Presence of novel retinoic acid-binding proteins in the lumen of rat epididymis.

The lumen of the rat epididymis was found to contain two binding activities for retinoic acid. The two proteins could be separated by chromatography on DEAE-cellulose but were essentially identical in binding specificity and molecular weight, as determined by gel filtration. The proteins were clearly distinct from cellular retinoic acid-binding protein and other known retinoid-binding proteins. The binding of various retinoids was examined by sucrose gradient centrifugation assay. The proteins were more discriminating than has been observed for cellular retinoic acid-binding protein. Of the retinoids examined, only 13-cis-retinoic acid showed appreciable ability to compete for binding of all-trans-retinoic acid. Retinol and retinal showed no ability to compete; various ring analogs of retinoic acid had little or no ability to compete. The presence of such binding activities in the lumen of the epididymis could mean that retinoic acid plays a role in sperm maturation and function, if retinoic acid proves to be the endogenous ligand.