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 lipop horin (I-125-HDLp-L). The iodination procedure did not change the affinity of the preparation for lipophorin. In the presence of increasing concentrat ions of membrane protein, corresponding increases in lipophorin binding wer e observed. The time-course of lipophorin binding to the membranes was affe cted by the lipophorin concentration in the medium, and at a low lipoprotei n concentration, a longer time was required for equilibrium to be reached. The specific binding of lipophorin to the midgut membrane was a saturable p rocess with a K-d=1.5+/-0.2x10(-7) M and a maximal binding capacity=127+/-1 7 ng lipophorin/mu g of membrane protein. Binding did not depend on calcium , was maximal around pH 5.5, was strongly inhibited by an increase in the i onic strength, and abolished by suramin. However, suramin did not completel y displace lipophorin that was previously bound to the membrane preparation . The lipid content of the lipophorin did not significantly affect the affi nity of the membrane preparation for lipoprotein. (C) 2000 Elsevier Science Ltd. All rights reserved.