Binding of lipophorin to the fat body of the migratory locust

Abstract

The interaction of locust high density lipophorin (HDLp) with pieces of fat body tissue was studied at 33°C using a radiolabelled ligand binding assay. Under the assay conditions, binding of tritium-labelled HDLp ([ 3H]HDLp) was demonstrated to correlate linearly with tissue concentration up to ∼ 7 mg of fat body protein per ml of incubation medium. The [ 3H]HDLp binding that was displaceable by a 20-fold excess of unlabelled HDLp (which is an approximation of the specific binding) reached equilibrium after ∼ 2 h, whereas low levels of non-displaceable binding increased linearly during this time interval. Analysis of the concentration dependent total binding of [ 3H]HDLp revealed the presence of a specific binding site with an equilibrium dissociation constant of K d = 3.1 (±0.5) × 10 −7 M and a maximal binding capacity of 9.8 (±0.5) ng μg −1 tissue protein. Competition experiments demonstrated that the affinity of unlabelled HDLp for the binding site is similar to the affinity of [ 3H]HDLp. Unlabelled low density lipophorin (LDLp), however, was shown to have an approx. 20-fold lower affinity for the binding site.