Subcellular distribution of "peripheral type" binding sites for [3H]Ro5-4864 in guinea pig lung. Localization to the...

Binding of [3H]Ro5-4864, a specific ligand for "peripheral type" benzodiazepine receptors, was determined in subcellular fractions of guinea pig lung. Even though the level of binding was predominant in the mitochondrial fraction, nuclear and cytosolic fractions also contained significantly measurable amounts of binding sites. The presence of binding sites in the microsomal fraction and in a fraction intermediate in density between the mitochondria and microsomes depended on which buffer was used to homogenize the tissue. If calcium-containing mannitol buffer was used, binding was negligible in the postmitochondrial organelles. However, in the case of sucrose buffer which did not contain any calcium, the postmitochondrial organelle fractions contained measurable amounts of binding sites. Most probably, these binding sites were of mitochondrial and nuclear origin. Furthermore, binding sites in the mitochondria were associated with the succinic dehydrogenase-enriched mitochondrial inner membrane, but not with the monoamine oxidase- and cholinephosphotransferase-enriched outer mitochondrial membrane. Furthermore, several proteolytic enzymes caused a decrease in binding of the ligand to the mitochondrial membrane only under hypotonic conditions and not under isotonic conditions, suggesting that the location of the receptors is inside the mitochondria.