Retinol-binding proteins in human testis cytosol.

In the cytosol fraction of human testes a specific binding protein for retinol has been isolated by sephadex column chromatography and by gradient centrifugation. The protein has a sedimentation coefficient of 2S, and its molecular weight is estimated by gel filtration to be approximately 16,000 daltons. Competitive binding studies with labeled retinol and 200-fold molar excess of unlabeled retinal, retinoic acid, retinyl palmitate and retinyl acetate indicate that the isolated protein is specific for retinol. Retinyl esters bind to the protein as effectively as retinol; however retinal binds to a lesser degree than retinol esters. In addition, a lower molecular weight protein of approximately 4,500 daltons was found in the cytosol that has comparatively lower affinity for retinol binding than the 16,000-dalton protein.