2 CELLULAR AND SUBCELLULAR LOCATIONS FOR THE PERIPHERAL-TYPE BENZODIAZEPINE RECEPTOR IN RAT-LIVER

Determination of ligand binding properties of the peripheral benzodiaz epine receptor (PBBS) in liver, in hepatocytes, and in nonparenchymal cells demonstrated the presence of receptor-specific high-affinity bin ding in both hepatocyte and nonhepatocyte cells. Density gradient cent rifugation showed that the high-affinity receptor in hepatocytes was l ocalised to mitochondria, whereas in nonhepatocytes it was not mitocho ndrial, but with a possible biliary epithelial cell plasma-membrane lo cation. Both receptors showed the peripheraltype specific high affinit y binding of PK 11195 and Ro5 4864 and could be photolabelled as 18 kD a proteins with [H-3]PK 14105. Immunocytochemistry showed the presence of acyl-CoA binding protein, a putative endogenous ligand for the rec eptor, in both cell locations. Some other properties of the PBBS were investigated in liver. Diphosphatidyl glycerol had a strong inhibitory effect on receptor binding in both liver and adrenal, with Ro5 4864 m ore sensitive to inhibition than PK 11195. However, whereas soybean li pid and phosphatidyl serine increased the binding of both ligands to a drenal receptor, these lipids had no effecton liver, suggesting that l iver PBBS may differ from the well-characterised adrenal PBBS in some of its protein conformation. Modulators of mitochondrial respiration t hat also influence intermembrane contact site formation were found to elicit no marked stimulatory or inhibitory effects on PBBS ligand bind ing in liver, a result also found for adrenal mitochondria, suggesting that the extent of contact site formation does not influence ligand b inding and that the hepatocyte receptor may not play a role in regulat ing mitochondrial respiration. These two cellular and subcellular loca tions of the PBBS in liver and the different effect of phospholipids c ompared to other peripheral tissues may be important for the role(s) o f PBBS in liver and also for the multiple roles ascribed to the recept or and to peripheral-type benzodiazepine ligands.