Affinity Labeling of the Androgen Receptor in Rat Prostate Cytosol with 17β-[(Bromoacetyl)oxy]-5α-androstan-3-one

An androgen affinity label,17β-[(bromoacetyl)-oxy]-5a-androstan-3-one, has been synthesized in both radioactive and nonradioactive forms. The affinity label (170 Ci/mmol) was characterized and found to have a high degree of purity. Affinity labeling of the androgen receptor from rat ventral prostate was androgen specific and appeared to be directed at the steroid binding site of the protein. Covalent binding was achieved at 0 °C; however, heat treatment at 23 °C for 30 min enhanced covalent binding by 31%. The covalently bound steroid was resistant to extraction with organic solvents and precipitation with trichloroacetate. The Stokes radius (4.2 nm) and sedimentation coefficient (4.5 S) were identical with those found for receptor bound noncovalently to dihydrotestosterone. Gel electrophoresis of the affinity-labeled receptor under denaturing conditions revealed a molecular weight of 86000. The same molecular weight was observed for the receptor from rat seminal vesicle. This affinity label will be useful in future studies on the structure and function of androgen receptors.