Characterization of lipophorin binding to the midgut of larval Manduca sexta

Lipophorin binding to the midgut of Manduca sexta larvae was characterized in a midgut membrane preparation, using iodinated larval high-density lipophorin ((125)I-HDLp-L). The iodination procedure did not change the affinity of the preparation for lipophorin. In the presence of increasing concentrations of membrane protein, corresponding increases in lipophorin binding were observed. The time-course of lipophorin binding to the membranes was affected by the lipophorin concentration in the medium, and at a low lipoprotein concentration, a longer time was required for equilibrium to be reached. The specific binding of lipophorin to the midgut membrane was a saturable process with a K(d) = 1.5+/-0.2x10(-7) M and a maximal binding capacity = 127+/-17 ng lipophorin/microg of membrane protein. Binding did not depend on calcium, was maximal around pH 5.5, was strongly inhibited by an increase in the ionic strength, and abolished by suramin. However, suramin did not completely displace lipophorin that was previously bound to the membrane preparation. The lipid content of the lipophorin did not significantly affect the affinity of the membrane preparation for lipoprotein.