PURIFICATION AND CHARACTERIZATION OF A UTERINE RETINOL-BINDING PROTEIN IN THE BITCH

A major canine endometrial secreted protein (cP6, 23000-M(t)) was puri fied by ion-exchange and gel-filtration chromatography and characteriz ed by two-dimensional gel electrophoresis. Anti[human retinol-binding protein (hRBP)] serum identified cP6 on immunoblot analysis and immuno precipitated cP6 from culture medium. This major protein was also show n to bind [H-3]retinol. N-terminal and internal amino acid sequences w ere determined and compared with previously identified protein, RNA, o r DNA sequences. N-terminal analysis revealed that cP6 had high identi ty and similarity to serum retinol-binding proteins (RBPs), while inte rnal sequence analysis showed a strong similarity to rat androgen-depe ndent epididymal protein and beta-lactoglobulins. Amino acid analysis, however, showed significant differences between these proteins and cP 6 in both total amino acid content and certain selected amino acids. I mmunohistochemical analysis showed staining for REP only in the uterin e luminal epithelium. These studies suggest that bitch endometrium sec retes a family of proteins (cP6), some of which bind [H-3]retinol, are immunologically related to the REP family, and have N-terminal and in ternal sequences with a high similarity to REP, beta-lactoglobulins an d other members of the lipocalin family. This family of proteins may b e important in early development for supplying retinol or derivatives to the developing embryo.