Characterization of the retinol and retinoic acid binding proteins in the human prostate

A retinol-binding protein has been detected in the cytosol of human prostates with benign hyperplasia. The binding was of high affinity and specific for retinol (Kd = 35 nM), with other retinoids such as trans-retinoic acid, retinal, and the synthetic analogues, all-trans-9-(4-methoxy-2,3,6-trimethylphenyl)-3,7-dimethyl-2,4,6,8-nona tetraenoic acid and p-[(E)-2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)-1-propenyl] benzoic acid, showing little or no competition. The retinol binding, which sedimented as a 2S component on sucrose density gradients, was also unaffected by the addition of excess unlabeled steroid hormones. Furthermore, pretreatment of the cytosol proteins with heat and/or trypsin totally abolished the retinol binding. Parallel experiments with trans-retinoic acid suggest that the hyperplastic prostate possesses a second retinoid-binding site which is specific for retinoic acid and distinct from the retinol-binding component. Experiments with serum from patients with benign prostate hyperplasia revealed no binding at the 2S sedimentation position; this suggests that the retinoid-binding proteins were exclusively associated with prostatic tissue and were not therefore derived from serum.