Immunochemical Heterogeneity of Human Plasma Apolipoprotein B. II. Expression of Apolipoprotein B Epitopes on Native Lipoproteins

Eleven hybridoma antibodies specific for apoprotein B were used to examine the expression of apoprotein B epitopes on native plasma lipoproteins using fluid phase radioimmunoassays. Heterogeneity of apoprotein B in low density lipoprotein (LDL) of density 1.019-1.063 g/ml was indicated by maximum binding of radioiodinated LDL in the presence of excess antibody that ranged from 45 to 100%. Affinity constants for LDL ranged from 0.29-3.0 X 10(9) M-1. Each of the 11 antibodies recognized apoprotein B epitopes that were expressed by very low, intermediate, and low density lipoproteins, VLDL, IDL, and LDL, respectively, since at high concentrations, each could fully displace the binding of radioiodinated LDL. In contrast, these same apoprotein B epitopes were not present on high density lipoproteins. Logit transformation analysis of competitive inhibition titrations demonstrated at least three patterns of epitope expression. The first pattern, identified by five of the antibodies, was characterized by indistinguishable expression of each of the apoprotein B epitopes on VLDL, IDL, and LDL. The second pattern, identified by three antibodies, was characterized by identical expression of the apoprotein B epitopes on VLDL and IDL, but of differing affinities than the related epitope on LDL. Three antibodies identified a third pattern of epitope expression which was was characterized by identical expression on IDL and LDL, but with differing affinities for the antibodies than the same epitopes on VLDL. These observations suggest that the organization of apoprotein B in VLDL, IDL, and LDL is similar, but not identical.