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 (KD) was 2.1 ± 0.3 nM, and density of binding sites (Bmax) was 920 ± 105 fmol/mg protein. The KD 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 (KI) 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 (KI > 10-5 M).
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