The peripheral-type benzodiazepine receptor. Localization to the mitochondrial outer membrane.

Abstract

We have investigated the subcellular localization of the peripheral-type benzodiazepine receptor in rat adrenal gland using the high affinity ligand 3H-labeled l-(2-chlorophenyl)-Nmethyl-(l-methylpropyl)-3-isoquinoline carboxamide (r3H]PK1 1195).The autoradiographic pattern of [3H]PK1 1195 binding sites in tissue sections of adrenal gland is similar to the histochemical distribution of the mitochondrial marker enzymes, cytochrome oxidase and monoamine oxidase, which are present in high concentrations only in the cortex.Subcellular fractionation studies of homogenates of adrenal gland indicate that the recovery and enrichment of [3H]PK11195 binding sites in the nuclear, mitochondrial, microsomal, and soluble fractions correlate closely with cytochrome oxidase activity, but not with markers for the nuclei, lysosomes, peroxysomes, endoplasmic reticulum, plasma membrane, or cytoplasm, indicating an association of the peripheraltype benzodiazepine receptor with the mitochondrial compartment.Titration of isolated mitochondria with digitonin results in the simultaneous release of the peripheral-type benzodiazepine receptor and of monoamine oxidase, but not cytochrome oxidase, indicating association of the peripheral-type benzodiazepine receptor with the mitochondrial outer membrane. Scatchard analysis and drug displacement studies of the binding of [3H]PK11195 to intact mitochondria and to the outer membrane-enriched digitonin extract further confirm the localization of the peripheral-type benzodiazepine receptor to the mitochondrial outer membrane.Benzodiazepines are widely used anxiolytic and anticonvulsant drugs, for which binding sites were initially identified in nervous tissue (1,2).These binding sites are closely linked to GABA receptors and modulate the GABA-regulated anion channel (3-6).In addition to these GABA receptor-linked, "central-type" benzodiazepine receptors, a second type of benzodiazepine receptor has been identified, which is sparse in nervous tissue, but prominent in many other tissues (1, 7-10).These "peripheral-type" benzodiazepine receptors are not linked to GABA-regulated anion channels (7, 8).Anxiolytic or anticonvulsant drugs, which bind with high affinity to the * This work was supported