Use of monoclonal antibodies in studies of lipoprotein structure and function.

In order to probe the structure and function of apoB, monoclonal antibodies directed against human LDL were produced. The seven antibodies tested were found to be directed against five individual antigenic determinants of apoB in holo-LDL. One of the antibodies inhibited the binding of 125I-LDL to the apoB, E receptor of cultured human fibroblasts. Limited proteolysis of LDL results in the production of relatively stable lipoprotein "core" particles that retain many physical, chemical and immunological properties of the native lipoprotein. Perturbations of the structure of apoB by limited proteolysis of LDL had selective effects on the expression of various epitopes in the resulting LDL "cores" - the expression of some was unaffected, others were abolished. In addition, the various epitopes could be localized by immunoblotting to different proteolytic fragments of apoB present in the LDL "cores". Limited proteolysis also provided water soluble cleavage peptides, some of which exhibited immunoreactivity. Digested LDL "core" particles and cleavage peptides both interacted with the LDL receptors of cultured human fibroblasts. These experiments suggest that apoB may consist of repeating subunits of similar structure. The immunoreactivity of LDL may be determined not only by the structure of the protein moiety itself, but also by other lipoprotein constituents. Studies utilizing both polyclonal and monoclonal anti-apoB antibodies have shown that apoB reacts differently in different types of lipoprotein particles (i.e., VLDL, IDL and LDL). We studied the immunological reactivities of apoB within the LDL class. The results showed that the expression of epitopes in different LDL preparations indeed varied, and that the number of epitopes expressed, appeared to be influenced by the lipid composition of LDL.