Molecular characterization of the Schwann cell myelin protein, SMP: structural similarities within the immunoglobulin superfamily

The Schwann cell myelin protein (SMP), previously defined in quail and chick by a monoclonal antibody, is in vivo exclusively expressed by myelinating and nonmyelinating Schwann cells and oligodendrocytes. The isolation of the complete nucleotide sequence of SMP is reported here. The predicted polypeptide chain reveals that SMP is a transmembrane molecule of the immunoglobulin superfamily showing sequence similarities with several surface glycoproteins expressed in the nervous and immune systems. In spite of a 43.5% overall sequence identity between rat myelin-associated glycoprotein (MAG) and quail SMP, SMP does not seem to be the avian homolog of MAG, since their expression, regulation, and functions are significantly different. Unusual sequence arrangements shared by SMP, MAG, and two lymphoid antigens suggest the existence of a particular subgroup in the immunoglobulin superfamily.

In the GluR1 glutamate receptor subunit a glutamine to histidine point mutation suppresses inward rectification but not calcium permeability

Recent papers have described glutamine to arginine point mutations of the cloned AMPA/Kainate receptor subunits that alter current-voltage relationship and suppress Ca2+ permeability, thus linking these two characteristics. We describe a glutamine to histidine mutation at the same position, which alters current-voltage relationship but retains Ca2+ permeability, thus dissociating the two properties.

Structure of the polyglutamyl side chain posttranslationally added to alpha-tubulin

Polyglutamylation, a new posttranslational modification of tubulin identified originally on the acidic alpha variants by Eddé et al. (Eddé, B., Rossier, J., Le Caer, J. P., Desbruyeres, E., Gros, F., and Denoulet, P. (1990) Science 247, 83-85), consists of the successive addition of glutamyl units to the Glu445. To characterize their linkage mode mouse tubulin was posttranslationally labeled with [3H]glutamate. After digestion of [3H]tubulin with thermolysin, up to eight radioactive peaks were separated on an anion exchange column (DEAE). Combined use of Edman degradation sequencing and mass spectrometry analysis of the first 6 one indicated that they all correspond to the same COOH-terminal sequence 440VEGEGEEEGEE450 bearing one to six glutamyl units on the Glu445. The first glutamyl residue is amide-linked to the gamma-carboxyl group of Glu445, but the additional residues can be linked to the gamma- or alpha-carboxyl groups of the preceding one. All possible linkages for the biglutamylated tubulin peptides (gamma 1 alpha 2, gamma 1 gamma 2) and triglutamylated (gamma 1 alpha 2 alpha 3, gamma 1 alpha 2 gamma 3, gamma 1 alpha 2 gamma 2, gamma 1 gamma 2 alpha 3, gamma 1 gamma 2 gamma 3) were synthesized. These different peptides were successfully separated on a C18 5-micron reverse phase column. We found that the bi- and triglutamylated tubulin peptides behave as the gamma 1 alpha 2 and gamma 1 alpha 2 alpha 3 synthetic peptides, respectively. These results indicate that the second and third glutamyl residues of the polyglutamyl side chain are amide-linked to the alpha-carboxyl group of the preceding unit.

The small GTP-binding protein Rho1p is localized on the Golgi apparatus and post-Golgi vesicles in Saccharomyces cerevisiae

In Saccharomyces cerevisiae the ras-related protein Rho1p is essentially the only target for ADP-ribosylation by exoenzyme C3 of Clostridium botulinum. Using C3 to detect Rho1p in subcellular fractions, Rho1p was found primarily in the 10,000 g pellet (P2) containing large organelles; small amounts also were detected in the 100,000 g pellet (P3), and cytosol. When P2 organelles were separated in sucrose density gradients Rho1p comigrated with the Kex-2 activity, a late Golgi marker. Rho1p distribution was shifted from P2 to P3 in several mutants that accumulate post-Golgi vesicles. Rho1p comigrated with post-Golgi transport vesicles during fractionation of P3 organelles from wild-type or sec6 cells. Vesicles containing Rho1p were of the same size but different density than those bearing Sec4p, a ras-related protein located both on post-Golgi vesicles and the plasma membrane. Immunofluorescence microscopy detected Rho1p as a punctate pattern, with signal concentrated towards the cell periphery and in the bud. Thus, in S. cerevisiae Rho1p resides primarily in the Golgi apparatus, and also in vesicles that are likely to be early post-Golgi vesicles.

Targeting of frog prodermorphin to the regulated secretory pathway by fusion to proenkephalin

We have investigated the sorting and processing of the amphibian precursor prepro-dermorphin in mammalian cells. Dermorphin, a D-alanine-containing peptide with potent opioid activity, has been isolated from the skin of the frog Phyllomedusa sauvagei. The maturation of this peptide from the precursor involves several posttranslational steps. Recombinant vaccinia viruses were used to infect AtT-20, PC12, and HeLa cells to study the sorting and processing of prepro-dermorphin. While this precursor was not processed in any of the examined cell lines, AtT-20 cells were able to process approximately 40% of a chimeric precursor consisting of the first 241 amino acids of prepro-enkephalin fused to a carboxy-terminal part of pro-dermorphin. By immunogold-EM, we could show that the chimeric protein, but not pro-dermorphin, was sorted to dense-core secretion granules. The processing products could be released upon stimulation by 8-Br-cAMP. We conclude that the pro-enkephalin part of the fusion protein contains the information for targeting to the regulated pathway of secretion, while this sorting information is missing in pro-dermorphin. This indicates that sorting mechanisms may differ between amphibian and mammalian cells.

Primary structure of two isoforms of the vitellogenesis inhibiting hormone from the lobster Homarus americanus

The amino acid sequence of two isoforms of the Vitellogenesis Inhibiting Hormone from the lobster Homarus americanus (one biologically active and one inactive in a heterologous bioassay) has been established by gas-phase microsequencing and fast-atom bombardment mass spectrometry. These two isoforms, isolated from sinus glands display the same sequence of 77 amino acid residues (m.w.: 9135 Da) and have a free N-terminus. Structurally related to Crustacean Hyperglycemic Hormone and Molt Inhibiting Hormone, the Vitellogenesis Inhibiting Hormone of the lobster clearly appears as an original member of the newly described family of neuropeptides, so far proper to crustaceans, which are involved in the control of major physiological functions.

A chimeric glutamate receptor subunit: discrete changes modify the properties of the channel

GluR1 and GluR2 are two highly homologous subunits of the glutamate AMPA receptor but with different functional properties. In ligand gated channels the transmembrane domain II is thought to form the wall of the ionic pore and determine the electrical properties. A chimeric AMPA receptor subunit was constructed by replacing the region comprising transmembrane domains I and II in GluR1 by the corresponding region of GluR2. Alone or forming an heteromer with GluR1, the resulting chimera has the properties of GluR2. Sequence comparison suggests that an arginine at position 600 in the chimera instead of a glutamine in GluR1 is responsible for these properties.

A combination of posttranslational modifications is responsible for the production of neuronal alpha-tubulin heterogeneity

We describe the presence of alpha-tubulin and MAP2 acetyltransferase activities in mouse brain. The enzyme(s) copurified with microtubules through two cycles of assembly-disassembly. Incubation of microtubule proteins with [3H]acetyl CoA resulted in a strong labeling of both alpha-tubulin and MAP2. To determine the site of the modification, tubulin was purified and digested with Glu-C endoproteinase. A unique radioactive peptide was detected and purified by HPLC. Edman degradation sequencing showed that this peptide contained epsilon N-acetyllysine at position 40 of the alpha-tubulin molecule. This result demonstrates that mouse brain alpha-tubulin was acetylated at the same site as in Chlamydomonas. Isoelectric focusing analysis showed that acetylated alpha-tubulin was resolved into five isoelectric variants, denoted alpha 3 and alpha 5 to alpha 8. This heterogeneity is not due to acetylation of other sites but results from a single acetylation of Lys40 of an heterogeneous population of alpha-tubulin isoforms. These isoforms are produced by posttranslational addition of one to five glutamyl units. Thus, neuronal alpha-tubulin is extensively modified by a combination of modifications including acetylation, glutamylation, tyrosylation, and other yet unknown modifications.

Reduction of desensitization of a glutamate ionotropic receptor by antagonists

The glutamate receptor channel subtype that responds to both quisqualate (QA) and alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) was expressed in Xenopus oocytes injected with rat cerebral cortex mRNA. Voltage-clamp current responses to QA, AMPA, and glutamate (GLU) exhibited a rapid increase followed by a decrease to a desensitized steady state (DS). Perfusion with high agonist concentrations produced smaller DS responses than perfusion with low concentrations. During the DS, the current was increased by lowering of the concentration of agonist or by application of low concentrations of a competitive antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX). This paradoxical increase of the agonist-induced currents during the DS was also observed in cultured Purkinje cells with another competitive antagonist, 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX). Dose-response curves obtained in oocytes were bell shaped, with a negative slope for high concentrations of QA. DNQX shifted these bell-shaped curves to the right. Together, these results indicate that the agonists are able to reversibly inhibit the AMPA receptor. The classical desensitization model of Katz and Thesleff [J. Physiol. (Lond.) 138:63-80 (1957)] cannot account for our observations.

Expression in yeast of a cDNA clone encoding a transmembrane glycoprotein gp41 fragment (a.a. 591-642) bearing the major immunodominant domain of human immunodeficiency virus

A cDNA clone corresponding to the gp41 gene fragment nucl. 7573-7730 of the human immunodeficiency virus type 1 (HIV-1) was selected from a random HIV-1 genomic library expressed in yeast. This clone encodes a 52-residue long peptide (amino acid (a.a.)) 591-642) bearing the major immunodominant domain (a.a. 598-609) of the HIV-1 transmembrane glycoprotein gp41. Expression of the recombinant peptide pSE-env591-642 was driven by the alpha-mating factor leader sequence contained in a plasmid pSE-x allowing the synthesis and secretion of foreign gene product in Saccharomyces cerevisiae. Time-course analysis of the secretion into culture medium revealed an optimal production of the glycoprotein fragment at 28-30 h with no observable cytotoxicity. The secreted peptide is highly glycosylated with NH2-terminal heterogeneity probably due to different post-translational modifications. The secreted peptide shows an extreme antigenicity since in ELISA assays, as few as 5 microliters/well of crude supernatant are sufficient to obtain a strong detection by monoclonal antibodies or by 100% of sera from HIV-infected individuals. The purified glycopeptide pSE-env591-642 binds to a monoclonal antibody directed against the immunodominant epitope (a.a. 603-609) with an affinity similar to that of the complete glycoprotein gp160 (Kd values within the 10(-10) M range) and with a 100-fold higher affinity than that of a linear peptide fragment SP-env584-609. These results indicate that overexpression in yeast can efficiently provide an abundant source of highly antigenic gp41 protein fragment pSE-env591-642 which retains the antigenic properties of the native gp160 protein. Such a recombinant peptide should therefore be considered as a good candidate for antigen in HIV detection tests.

A peptide library expressed in yeast reveals new major epitopes from human immunodeficiency virus type 1

In order to characterize novel human immunodeficiency virus type 1 (HIV-1) continuous epitopes, we designed a simple method, based on recombinant DNA, providing a complete set of peptides derived from HIV-1. A library (4 x 10(4) clones) was first constructed in a new expression/secretion vector, using as inserts small fragments of HIV-1 DNA (50-150 bp) generated by random DNAse I cleavage. This peptide library, expressed in the yeast Saccharomyces cerevisiae, was screened with sera of HIV-1 infected individuals and human and murine anti-HIV-1 monoclonal antibodies. Plasmids from immunoreactive colonies were recovered and the sequences of the HIV-1 derived inserts were determined. By using human sera, we have detected classical HIV-1 epitopes and identified two novel major epitopes, which may be used to improve diagnostic tests, localized in the p24 core protein and in the endonuclease. In addition, four minor epitopes were also defined by screening the library with monoclonal antibodies: in the protease, in the p17 core protein, in gp120 and near the C-terminal of gp41. This method is general and can be used for any protein from which a cloned cDNA is available.

Characterization and large production of human monoclonal antibodies against the HIV-1 envelope

Peripheral blood lymphocytes from a volunteer immunized with a recombinant vaccinia virus VSC-25 expressing the gp160 env protein of HTLV-IIIB strain and from an asymptomatic HIV-infected individual were immortalized by Epstein-Barr (EBV). Clones which secrete human monoclonal antibodies from the two individuals (DZ, IgG1, lambda and C31, IgG1, kappa) were obtained and were stable for more than 2 years. The two monoclonals were directed against the gp160 env protein of HIV, DZ directed against the gp41 and C31 directed against the gp120. C31 was group-specific, whereas DZ was directed against the HTLV-IIIB and HTLV-RF strains. The epitope recognized by DZ was mapped to the carboxy terminus of the gp41, by expression of HIV DNA fragments in a yeast system and peptide analysis. The C31 epitope was not expressed by the yeast library and not present among the peptides which were tested. Monoclonal antibodies had no inhibitory effect in an HIV-induced cell fusion assay, but DZ showed a weak neutralizing activity against the HTLV-IIIB strain. Cloned EBV-transformed cell lines were fused to a murine myeloma, which allowed the heteromyeloma to be cultivated in serum-free medium. The monoclonal antibodies were produced in large quantity in a hollow-fibre reactor at defined culture conditions and purification procedures.

Electrophysiological and pharmacological properties of GluR1, a subunit of a glutamate receptor-channel expressed in Xenopus oocytes

A cDNA clone encoding an excitatory amino acid receptor was isolated from a rat brain cDNA library by Hollmann et al. (Nature, 342 (1989) 643-648). In Xenopus oocytes, this clone, GluR1, expressed a functional receptor-channel activated by kainate (KA), domoate (D), glutamate and quisqualate (QA). The apparent affinity (EC50) for QA (0.1 microM) was higher than that for KA (50 microM). The maximal response to QA was about 1/10 of that to KA. QA inhibited the KA induced current. The N-methyl-D-aspartate (non-NMDA) receptor antagonist 6,7-dinitroquinoxaline-2,3 dione (DNQX) competitively blocked the effects of both agonists. Currents induced by KA, QA and D in oocytes expressing GluR1 showed identical voltage sensitivities. GluR1 and KA receptor-channels expressed from rat striatum poly(A)+ RNA showed the same ionic selectivity, being permeable mostly to Na+ and K+. The current-voltage relationships of GluR1 showed a strong inward rectification, whereas those of KA receptor-channels expressed from poly(A)+ RNA from various rat brain regions were more linear.

A 15 kDa proteolipid found in mediatophore preparations from Torpedo electric organ presents high sequence homology with the bovine chromaffin granule protonophore

Upon SDS PAGE of isolated mediatophore, an acetylcholine-translocating protein, a doublet at 15 kDa was identified. Amino acid sequencing after CNBr cleavage gave a 17 residue-long peptide completely homologous with a sequence of the proton-translocating proteolipid from bovine chromaffin granules. A 51-mer oligodeoxynucleotide corresponding to this sequence was used to screen a library of electric lobe cDNAs constructed in lambda Zap II. A positive recombinant clone was isolated and found to encode the complete sequence of a 15.5 kDa protein highly homologous to the bovine chromaffin or yeast vacuolar ATPase proteolipid. In vitro translation of sense RNA transcripts of the clone indeed yielded a single 15 kDa proteolipid. Northern blot analysis showed that the 1.3 kb mRNA encoding this protein is significantly expressed in nervous tissues but not in electric organ or liver of Torpedo marmorata.

Detection of synenkephalin, the amino-terminal portion of proenkephalin, by antisera directed against its carboxyl terminus

Synenkephalin (SYN), the nonopioid amino-terminal portion of proenkephalin (PRO), is stable and well conserved in mammals and therefore a promising marker for PRO systems. We immunized rabbits with synthetic [Tyr63]SYN(63-70)-octapeptide, coupled by glutaraldehyde to bovine serum albumin. In radioimmunoassay (RIA) using antiserum no. 681, [Tyr63]SYN(63-70)-octapeptide as standard, and 125I-[Tyr63]SYN(63-70)-octapeptide as tracer, the IC50 was approximately 51 fmol/100-microliters sample at equilibrium or 12 fmol/100 microliters in disequilibrium, and the sensitivity was approximately 3 fmol/100 microliters. Cross-reactivity of the assay was 100% with [Cys63]SYN(63-70)-octapeptide and with bovine adrenal 8.6-kilodalton peptide digested with trypsin and carboxypeptidase B, but less than 0.1% with transforming growth factor-alpha, less than or equal to 2 x 10(-6) with Leu-Leu-Ala [SYN(68-70)-tripeptide], and much less than 10(-6) with all other peptides tested. Therefore in RIA this antiserum is specific for the free carboxyl terminus of SYN. Because the peptide detected after enzyme digestion is the complete SYN(63-70)-octapeptide, we refer to the RIA as an assay for SYN(63-70). Tissue extracts were made in 1 M acetic acid, dried, reconstituted in Tris-CaCl2, and digested sequentially with trypsin plus carboxypeptidase B. Extracts from bovine corpus striatum gave SYN(63-70) RIA dilution curves parallel to the standard curve both before and after digestion. Digestion increased the amount of immunoreactive SYN(63-70) in striatum by a factor of 1.5-2.0. The ratio of total immunoreactive [Met5]enkephalin to total immunoreactive SYN(63-70) (after sequential digestion) was approximately 6:1. At least 90% of the immunoreactive SYN(63-70) in extracts of bovine caudate nucleus eluted from Sephadex G-100 with an apparent molecular weight equal to that of bovine PRO(1-77). Using the new RIA we were able to detect and characterize SYN processing for the first time in extracts of whole rat brain, human globus pallidus, and human pheochromocytoma. Results in these tissues were similar to those in cattle, in that most stored SYN had been processed to a free carboxyl terminus. Since the C-terminal octapeptide of SYN is practically identical in all known mammalian PRO, antiserum no. 681 should be useful for detecting, measuring, and purifying SYN from various mammals, including human beings.

Posttranslational glutamylation of alpha-tubulin

The high degree of tubulin heterogeneity in neurons is controlled mainly at the posttranslational level. Several variants of alpha-tubulin can be posttranslationally labeled after incubation of cells with [3H]acetate or [3H]glutamate. Peptides carrying the radioactive moiety were purified by high-performance liquid chromatography. Amino acid analysis, Edman degradation sequencing, and mass spectrometric analysis of these peptides led to the characterization of a posttranslational modification consisting of the successive addition of glutamyl units on the gamma-carboxyl group of a glutamate residue (Glu445). This modification, localized within a region of alpha-tubulin that is important in the interactions of tubulin with microtubule-associated proteins and calcium, could play a role in regulating microtubule dynamics.

Co-secretion of carboxypeptidase H and insulin from isolated rat islets of Langerhans

The release of carboxypeptidase H activity from isolated rat islets was determined and compared to the secretion of immunoreactive insulin. Analysis of pancreatic islet cells sorted into beta and non-beta types indicated that approx. 80% of islet carboxypeptidase H activity is present in the beta cell. The release of both insulin and carboxypeptidase H was stimulated markedly by increasing the glucose concentration in the medium from 2.8 to 28 mM. The fractional release was in accordance with the observed cellular distribution of both proteins. The secretory response was biphasic with time, with an initial rapid transient phase of release within 5 min, followed by a more sustained response. The concentration-dependencies of glucose stimulation of release of insulin and carboxypeptidase H were similar, with a threshold for stimulation around 5.6 mM-glucose and maximal stimulatory response at 16.7-28 mM-glucose. The release of both proteins was inhibited by 20 mM-mannoheptulose, removal of Ca2+ from the medium and addition of 1 microM-noradrenaline. The combination of 10 mM-4-methyl-2-oxopentanoate and 10 mM-glutamine stimulated the release of carboxypeptidase H and insulin, as did 3-isobutyl-1-methylxanthine and 350 microM-tolbutamide in the presence of glucose. It is evident that carboxypeptidase H is released from the pancreatic beta-cell by an exocytotic process from the same intracellular compartment as insulin. The release of carboxypeptidase H by a constitutive process was at best equivalent to 0.4%/h, or less than 2% of the maximal rate of release via the regulated pathway. It is concluded that carboxypeptidase H can be used as a sensitive index of beta-cell secretion and an alternative marker to the insulin-related peptides.

Comparison of synenkephalin and methionine enkephalin immunocytochemistry in rat brain

Immunocytochemistry using an antiserum to the C-terminal octapeptide of synenkephalin, proenkephalin(63-70), was performed throughout the rat brain and revealed numerous immunopositive fibers and some cell bodies. The morphology and distribution of synenkephalin immunoreactivity was extremely similar to that of a commercial methionine enkephalin (Met-ENK) antiserum. Colchicine pretreatment allowed the immunostaining of cell bodies not otherwise possible without pretreatment, but did not affect the distribution of immunoreactive fibers. Using 6 microns serial sections, we were able to colocalize synenkephalin and Met-ENK immunoreactivities in gigantocellular neurons of the medullary reticular formation. Preabsorption of the antiserum with [Tyr63]proenkephalin(63-70) octapeptide (YEESHLLA) completely eliminated immunoreactivity in the rat brain, while preabsorption with all other peptides used had no detectable effect. We conclude that our antiserum to synenkephalin is specific for enkephalinergic cell bodies, fibers and terminals. The synenkephalin antiserum used in these studies may have advantages over other antisera utilized for immunocytochemical detection of proenkephalin gene expression.

Isolation of dermenkephalin from amphibian skin, a high-affinity delta-selective opioid heptapeptide containing a D-amino acid residue

The predicted amino acid sequence of the biosynthetic precursor of dermorphin, a highly potent and nearly specific mu-opioid peptide from amphibian skin, contains four repeats of the dermorphin progenitor sequence and one single copy of a different heptapeptide sequence. We have developed a specific enzyme immunoassay and used synthetic peptides to detect and purify the new predicted heptapeptide (2.4 micrograms/g dry skin) from the skin of the Phyllomedusa sauvagei frog from which dermorphin was originally isolated. The identity of the novel pro-dermorphin related peptide, Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2, was established by co-chromatography with synthetic peptides on reverse-phase HPLC, immunological analysis, gas-phase sequencing, mass spectrometry and by pharmacological assays. Opioid-binding assays in vitro demonstrated that both the natural and synthetic heptapeptides displayed exceptionally high selectivity and affinity towards the delta-opioid receptors. Because of its origin and its delta-opioid (enkephalin) activity and specificity, this novel D-amino acid containing peptide is named dermenkephalin.

Synthesis and benzodiazepine receptor affinities of rigid analogues of 3-carboxy-beta-carbolines: demonstration that the benzodiazepine receptor recognizes preferentially the s-cis conformation of the 3-carboxy group

1H-Indolo[3',2':4,5]pyrido[3,2-b]-2-penten-5-olide (6) and 1H,5H-indolo[3',2'-c]-6,7-dihydro-2-pyridone (7), rigid analogues of methyl 4-ethyl-beta-carboline-3-carboxylate (8) and N-methyl-4-ethyl-beta-carboline-3-carboxamide (9), respectively, were synthesized and their in vitro binding affinities to the central type benzodiazepine receptors were compared. The IC50 values of 6 and 8 were approximately equivalent (42 and 27 nM, respectively). The amide derivative 9, for which theoretical energy calculations indicate that the s-trans carbonyl conformation is the preferred one, displayed very low affinity (IC50 greater than 10(4) nM). However, when the carbonyl group of 9 was forced to adopt the s-cis conformation as in lactam 7, binding to the benzodiazepine receptor was largely restored (IC50 = 150 nM), indicating that the s-cis carboxy conformation at C-3 of beta-carbolines is preferentially recognized by this receptor. In vivo, compound 6 showed neither convulsant, proconvulsant, nor anticonvulsant activity in mice. Moreover, 6 did not antagonize methyl beta-carboline-3-carboxylate induced convulsions in mice. This lack of activity of 6 was attributed to its inability to cross the blood-brain barrier since no significant displacement of [3H]Ro 15-1788 from mouse brain benzodiazepine receptors by 6 could be observed in vivo.

Radiometric assay for carboxypeptidase H (EC 3.4.17.10) and other carboxypeptidase B-like enzymes

Carboxypeptidase H, EC 3.4.17.10, also known as enkephalin convertase, carboxypeptidase E, and crino carboxypeptidase B, is an important enzyme involved in the biosynthesis of bioactive peptides. To assay the enzyme, tissues are homogenized in at least 20 vol (ml/g) of 0.025 M Tris-HCl buffer, pH 8, with 5 mg/ml of bovine serum albumin. After centrifugation, the supernatant is brought to pH 5.6 and centrifuged again. Following a 20-min preincubation in 2 mM CoCl2, the supernatant is incubated with 0.1 mM (final concentration) of the radioactive substrate [3H]benzoyl-Phe-Ala-Arg. The 100-microliters assay is stopped by the addition of 680 microliters of acetonitrile/0.25 M HCl (0.7/1). The 1.5-ml tube is transferred into a scintillation vial and is flushed with 4 ml of Econofluor, a water-immiscible scintillation fluid. The product, [3H]benzoyl-Phe-Ala, recovered in the organic phase, is counted directly with no interference from the substrate remaining in the aqueous phase. The blank is below 1%. Expressed in nanomoles per minute per milligram of tissue, the activity of the soluble enzyme in rat is 0.34 for striatum, 21.0 for pancreatic islet, 16.6 for anterior pituitary, 46.0 for intermediate pituitary, and 10.9 for neural pituitary. In every case 25 microM guanidinoethylmercaptosuccinic acid, an active site-directed inhibitor of carboxypeptidase H, completely inhibits the activity.

Demonstration of the partial agonist profiles of Ro 16-6028 and Ro 17-1812 in mice in vivo

Benzodiazepine receptor occupancy by the full agonist, diazepam, and by the two putative partial agonists, Ro 16-6028 and Ro 17-1812, was measured by inhibition of in vivo [3H]Ro 15-1788 binding in mouse brain and was correlated with their pharmacological effects. The anticonvulsant effects of Ro 16-6028, Ro 17-1812 and diazepam (ED50 values) appeared at receptor occupancies of 40, 20 and less than 5%, respectively. Moreover, at the highest measurable receptor occupancy (90-100%), Ro 16-6028 and Ro 17-1812 did not induce any rotarod deficit whereas a complete deficit was observed with diazepam at 35% receptor occupancy.