Purification and partial characterization of rat epididymal retinoic acid-binding protein, and its immunohistochemical localization.

Two types of retinoic acid-binding proteins, designated as epididymal retinoic acid-binding protein (ERABP) types A and B, have been purified to homogeneity from rat genital organs. ERABP types A and B had molecular weights of 19,000 and 18,500 and isoelectric points of 5.72 and 5.90, respectively. The absorption spectrum of ERABP complex with retinoic acid had two peaks at 277 and 354 nm, and showed similar uncorrected fluorescence spectra to that of cellular retinoic acid-binding protein (CRABP). These ERABPs were identified as major androgen-dependent epididymal proteins by amino acid sequences analyses. Furthermore, immunohistochemical examinations revealed that the ERABPs exist in the epithelium of the proximal portion of the epididymis and in the lumen of epididymal canal and seminal tract leading from the epididymis. The existence of ERABP strongly suggests that retinoic acid might be involved in the maturation of spermatozoa.