Characterization and immunolocalization of bovine uterine retinol-binding protein

Endometrial explants obtained from cows between Days 13 and 29 of pregnancy were cultured for 24 h in modified minimum essential medium in the presence of [35S]methionine or [3H]leucine. Proteins synthesized and released into medium were analyzed by two-dimensional polyacrylamide gel electrophoresis and fluorography. Uterine luminal flushings were obtained from cyclic cows (Days 2-20 of estrous cycle) and early pregnant cows (Days 17-22). Endometrial tissues from cows on Days 17 and 29 of pregnancy were prepared for immunocytochemistry. A uterine secretory protein, which consisted of five isoelectric variants (pI 5.3-6.1) of identical molecular mass (23,000 Da), was shown to react immunologically with antiserum raised against bovine placental retinol-binding protein (bpRBP). Limited N-terminal sequence analysis of two major isoforms showed that the protein had nearly complete homology with bovine placental and plasma retinol-binding protein (RBP) over the first 25 amino acids. Through use of bpRBP antiserum, immunoreactive RBP was detected in uterine flushings collected from cows in the late luteal phase of the estrous cycle and early pregnancy by Western blotting, and in medium conditioned by uterine explants prepared at Days 13-29 of pregnancy by immunoprecipitation. Immunoreactive RBP was localized in endometrial surface and glandular epithelium on Days 17 and 29 of pregnancy by immunocytochemistry. These results demonstrate that RBP is a product of bovine uterine tissues. The uterine RBP may play an important role in vitamin A transport between maternal tissues and developing embryos.