Following the administration of [3H] testosterone to castrated rats in vivo, most of the radioactivity in the epididymal cytosol fraction was bound to macromolecules. The molecular weight of the androgen-macromolecular complexes was determined to about 90 000, and the frictional ratio (f/fo) was 1.62. The sedimentation coefficient was shown to be 4.6 S (± 0.2) and the complex was slightly retarded on a column of Sephadex G-100 (Einstein-Stokes radius 47–48 Å). On polyacrylamide gel electrophoresis it moved as a sharp zone with RF: 0.41.
Almost all the radioactivity bound to these macromolecules was identified as 5α-dihydrotestosterone (5α-DHT) (95%) and only a minor part (3–4%) as testosterone.
The receptor for 5α-DHT in the epididymal cytosol fraction was in its physico-chemical properties shown to be different from the androgenbinding proteins of the rat ventral prostate, and could not be demonstrated in non-target organs like liver and kidney. The receptor for 5α-DHT in the epididymal cytosol fraction did not form aggregates in hypotonic solutions, and was roughly of the same molecular size both at high and low salt concentrations.