INTERACTION OF AN INSECT LIPOPROTEIN WITH ITS BINDING-SITE AT THE FAT-BODY

A single type of high density lipophorin (HDLp) binding sites is prese nt at intact far body tissue and in fat body membranes of larval and a dult locusts. HDLp is bound with high affinity (K-d approximate to 10( -7) M). This interaction does not require divalent cations and is heat -labile because heat-treatment of fat body membranes results in a subs tantial reduction of the maximal binding capacity. In addition to unla beled HDLp and low density lipophorin (LDLp), human low density lipopr otein also seems to compete with radiolabeled HDLp for this binding si te, suggesting a relaxed specificity. Induction of lipid mobilization with adipokinetic hormone did not change the binding characteristics o f the fat body. An increase in the binding capacity of intact fat body tissue in the adult stage suggests that the number of cell surface bi nding sites is upregulated during development. However, the total numb er of HDLp binding sites appears to be constant, because larval and ad ult fat body membranes have similar binding capacities.