Expression of apolipoprotein B epitopes in very low density lipoprotein subfractions. Studies with monoclonal antibodies.

Epitope expression was studied in both denatured apolipoprotein B (apo B) on Western blots and in intact low density lipoprotein (LDL) and very low density lipoprotein subfractions VLDL1 (Sf120-400), VLDL2 (Sf60-120), and VLDL3 (Sf20-60) in competitive binding immunoassays with the aid of six monoclonal anti-LDL antibodies. The apo B in all lipoprotein fractions was shown to bind to all antibodies, but significant differences in apo B epitope expression were observed. On the average, the immunoreactivity of VLDL subfractions (expressed as binding affinity and as relative 125I-LDL displacing potency) decreased with increasing flotation rate. Similarly, VLDL1 was less immunoreactive than lipolyzed "remnants" of VLDL1 after treatment with bovine milk lipoprotein lipase. The results indicate that, even when lipoprotein fractions obtained from the same individual and having the same kind of apo B subspecies were compared, significant differences in immunoreactivity occurred due to the modulating effect of other lipoprotein components on apo B epitope expression.