J. Biol. Chem., Vol. 261, Issue 15, 6847-6852, 05, 1986
The effects of human low and high density lipoproteins on the binding of rat intermediate density lipoproteins to rat liver membranes
L Brissette and SP Noel
Upon incubation with rat liver membranes, radioiodinated rat intermediate
density lipoproteins (IDL) interacted with at least two binding sites
having a low and a high affinity as demonstrated by the curvilinear
Scatchard plots obtained from the specific binding data. The purpose of our
work was to identify the nature of these binding sites. Human low density
lipoproteins (LDL), contain apolipoprotein B only, and human high density
lipoproteins (HDL3), containing neither apolipoprotein B nor E, were both
capable of decreasing the specific binding of rat 125I-IDL. The Scatchard
analysis clearly revealed that only the low affinity component was affected
by the addition of these human lipoproteins. In fact, the low affinity
binding component gradually decreased as the amount of human LDL or HDL3
increased in the binding assay. At a 200-fold excess of human LDL or HDL3,
the low affinity binding was totally masked, and the Scatchard plot of the
specific 125I-IDL binding became linear. Only the high affinity binding
component was left, enabling a precise measurement of its binding
parameters. In a series of competitive displacement experiments in which
the binding assay contained a 200-fold excess of human LDL or HDL3, only
unlabeled rat IDL effectively displaced the binding of rat 125I-IDL. We
conclude that the low affinity binding of rat IDL to rat liver membranes is
due to weak interactions with unspecified lipoprotein binding sites. The
camouflage of these sites by human lipoproteins makes possible the study of
IDL binding to the high affinity component which likely represents the
combined effect of IDL binding to both the remnant and the LDL receptors.