Characterization of the Retino! and Retinoic Acid Binding Proteins in the Human Prostate1

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 (K¿= 35 nM), with other retinoids such as frans-retinoic acid, retinal, and the synthetic analogues, all-/rans-9-(4-methoxy-2,3,6-trimethylphenyl)-3,7dimethyl-2,4,6,8-nonatetraenoic acid and p-[(E)-2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)-1 -propenyl]benzoic acid, snowing 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 frans-retinoic acid suggest that the hyperplastic prostate possesses a second retinoid-binding site which is specific for retinoic acid and distinct from the retinolbinding component. Experiments with serum from patients with benign prostate hyperplasia revealed no binding at the 2S sedi mentation position; this suggests that the retinoid-binding pro teins were exclusively associated with prostatic tissue and were not therefore derived from serum.