Immunocytochemical localization of a non-vitellogenin, estrogen-induced plasma protein in the hepatocyte of the American bullfrog, Rana catesbeiana.

Recently, a non-vitellogenin, estrogen-induced frog plasma protein of unknown function and site of synthesis, which has been given the temporary name, protein-RcX, was isolated and partially characterized by Mitchell et al. In the present study, the protein A-gold immunocytochemical technique was applied to investigate its site of secretion; this was found to be the hepatocyte of the estradiol-17 beta-treated adult, male American bullfrog, Rana catesbeiana. Specific immunolabeling for protein-RcX was present over those intracellular compartments involved in protein secretion, i.e., the rough endoplasmic reticulum, Golgi apparatus and secretory granules, in addition, lysosomes were also labeled. No specific labeling for this protein was observed on hepatocytes of normal, non-estrogen treated, adult, male bullfrogs. Further, the labeling was abolished when plasma containing protein-RcX was added to the antibody prior to incubation but remained when purified vitellogenin was added. These observations support the hypothesis that protein-RcX is a non-vitellogenin, estrogen-induced plasma protein which is synthesized and secreted in parallel with vitellogenin by the hepatocyte of the estrogen-treated frog.