amino acid composition, and provoked the production of monospecific antisera. 125I-ApoA-II (iodinated by lactoperoxidase, purified by Sephadex G-75 chromatography) migrated with “cold” ApoA-II as a single band on disc gel electrophoresis in SDS. Its specific radioactivity was 5–12 mCi/µg. In assays, (0.05 M barbital buffer, 0.01% Triton X-100, pH 8.6) over 90% of 125I-ApoA-II was bound by excess first antibody and over 95% was displaced by excess “cold” ApoA-II. Low density lipoprotein, very low density lipoprotein, ApoA-I, ApoC-II, and ApoC-III displaced no counts. Intraassay and interassay coefficients of variation for lipoprotein or plasma samples were 7 ± 4 and 11 ± 6%, respectively. As little as 1.0 ng of ApoA-II was detectable with a precision of 10%. ApoA-II made up 20–25% of the proteins of HDL (d 1.083–1.19), HDL2 (d 1.083–1.124), and HDL3 (d 1.124–1.19) on column chromatography. The ApoA-II contents of these HDL fractions were also 20–25% by radioimmunoassay. Similar results were obtained whether assays were carried out on intact or delipidated HDL samples. Thus, in contrast with ApoA-I (only 10% of which is detectable), all of the ApoA-II contents of intact HDL are detected with accuracy by this assay. Plasma levels of ApoA-II in young normolipemic subjects were approximately 40 mg/dl (n = 29). In these subjects, over 98% of ApoA-II was found in the d 1.063–1.21 density fractions.

Keywords

• high density lipoprotein structure
• density distribution of apolipoprotein A-II