Lipophorin interaction with the midgut of Rhodnius prolixus: characterization and changes in binding capacity

Several classes of lipids are transported in insect hemolymph by lipophorin, a major hemolymphatic lipoprotein. The binding of lipophorin to the midgut of the hematophagous insect Rhodnius prolixus was characterized in a midgut membrane preparation, using purified lipophorin radiolabelled in protein moiety ((125)I-HDLp). Lipophorin specific binding to membranes achieved equilibrium after 30-40 min, was sensitive to pH, and was maximal at pH 7.0. In the presence of increasing concentrations of membrane protein, corresponding increases in lipophorin binding were observed. The specific binding of lipophorin to the membrane preparation was a saturable process, with K(d)=0.9+/-0.06 x 10(-7) M and a maximal binding capacity of 70+/-11 ng lipophorin/microg of membrane protein. Lipophorin binding did not depend on calcium, but it was affected by ionic strength and was inhibited in the presence of increasing salt concentrations. Suramin interfered with lipophorin binding to the midgut receptor, and it was abolished in the presence of 2 mM suramin, but at concentrations between 0.05 and 0.2 mM it was slightly increased. Condroitin 4-sulfate also affected lipophorin binding, which was reduced to 56% of control. Pre-incubation of the midgut membrane preparation with trypsin or at high temperature inhibited binding. Midgut capacity to bind lipophorin varied at different days after blood meal. It was highest at second day after feeding, and then gradually decreased.