Interactions of lipids with peripheral-type benzodiazepine receptors

Peripheral-type benzodiazepine receptors (PBRs) are present at high densities in the rat kidney distal tubule. [3H]RO 5-4864 binding to PBRs in kidney membranes is inhibited by several unidentified low molecular weight hydrophobic compounds in urine and serum. We tested representative hydrophobic compounds from several lipid classes for ability to inhibit binding to rat kidney PBRs of two high affinity ligands, [3H]RO 5-4864 and [3H]PK 11195. Unsaturated fatty acids and alcohols inhibited [3H]RO 5-4864 binding with half-maximal inhibition occurring at 3 X 10(-6) M to 10(-4) M. Inhibitory potency increased with the degree of unsaturation. Phospholipids inhibited [3H]RO 5-4864 in the same concentration range, with inhibitory potency in this case dependent both upon an unsaturated fatty acid moiety and upon the polar head group. Phosphatidylethanolamine was the most potent phospholipid tested (IC50 = 2 X 10(-6) M), whereas phosphatidylcholine was not inhibitory. Although phospholipids inhibited both [3H]RO 5-4864 and [3H]PK 11195 binding equally, unsaturated fatty acids had a much greater inhibitory effect upon [3H]RO 5-4864 than upon [3H]PK 11195 binding. Similar effects were obtained with digitonin-solubilized PBRs. These data demonstrate that in our experiments PBR binding was inhibited by specific lipids and that binding of proposed agonist (RO 5-4864) and antagonist (PK 11195) ligands was differentially affected by unsaturated fatty acids.