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Biomedical subjects

G R Marshall

Publications and source records attributed to G R Marshall.

At least 19 recordsLinked to original sources
PubMed1995

Identification of 3,5-dihydro-2-aryl-1H-pyrazolo[3,4-c]quinoline-1,4(2H)-diones as novel high-affinity glycine site N-methyl-D-aspartate antagonists.

Almost all of the existing known antagonists at the glycine site of the N-methyl-D-aspartate (NMDA) receptor have a low propensity for crossing the blood-brain barrier. It has been suggested that in many cases this may be due to the presence of a carboxylic acid which is a common feature of most of the potent full antagonists at this receptor. In this study, 2-aryl-1H-pyrazolo[3,4-c]quinoline-1,4(2H)-diones were found to have high-affinity binding at the glycine receptor. In particular, structure-activity studies identified 7-chloro-3,5-dihydro-2-(4-methoxyphenyl)-1H-pyrazolo[3,4-c]quinoline- 1,4(2H)-dione as the most potent of a series of analogues with an IC50 of 3.3 nM. The measured pKa values in this class of compounds (typically 4.0) indicate they are of equivalent acidity to carboxylic acids. Functional antagonism was demonstrated by inhibition of NMDA-evoked responses in rat cortical slices. Anticonvulsant activity in DBA/2 mice was achieved after dosing by direct injection into the cerebral ventricles, but no activity was seen after systemic administration, suggesting low brain penetration with this class of antagonists.

Animals↗
PubMed1995

Conformational mimicry: synthesis and solution conformation of a cyclic somatostatin hexapeptide containing a tetrazole cis amide bond surrogate.

Potent, cyclic hexapeptide analogues of somatostatin are generally believed to adopt some common secondary structural features: a II' beta turn at one end of the cycle, and a type VI turn with a cis amide bond at the other. A proposed cis amide surrogate, the 1,5-disubstituted tetrazole, has been placed into a cyclic hexapeptide analog of somatostatin in order to constrain the putative cis amide bond. The final cyclization was done by either chemical or enzymatic means. The product, cyclo(Ala6-Tyr7-D-Trp8-Lys9-Val10-Phe11-psi[CN4] ), was found to have 83% of the activity of somatostatin. Solution nmr analysis in DMSO/water revealed that the backbone as well as side chain chi1 and chi2 were well ordered. Relaxation matrix methods were used to extract distance restraints from the nuclear Overhauser effect spectroscopy data set, and these were used in a systematic search of torsional space to identify structures consistent with the nmr data. Restrained minimizations of these structures using a number of different force fields produced structures having the expected beta II' turn at D-Trp8-Lys9 and a beta VIa turn in the Phe11-psi[CN4]-Ala6 portion of the molecule. The similarity of the minimized structures to those previously reported for cyclic hexapeptide analogues of somatostatin confirms the similarity of the tetrazole geometry to that of the cis amide in solution.

Amides↗
PubMed1995

Conformationally readdressed CCK-B/delta-opioid peptide ligands.

The sequence of a cholecystokinin (CCK) related peptide was modified to obtain analogues, which interact selectively either with CCK-B, or with delta-opioid receptors. Two kinds of peptides were designed, namely, the cyclic peptides of the H-Tyr-cyclo (D-Pen-Gly-Trp-L/D-3-transmercaptoproline)-Asp-Phe-NH2 sequence (compounds 1a and 1b, respectively), and the linear peptides of the H-Tyr-D-Val-Gly-Trp-L/D-3-trans-methylmercaptoproline-Asp-Phe- NH2 sequence (compounds 2a and 2b, respectively). The only difference between the chemical structures of the linear analogues compared to the cyclic ones is that one covalent bond has been eliminated and a sulfur atom is replaced by a methyl group. Molecular modeling showed that, among low-energy conformers of cyclic compounds 1, there are three-dimensional structures compatible to the model for delta-receptor-bound conformer, suggested earlier [G. V. Nikiforovich, V.J. Hruby, O. Prakash, and C.A. Gehrig (1991) Biopolymers, vol. 31, pp. 941-955]. Results of binding assays fully supported the rationale for the design of compounds 1 and 2. The cyclic analogue 1a has Ki values of 4.5 and > 5000 nM at delta- and mu-opioid receptors, respectively; and IC50 values of 1.6 and > 10,000 nM for CCK-A and CCK-B receptors, respectively. The results of this study demonstrate a possibility to redirect a peptide sequence that interacts with one type of receptors (CCK-B receptors) toward interaction with another type (delta-opioid receptors) belonging to a different physiological system. This redirection could be performed by changing the conformational properties of the peptide with very minimal changes in its chemical structure.

Amino Acid Sequence↗
PubMed1995

DBMAKER: a set of programs to generate three-dimensional databases based upon user-specified criteria.

DBMAKER is a program that, in conjunction with CONCORD, generates three-dimensional structural databases. Numerous user-defined parameters monitor content, composition, size and connectivity information, but allow the program to generate random compounds within the scope of these constraints. SMILES string representations are generated, and conversion to 3D is performed by CONCORD. This assures high-quality 3D structures and portability to numerous proprietary storage formats. Methods are described to maintain compound registration, allowing database expansion as required without duplication.

Computer Simulation↗
PubMed1995

Tracheal cytotoxin structural requirements for respiratory epithelial damage in pertussis.

The respiratory epithelial pathology of pertussis (whooping cough) can be reproduced by tracheal cytotoxin (TCT), a disaccharide-tetrapeptide released by Bordetella pertussis. TCT is a muramyl peptide, a class of peptidoglycan-derived compounds which have many biological activities including adjuvanticity, somnogenicity, pyrogenicity, and cytotoxicity. The structural requirements for muramyl peptides to produce some of these biological effects have been partially characterized. Using in vitro assays with respiratory epithelial cells and tissue, we have previously determined that the disaccharide moiety of TCT is not involved in toxicity and that the side-chain functional groups of diaminopimelic acid (A2pm) are crucial for toxicity. In this study, we determine the importance of every amino acid, functional group and chiral centre in the peptide portion of TCT. Although lactyl tetrapeptides are the most toxic of the TCT fragments, producing dose-response curves identical to TCT, the smallest analogues of TCT which are active in our assay are of the form X-gamma-(D)-Glu-meso-A2pm, where X may be an amino acid or a blocking group. Within this active substructure, main-chain chirality and all functional groups are essential for toxicity. This definition of the core region of TCT indicates that the TCT interaction site is unlike almost all other muramyl peptide interaction sites for which structure-activity data are available.

Amino Acid Sequence↗
PubMed1995

Follicle-stimulating hormone amplifies the population of differentiated spermatogonia in the hypophysectomized testosterone-replaced adult rhesus monkey (Macaca mulatta).

Although testosterone supports all phases of spermatogenesis in primates, FSH is obligatory for quantitatively normal spermatogenesis. To further investigate the mechanism of action of FSH on spermatogenesis, eight adult male rhesus macaques were hypophysectomized and supplemented daily with cortisone acetate (5 mg/kg BW, sc) and T4 (50 mg/animal, orally). Complete pituitary ablation was established by 1) a decline in mean testicular volume to 8% of the prehypophysectomy value; 2) a failure to secrete gonadotropins in response to 50 micrograms GnRH, iv; and 3) an absence of pituitary in the sella turcica on postmortem examination. Testosterone-filled SILASTIC brand capsules (Dow Corning) were implanted sc to restore testicular testosterone to normal levels. Once the testes had achieved maximum growth under testosterone stimulation alone, the animals were implanted with indwelling venous catheters. In four animals, a pulsatile infusion of human FSH (one pulse of 4 IU/kg BW every 3 h) was administered for 12 days, and the other four monkeys received vehicle. Testosterone replacement continued throughout the experiment. At the termination of the 12 days of FSH stimulation or vehicle administration, the right testes were removed. The left testes were removed 22 days later to investigate whether testosterone was able to maintain the effects, if any, of FSH stimulation. Portions of each testis were fixed in Bouin's solution and subsequently prepared for histological examination, whereas other portions were frozen in liquid nitrogen for determination of testicular testosterone content. Five hundred cross-sections of seminiferous tubules in periodic acid-Schiff-hematoxylin-stained histological sections were randomly selected from each testis. The stage of the seminiferous epithelial cycle in these sections was identified, and the germ cells and Sertoli cells were counted in each. All cell counts were corrected by the Abercrombie method and expressed per cross-section of seminiferous tubule. Treatment with FSH for 12 days failed to influence the numbers of either Sertoli cells or Ad and Ap stem spermatogonia. In striking contrast, the number of all four generations of differentiated (B1, B2, B3, and B4) spermatogonia were significantly amplified by stimulation with human FSH for 12 days. As reflected by the analysis of the left testes collected 22 days after termination of the gonadotropin treatment, the progeny of these B spermatogonia were not maintained in the absence of FSH. In conclusion, the results of the present study indicate that the action of FSH to quantitatively maintain spermatogenesis in the rhesus monkey is mediated by a selective amplification of B spermatogonia.

Animals↗
PubMed1994

HIV-1 protease inhibitors: ketomethylene isosteres with unusually high affinity compared with hydroxyethylene isostere analogs.

HIV protease is a member of the aspartic proteinase family of proteolytic enzymes which include pepsin and renin. In contrast to the enhanced affinity seen with renin and pepsin upon conversion of the transition-state isostere, ketomethylene, to the hydroxyethylene, a set of HIV protease inhibitors showed a reduction in affinity. This implies that interactions with the active site of other segments of the inhibitor than those of the transition-state analog must predominate in the case of HIV protease, and that observations made on mammalian aspartic proteinases do not necessarily apply to viral aspartic proteinases.

Amides↗
PubMed1994

3D-QSAR of human immunodeficiency virus (I) protease inhibitors. III. Interpretation of CoMFA results.

Comparative Molecular Field Analysis (CoMFA), a three-dimensional quantitative structure-activity relationship (3D-QSAR) paradigm (Cramer, R.D.; et al. (1988), J. Am. Chem. Soc., 110, 5959-5967), correlates variations in the (experimental) biological activity with 3D variance in the steric and electrostatic field of modeled compounds. Of general interest to the drug design area is the interpretation of CoMFA results, in order to gain maximum benefit from an established 3D-QSAR model. CoMFA studies report results using the standard deviation (stdev) times(*) coefficient (beta) field and its contributions to make SAR statements. This field is the scalar product of the absolute stdev of the CoMFA field at a lattice point and the QSAR equation coefficient (beta) at the same point. Negative beta values yield detrimental contributions, while positive beta values are considered beneficial. The QSAR equation is based on actual field values, therefore both positive and negative field values can have beneficial effect to the target property (Y), depending on the sign of beta. The results of a CoMFA model on 59 HIV-1 protease (HIV-PR) inhibitors (Waller, C.L.; et al. (1993), J. Med. Chem., 36, 4152-4160) were compared with the HIV-PR crystal structure to analyze the correspondence between CoMFA fields and ligand binding regions in the enzyme. Local steric and electrostatic interactions were analyzed in terms of various field values and beta coefficients. While redundant for some regions, other field contours besides stdev* beta bring additional information. Using this method, we observed a unique region with negative beta values for the electrostatic field (based on a -1 charged probe) located opposite of the scissile bond, between P1 and P1', where steric stdev* beta values are positive. Four hydrophobic residues in the HIV-PR crystal delimit the region, which is suggested as a new potential hydrophobic binding site for the inhibitors. The same region was confirmed using the stdev* beta contours of a HINT (Kellogg, G.; et al. (1991), J. Comput.-Aided Mol. Design, 5, 545-552) calculation on the same model. The steric, electrostatic and lipophilic fields of the CoMFA and HINT models are presented in various forms, and the information extracted is detailed.

Binding Sites↗
PubMed1992

2-Carboxytetrahydroquinolines. Conformational and stereochemical requirements for antagonism of the glycine site on the NMDA receptor.

2-Carboxy-1,2,3,4-tetrahydroquinoline derivatives, derived from kynurenic acid, have been synthesized and evaluated for in vitro antagonist activity at the glycine site on the NMDA receptor. 2,3-Dihydrokynurenic acids show reduced potency relative to the parent lead compounds (Table I) possibly as a result of conformational effects. Removal of the 4-oxo group results in further reduced potency, but introduction of a cis-carboxymethyl group to the 4-position restores antagonist activity (Tables III and IV). Replacement of the keto group of 5,7-dichloro-2,3-dihydrokynurenic acid with other alternative H-bonding groups, for example cis- and trans-benzyloxycarbonyl and cis- and trans-carboxamido (Table V), gives comparable activity, but there is negligible stereoselectivity. A significant increase in potency and stereoselectivity is seen within the 4-acetate series (Table VI). The trans-4-acetic acid is significantly more potent than the corresponding lead kynurenic acid and has 100-fold greater affinity than the cis isomer. The results are consistent with a requirement in binding for a pseudoequatorially placed 2-carboxylate and clearly demonstrate the importance for binding of a correctly positioned hydrogen-bond-accepting group at the 4-position. The high-affinity binding of an anionic group in the 4-substituent binding pocket suggests that the glycine site and the neurotransmitter recognition (NMDA) site may have some features in common.

Animals↗
PubMed1992

Facilitated synthesis of peptaibols: alamethicin via enzymatic segment condensation.

We have used a combined chemical-enzymatic approach to facilitate the total synthesis of the 20-residue peptaibol, alamethicin. The 1-11 segment of alamethicin, having a C-terminal Gly, and the 12-20 segment, having an N-terminal Leu, were prepared by well-established chemical methods, and then coupled using papain to afford a 54% yield of alamethicin in straightforward fashion. In contrast to the reported chemical syntheses of alamethicin requiring side-chain protection at Glu,18 the papain-catalyzed coupling proceeded readily and selectively using a C-terminal segment having a free gamma-carboxyl group at this position. Several alamethicin partial sequences were obtained via enzymatic formation of the Gly11-Leu12 bond. The high efficiency of this route is illustrated by the enzymatic assembly of the 1-17 alamethicin fragment on a 400-mg scale in 62% yield. An alternative route to alamethicin through enzymatic formation of the Ala6-Gln7 bond was less successful because of a low yield in the final coupling.

Alamethicin↗
PubMed1992

The utility of side-chain cyclization in determining the receptor-bound conformation of peptides: cyclic tripeptides and angiotensin II.

The effect of side-chain cyclization on accessible backbone conformations of tripeptides, X-Ala-Y (X and/or Y = Cys, Hcy (Hcy: homocysteine), cis 4-mercaptoproline (MPc), and trans 4-mercaptoproline (MPt)), was elucidated using two variants of systematic conformational search. In addition to cyclization through a disulfide bond, the thioether (-S-CH2-) and amide (-CO-NH-) side-chain analogues of Cys-Ala-Cys and Hcy-Ala-Hcy were evaluated. The number of valid backbone conformations and the allowed phi, psi space were evaluated for each compound, and the ability of the cyclic tripeptides to accommodate beta-turn conformations was examined in order to assess the value of cyclization in limiting conformational freedom. Based on the number of conformations, cyclization was highly effective in reducing the backbone degree of freedom: in order of decreasing number of conformations, Ala-Ala-Ala 1 >> Hcy-Ala-Hcy 2 >> Cys-Ala-Hcy 3 approximately equal to Hcy-Ala-Cys 4 >> MPc-Ala-Hcy 5, 7 > Cys-Ala-Cys 6 > MPc-Ala-Cys 8 > Hcy-Ala-MPt 9 > Cys-Ala-MPt 10 approximately equal to MPc-Ala-MPt 11. Although Hcy-Ala-Hcy 2 had the greatest number of conformations of the cyclic peptides studied, it was still greatly constrained relative to its linear analogue 1. The bicyclic ring system introduced by MP was even more effective in constraining the cycle, having greater impact at position 3 than at position 1. Under the conditions of the study, cyclization of MP-containing analogues could be effected only with the cis isomer (MPc) at position 1 and/or the trans isomer (MPt) at position 3. Sterically allowed conformations of Ala2 for the cyclic tripeptides 2-4 were generally similar to those of the linear tripeptide 1, while those of Cys-Ala-Cys 6 and MPc-Ala-Hcy 7 were restricted to a smaller region of phi 2, psi 2 space: the right- and left-handed alpha-helical conformation and the beta-conformation. This trend was even more pronounced for Hcy-Ala-MPt 9, Cys-Ala-MPt 10, and MPc-Ala-MPt 11, in which Ala2 was severely restricted to a very small region of phi, psi space: the left-handed alpha-helical conformation for 9-11, plus the beta conformation for 9. This suggests that MP at the 3-position is incompatible with a right-handed alpha-helical conformation at position 2.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acid Sequence↗
PubMed1992

Effects of orchidectomy on gonadotropin and inhibin subunit messenger ribonucleic acids in the pituitary of the rhesus monkey (Macaca mulatta).

Our research programs required the preparation of hypophysectomized and orchidectomized rhesus monkeys. This afforded us the possibility to characterize and compare levels of the gonadotropin and inhibin subunit mRNAs in pituitaries from intact and castrate monkeys. Eighteen adult male monkeys, four of which had been bilaterally orchidectomized 5-9 months previously, were used in this study. Plasma concentrations of LH and FSH were, respectively, 188.5 +/- 5.3 and 246.8 +/- 25.2 ng/ml in the castrate monkeys and 25.8 +/- 4.5 and 4.1 +/- 1.1 ng/ml (mean +/- SEM) in the intact animals. Total pituitary RNA was hybridized to cDNA probes for cynomolgus monkey gonadotropin subunits (FSH beta, LH beta, and the common alpha-subunit) and for human inhibin subunits (alpha, beta B, and beta A) by Northern blot analysis, and mRNA levels were normalized by subsequent hybridization to cyclophilin. Each of the gonadotropin subunit probes hybridized to a single RNA species with the approximate sizes of 1.6 kilobases (kb; FSH beta), 0.7 kb (LH beta), and 0.8 kb (alpha). Levels of LH beta and alpha-subunit mRNAs in pituitaries from castrate monkeys were about 5- and 2-fold higher, respectively, than those in pituitaries from intact monkeys. FSH beta mRNA, on the other hand, was elevated about 27-fold in castrate monkeys [mean +/- SEM, 3176 +/- 408 cpm bound (n = 4 castrate) and 116 +/- 30 cpm bound (n = 8 intact]). Inhibin beta B-subunit mRNA was present in the monkey pituitary as a doublet of about 5 kb, and it was approximately twice as abundant in intact pituitaries as in castrate pituitaries. Hybridizations involving inhibin beta A cDNA revealed a faint band in the region expected for monkey beta A mRNA (6.5 kb) in three of six RNA samples from intact monkeys and a 0.3- to 0.4-kb mRNA species. mRNA encoding the inhibin alpha-subunit was undetectable by Northern blot hybridization. These results indicate that the postpubertal testis imposes an inhibition on the expression of the genes encoding FSH beta, LH beta, and glycoprotein hormone alpha-subunit and that this suppression of the FSH beta gene in the monkey is much greater than that in the rat. In addition, the monkey pituitary may be a source of activin, which may act locally to modulate FSH gene expression and secretion.

Animals↗
PubMed1992

Kynurenic acid analogues with improved affinity and selectivity for the glycine site on the N-methyl-D-aspartate receptor from rat brain.

The glycine site on the N-methyl-D-aspartate (NMDA) subtype of receptors for the excitatory neurotransmitter glutamate is a potential target for the development of neuroprotective drugs. We report here two chemical series of glycine site antagonists derived from kynurenic acid (KYNA), with greatly improved potency and selectivity. Disubstitution with chlorine or bromine in the 5- and 7-positions of KYNA increased affinity for [3H]glycine binding sites in rat cortex/hippocampus P2 membranes, with a parallel increase of potency for antagonism of NMDA-evoked responses in the rat cortical wedge preparation. The optimal compound was 5-I,7-Cl-KYNA, with an IC50 for [3H]glycine binding of 29 nM and an apparent Kb in the cortical wedge preparation of 0.41 microM. Reduction of the right-hand ring of 5,7-diCl-KYNA reduced affinity by 10-fold, but this was restored by substitution in the 4-position with the trans-phenylamide and further improved in the trans-benzylamide. The optimal compound was the transphenylurea (L-689,560), with an IC50 of 7.4 nM and an apparent Kb of 0.13 microM. Both series of compounds displayed a high degree of selectivity for the glycine site, having IC50 values of greater than 10 microM versus radioligand binding to the glutamate recognition sites of NMDA, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), and kainate receptors and the strychnine-sensitive glycine receptor. Selectivity versus AMPA receptor-mediated responses was also apparent in the rat cortical wedge and in patch-clamp recordings of cortical neurons in culture. Experiments using [3H]dizocilpine (MK-801) binding indicated that 5,7-diBr-KYNA, 5,7-diCl-KYNA, 5-I,7-Cl-KYNA, and L-689,560 all behaved as full antagonists and were competitive with glycine. Patch-clamp recordings of cortical neurons in culture also indicated that NMDA-induced currents were antagonized by competition for the glycine site, and gave no evidence for partial agonist activity. pKi values for 5,7-diBr-KYNA and L-689,560 in these experiments were 7.2 and 7.98, respectively, similar to the affinities of these compounds in the glycine binding assay. The high affinity and selectivity of these new derivatives make them useful tools to investigate the function of the glycine site on the NMDA receptor.

Animals↗
PubMed1992

Effects of mono-, di-, and triamines on the N-methyl-D-aspartate receptor complex: a model of the polyamine recognition site.

Systematic series of monoamines, diamines, and triamines were used to define the structural requirements for interaction at the polyamine recognition site of the N-methyl-D-aspartate receptor complex. Effects of amines on binding of [3H]MK-801 to washed synaptic plasma membranes were measured in the presence of L-glutamate and glycine (100 microM each), in the absence or presence of spermine (10 microM). Linear aliphatic monoamines of methylene chain length up to 12 (dodecylamine) did not interact with the polyamine recognition site. Nonspecific inhibition of binding was observed at high concentrations of the longer monoamines. alpha,omega-Diamines of methylene chain length 2 (1,2-diaminoethane, DA2) through 12 (1,12-diaminododecane, DA12) had varying actions, depending on chain length. The shortest diamines (DA2 and DA3) acted as weak partial agonists, enhancing the binding of [3H[MK-801. Intermediate-length diamines (DA4-DA7) were selective polyamine antagonists, having little or no effect on binding of [3H]MK-801 measured in the absence of spermine but inhibiting binding measured in the presence of spermine. The longest diamines tested (DA8-DA12) acted as inverse agonists; they inhibited binding in the absence or presence of spermine, and this inhibition was blocked by the selective polyamine antagonist diethylenetriamine. Computer modeling of conformations of the diamines quantitatively documented that 1) these molecules are flexible and 2) long diamines may easily adopt conformations with inter-nitrogen distances mimicking those of short diamines. The cis and trans isomers of 1,4-diaminocyclohexane are inflexible, conformationally restricted diamines with markedly different actions. The cis isomer was a partial agonist and the trans isomer was an antagonist at the polyamine recognition site. Triamines of general structure NH2(CH2)3NH(CH2)xNH2 (TRI[3,x]), in which x = 3-12, were synthesized and tested for activity at the polyamine recognition site. Despite the large range of size, TRI[3,3] through TRI[3,9] were all fully polyamine agonists of similar potency. TRI[3,10] was a partial agonist, whereas TRI[3,12] inhibited binding of [3H]MK-801. Diethylenetriamine did not attenuate the effect of TRI[3,12]. Based on the results of the radioligand binding studies and the computer analysis, a model of the polyamine recognition site is proposed.

Amines↗
PubMed1991

Kynurenic acid derivatives. Structure-activity relationships for excitatory amino acid antagonism and identification of potent and selective antagonists at the glycine site on the N-methyl-D-aspartate receptor.

Derivatives of the nonselective excitatory amino acid antagonist kynurenic acid (4-oxo-1,4-dihydroquinoline-2-carboxylic acid, 1) have been synthesized and evaluated for in vitro antagonist activity at the excitatory amino acid receptors sensitive to N-methyl-D-aspartic acid (NMDA), quisqualic acid (QUIS or AMPA), and kainic acid (KA). Introduction of substituents at the 5-, 7-, and 5,7-positions resulted in analogues having selective NMDA antagonist action, as a result of blockade of the glycine modulatory (or coagonist) site on the NMDA receptor. Regression analysis suggested a requirement for optimally sized, hydrophobic 5- and 7-substituents, with bulk tolerance being greater at the 5-position. Optimization led to the 5-iodo-7-chloro derivative (53), which is the most potent and selective glycine/NMDA antagonist to date (IC50 vs [3H]glycine binding, 32 nM; IC50's for other excitatory amino acid receptor sites, greater than 100 microM). Substitution of 1 at the 6-position resulted in compounds having selective non-NMDA antagonism and 8-substituted compounds were inactive at all receptors. The retention of glycine/NMDA antagonist activity in heterocyclic ring modified analogues, such as the oxanilide 69 and the 2-carboxybenzimidazole 70, suggests that the 4-oxo tautomer of 1 and its derivatives is required for activity. Structurally related quinoxaline-2,3-diones are also glycine/NMDA antagonists, but are not selective and are less potent than the 1 derivatives, and additionally show different structure-activity requirements for aromatic ring substitution. On the basis of these results, a model accounting for glycine receptor binding of the 1 derived antagonists is proposed, comprising (a) size-limited, hydrophobic binding of the benzene ring, (b) hydrogen-bond acceptance by the 4-oxo group, (c) hydrogen-bond donation by the 1-amino group, and (d) a Coulombic attraction of the 2-carboxylate. The model can also account for the binding of quinoxaline-2,3-diones, quinoxalic acids, and 2-carboxybenzimidazoles.

Animals↗
PubMed1991

Conformational mimicry. II. An obligatory cis amide bond in a small linear peptide.

The structure of Z-Pro psi [CN4]-Ala-OBzl has been determined by X-ray crystallographic techniques. The structure crystallizes in space group P2(1) with cell constants a = 22.176(3) A, b = 6.141(1)A, c = 8.275(1) A, beta = 98.31(1), and Z = 2. The structure has been refined to a residual of 0.038 for 2538 independent data. The amide bond between the prolyl and alanyl residues is cis, a result of the presence of the tetrazole ring system, as is the urethane bond linking the benzyloxycarbonyl and the prolyl groups. A comparison of the structures in this study to other structures containing cis amide bonds shows that the tetrazole ring system, when incorporated into peptides, mimics a cis amide bond. Changes in the distance between the alpha-carbons adjacent to the tetrazole rings in the linear peptide as compared with the bicyclic diketopiperazine required a reassessment of the conformational mimicry with the cis amide bond.

Amides↗