Binding Kinetics and Physical Properties of Androgen Receptor in Androgen-Dependent Shionogi Mammary Carcinoma 115

Abstract

The characteristics of the androgen receptor in the cytoplasmic fraction of Shionogi carcinoma 115 were studied in vitro by means of charcoal adsorption assay, sucrose density gradient centrifugation, and polyacrylamide gel electrophoresis. The equilibrium dissociation constant for [³H]dihydrotestosterone (K_d=(2-5)×10^-11M) was estimated from independently determined rates of association and dissociation, and was lower by one order of magnitude than the value obtained by saturation analysis (K_d=(2-8)×10^-10M). Evaluation of the effect of temperature on receptor binding of androgen allowed the estimation of several thermodynamic parameters, including activation energies of association (4 kcal/mol) and dissociation (14 kcal/mol), the apparent free energy (-13 kcal/mol), enthalpy (-9 kcal/mol), and entropy (+14 cal/mol per K). The receptor was greatly stabilized when bound with androgen. The results indicate how the lability of the unbound receptor and slow rate of dihydrotestosterone-receptor interaction can influence the estimation of dissociation constants by usual saturation analysis. The sedimentation coefficient of androgen receptor in freshly prepared cytosol was 6S, and became 7S after storage for 2 months at -80°C. The 7S conversion of the receptor was reversed by treatment with heparin. In all cases, a single 5S peak was obtained in the presence of 0.5M KCl. On electrophoresis in heparin-containing polyacrylamide gel, protein-bound radioactive androgen migrated as a single peak (R_f=0.5 in 5% gel). Differences in reported values for the sedimentation coefficient of androgen receptor in cytosol of Shionogi carcinoma 115 appear to be derived largely from aggregation of the receptor protein during the assay procedure.