2e). Identifying novel ligands for the Ro 61-8048 in vitro rPGRMC1-associated binding site activity through LAGS binding site activity The low expression PSI-7977 clinical trial of binding site activity in rPGRMC1-transfected COS-7 cells and relatively high level of non-specific binding in extracts (~50% of specific and non-specific binding), precluded this system from extensive and effective screening for novel rPGRMC1 ligands. However, the binding of dexamethasone to rat liver microsomes (LAGS activity)

gave reproducible saturable binding characteristics with a kD of 51 nM and maximal binding site concentration of 8.3 pmoles/mg of microsomal protein (Fig. 3a); was subject to relatively low non-specific binding (~5% of specific and non-specific binding); was sufficiently abundant and binding was competed by progesterone and a range of other ligands (Fig. 3b, Table 1), but not by the sigma receptor ligand haloperidol [25]. Early work by Meyer et al identified a progesterone binding protein in pig liver microsomes with no competition

for binding by dexamethasone (IC50% > 100 μM) [26], but competition by haloperidol [27]. There may be species differences between pig and rat which makes comparison complicated. Belnacasan cost However, a sigma-related binding site has been shown to be expressed in rat liver microsomes, which binds both progesterone and haloperidol [28]. Our data suggest that dexamethasone and progesterone share a binding site in rat liver microsomes, but on the basis that there is no competition for binding by haloperidol, this is not the sigma-related binding site. Therefore, the use of dexamethasone as a ligand either for the LAGS is preferred over progesterone. Table 1 IC50% values for competing radiolabelled dexamethasone from specific binding to rat liver microsomes. Cold Competitor IC50% (10-6 M) dexamethasone 0.098 ± 0.003 progesterone 0.081 ± 0.010 clotrimazole 40 ± 12 metyrapone 310 ± 52 haloperidol > 10000 Data are the mean and standard deviation of

at least 3 separate microsomal (isolated from different animals) determinations. Figure 3 Radiolabelled dexamethasone interacts in a specific and saturable manner with rat liver microsomes and binding is competed by selected compounds. Male rat liver microsomes were incubated in duplicate with increasing concentrations of radiolabelled dexamethasone (ligand) with or without excess unlabelled dexamethasone and allowed to reach equilibrium on ice. A small volume of each incubation was removed to determine the total ligand concentration ([L0]) prior to removal of free unbound ligand by dextran/charcoal adsorption. Specifically bound ligand at equilibrium ([LRe]) was calculated by subtracting radioactive counts present in samples which also contained excess unlabelled dexamethasone after dextran-charcoal adsorption (and was typically < 5%).