RAT EPIDIDYMAL RETINOIC ACID-BINDING PROTEIN - DEVELOPMENT OF A RADIOIMMUNOASSAY, ITS TISSUE DISTRIBUTION, AND ITS CHANGES IN SELECTED ANDROGEN-DEPENDENT ORGANS AFTER ORCHIECTOMY

Rat epididymal retinoic acid-binding Protein (EP-RABP) has been purified to apparent homogeneity from extracts of the epididymis by HPLC. N-Terminal amino acid sequence analysis revealed that the HPLC-purified protein consisted of two molecular variants, in that one has three extra N-terminal amino acids of NH2-TEG. These two molecular variants were subsequently separated by high performance electrophoresis chromatography. A specific and sensitive RIA has been developed to quantify this protein in various organ extracts of both male and female rats. Rat EP-RABP is a male-specific protein, since it was detected in male organ extracts, including epididymis, testis, prostate, seminal vesicles, liver, spleen, and brain, with a negligible concentration in the female liver and spleen. It was noted that this protein was absent in the systemic circulation of both male and female rats. It was first detected in the epididymis and testis of rats at 22 and 27 days of age, respectively. Both the concentrations (micrograms per g tissue) and the organ content (micrograms per organ pairs) of this protein in the testis and epididymis increased progressively with an increase in age and reached the highest levels at 60 and 120 days of age, respectively. At 120 days of age, its concentrations in all organs examined, with the exception of the epididymis, showed a dramatic decrease compared to levels in rats at 60 days of age. Orchiectomy decreased its concentrations in the caput, corpus, and cauda epididymis and in the ventral prostate, but had no apparent effect on seminal vesicles. Administration of dihydrotestosterone to castrated rats restored only 30% of the level of this protein in the caput epididymis compared to that in intact animals, but had no apparent effect on the corpus, cauda epididymis, or ventral prostate. These observations suggest that this protein is under multihormonal control in the epididymis and selected androgen-dependent organs.