Characterization and immunocytochemical localization of lipophorin binding sites in the oocytes of Rhodnius prolixus.

Purified lipophorin, metabolically labelled with 32P exclusively in the phospholipid moiety, was used to study the process of phospholipid delivery to the oocyte. The kinetics of phospholipid transfer "in vitro," from lipophorin to the oocytes, was linear at least up to 4 h and was impaired by low temperature. A net transfer of phospholipids from lipophorin particles to the oocytes was observed. The rate of phospholipid uptake was dependent on the concentration of lipophorin in the medium and was shown to be a saturable process. The addition of a molar excess of purified unlabelled lipophorin to the culture medium resulted in a substantial decrease in the transfer of [32P]phospholipids, but no reduction occurred in the presence of a molar excess of albumin. The lipophorin binding sites were localized in the oocytes by immunogold techniques using two different protocols for oocyte fixation. Strong labelling was observed especially at the microvilli. No labelling was detected in the yolk granules.