ABSTRACT

The recent isolation and characterization of two retinoic add-binding proteins, different from CRABP, from the lumen of the epididymis brought forward the question of unique, epididymis-specific retinoid-binding proteins (99,100). Several tissue-specific retinoid-binding proteins were previously characterized in the eye and intestine (101,102). and Chytil (99) first described the retinoic binding activity of these proteins and named them epididymal binding proteins 1 and 2 (EBP-1 and 2). These proteins were found to be identical with the previously described epididymal prealbumin proteins B and C, for which cDNAs have been cloned (103,104). Further work showed that EBP-1 and 2 are isoforms of the major androgen-dependent B protein (105). The isoforms are of similar size (18.5 kD) differing in length by three amino acids and somewhat larger than CRABP (15 kD). The major androgen-dependent B-protein isoforms belong to the «2~-t-microglobulin superfamily (106), which also comprises serum retinolbinding protein. The expression of these proteins is controlled by testosterone; hence, in hypophysectomized animals this expression is strongly reduced (103,104). Synthesis of both isoforms occurs in the caput epididymis, where they enter the lumen and attach to the sperm cells. It is not clear as to whether they carry retinoic acid from the epithelial cell into the lumen or bind retinoic acid already present in the epididymal lumen, similar to the way ABP functions. The presence in caput epididymis of high levels of CRBP, which can serve to accumulate high concentrations of retinol and make it available for oxidation to retinoic acid, would allow these proteins lo become charged with retinoic acid before entering the lumen.