Effect of lipid transfer activity and triglyceride hydrolysis on apolipoprotein B immunoreactivity in modified low density lipoproteins.

Consequences of alterations in the size and the lipid composition of low density lipoproteins (LDL) on apolipoprotein (apo) B immunoreactivity were analyzed using two distinct anti-apoB monoclonal antibodies (Mabs), i.e., 4G3, which recognizes an epitope closed to the binding site to the LDL receptor, and 2D8, which is directed against a distal region. Inhibition analysis revealed that the lipid transfer-mediated triglyceride enrichment of LDL isolated from 12 native human plasmas is associated with significant reductions in the expression of 2D8 and 4G3 epitopes (P < 0.05 in both cases). In contrast, triglyceride hydrolysis of triglyceride-rich LDL significantly increased apoB immunoreactivity as compared with non-lipolyzed counterparts (P < 0.05 with 2D8 and 4G3 Mabs). Among all the modified LDL fractions studied (n = 36), immunoreactivity of 2D8 and 4G3 epitopes correlated negatively and significantly with the triglyceride content (P < 0.01 in both cases), but with neither the size nor the other lipid parameters of LDL particles. Furthermore, changes in the triglyceride content of LDL correlated significantly with changes in apoB immunoreactivity after in vitro treatment with either lipid transfer activity alone (P < 0.01 with 2D8 and 4G3 Mabs) or lipid transfer activity combined with triglyceride hydrolysis (P < 0.01 with 2D8 and 4G3 Mabs). Finally, both the triglyceride content of native LDL and the total triglyceride level in 12 normolipidemic human plasmas correlated negatively and significantly with the expression of 2D8 epitope (P < 0.03 in both cases) and 4G3 epitope (P < 0.02 in both cases). It is concluded that triglycerides constitute a major determinant of the immunoreactivity of 2D8 and 4G3 apoB epitopes in LDL.