Binding of [3H]-4,5-secodihydrotestosterone by androgen receptor

Specific binding of [5,6-3H2]-17 beta-hydroxy-4,5-secoandrostan-3-one (or [3H]-4,5-secodihydrotestosterone, IX) to rat prostate cytosol receptors was observed and confirmed by two independent methods: by adsorption to dextran-charcoal and by polyacrylamide gel electrophoresis. The electrophoresis of [3H]-4,5-secodihydrotestosterone--cytosol mixtures showed a single radio-activity peak at different radioligand concentrations, displaceable by nonradioactive 4,5-secodihydrotestosterone; the mobility of the peak corresponds to a protein with a relative molecular weight of 90 000-130 000. The apparent association constant of the compound with respect to adsorption to dextran-charcoal is 1.48 X 10(9) l X mol-1, similar to the value observed with dihydrotestosterone (4.8 X 10(8) l X mol-1).