Stabilization of the peripheral-type benzodiazepine acceptor by specific phospholipids

Acute cognitive effects of standardised Ginkgo biloba extract complexed with phosphatidylserine

Recent data suggest that the complexation of standardised Ginkgo biloba extract (GBE) with soy-derived phospholipids enhances the bioavailability of GBE's active components. The current study therefore aimed to assess the comparative cognitive and mood effects of a low dose of GBE and products complexing the same extract with either phosphatidylserine or phosphatidylcholine. The study utilised a placebo-controlled, multi-dose, double-blind, balanced-crossover design. Twenty-eight healthy young participants received 120 mg GBE, 120 mg GBE complexed with phosphatidylserine (Virtiva), 120 mg GBE complexed with phosphatidylcholine and a matching placebo, on separate days 7 days apart. Cognitive performance was assessed using the Cognitive Drug Research (CDR) computerised test battery and Serial Subtraction tasks immediately prior to dosing and at 1, 2.5, 4 and 6 h thereafter. The primary outcome measures were the four aspects of cognitive performance, which have previously been derived by factor analysis of CDR subtests. Levels of terpenoids (bilobalide, ginkgolide A and ginkgolide B) were concomitantly assessed in plasma samples taken pre-dose and at 3 and 6.5 h post-dose.In keeping with previous research utilising the same methodology, 120 mg of GBE was not associated with markedly improved performance on the primary outcomes. However, administration of GBE complexed with phosphatidylserine resulted both in improved secondary memory performance and significantly increased speed of memory task performance across all of the post-dose testing sessions. Enhancement following GBE complexed with phosphatidylcholine was restricted to a modest improvement in secondary memory performance which was restricted to one post-dose time point. All three treatments were associated with improved calmness. There were no significant differences in post-dose levels of terpenoids between the Ginkgo containing treatments, although this latter finding may be attributable to methodological factors. Complexation with phosphatidylserine appears to potentiate the cognitive effects associated with a low dose of GBE. Further research is required to identify whether this effect is due to the complexation of the extracts, their mere combination, or the separate psychopharmacological actions of the two extracts.

Peripheral-type benzodiazepine receptors

1. The pharmacological effects of benzodiazepines are mediated through a class of recognition sites associated with the gamma-aminobutyric acid A receptor. A second class of benzodiazepine binding sites is found in virtually all mammalian peripheral tissues and is therefore called the peripheral type benzodiazepine receptor (PBR). 2. The first section of this review describes the tissue and subcellular distribution of the PBR in mammalian tissues and analyzes its many putative endogenous ligands. 3. The next section deals with the pharmacological, structural and molecular characterization of the PBR that has taken place in the past few years. 4. The final section describes the possible physiological role(s) of the PBR and identifies future work that would help deepen our understanding of the PBR and its function.

Size-exclusion chromatographic reconstitution of the bovine brain benzodiazepine receptor. Effects of lipid environment on the binding characteristics

The benzodiazepine receptor from calf brain was solubilized with sodium deoxycholate (2 mg/ml) in the presence of 0.5 M KCl and protease inhibitors, and bound flunitrazepam with an equilibrium dissociation constant (Kd) of 2.7 +/- 1.2 nM and with 0.40 +/- 0.04 pmol binding sites per mg protein (Bmax). Up to 60% of the benzodiazepine binding sites (average 25%) could be reconstituted in lipid vesicles, upon size-exclusion chromatography of protein-detergent-lipid mixtures on Sephadex G-50 Medium for detergent depletion. The flunitrazepam affinity for the reconstituted receptor varied with the lipid composition (Kd 1.4-4 nM). Freezing and thawing increased the size of the small proteoliposomes obtained by chromatographic reconstitution and, on the average, doubled the number of operative flunitrazepam binding sites. When the proteoliposomes were stored at -20 degrees C or -80 degrees C or in lyophilized state, the receptor retained its benzodiazepine binding affinity and Bmax over a period of 2 months.

Two cellular and subcellular locations for the peripheral-type benzodiazepine receptor in rat liver

Determination of ligand binding properties of the peripheral benzodiazepine receptor (PBBS) in liver, in hepatocytes, and in nonparenchymal cells demonstrated the presence of receptor-specific high-affinity binding in both hepatocyte and nonhepatocyte cells. Density gradient centrifugation showed that the high-affinity receptor in hepatocytes was localised to mitochondria, whereas in nonhepatocytes it was not mitochondrial, but with a possible biliary epithelial cell plasma-membrane location. Both receptors showed the peripheral-type specific high-affinity binding of PK 11195 and Ro5 4864 and could be photolabelled as 18 kDa proteins with [3H]PK 14105. Immunocytochemistry showed the presence of acyl-CoA binding protein, a putative endogenous ligand for the receptor, 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 more sensitive to inhibition than PK 11195. However, whereas soybean lipid and phosphatidyl serine increased the binding of both ligands to adrenal receptor, these lipids had no effect on liver, suggesting that liver PBBS may differ from the well-characterised adrenal PBBS in some of its protein conformation. Modulators of mitochondrial respiration that also influence intermembrane contact site formation were found to elicit no marked stimulatory or inhibitory effects on PBBS ligand binding in liver, a result also found for adrenal mitochondria, suggesting that the extent of contact site formation does not influence ligand binding and that the hepatocyte receptor may not play a role in regulating mitochondrial respiration. These two cellular and subcellular locations of the PBBS in liver and the different effect of phospholipids compared to other peripheral tissues may be important for the role(s) of PBBS in liver and also for the multiple roles ascribed to the receptor and to peripheral-type benzodiazepine ligands.

Rapid purification of cotton seed membrane-bound N-acylphosphatidylethanolamine synthase by immobilized artificial membrane chromatography

N-Acylphosphatidylethanolamine synthase (NAPES) is a membrane-bound enzyme present in cotton seedlings at a concentration of < or = 0.02% of the total protein. NAPES was purified to electrophoretic homogeneity in a single chromatographic step using immobilized artificial membrane (IAM) chromatography. The IAM column used for NAPES purification was etherIAM.PEC10/C3 and this surface contains a monolayer of immobilized phosphatidyl-ethanolamine (PE). Since PE is an analogue of the natural substrate for NAPES, etherIAM.PEC10/C3 columns function as an affinity column for this enzyme. Detergent-solubilized microsomal proteins from cotton were loaded on to the etherIAM.PEC10/C3 column and eluted with buffered mobile phases containing 0.2 mM dimyristoylphosphatidylethanolamine (DMPE) and 2 mM dodecylmaltoside. Little NAPES functional activity eluted if DMPE was removed from the mobile phase. Mobile phase DMPE is also a substrate for NAPES, both the mobile phase and IAM surface contains NAPES substrates. Mobile phase DMPE may function as both a surfactant-type affinity displacing ligand effecting protein elution and also a stabilizing factor of NAPES functional activity. The loading capacity on semi-preparative etherIAM.PEC10/C3 (6.5 x 1.0 cm) columns was ca. 5 mg of total detergent solubilized microsomal proteins, and protein recovery was quantitative. This one-step IAM purification of NAPES resulted in a single band on silver-stained polyacrylamide gels, and 3940 fold increase in NAPES specific activity. The molecular mass of the purified NAPES protein is 64,000. 125I labeled [12-(4-azidosalicyl)amino]dodecanoic acid is a photoreactive fatty acid substrate of NAPES that was used to confirm protein purity.

Trace amounts of Triton X-100 modify the inhibitor sensitivity of the mitochondrial porin

Transport properties of mitochondrial porin were investigated on the basis of changes in the activity of hexokinase utilizing external ATP. Production of glucose 6-phosphate is inhibited by polyanion both in intact brain mitochondria and in contact point vesicles. Hexokinase activity is restored by solubilization of the enzyme by high ionic strength or 0.5-1% Triton X-100. In very low concentrations (0.001-0.005%) Triton does not mobilize hexokinase from its binding sites but it is able to release polyanion-inhibition completely. This finding provides an explanation for the discrepancy observed in the transport properties of porin when studied 'in situ' or in artificial lipid membranes.