The antigenic structure of apolipoprotein A-I in human high density lipoproteins. Radioimmunoassay using surface-specific antibodies

Only a small proportion of the apolipoprotein A-I (apoA-I) in human plasma or high density lipoprotein (HDL) is normally detectable by radioimmunoassay without prior physical or chemical treatment, probably because some antigenic sites are masked by lipid. We have developed a novel radioimmunoassay, which allows measurement of all the apoa-I in plasma or HDL without initial chemical or physical treatment using anti-apoA-I antibodies directed to those antigenic determinants of apoA-I which are normally exposed on the HDl surface. These surface-specific antibodies were isolated from a whole anti-apoA-I antiserum using a Sepharose affinity column to which the lipoprotein fraction (d<1.21) from a normal plasma pool had been coupled. Using this assay, the quantities of apoA-I measured in HDL and apoHDL were indistinguishable and apoA-I constituted 62% of the protein in apoHDL. Applying the technique to the whole plasma of 16 normolipidemic subjects we found mean (±S.D.) plasma apoA-I concentrations of 129±34.0 mg/dl for men and 154±28.8 for women, which correlate closely with values obtained independently by electroimmunoassay using a whole antiserum (128±29.8 for men, 160±21.3 for women; r=0.92, p<0.001). Moreover, the individual apoA-I concentrations are highly correlated with plasma concentrations of α-cholesterol (r=0.892, p<0.001). Since delipidation of HDL does not increase the amount of apoA-I detected by surface-specific antiapoA-I antibodies, it is suggested that individual apoA-I molecules are arranged similarly in HDL with the same antigenic sites exposed in each apoA-I molecule.