Detection of unique antigenic determinants on human plasma low density lipoprotein and on delipidated apolipoprotein B

Modulation of the expression of human LDL-Apo B-100 epitopes by lipids and apolipoproteins

The purpose of the present study was to determine the immunochemical properties of apolipoprotein (apo) B-100 associated with low density lipoprotein (LDL) in relation to lipid and apolipoprotein composition. LDLs were isolated by sequential ultracentrifugation (1.019 < d < 1.050 g/mL) from two healthy volunteers and 21 dyslipidemic patients to obtain heterogeneous samples of LDL. Lipid (free cholesterol, cholesteryl esters, triglycerides, and phospholipids) and apolipoprotein contents (apo B, apo C-III, apo E) were determined in each LDL sample. Immunoreactivities of apo B were tested in solid-phase competitive-binding radioimmunoassays using seven monoclonal anti-LDL antibodies that reacted with defined epitopes of apo B-100. The relation between lipid and/or protein variables and the immunoreactivity of apo B was evaluated by successive use of Spearman's rank simple correlation, partial correlation, and canonical correlation analyses. The canonical correlation analysis showed that apo B-100 immunoreactivity on LDL is highly dependent on lipid and apolipoprotein composition simultaneously. The results confirmed the influence of surface and core lipids on the expression of the apo B-100 epitopes, independent of their location on the molecule. However, the lipid requirement of LDL strongly influences the expression of epitopes mapped in the LDL receptor-recognition domain. In contrast to apo E, apo C-III does not seem to influence the expression of the apo B-100 epitopes in the LDL range studied.(ABSTRACT TRUNCATED AT 250 WORDS)

The development of glutathione S-transferase subunits in rat liver. Sensitive detection of the major subunit forms of rat glutathione S-transferase by using an e.l.i.s.a. method

The development of the subunits of glutathione S-transferase in rat liver shows that there is a co-ordinated development of the Ya, Yb1, Yb2 and Yc subunits but that the Yf and Yk subunits show unique patterns of development. The Yk subunit is the only form that is expressed at relatively high levels during the foetal period as well as during the adult period. In contrast with all other forms, the Yf subunit in the rat declines rapidly during the last few days before parturition and is virtually undetectable in hepatocytes of adult animals. The expression of the Yf subunit in foetal liver presents a 'patchy' appearance that is similar to that induced by the administration of lead acetate and may reflect cell-cycle-associated regulation of expression.

Rat monoclonal antibodies to rabbit and human serum low-density lipoprotein

A total of 16 hybrid myeloma clones secreting monoclonal antibodies (McAb) to rabbit or human serum low-density lipoprotein (LDL) were derived from the fusion of spleen cells from LOU or DA rats immunized with rabbit or human LDL and the rat myeloma lines Y3 Ag1.2.3 or YB2/0. Anti-(rabbit LDL) McAb showed limited reactivity with LDL from human, rhesus-monkey, rat and mouse serum. Six out of seven anti-(human LDL) McAb reacted with rhesus-monkey LDL, and only one showed partial cross-reaction with rabbit LDL. Binding-competition experiments indicated that the epitopes recognized by the anti-(rabbit LDL) IgG could be grouped into two major clusters: McAb in the first cluster reacted either with apo-(lipoprotein B-100) (apoB-100) and apo-(lipoprotein B-74) (apoB-74) or with apoB-100 but not with apo-(lipoprotein B-48) (apoB-48), the lower-Mr form of apoB of intestinal origin; the McAb in the second cluster all reacted with apoB-48 in addition to apoB-100 or apoB-100 and apoB-74. The six anti-(human LDL) IgG bound to separate epitopes on LDL. Further data on the epitope specificity of these McAb were obtained by antibody blotting after partial proteolysis of apoB-100 with trypsin or staphylococcal V8 proteinase, and the data confirmed the results obtained with the binding-competition experiments. One McAb to rabbit LDL inhibited the binding of LDL to the fibroblast LDL receptor (50% inhibition at a McAb/LDL molar ratio of 10). A similar result was produced by two other McAb at higher concentrations of antibody.

The production and use of monoclonal antibodies to measure apolipoprotein B by a competitive enzyme-linked immunoassay

The production of monoclonal antibodies to human lipoproteins is described. One of these antibodies, which was shown to be specific to apolipoprotein B, was used to develop a competitive enzyme-linked immunoassay for apolipoprotein B in serum samples. The antibody selected recognises apolipoprotein B in both low density and very low density lipoprotein particles, but there is no cross-reactivity with high density lipoprotein. There is no requirement for labelling of antigen or antibody used in the assay, and results obtained correlate well (r = 0.88) with measurements on serum samples using a radial immunodiffusion assay for apolipoprotein B.

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.