Involvement of tyrosyl residues in the binding of benzodiazepines to their brain receptors

Involvement of a disulfide bond in the binding of flunitrazepam to the benzodiazepine receptor from bovine cerebral cortex

The effects of chemical modification of a disulfide bond(s) (-SS-) or sulfhydryl group(s) (-SH) on the [3H]-flunitrazepam ([3H]FNZ) binding to membrane-bound or immunoprecipitated benzodiazepine (BZD) receptors (BZD-R) from bovine cerebral cortex were examined. Reduction of -SS- with dithiothreitol (DTT) brought about a reversible, time- and dose-dependent inhibition of [3H]FNZ binding to the membrane-bound BZD-R. Alkylation of the membranes with the -SH-modifying reagent iodoacetamide (IAA) or 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) produced a slight inhibition of [3H]FNZ binding in a dose-dependent manner. Scatchard analysis of saturation curves of [3H]FNZ binding in the presence and absence of 5 mM DTT revealed changes in affinity without modification in the maximal binding capacity, thus indicating a competitive mode of interaction. DTT pretreatment of both the membrane-bound and the immunoprecipitated BZD-R led to [3H]FNZ binding inhibition. Consistent with the modification of a binding site is the observation that reduction of -SS- does not bear on the binding affinity, but rather reduces the number of sites. Complete protection from DTT inhibition of [3H]FNZ binding by FNZ (an agonist) or by Ro 15-1788 (an antagonist) suggests the presence of -SS- at, or very close to, the BZD recognition binding site. No protection against IAA or DTNB inhibition was provided by FNZ. Photoaffinity labeling experiments with [3H]FNZ revealed a clear-cut band of 50 kDa in native and alkylated membranes but an extremely weak label in 5 mM DTT/IAA-treated membranes. The present results provide evidence for the participation of a disulfide bond in the recognition binding site of the bovine cerebral cortex BZD-R.

Organization of mitochondrial UDP-glucose:dolichylmonophosphate glucosyltransferase in the outer membrane

Previous studies have shown the existence of an autonomous mitochondrial UDP-glucose: dolichylmonophosphate glucosyltransferase, located in mitochondrial outer membrane of liver cells. To improve our knowledge about the topographical aspects of glycosylation in mitochondria, we have investigated the organization of this enzyme in intact mitochondria, using controlled proteolysis with trypsin and sensitivity towards amino-acid specific reagents. Our data provides evidence: --for a mitochondrial glucosyltransferase facing the cytoplasmic side of the outer membrane --and for the involvement of histidine and tryptophan residues as well as sulfhydryl groups in the catalytic activity of the enzyme.

Benzodiazepine agonists protect a histidine residue from modification by diethyl pyrocarbonate whereas propyl beta-carboline does not

The pH sensitivity of benzodiazepine binding suggests that a histidine residue may be present in, or close to the benzodiazepine binding site. This was confirmed by the selective modification of histidine residues using diethyl pyrocarbonate which was found to block both benzodiazepine and beta-carboline binding. In order to assess whether this histidine residue is located in or adjacent to the benzodiazepine and beta-carboline binding sites, experiments were performed using either benzodiazepine or beta-carboline to protect against diethyl pyrocarbonate treatment. It was found that benzodiazepine agonists, but not propyl beta-carboline protect the benzodiazepine binding sites from diethyl pyrocarbonate modification.

Glycoprotein as a constituent of purified gamma-aminobutyric acid/benzodiazepine receptor complex: structures and physiological roles of its carbohydrate chain

The effect of treatments with various enzymes and chemically modifying agents on [3H]muscimol binding to a purified gamma-aminobutyric acid (GABA)/benzodiazepine receptor complex from the bovine cerebral cortex was examined. Treatments with pronase, trypsin, guanidine hydrochloride, and urea significantly decreased the binding of [3H]muscimol, but dithiothreitol, N-ethylmaleimide, reduced glutathione, oxidized glutathione, cysteine, and cystine had no significant effect. These results indicate that the GABA receptor indeed consists of protein, but -SH and -S-S- groups in the protein are not involved in the exhibition of the binding activity. On the other hand, column chromatography using concanavalin A-Sepharose eluted protein having [3H]muscimol binding activity and staining of glycoprotein using an electrophoresed slab gel indicated the existence of two bands originating from the subunits of the GABA/benzodiazepine receptor complex. Furthermore, treatments with various glycosidases such as glycopeptidase A, beta-galactosidase, and alpha-mannosidase significantly increased the binding of [3H]muscimol. These results strongly suggest that GABA/benzodiazepine receptor complex is a glycoprotein and that its carbohydrate chain may be a hybrid type. Treatment with beta-galactosidase resulted in the disappearance of the low-affinity site for [3H]muscimol binding and in an increase of Bmax of the high-affinity site, without changing the KD value. These results suggest that the carbohydrate chain in the receptor complex may have a role in exhibiting the low-affinity binding site for GABA. The observation that the enhancement of [3H]muscimol binding by treatments with beta-galactosidase and glycopeptidase A were much higher than that with alpha-mannosidase may also indicate a special importance of the beta-galactosyl residue in the inhibition of GABA receptor binding activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Partial chemical characterization of cyclopyrrolones ([3H] suriclone) and benzodiazepines ([3H]flunitrazepam) binding site: differences

Rat hippocampus membranes were treated with several protein modifying reagents (iodoacetamide, N-ethylmaleimide, tetranitromethane and N-acetylimidazole). The effects of these treatments on the binding sites of cyclopyrrolones ([3H] suriclone), a new chemical family of minor tranquilizers, and benzodiazepines ([3H] flunitrazepam) were investigated. Here we show that both ligands are similarly sensitive to cysteine alkylation: [3H] suriclone and [3H] flunitrazepam binding are reduced by iodoacetamide and slightly increased by N-ethylmaleimide. On the contrary they are clearly differenciated by tyrosine modification: [3H] suriclone binding is not changed whereas [3H] flunitrazepam binding is increased by tetranitromethane and decreased by N-acetylimidazole. Our present findings and published evidence suggest cyclopyrrolones and benzodiazepines bind to distinct sites or to different allosteric forms of the benzodiazepine receptor.

Characterization of gamma-aminobutyric acid-benzodiazepine receptor complexes by protection against inactivation by group-specific reagents

The chemical topography of the gamma-aminobutyric acid (GABA) and benzodiazepine (BZ) receptors was investigated in a thoroughly washed cortical membrane preparation of the rat. Chemical modification by several amino- and tyrosyl-selective reagents and the protection from it by direct and allosteric ligands of the GABA-BZ receptor complex were used to identify the residues at the binding sites. Inhibition of specific GABA binding by p-diazobenzenesulfonic acid (DSA), tetranitromethane (TNM), and N-acetylimidazole and the selective and complete protection from it by GABA and muscimol suggest the presence of a tyrosine residue at the GABAA site. TNM, like DSA, selectively decreased the number of the low-affinity GABA receptors, and this could be completely protected only by GABA concentrations that can saturate the low-affinity sites. TNM pretreatment also abolished the muscimol enhancement of [3H]diazepam binding, which suggests that the low-affinity GABA receptor sites are responsible for this enhancement. Inhibition of GABA binding by pyridoxal-5-phosphate (PLP) and the selective protection by GABA and muscimol support the presence of a lysine residue at the GABAA receptor site. Complete and selective protection from diethylpyrocarbonate (DEP) inhibition of [3H]diazepam binding by flurazepam suggests the presence of a histidine residue at the BZ site. Flurazepam selectively protected from inhibition of [3H]diazepam binding by N-bromosuccinimide and N-acetylimidazole, but not that by DSA and TNM, which does not allow a unanimous conclusion regarding the presence of tyrosine or tryptophan residues at the BZ site.

Involvement of some amino acid residues in the enzymatic activity of solubilized cerebral fucosyltransferase

We have checked the effect of some chemical reagents specific for amino acid residues on the activity of a solubilized cerebral glycoprotein:fucosyltransferase. Diethylpyrocarbonate, 2,3-butanedione and tetranitromethane specific for histidyl, arginyl, and tyrosyl residues respectively, were strong inhibitors of the enzymatic activity This led us to conclude that these amino acid residues are "essential residues" in the cerebral fucosyltransferase activity.

Heterogeneity in the physicochemical properties of deoxycholate-solubilized benzodiazepine receptors from calf cerebral cortex

The hydrodynamic behaviour of benzodiazepine receptors solubilized by deoxycholate from calf cerebral cortex reveals two molecular forms. The Stokes radii are 46.5 A and 67.2 A, and the sedimentation coefficients are 10.9 S and 14.6 S. The calculated apparent molecular weights and frictional ratios suggest either two nearly globular proteins of ca. 200K and 400K daltons each, or two ca. 300K daltons proteins which differ significantly in their degree of asymmetry. The benzodiazepine binding site is located on ca. 51K daltons component(s) in both forms.

Diethylpyrocarbonate modification of benzodiazepine receptors from calf cerebral cortex

Diethylpyrocarbonate (DEP), an amino acid modifying reagent, causes complete inactivation of particulate and deoxycholate-solubilized benzodiazepine-receptors from calf cerebral cortex. No heterogeneity was observed in DEP-sensitivity of the receptors. Protection from DEP-induced inactivation was provided by the centrally active benzodiazepines, diazepam and nitrazepam, but not by the peripherally active Ro5-4864, suggesting that DEP modifies a residue which is essential for the central actions of benzodiazepines. GABA did not protect against inactivation or influence the protection afforded by diazepam, indicating that the DEP-modifiable residue is independent of GABA binding sites, or that GABA binding sites are also sensitive to DEP. DEP-induced inactivation of benzodiazepine-receptors proceeds much faster at pH 10.1 than at pH 8.1 or 6.0, indicating the modification of a high pKa side group, possibly the phenol of a tyrosyl residue. This postulation is in accord with our previous findings with the modifying reagents tetranitromethane and N-acetylimidazole.

Glycoprotein properties of benzodiazepine receptors from calf cerebral cortex

Several lectins (concanavalin A, wheat germ agglutinin, wax bean agglutinin, and phytohemagglutinin) induce aggregation and precipitation of deoxycholate-solubilized benzodiazepine receptors from calf cerebral cortex. These receptors were also retained on columns of immobilized lectins. Photoaffinity labeling of the soluble benzodiazephine receptors prevented their interaction with lectins. Periodate treatment caused inactivation of benzodiazepine receptors which was partially prevented by diazepam and/or gamma-aminobutyric acid. The possible association of the drug-binding site and the carbohydrate moiety is discussed.