LIPOPHORIN AS A YOLK PRECURSOR IN HYALOPHORA-CECROPIA - UPTAKE KINETICS AND COMPETITION WITH VITELLOGENIN

Vitellogenic follicles of Hyalophora cecropia were incubated in metabolically radiolabeled, high‐density lipophorin isolated from pharate adult hemolymph by KBr density gradient centrifugation. The follicles transferred this probe from the incubation medium to the cortical yolk spheres in the oocyte by an energy‐dependent and saturable mechanism. Vitellogenin and high‐density lipophorin competed with each other for uptake, and are therefore concentrated by the follicle with a common mechanism. Microvitellin and lipophorin, in contrast, did not compete for uptake. The Kuptake for the accumulation of high‐density lipophorin was substantially higher than the value estimated earlier for vitellogenin (133 μM vs. 18 μM). This relationship helps explain why the shared concentrating mechanism does not deplete the lipid transport capacity of the hemolymph, and how a low vitellogenin: lipophorin molar ratio in the hemolymph yields a high ratio in the mature egg. Copyright © 1990 Wiley‐Liss, Inc.