Purification, characterization and immunolocalization of bovine and ovine placental retinol-binding protein

A retinol-binding protein (RBP) synthesized and secreted by bovine and ovine allantois in vitro was purified from culture medium. The protein consisted of three isoelectric variants (pI 5.3-6.1) of identical molecular weight of about 23,000 as determined by two-dimensional polyacrylamide gel electrophoresis. Thirty-one of the first 34 N-terminal amino acids of the purified protein from both sources were sequenced and shown to have complete homology with bovine plasma RBP. The UV absorption spectrum and fluorescence excitation and emission spectra of the purified bovine and ovine placental RBP indicated the presence of bound retinol. Rabbit antiserum was raised against purified bovine placental RBP. Metabolic labelling, immunoprecipitation studies demonstrated that the protein was synthesized by bovine and ovine extraembryonic membranes (allantois, chorion and amnion). The protein was also detected in allantoic and amniotic fluids by immunoblotting. With immunocytochemical procedures, the placental RBP was immunolocalized in trophectodermal cells of the chorion, endodermal cells lining the allantois and ectodermal cells lining the amnion. Results from this study suggest that bovine and ovine placental membrane epithelia synthesize and secrete REP. Transport, storage, and metabolism of retinol mediated by placental REP may be essential for normal embryonic development during pregnancy.