The subcellular location in rat kidney of the peripheral benzodiazepine acceptor

In rat kidney high‐affinity binding sites for [3H]Ro‐5‐4864 and [3H]PK‐11195 with the properties of the peripheral‐type acceptor were found enriched in mitochondrial (M) and light‐mitochondrial‐lysosomal (L) fractions on differential centrifugation. When the combined M and L fractions were subjected to sucrose density gradient centrifugation, these binding sites were found enriched at a density of 1.155 g/ml coincident with a population of light mitochondria, whereas a population of heavier mitochondria (ϱ= 1.175 g/ml) had few or no binding sites. Transmission electron microscopy showed that whereas the heavier mitochondria appeared highly pure and intact, the lighter mitochondria appeared less intact and to be contaminated with vesicular structures. After fractionation of the light mitochondria and vesicles by centrifugation, both fractions showed the same ratio of [3H]Ro‐4864 binding sites to monoamine oxidase activity consistent with the vesicles being of mitochondrial outer‐membrane origin. Digitonin pre‐treatment had no effect on the density of acceptor‐rich fractions on sucrose density gradient centrifugation. However, pretreatment with succinate/iodophenylnitrophenylphenyltetrazolium (INT) perturbed equally the density of acceptor‐rich fractions and mitochondrial marker enzymes. When mitochondrial fractions were subjected to sonication prior to density gradient centrifugation the binding sites were now found highly enriched in a much lighter fraction coincident with the monoamine oxidase activity and thus consistent with being outer‐membrane vesicles. When a mitochondrial fraction was subjected to hypotonic treatment before assay no evidence for activation/ unmasking of binding sites was found. The hypotonic treatment did not release any inhibitor of the binding sites. These results are consistent with the peripheral benzodiazepine acceptor having an outer‐membrane location on a sub‐population of rat kidney mitochondria. Those mitochondria showing high levels of the acceptor are either light mitochondria or appear more susceptible to osmotic damage than those mitochondria in which the acceptor is absent or at low levels.