Purification, biochemical characterization, binding activity, and selectivity of a glutamate binding protein from bovine brain

Purification and biochemical characterization of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate-sensitive L-glutamate receptors of pig brain

Two preparations of glutamate receptors were purified from the synaptic junctions of pig brain by a combination of detergent solubilization, anion-exchange chromatography, wheat-germ agglutinin affinity chromatography and sedimentation through sucrose gradients. These preparations were enriched in specific L-[3H]glutamate binding activity (> 5000 pmol of glutamate binding sites/mg of protein), and the rank order of ligand affinity for binding to these preparations was: quisqualate > 6-cyano-7- nitroquinoxaline-2,3-dione > alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid (AMPA) > L-glutamate > kainate > > N-methyl-D-aspartate approximately L-2-amino-4-phosphonobutyrate. SDS/PAGE analysis revealed that more than 80% of the protein in either of these preparations appeared as a single protein band of 106 kDa. Two-dimensional gel electrophoresis further revealed that these 106 kDa proteins consisted of a series of acidic proteins which were recognized by antibodies against rat AMPA receptor subunits. These 106 kDa proteins were also recognized by wheatgerm agglutinin and concanavalin A; in addition, peptide N-glycosidase F treatment of these preparations decreased their size to 99 kDa. Our results suggest that the putative glutamate receptors isolated here are likely to belong to the AMPA subtype of glutamate receptors in pig brain. Using the purification procedure reported here, 5 micrograms of AMPA receptor proteins can be isolated from 250 g of pig brain tissue.

Partial purification of [3H]glutamate-associating-proteins with sensitivity to displacement by N-methyl-D-aspartate from rat brain

An attempt was made to solubilize and isolate [3H]L-glutamic acid (Glu) binding sensitive to displacement by N-methyl-D-aspartic acid (NMDA) from rat brain. Brain synaptic membranes were solubilized by deoxycholic acid, followed by gel filtration with Sephadex G-25. In these turbid supernatants, significant but fragile binding was detected with a variety of radioligands related to ionotropic subclasses of receptors for excitatory amino acids. These included [3H]5-methyl-10,11-dihydro-5H-dibenzo-[a,d]cyclohepten-5,10-imine (MK-801), [3H]glycine, [3H]spermidine, [3H]Glu, [3H]DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic and [3H]kainic acids. Re-solubilization of turbid supernatants by Triton X-100 resulted in detection of [3H]Glu binding which was only stable for 24 h, with [3H]MK-801 binding being entirely lost. In these clear preparations after re-solubilization, Glu was exclusively effective in completely displacing [3H]Glu binding with other ligands being partially active. Furthermore, [3H]Glu binding displaceable by NMDA was eluted with 0.5 M KCl together with [3H]Glu binding insensitive to NMDA on DEAE-Toyopearl column chromatography, while fractions eluted with 0.2 M KCl had NMDA-insensitive [3H]Glu binding only. Chromatography on chelate (Zn)-Toyopearl resin resulted in elution of both NMDA-sensitive and NMDA-insensitive [3H]Glu binding with 10 mM EDTA. High performance liquid chromatography revealed that NMDA-sensitive [3H]Glu binding was detected at retention times of 10-20 min when eluted from an Asahipak ES-502N column with NaCl at linearly graded concentrations up to 0.5 M. In order to detect NMDA-sensitive [3H]Glu binding, however, the whole procedures needed to be completed within 24 h after re-solubilization. Accordingly, the identity of the NMDA-sensitive [3H]Glu binding partially purified here is still unclear at present. The NMDA recognition domain could be more stable than the NMDA channel domain on the NMDA receptor ionophore complex under aqueous conditions.

Characterization of a presynaptic glutamate receptor

Glutamate receptors mediate excitatory neurotransmission in the brain and are important in the formation of memory and in some neurodegenerative disorders. A complementary DNA clone that encoded a 33-kilodalton protein (GR33) was obtained by screening a library with an antibody generated against glutamate binding proteins. The sequence of GR33 is identical to that of the recently reported presynaptic protein syntaxin. When GR33 was expressed in Xenopus oocytes, it formed glutamate-activated ion channels that are pharmacologically similar to those of N-methyl-D-aspartate receptors but with different electrophysiological properties. Mutation of the leucine 278 residue in the single putative transmembrane segment of GR33 affects the properties of the channel. Thus, in vivo GR33 may be a presynaptic glutamate receptor.

Molecular aspects of glutamate receptors and sodium-calcium exchange carriers in mammalian brain: Implications for neuronal development and degeneration

N-Methyl-D-aspartate (NMDA) and L-glutamate activate membrane receptors that produce substantial permeation of Na+, K+ and Ca2+ through the neuronal membrane. These ionic fluxes are intimately linked to processes that regulate neuronal survival, growth and differentiation. Intracellular free Ca2+ concentrations are thought to be particularly important determinants of the vulnerability of neurons to excessive excitatory stimulation produced through activation of NMDA receptors. In order to understand the molecular events involved in both NMDA receptor activation and regulation of intracellular Ca2+ levels, we have purified and reconstituted the protein complexes that form the NMDA/glutamate receptors in rat brain synaptic membranes and those that constitute the Na(+)-Ca2+ antiporters in bovine brain synaptic membrane. The molecular properties of these protein complexes are described, and information from the most recent studies of exploration of the molecular structures of these receptors and transport carriers is summarized.

Solubilisation and characterisation of a putative quisqualate-type glutamate receptor from chick brain

The brains of 1-day-old chicks were shown to be a rich source of binding sites with the pharmacological characteristics expected of a quisqualate-type glutamate receptor. alpha-[3H]Amino-3-hydroxy-5-methylisoxazolepropionate ([3H]AMPA) bound with KD and Bmax values, measured at 0 degree C in the presence of the chaotrope potassium thiocyanate, of 55 nM and 2.6 pmol/mg protein. The regional localisations of [3H]AMPA and [3H]kainate binding sites were manifestly different. The membrane-bound [3H]AMPA binding sites were efficiently solubilised by N-octyl-beta-D-glucopyranoside (1%) in the presence of 0.2 M thiocyanate. In the detergent extract the affinity was 69 nM and there was an apparent increase in the number of sites (Bmax, 4.6 pmol/mg protein). The rank order of potency for competitive ligands in displacing [3H]AMPA binding was quisqualate approximately AMPA greater than 6-cyano-7-nitroquinoxaline-2,3-dione greater than L-glutamate greater than kainate and was identical for the membrane-bound and solubilised sites. Dissociation was biphasic with rate constants of 0.117 min-1 and 0.015 min-1. The association rate constants for [3H]AMPA at the solubilised sites were 1.45 x 10(6) M-1 min-1 and 6.55 x 10(6) M-1 min-1. The kinetically derived KD values were 80.7 nM and 2.3 nM. The detection of higher affinity binding sites by kinetic analysis but not by equilibrium binding may be explained by the greater sensitivity of dissociation data to small populations of high-affinity sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Solubilization of stereospecific and quisqualate-sensitive activity of [3H]glutamate binding in the pituitary of the rat

Binding activity of a putative central excitatory neurotransmitter, L-glutamic acid, was solubilized from the pituitary glands of the rat by treatment of the membranous homogenates with a nonionic detergent, Nonidet P-40. The binding activity of [3H]glutamic acid increased linearly with increasing concentrations of the solubilized proteins, up to 15 micrograms. The binding activity reached an equilibrium within 10 min at 2 degrees C, while the time required to attain equilibrium at 30 degrees C was 60 min. Addition of an excess of nonradioactive glutamic acid rapidly decreased the activity detected at 30 degrees C, to the nonspecific binding level. Scatchard analysis of these data revealed that the solubilized binding activity consisted of a single component with a Kd of 0.34 microM and a Bmax of 53.6 pmol/mg protein. L-Glutamic but not D-glutamic acid inhibited the binding activity in a concentration-dependent manner, at the concentration range greater than 10(-8) M. An agonist for a certain subclass of the central glutamate receptors, quisqualic acid, significantly inhibited the solubilized activity, whereas the other two agonists, such as N-methyl-D-aspartic acid and kainic acid, had no significant effect. Reduction of the incubation temperature from 30 degrees C to 2 degrees C resulted in a drastic attenuation of the binding activity due to a decrement in the number of apparent binding sites. These results suggest that the binding activity of [3H]glutamic acid in the pituitary may be derived from a quisqualate-sensitive membranous constituent with a stereospecific high affinity for the central neurotransmitter.

Solubilization of quisqualate-sensitive [3H]glutamate binding activity from rat retina

Binding activity of a putative central neurotransmitter, L-glutamic acid, was examined in the supernatant preparations solubilized from rat retinal membranes by Nonidet P-40. [3H]Glutamate binding activity increased linearly with increasing concentrations of the solubilized proteins up to 15 micrograms. The binding activity reached an equilibrium within 10 min at 2 degrees C, while increasing with incubation time up to 60 min at 30 degrees C. Addition of an excess of nonradioactive glutamate rapidly decreased the activity at 30 degrees C. Scatchard analysis revealed that the solubilized retinal binding activity consisted of a single component with a KD of 0.25 microM and a Bmax of 57.4 pmol/mg protein. The solubilized binding activity exhibited a stereospecificity and a structure selectivity to L-glutamate, and was abolished by quisqualate, L-glutamate diethyl ester, and DL-2-amino-3-phosphonopropionate. None of the other agonists and antagonists for the central excitatory amino acid receptors affected the binding activity. Reduction of incubation temperature from 30 degrees C to 2 degrees C resulted in a drastic attenuation of the binding activity due to decrement of the number of the apparent binding sites. Cation-exchange column chromatography revealed that unidentified radioactive material was in fact formed during the incubation of [3H]glutamate with the retinal preparations at 30 degrees C. These results suggest that retinal [3H]glutamate binding activity may be derived at least in part from the quisqualate-sensitive membranous enzyme with a stereospecific and structure-selective high affinity for the central neurotransmitter.

Microbial methodological artifacts in [3H]glutamate receptor binding assays

Incubation of radiolabeled L-glutamic acid, a putative central excitatory neurotransmitter, in 50 mM Tris-acetate buffer (pH 7.4) at 30 degrees C in the absence of brain synaptic membranes resulted in a significant adsorption of the radioactivity to glass fiber filters routinely employed to trap the bound ligand in receptor binding assays. The adsorption was not only eliminated by the inclusion of L-isomers of structurally related amino acids, but also inhibited by that of most presumed agonists and antagonists for the brain glutamate receptors. This displaceable adsorption was a temperature-dependent nonreversible, and saturable phenomenon. Scatchard analysis of these data revealed that the adsorption consisted of a single component with an apparent dissociation constant of 73 nM. The displaceable adsorption was significantly attenuated by a concurrent incubation with papain, pronase E, and phospholipase C. A significant amount of the radioactivity was detected in the pass-through fraction of the Dowex column following an application of the reaction mixture incubated with purified [3H]glutamate at 30 degrees C for 60 min in the absence of membranous proteins added. Complete abolition of the displaceable adsorption resulted from the use of incubation buffer boiled at 100 degrees C as well as filtered through a nitrocellulose membrane filter with a pore size of 0.45 micron immediately before use. These results suggest that the displaceable adsorption may be attributable to the radioactive metabolite of [3H]glutamate by microorganisms contaminating the Tris-acetate buffer. This might in part contribute to some of the controversial results with regard to receptor binding studies on acidic amino acids.

Solubilisation of a glutamate binding protein from rat brain

Rat brain synaptic plasma membranes were solubilised in either 1% Triton X-100 or potassium cholate and subjected to batch affinity adsorption on L-glutamate/bovine serum albumin reticulated glass fibre. The fibre was extensively washed, and bound proteins eluted with 0.1 mM L-glutamate in 0.1% detergent, followed by repeated dialysis to remove the glutamate from the eluted proteins. Aliquots of the dialysed extracts were assayed for L-[3H]glutamate binding activity in the presence or absence of 0.1 mM unlabelled L-glutamate (to define displaceable binding). Incubations were conducted at room temperature and terminated by rapid filtration through nitrocellulose membranes. Binding to solubilised fractions could be detected only following affinity chromatography. Binding was saturable and of relatively low affinity: KD = 1.0 and 1.8 microM for Triton X-100 and cholate extracts, respectively. The density of binding sites was remarkably high: approximately 18 nmol/mg protein for Triton X-100-solubilised preparations, and usually double this when cholate was employed. Analysis of structural requirements for inhibition of binding revealed that only a very restricted number of compounds were effective, i.e., L-glutamate, L-aspartate, and sulphur-containing amino acids. Binding was not inhibited significantly by any of the selective excitatory amino acid receptor agonists--quisqualate, N-methyl-D-aspartate, or kainate. The implication from this study is that the glutamate binding protein is similar if not identical to one previously isolated and probably is not related to the pharmacologically defined postsynaptic receptor subtypes, unless solubilisation of synaptic membranes resulted in major alterations to binding site characteristics. Since solubilisation with Triton X-100 is known to preserve synaptic junctional complexes, it seems likely that the origin of the glutamate binding protein may be extrajunctional, although its functional role is unknown.

Differences between seizure-prone and non-seizure-prone mice with regard to glutamate and GABA receptor binding in the hippocampus and other regions of the brain

Quisqualate-preferring glutamate receptors were determined in membranes from frontal cortex, occipital cortex, hippocampus and cerebellum, from seizure-prone DBA/2J BOM and seizure-resistant C57/BL mice. The animals were studied 21, 27 and 40 days postnatally, i.e., before, during and after the age at which DBA mice are most susceptible to seizures. Radio-binding assays were performed using [3H]AMPA in the presence of 100 nM glutamate. Except for the occipital cortex, where no significant differences between the two strains were observed, all areas of the brain of DBA mice exhibited significantly (P less than 0.001, t test) higher AMPA binding than the corresponding areas of C57/BL mice at 27 days of age. At pre- and post-susceptible ages, the two strains showed no significant differences in the hippocampus and occipital cortex. A significant difference was observed, however, in the frontal cortex and cerebellum at the ages of 21 and 40 days, respectively, although this difference was considerably less than at 27 days. In addition to determination of glutamate receptors, GABA-receptor binding was also studied in membranes from the same cerebral areas and at the above-mentioned ages. Binding characteristics, using [3H]GABA as the ligand, were essentially identical in the two strains at all ages investigated, i.e., both low and high affinity GABA receptors could be identified with KD values of 6-16 nM and 100-800 nM, respectively.

Multiple Cl(-)-independent binding sites for the excitatory amino acids: glutamate, aspartate and cysteine sulfinate in rat brain membranes

As we have recently reported that Cl(-)-dependent glutamate (GLU) binding reflects GLU accumulation into membrane vesicles, the characteristics, kinetics and pharmacological specificities of L-[3H]glutamate (L-[3H]GLU) binding to crude rat brain synaptic membranes, were investigated in Cl(-)-free medium. L-[3H]GLU binding was systematically compared to that of L-[3H]cysteine sulfinate (L-[3H]CSA) and L-[3H]ASP), two other putative excitatory amino acids. A high affinity site was determined for each of these radioactive ligands (L-[3H]GLU: Kd = 0.14 microM, Bm = 3.4 pmol/mg protein; L-[3H]CSA: Kd = 0.07 microM, Bm = 2.2 pmol/mg protein; L-[3H]ASP: Kd = 5.8 microM, Bm = 31.2 pmol/mg protein). The pharmacological specificity of these Cl(-)-independent binding sites indicate the existence of at least 3 distinct high affinity sites, all different from the Cl(-)-dependent GLU binding 'site': one having a similar affinity for GLU and CSA, a second one preferring CSA, and a third one preferring ASP. Among the large quantity of structural analogs of the neuroexcitatory amino acids tested, only endogenous compounds (GLU, ASP and CSA) (except hydroxylamine-o-sulfate) were able to interact efficiently. No inhibition by classical agonists and antagonists (such as N-methyl-D-aspartate, quisqualate, kainate, 2-amino-4-phosphonobutyrate, or 2-amino-5-phosphonovalerate) was found. In addition to their high specificity, these Cl(-)-independent sites possess most other biochemical characteristics of receptor proteins.

Presence of proteins and glutamate as major constituents of the venom of the spider Araneus gemma

The venom from the spider Araneus gemma contains an inhibitor of physiologic glutamate receptors and of glutamate binding sites in brain synaptic membranes. In the present study the chemical composition of the venom was examined in order to determine the presence of constituents that may have physiologically important actions on the prey. The milked venom contains high concentrations of protein (approximately equal to 200 micrograms/microliter) and of glutamate (130-425 mM) and very low concentrations of epinephrine, epinine, dopamine and 3,4-dihydroxyphenylacetic acid. There is also marked heterogeneity in the venom peptides detected by SDS gel electrophoresis. The presence of free glutamate may be very important for the actions of the glutamate receptor inhibitor in the venom.

Identification of a Cl-/Ca2+-dependent glutamate (quisqualate) binding site in bovine pineal organ

The presence of a high concentration of glutamic acid, a transmitter shown to have excitatory action in the pineal organ, prompted us to search for and to characterize glutamate receptor site in the bovine pineal organ. By using 10 nM- 100 microM of labeled and unlabeled L-glutamate and by employing the LIGAND computer program, we found a glutamate binding site with a dissociation equilibrium constant (KD) of 0.534 microM and a receptor density (Bmax) of 4.84 pmol/mg protein. This pH- and temperature-dependent binding site showed stereospecificity, was activated by Ca2+, and displayed affinity for both glutamate agonists and antagonists. The IC50 values for L-glutamate, L-aspartate, L-cysteate, L-cysteine sulfinate, quisqualate, and (+/-) ibotenate were 0.5, 2, 12, 16, 25, and 30 microM, respectively, whereas those for D-aspartate, L-alpha-aminoadipate, L-homocysteate, and DL(+/-) 2-amino-4-phosphonobutyrate were greater than 100 microM. Kainate, N-methyl-D-aspartate, and L-glutamic acid diethyl ester were inactive. Based on these results, the presence of a quisqualate-type, Cl-/Ca2+-dependent glutamate binding site in the pineal organ is suggested, and a possible neuroexcitatory role for glutamic acid, aspartic acid, and certain sulfur-containing amino acids is also implied. The precise nature of this excitatory effect in modulating the function(s) of the pineal organ and the synthesis of its hormone(s) remains to be elucidated.

Solubilization of quisqualate-sensitive L-[3H]glutamate binding sites from guinea pig brain membranes using a zwitterionic detergent

L-[3H]Glutamate binding sites were solubilized with a zwitterionic detergent 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane-sulfonate (CHAPS) plus ammonium thiocyanate from guinea pig synaptosomal membranes. The binding of L-[3H]glutamate to the solubilized binding sites was saturable and reversible. Scatchard analysis suggested the existence of two different classes of binding sites with KDs of 63.8 and 644 nM. The L-[3H]glutamate binding was displaced by excitatory amino acids with such an order of potency that L-glutamate much greater than D-glutamate congruent to L-aspartate greater than D-aspartate. Quisqualate effectively displaced the glutamate binding in biphasic manner. L-Glutamic acid diethyl ester, the quisqualate receptor antagonist, also showed a moderate displacing ability. Other neuroactive amino acid analogues displaced the glutamate binding only weakly, except for L- and D-homocysteic acids which had moderate potency. It is very likely from these results that the glutamate binding sites solubilized in this study are relevant to the physiological glutamate receptors especially of quisqualate-type.

Effects of anti-glutamate-binding protein antibodies on synaptic membrane ion flux, glutamate transport and release, and L-glutamate binding activities

Antibodies (Abs) raised against the L-glutamate-binding protein (GBP) purified from bovine brain were used to define the possible physiologic activity of GBP in synaptic membranes. Three processes were examined for their sensitivity to the Abs: the excitatory amino acid stimulation of thiocyanate (SCN-) flux, the transport of L-glutamic acid across the synaptic membrane, and the depolarization-induced release of L-glutamate. Only the amino acid-induced changes in ion flux were inhibited by the anti-GBP Abs. The change in membrane potential produced by exposure of synaptic membranes to excitatory amino acids was measured as the increase in the uptake of the lipophilic anion SCN-. The L-glutamate-induced SCN- influx was 40 times more sensitive to inhibition by the anti-GBP Abs than the stimulation of ion flux by kainate, and 60 times more sensitive than that produced by quisqualate. The anti-GBP Abs did not inhibit the activation of ion flux produced by N-methyl-D-aspartate. The inhibition of glutamate-stimulated ion fluxes by the Abs was complete, whereas the inhibition of L-glutamate binding to either the rat or bovine brain GBP was not. The results obtained indicated that although the majority of the anti-GBP Abs were not directed against the glutamate recognition site of the GBP and of presumed synaptic membrane receptors, they were effective in blocking the activation of receptor-associated ion channels. Thus, the GBP may be considered a component of some excitatory amino acid receptor complexes.

Complex structure of quisqualate-sensitive glutamate receptors in rat cortex

Multiplicity of excitatory amino acid receptors has previously been demonstrated. The functional size of the quisqualate-type receptors was investigated by high energy radiation inactivation analysis of the binding of [3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid ([3H]AMPA) to rat cortical membranes. The AMPA binding site is coupled to a high molecular weight modulatory unit, which down-regulates the affinity of the binding site for quisqualate, glutamate and AMPA. The estimated molecular target size of the AMPA binding site is different from the reported molecular weight of the glutamate binding site isolated from bovine brain. Furthermore, enhanced [3H]AMPA binding induced by chaotropic ions seems not to be a consequence of removal of the modulatory unit.

Molecular size of the high-affinity glutamate-binding site on synaptic membranes from rat brain

We have used radiation inactivation as a means of determining the molecular size of the high-affinity glutamate-binding site on rat brain synaptic membranes. The molecular size was 75,000 +/- 15,000 in the absence of glutamate and 263,000 +/- 34,000 in the presence of glutamate. These data may be interpreted as suggesting that the high-affinity glutamate-binding site is comprised of a number of subunits. The minimum sub-unit size detected by this method was 75,000 +/- 15,000.

Association of [14C]glutamate with the protein components of synaptic membrane preparations

Calf brain synaptic plasma membranes were saturated under extracellular conditions with [14C]glutamic acid and the resulting labelled membranes fractionated with 0.9% NaCl, distilled water, n-butanol-water, 0.05 mol/L NaOH and 0.5% Triton X-100 solutions in this order. Triton X-100 was the most effective solubilizer, liberating altogether about 24% of the membrane proteins, but only 4-7% of the label, while NaOH was the most potent solubilizer for the protein-bound label (50-70%). Slab gel electrophoresis showed significant qualitative differences in the banding patterns of the protein extracts, but only two fractions, a low-molecular weight (around 15 kd) and a high-molecular weight (greater than 200 kd) fraction bound [14C]glutamate.

Spider venoms inhibit L-glutamate binding to brain synaptic membrane receptors

The venoms from three spider species, Araneus gemma, Neoscona arabesca, and Argiope aurantia, were shown to inhibit the high-affinity, sodium-independent L-glutamate-binding sites in rat brain synaptic membranes. The same three venoms caused concentration-dependent inhibition of the activity of the glutamate-binding glycoprotein purified from rat brain synaptic membranes. The venom milked from the glands of Araneus gemma was the most active inhibitor of L-glutamate binding, causing 60-80% inhibition of both synaptic membrane and purified protein binding activity at 0.01 unit. The inhibitory activity of this venom was associated with a single protein peak obtained from gel permeation chromatography of the venom. Finally, the effect of the venom from Araneus gemma on the synaptic membrane glutamate-binding sites was slowly reversible. These observations indicate that the spider venoms have a direct effect on the recognition sites for L-glutamic acid in brain synaptic membranes and that these sites are related to the physiologic glutamate receptors.

Functional characteristics of L-glutamate, N-methyl-D-aspartate and kainate receptors in isolated brain synaptic membranes

L-Glutamic acid (L-Glu) and other excitatory amino acids and amino acid analogs enhanced [35S]thiocyanate (SCN-) uptake in isolated-resealed synaptic membrane vesicles. The SCN- uptake was used as a measure of membrane depolarization to evaluate the characteristics of functional excitatory amino acid receptors in the synaptic membranes. N-Methyl-D-aspartate (NMDA) and L-Glu produced additive effects on SCN- accumulation indicating the presence of distinct L-Glu and NMDA receptors. On the other hand, kainic acid (KA) and L-Glu shared either common receptor sites or ion channels. The effects of antagonists on NMDA, L-Glu, and KA stimulation of SCN- influx were consistent with previously reported electrophysiologic observations in intact neurons.

Antibodies against the bovine brain glutamate binding protein

Antibodies against the purified bovine brain glutamate binding protein (GBP) were raised in rabbits. Both nonderivatized and dinitrobenzene-derivatized GBP produced strong immunological responses in rabbits. Using the enzyme-linked immunosorbent assay (ELISA), we have quantified the antibody production and determined the specificity of the antibodies. Bovine brain GBP and the analogous protein from rat brain interacted most strongly with the antibodies. A bacterial glutamate-aspartate binding protein, as well as the enzymes glutamate dehydrogenase (EC 1.4.1.3), glutamine synthetase (EC 6.3.1.2), and gamma-glutamyl transpeptidase (EC 2.3.2.2), showed little or no cross-reactivity with the anti-GBP antibodies. A crude bacterial glutamate decarboxylase (EC 4.1.1.15) preparation gave a small to moderate cross-reaction with the anti-GBP antibodies. The sensitivity of the ELISA assay and the specificity of the antibodies were such that GBP levels as low as 3-10 ng could be detected.