Because of its physical properties, apolioprotein B (apo B) has remained poorly characterized. In an attempt to elucidate apo B structure, the Fab fragments of 3 different monoclonal anti-human apo B anti-bodies were tested in a quantitative assay for their binding to human low density lipoprotein (LDL). In each case the assay gave a linear Scatchard plot with a maximum of 1 Fab fragment bound to a single LDL particle. This result favors an LDL model containing a single large M r apo B protein, which is not composed of multiple, identical, small M r subunits.
