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G H Loew

Publications and source records attributed to G H Loew.

At least 55 records · Page 3Linked to original sources
PubMed1994

Evidence for central benzodiazepine receptor heterogeneity from behavior tests.

To explore behavioral selectivity as a consequence of multiple receptor subtypes, four benzodiazepine receptor ligands, flunitrazepam, CGS 9896, zolpidem, and AHR 11797, were tested at five in vivo endpoints: anticonvulsant action, anxiolysis/anxiogenesis as determined in the plus-maze test, locomotor activity, changes in food consumption, and hypothermia. All compounds produced hypothermia. In the plus-maze test, flunitrazepam, CGS 9896, and a low dose of zolpidem (0.05 mg/kg) increased the time spent in the open arms, although AHR 11797 and higher doses of zolpidem decreased time spent in the open arms. Flunitrazepam and zolpidem greatly reduced, CGS 9896 slightly reduced, and AHR 11797 did not affect locomotor activity. Flunitrazepam and CGS 9896 increased food consumption, but AHR 11797 and zolpidem had no effect. Only flunitrazepam fully protected the animals from pentylenetetrazol-induced seizures. The qualitative differences in the effects of these compounds observed are difficult to explain by activation of a single benzodiazepine receptor subtype. As Ro15-1788 antagonized all the observed effects, these compounds act through multiple central benzodiazepine receptors.

Animals↗
PubMed1994

Molecular determinants of recognition and activation at the cerebellar benzodiazepine receptor site.

Semiempirical quantum mechanical and molecular mechanics calculations were carried out to identify and characterize the steric and electronic properties that modulate ligand recognition and activation of the cerebellar GABAA/benzodiazepine (BDZ) receptor. For this hypothesis development, thirteen compounds belonging to structurally diverse chemical families were selected for study. Among the compounds selected were nine that bind and four that do not bind with appreciable affinity to this receptor and some that are known agonists, antagonists and inverse agonists, as measured by their modulation of GABA (gamma-aminobutyric acid) enhanced chloride ion flux in cerebellum. The stereoelectronic requirements for recognition deduced from commonalities among the ligands are the presence of at least two of three hydrogen bonding centers, and a lipophilic aromatic ring, in a specific spatial relationship. The results suggest that the selectivity for the cerebellar or Type I subtype, demonstrated by some of these ligands, could be failure to meet the requirements for binding at other receptors because of the absence of one of the proton accepting centers or the larger surface area and volume of these ligands. The requirement for activation, deduced from comparisons of agonist, antagonist, and inverse agonist properties is the presence of an electron accepting aromatic ring in a specific geometric arrangement with respect to the components of recognition. The validity of the '3D-Pharmacophore' developed was probed by using it for predictions of the behavior of 11 additional compounds not used for its development.

Animals↗
PubMed1994

Molecular dynamics simulations of phospholipid bilayers.

Molecular dynamics (MD) simulations at 37 degrees C have been performed on three phospholipid bilayer systems composed of the lipids DLPE, DOPE, and DOPC. The model used included 24 explicit lipid molecules and explicit waters of solvation in the polar head group regions, together with constant-pressure periodic boundary conditions in three dimensions. Using this model, a MD simulation samples part of an infinite planar lipid bilayer. The lipid dynamics and packing behavior were characterized. Furthermore, using the results of the simulations, a number of diverse properties including bilayer structural parameters, hydrocarbon chain order parameters, dihedral conformations, electron density profile, hydration per lipid, and water distribution along the bilayer normal were calculated. Many of these properties are available for the three lipid systems chosen, making them well suited for evaluating the model and protocols used in these simulations by direct comparisons with experimental data. The calculated MD behavior, chain disorder, and lipid packing parameter, i.e. the ratio of the effective areas of hydrocarbon tails and head group per lipid (a(t)/ah), correctly predict the aggregation preferences of the three lipids observed experimentally at 37 degrees C, namely: a gel bilayer for DLPE, a hexagonal tube for DOPE, and a liquid crystalline bilayer for DOPC. In addition, the model and conditions used in the MD simulations led to good agreement of the calculated properties of the bilayers with available experimental results, demonstrating the reliability of the simulations. The effects of the cis unsaturation in the hydrocarbon chains of DOPE and DOPC, compared to the fully saturated one in DLPE, as well as the effects of the different polar head groups of PC and PE with the same unsaturated chains on the lipid packing and bilayer structure have been investigated. The results of these studies indicate the ability of MD methods to provide molecular-level insights into the structure and dynamics of lipid assemblies.

Algorithms↗
PubMed1994

Comparison of a new class of pyrrole containing benzodiazepine ligands to the pyrazoloquinolinones CGS 9896, 9895, and 8216.

Four pyrazoloquninolinone compounds, variations of known high affinity ligands for the GABA A/Benzodiazepine receptors (BDZRs), were synthesized and their affinities for BDZRs in cerebellum and spinal cord measured and compared to the parent compounds, CGS 8216, CGS 9895, and CGS 9896. Using the techniques of computational chemistry, specific properties of both types of compounds were calculated and evaluated for the extent to which they fulfill the minimum postulated requirements for recognition of a cerebellum, "Type I", BDZR embodied in our current three dimensional pharmacophore. Additional properties were also calculated and examined as possible further determinants of recognition of the receptor subtype.

Animals↗
PubMed1993

The reaction of atomic oxygen with methanethiol. A theoretical study of the structures and the potential energy surface.

Ab initio calculations at the Hartree-Fock, MP2 and MP4 levels were performed to find structures of the equilibrium and transition states and the reaction energies and energies of activation of several competing reaction pathways of O (3P)+CH3SH. A 6-31G* basis set was used in all calculations. The mechanism of hydrogen atom abstraction from the S-H group methanethiol was found to be very competitive with the oxygen atom addition to the sulfur atom.

Computer Simulation↗
PubMed1993

Characterization of the bioactive form and molecular determinants of recognition of cyclic enkephalin peptides at the delta-opioid receptor.

An extensive and systematic search strategy to determine the conformational profile of 12 cyclic disulfide-bridged opioid peptides with varying affinities at the delta receptor has been carried out to identify the structure that is recognized by the delta receptor for each analogue. The methods and procedures used here for the conformational search have already been validated for [D-Pen2,D-Pen5] enkephalin (DPDPE), one member of this family. Use of these methods led to a low-energy solution conformation of DPDPE in excellent agreement with all the geometric properties deduced from its solution nmr spectra. Each of the analogue was subjected to the same procedure, involving a combination of molecular dynamics simulations at high and low temperature. The study was repeated in two environmental conditions, an apolar environment, simulated by using a distance-dependent dielectric constant, and a polar environment by embedding the peptides in a high constant dielectric (epsilon = 80). An automated comparison of the different conformers based on their backbone rms and average distance between the key aromatic moieties was followed by graphic analysis using maximum structural overlap. The cross-comparison of the conformations for each analogue revealed a unique conformer that may be recognized by the delta receptor for each high-affinity analogue that permitted maintaining the critical elements required for recognition in a simple spatial orientation, while maximizing similarity in other regions.

Amino Acid Sequence↗
PubMed1993

Strategies for indirect computer-aided drug design.

This review is intended to describe some of the methods and procedures used for computer-aided drug design when the structure of the macromolecular target is unknown, as is the case for CNS active drugs. Strategies and methods used in computer-aided design of drugs in such instances must be "indirect," i.e., focusing on the characterization of the ligands themselves. This situation is different from one in which the three-dimensional structure of the macromolecular target for a drug is known, for example, for drugs that are enzyme inhibitors, allowing "direct" characterization of ligand-receptor interactions. Two qualitatively different "indirect" approaches are described here. One, called 2D-QSAR, is briefly reviewed. It is based on delineating regression relationships between a specified biological end point and properties of the compounds eliciting it. The other, based on pharmacophore development, constitutes the main part of this review. Several levels of pharmacophore development are described, which differ in the extent to which they encompass fundamental molecular properties that are determinants of receptor recognition and activation. The strengths and limitations of each procedure are discussed and illustrated by examples. Two methods for obtaining model receptor structures are then briefly described. Both rely on the prior success of the indirect methods in obtaining ligand properties that modulate receptor recognition and activation. These emerging capabilities have the potential to bridge the gap between indirect and direct methods of drug design, since, if successful, the design process can continue in a direct mode using explicit characterization of drug-receptor interactions. Strategies for hypothesis validation and use of hypothesis for drug design and discovery are also briefly reviewed. The final sections of this review describe specific computational tools such as molecular mechanics and quantum mechanical methods used to characterize and identify relevant molecular properties and indicate some areas for future development of computational chemistry methods that could increase its effectiveness in the design of novel drugs.

Computer Simulation↗
PubMed1993

Zinc selectively inhibits flux through benzodiazepine-insensitive gamma-aminobutyric acid chloride channels in cortical and cerebellar microsacs.

The effects of Zn2+ on the activity of gamma-aminobutyric acid (GABA)A receptor-Cl- ionophore complexes found in adult rat cortex and cerebellum were tested by measuring 36Cl- influx into microsacs. In both preparations, the concentration-response curves were biphasic, with 25% of the cerebellar and 20% of the cortical Cl- flux being blocked by less than 10 microM Zn2+ and 45% of the cerebellar and 50% of the cortical flux being blocked by concentrations of Zn2+ exceeding 10 microM. Zn2+ (100 microM) did not affect basal Cl- flux but inhibited that stimulated by 100 microM GABA in a noncompetitive manner. The ability of 1 microM flunitrazepam to enhance Cl- flux was unaffected by 100 microM Zn2+. These results demonstrate that, in adult rat cerebellum and cortex, there are three populations of GABAA receptors, two that are sensitive to Zn2+ and insensitive to benzodiazepines (BDZ) and the remainder that are the reverse, i.e., insensitive to Zn2+ but fully sensitive to BDZ enhancement. This result is consistent with the idea that Zn2+ blocks only those GABAA receptor-Cl- ionophore complexes that lack a gamma subunit, which is required for modulation by BDZ. The results obtained in this study also show that the proportion of Zn(2+)-sensitive GABA receptors is substantial, suggesting that they play an important role in the functioning of the adult central nervous system.

Animals↗
PubMed1993

Molecular mechanism of delta-selectivity of indole analogs of nonpeptide opioids.

A combined experimental and computational approach was used to understand the mechanism of delta-receptor selectivity of a series of nonpeptide opioids. Six pairs of fused ring opioids/indole derivatives were studied. Receptor-binding assays using [3H][D-Ala2-MePhe4-Gly-ol]-enkephalin (mu), [3H][D-Pen2-D-Pen5]-enkephalin (delta), and [3H]U-69593 (kappa) were performed in guinea pig whole-brain membranes. Agonist activity was determined in norbinaltorphimine- or beta-funaltrexamine-treated guinea pig ileum (mu and kappa) and beta-funaltrexamine-treated mouse vas deferens (delta). Steric and electronic properties were calculated for each compound. Although the parent compounds were selective for the mu-receptor, the indole analogs displayed selectivity for the delta-site because of a decrease in mu-affinity accompanied by an increase in delta-affinity. The indole analogs displayed little or no activity at the delta-receptor. The role of the indole in enhanced delta-recognition is likely interaction with a lipophilic site in the receptor. The diminished mu-affinity of the indole analogs is a result of the loss of the carbonyl oxygen as the proton-accepting center, which we have previously determined to be important for recognition of the mu-receptor.

Amino Acid Sequence↗
PubMed1992

Molecular dynamics simulations of the resting and hydrogen peroxide-bound states of cytochrome c peroxidase.

The current hypothesis for the formation of the catalytically active compound I of peroxidases from the resting state and peroxide involves formation of a reversible "inner-sphere" complex in which the peroxide is bound to the heme iron. It is this precursor that is postulated to then form compound I. However, this crucial putative transient intermediate has not yet been definitively detected or characterized by experimental methods. We report here the use of energy minimization and molecular dynamics simulation together with the known X-ray structure of cytochrome c peroxidase to investigate the nature of this complex and comparisons of it with the resting state in which a water is bound as a ligand. Among the properties monitored in these simulations are the mode of binding of the peroxide to the heme iron, its interactions with neighboring amino acid residues, and the extent to which the binding of the peroxide perturbs both the local environment around the heme unit and more distant regions. The results of this study indicate that solvated, full protein dynamics is required to obtain reliable results for the known resting-state complex and hence for the uncharacterized peroxide complex. In this complex, the peroxide binds to the heme iron in a dynamically averaged end-on fashion, rather than a bridged structure, with approximately equal probability of each oxygen serving as the ligand to the iron. Binding of the peroxide as a ligand disrupts the H-bonded network of waters in the distal binding pocket which are present in the resting state, but there is no dramatic perturbation of the nearby amino acid residues.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acids↗
PubMed1992

Thermodynamic analysis of binding to the cerebellar type I benzodiazepine receptor.

The temperature dependence of binding to the type I benzodiazepine receptor in rat cerebellum was determined using [3H]Ro15-1788 and regression analysis techniques. The ligands chosen were from diverse chemical families and display different pharmacological properties. Included were the agonists flunitrazepam, CL 218,872, zolpidem and alpidem; the antagonists Ro15-1788 and propyl-beta-carboline-3-carboxylate (beta-CCP); the inverse agonists ethyl-beta-carboline-3-carboxylate (beta-CCE) and methyl-6,7-dimethoxy-beta-carboline-3-carboxylate (DMCM); and the selective muscle relaxant AHR-11797. Assays were performed at 0 degrees C, 20 degrees C and 37 degrees C and the K(i) at each temperature was used to construct a van't Hoff plot for each compound. The binding of all ligands, with the exception of DMCM, was enthalpy-driven. However, enthalpy alone does not determine the rank order of affinity. There was no relationship between the thermodynamic behavior of binding and the observation of agonism, antagonism or inverse agonism, indicating that activation and recognition are distinct steps in this receptor system.

Animals↗
PubMed1992

Evidence for more than two central benzodiazepine receptors in rat spinal cord.

Competitive binding assays were performed in rat spinal cord membranes at 0 degrees C against [3H]Ro15-1788. The displacement curves of Ro15-1788 and alpidem produced pseudo-Hill slopes of 1.0 and 0.5, respectively. In addition, 5-10% of the specifically bound [3H]Ro15-1788 could not be displaced by greater than 100 microM of alpidem. These two pieces of evidence strongly suggest the presence of at least three central benzodiazepine binding sites in the spinal cord.

Animals↗
PubMed1992

Characterization of low-energy conformational domains for Met-enkephalin.

An extensive exploration of the conformational hypersurface of Met-enkephalin has been carried out, in order to characterize different low-energy conformational domains accessible to this pentapeptide. The search strategy used consisted of two steps. First, systematic nested rotations were performed using the ECEPP potential. Ninety-two low-energy structures were found and minimized using the CHARMm potential. High and low-temperature molecular dynamics trajectories were then computed for the lowest energy structures in an interative fashion until no lower energy conformers could be found. The same search strategy was used in these studies simulating three different environments, a distance-dependent dielectric epsilon = r, and two constant dielectrics epsilon = 10 and epsilon = 80. The lowest energy structure found in a distance-dependent dielectric is a Gly-Gly beta-II'-type turn. All other structures found for epsilon = r within 10 kcal/mol of this lowest energy structure are also bends. In the more polar environments, the density of conformational states is significantly larger compared to the apolar media. Moreover, fewer hydrogen bonds are formed in the more polar environments, which increases the flexibility of the peptide and results in less structured conformers. Comparisons are made with previous calculations and experimental results.

Amino Acid Sequence↗
PubMed1992

Opioid agonists binding and responses in SH-SY5Y cells.

SH-SY5Y (human neuroblastoma) cultured cells, known to have mu-opioid receptors, have been used to assess and compare the ability of eight representative mu-selective compounds from diverse opioid families to recognize and activate these receptors. A wide range of receptor affinities spanning a factor of 10,000 was found between the highest affinity fentanyl analogs (Ki = 0.1nM) and the lowest affinity analog, meperidine (Ki = 1 microM). A similar range was found for inhibition of PGE1-stimulated cAMP accumulation with a rank order of activities that closely paralleled binding affinities. Maximum inhibition of cAMP accumulation by each compound was about 80%. Maximum stimulation of GTPase activity (approximately 50%) was also similar for all compounds except the lowest affinity meperidine. Both effects were naloxone reversible. These results provide further evidence that mu-receptors are coupled to inhibition of adenylate cyclase and that the SH-SY5Y cell line is a good system for assessment of mu-agonists functional responses.

Alprostadil↗
PubMed1992

Comparison of behavioral and central BDZ binding profile in three rat lines.

The performance of three widely used rat lines (Sprague-Dawley, Wistar, and Long Evans hooded) were evaluated in behavioral test systems that are sensitive to benzodiazepines. The in vivo effects of flunitrazepam and the brain [3H]Ro 15-1788 binding were determined and compared in these rat lines. The behavioral end points evaluated in this study were anxiolysis, measured using the automated elevated plus-maze; sedation by modification of locomotor activity; hyperphagia following food deprivation; protection for pentylenetetrazol-induced convulsions; and hypothermia. There were comparable results in the hypnotic, hypothermic, anticonvulsant, and feeding tests in these lines following flunitrazepam administration. However, the behavior of the Long Evans hooded rat was most amenable to the detection of drug-induced changes in the anxiety test. There was no difference in the maximum number of binding sites (Bmax) or the affinity (Ki) of the Ro 15-1788 or flunitrazepam binding in either the cerebellum or whole brain (minus cerebellum) in the three rat lines as determined by the competitive binding against [3H]Ro 15-1788. Thus, while these rat lines exhibited similar behavioral profiles in most tests the modest differences in the baseline responses and the ability to detect anxiolysis at lower doses of flunitrazepam observed with Long Evans hooded rats makes them particularly suited for these types of studies.

Animals↗
PubMed1992

Homology modeling of a heme protein, lignin peroxidase, from the crystal structure of cytochrome c peroxidase.

A 3-dimensional model of lignin peroxidase (LiP) was constructed based on its sequence homology with other peroxidases, particularly cytochrome c peroxidase, the only protein with a known crystal structure in the peroxidase family. The construction of initial conformations of insertions and deletions was assisted by secondary structure predictions, amphipathic helix predictions, and consideration of the specific protein environment. A succession of molecular dynamics simulations of these regions with surrounding residues as constraints were carried out to relax the bond lengths and angles. Full protein molecular dynamics simulations with explicit consideration of bound waters were performed to relax the geometry and to identify dynamically flexible regions of the successive models for further refinement. Among the important functionally relevant structural features predicted are: (i) four disulfide bonds are predicted to be formed between Cys3 and Cys15, Cys14 and Cys285, Cys34 and Cys120 and Cys249 and Cys317; (ii) a glycosylation site, Asn257, was located on the surface; (iii) Glu40 was predicted to form a salt bridge with Arg43 on the distal side of the heme and was considered as a possible origin for the pH dependence of compound I formation; and (iv) two candidate substrate binding sites with a cluster of surface aromatic residues and flexible backbones were found in the refined model, consistent with the nature of known substrates of LiP. Based on these predicted structural features of the model, further theoretical and experimental studies are proposed to continue to elucidate the structure and function of LiP.

Amino Acid Sequence↗
PubMed1992

Assessment of delta-opioid receptor activation by a series of peptides in cultured cells.

This study was designed to investigate the relative ability of a series of cyclic opioid peptides to initiate the first activation steps following their binding of delta-opioid receptors. The extent of stimulation of low Km guanosine-triphosphatase (GTPase activity) and inhibition of hormonally-stimulated cAMP accumulation in the NG108-15 (neuroblastoma-glioma) hybrid cell line were determined and compared for six closely related peptides. In addition, their binding affinity was assessed by competition with 3H-[D-Pen2D-Pen5]-enkephalin (3H-DPDPE) in membranes from these cells. All peptides tested elicited comparable maximal effects for both functional responses. Different potencies in stimulating the low Km GTPase was observed at sub-maximal agonist concentrations, although the shallow dose-response behavior did not allow accurate determination of ED50s. Estimation of ED50s for inhibition of cAMP accumulation could be made by curve fitting and were similar for four of these peptides, while DCDPE and 3R-methylDCDPE, the highest affinity analogs, were considerably more potent. In general, the observed differences in hormonal activity somewhat parallel the rank order of binding affinities, but no strict relationship was found between receptor binding and activation.

Amino Acid Sequence↗