Characterization of peripheral benzodiazepine binding sites in human term placenta

Peripheral benzodiazepine binding sites were characterized in human term placental membranes using [³H]PK 11195, which is a ligand specific for peripheral benzodiazepine binding sites. Binding of [³H]PK 11195 to human term placental membranes was found to be saturable. Scatchard analysis revealed a single population of binding sites (r = 0.98). Equilibrium dissociation constant (K_D) was 2.1 ± 0.3 nM, and density of binding sites (B_max) was 920 ± 105 fmol/mg protein. The K_D value calculated from kinetic experiments was 3.6 ± 0.2 nM. The ability of various drugs to displace [³H]PK 11195 from human term placental binding sites was tested: the inhibition constants (K_I) for PK 11195, Ro 5-4864, and diazepam were 2.9, 11.8, and 177 nM, respectively, whereas clonazepam, methyl-β-carboline-3-carboxylate, Ro 15-1788, chlordiazepoxide, atropine, and estradiol were inefficient in displacing [³H]PK 11195 (K_I > 10^-5 M).