19F nuclear magnetic resonance investigation of stereoselective binding of isoflurane to bovine serum albumin

X-Ray Crystallographic Studies for Revealing Binding Sites of General Anesthetics in Pentameric Ligand-Gated Ion Channels

X-ray crystallography is a powerful tool in structural biology and can offer insight into structured-based understanding of general anesthetic action on various relevant molecular targets, including pentameric ligand-gated ion channels (pLGICs). In this chapter, we outline the procedures for expression and purification of pLGICs. Optimization of crystallization conditions, especially to achieve high-resolution structures of pLGICs bound with general anesthetics, is also presented. Case studies of pLGICs bound with the volatile general anesthetic isoflurane, 2-bromoethanol, and the intravenous general anesthetic ketamine are revisited.

Solution NMR Studies of Anesthetic Interactions with Ion Channels

NMR spectroscopy is one of the major tools to provide atomic resolution protein structural information. It has been used to elucidate the molecular details of interactions between anesthetics and ion channels, to identify anesthetic binding sites, and to characterize channel dynamics and changes introduced by anesthetics. In this chapter, we present solution NMR methods essential for investigating interactions between ion channels and general anesthetics, including both volatile and intravenous anesthetics. Case studies are provided with a focus on pentameric ligand-gated ion channels and the voltage-gated sodium channel NaChBac.

19F-NMR Diastereotopic Signals in Two N-CHF₂ Derivatives of (4S,7R)-7,8,8-Trimethyl-4,5,6,7-tetrahydro-4,7-methano-2H-indazole

In this paper, we report the anisochrony of the fluorine atoms of a CHF₂ group when linked to a pyrazole ring. The pyrazole is part of (4S,7R)-7,8,8-trimethyl-4,5,6,7-tetrahydro-4,7-methano-2H-indazole also known as (4S,7R)-campho[2,3-c]pyrazole, which has two stereogenic centers. Gauge-Independent Atomic Orbital (GIAO)/Becke, 3-parameter, Lee-Yang-Parr (B3LYP)/6-311++G(d,f) calculated 19F chemical shifts of the minimum energy conformations satisfactorily agree with the experimental data. The energy differences between minima need to consider solvent effects (continuum model) to be satisfactorily reproduced.

19F Nuclear Magnetic Resonance Spectrometric Determination of the Partition Coefficients of Flutamide and Nilutamide (Antiprostate Cancer Drugs) in a Lipid Nano-Emulsion and Prediction of Its Encapsulation Efficiency for the Drugs

To design a useful lipid drug carrier having a high encapsulation efficiency (EE%) for the antiprostate cancer drugs flutamide (FT) and nilutamide (NT), a lipid nano-emulsion (LNE) was prepared with soybean oil (SO), phosphatidylcholine (PC), and sodium palmitate, and the partition coefficients (K _ps) of the drugs for the LNE were determined by ¹⁹F nuclear magnetic resonance (NMR) spectrometry. The ¹⁹F NMR signal of the trifluoromethyl group of both drugs showed a downfield shift from an internal standard (trifluoroethanol) and broadening according to the increase in the lipid concentration due to their interaction with LNE particles. The difference in the chemical shift (Δδ) of each drug caused by the addition of LNE was measured under different amounts of LNE, and the K _p values were calculated from the Δδ values. The results showed that FT has higher lipophilicity than NT. The total lipid concentration (SO + PC) required to encapsulate each drug into LNE with an EE% of more than 95% was calculated from the K _p values as 93.3 and 189.9 mmol/L for FT and NT, respectively. For an LNE prepared with the total lipid concentration of 215 mmol/L, the predicted EE% values were 98 and 96% for FT and NT, respectively, while the experimental EE% values determined by a centrifugation method were approximately 99% for both drugs. Thus, the ¹⁹F NMR spectrometric method is a useful technique to obtain the K _p values of fluorinated drugs and thereby predict the theoretical lipid concentrations and prepare LNEs with high EE% values.

Stereoselectivity of isoflurane in adhesion molecule leukocyte function-associated antigen-1

BACKGROUND

Isoflurane in clinical use is a racemate of S- and R-isoflurane. Previous studies have demonstrated that the effects of S-isoflurane on relevant anesthetic targets might be modestly stronger (less than 2-fold) than R-isoflurane. The X-ray crystallographic structure of the immunological target, leukocyte function-associated antigen-1 (LFA-1) with racemic isoflurane suggested that only S-isoflurane bound specifically to this protein. If so, the use of specific isoflurane enantiomers may have advantage in the surgical settings where a wide range of inflammatory responses is expected to occur. Here, we have further tested the hypothesis that isoflurane enantioselectivity is apparent in solution binding and functional studies.

METHODS

First, binding of isoflurane enantiomers to LFA-1 was studied using 1-aminoanthracene (1-AMA) displacement assays. The binding site of each enantiomer on LFA-1 was studied using the docking program GLIDE. Functional studies employed the flow-cytometry based ICAM binding assay.

RESULTS

Both enantiomers decreased 1-AMA fluorescence signal (at 520 nm), indicating that both competed with 1-AMA and bound to the αL I domain. The docking simulation demonstrated that both enantiomers bound to the LFA-1 "lovastatin site." ICAM binding assays showed that S-isoflurane inhibited more potently than R-isoflurane, consistent with the result of 1-AMA competition assay.

CONCLUSIONS

In contrast with the x-ray crystallography, both enantiomers bound to and inhibited LFA-1. S-isoflurane showed slight preference over R-isoflurane.

Critical role of water in the binding of volatile anesthetics to proteins

Numerous small molecules exhibit drug-like properties by low-affinity binding to proteins. Such binding is known to be influenced by water, the detailed picture of which, however, remains unclear. One particular example is the controversial role of water in the binding of general anesthetics to proteins as an essential step in general anesthesia. Here we demonstrate that a critical amount of hydration water is a prerequisite for anesthetic-protein binding. Using nuclear magnetic resonance, the concurrent adsorption of hydration water and bound anesthetics on model proteins are simultaneously measured. Halothane binding on proteins can only take place after protein hydration reaches a threshold hydration level of ∼0.31 g of water/g of proteins at the relative water vapor pressure of ∼0.95. Similar dependence on hydration is also observed for several other anesthetics. The ratio of anesthetic partial pressures at which two different anesthetics reach the same fractional load is correlated with the anesthetic potency. The binding of nonimmobilizers, which are structurally similar to known anesthetics but unable to produce anesthesia, does not occur even after the proteins are fully hydrated. Our results provide the first unambiguous experimental evidence that water is absolutely required to enable anesthetic-protein interactions, shedding new light on the general mechanism of molecular recognition and binding.

Molecular mechanisms of drug action: an emerging view

Volatile anesthetics serve as useful probes of a conserved biological process that is essential to the proper functioning of the central nervous system. A kinetic and thermodynamic analysis of their unusual pharmacological and physiological characteristics has led to a general, predictive theory in which small molecules that adsorb to membranes modulate ion channel function by altering physical properties of membrane bilayers. A kinetic model that is both parsimonious and falsifiable has been developed to test this mechanism. This theory leads to predictions about the structure, function, origin, and evolution of synapses, the etiology of several diseases and disease symptoms affecting the brain, and the mechanism of action of several drugs that are used therapeutically. Neuronal membranes may offer an appealing drug target, given the large number of compounds that adsorb to interfaces and hence membranes.

The interaction of fatty acid amide hydrolase (FAAH) inhibitors with an anandamide carrier protein using (19)F-NMR

It has been reported that the endocannabinoid anandamide (AEA) binds to a class of fatty acid-binding proteins and serum albumin which can serve as carrier proteins and potentiate the cellular uptake of AEA and its intracellular translocation. Here, we employed (19)F nuclear magnetic resonance spectroscopy to study the interactions of serum albumin with two inhibitors of fatty acid amide hydrolase (FAAH), the enzyme involved in the deactivation of anandamide. We found that, for both inhibitors AM5206 and AM5207, the primary binding site on serum albumin is drug site 1 located at subdomain IIA. Neither inhibitor binds to drug site 2. While AM5207 binds exclusively to drug site 1, AM5206 also interacts with other fatty acid-binding sites on serum albumin. Additionally, AM5206 has an affinity for serum albumin approximately one order of magnitude higher than that of AM5207. The data suggest that interactions of FAAH inhibitors with albumin may provide added advantages for their ability to modulate endocannabinoid levels for a range of applications including analgesia, antiemesis, and neuroprotection.

Crystal structure of isoflurane bound to integrin LFA-1 supports a unified mechanism of volatile anesthetic action in the immune and central nervous systems

Volatile anesthetics (VAs), such as isoflurane, induce a general anesthetic state by binding to specific targets (i.e., ion channels) in the central nervous system (CNS). Simultaneously, VAs modulate immune functions, possibly via direct interaction with alternative targets on leukocytes. One such target, the integrin lymphocyte function-associated antigen-1 (LFA-1), has been shown previously to be inhibited by isoflurane. A better understanding of the mechanism by which isoflurane alters protein function requires the detailed information about the drug-protein interaction at an atomic level. Here, we describe the crystal structure of the LFA-1 ligand-binding domain (I domain) in complex with isoflurane at 1.6 A. We discovered that isoflurane binds to an allosteric cavity previously implicated as critical for the transition of LFA-1 from the low- to the high-affinity state. The isoflurane binding site in the I domain involves an array of amphiphilic interactions, thereby resembling a "common anesthetic binding motif" previously predicted for authentic VA binding sites. These results suggest that the allosteric modulation of protein function by isoflurane, as demonstrated for the integrin LFA-1, might represent a unified mechanism shared by the interactions of volatile anesthetics with targets in the CNS.

Volatile anesthetic binding to proteins is influenced by solvent and aliphatic residues

The main objective of this work was to characterize VA binding sites in multiple anesthetic target proteins. A computational algorithm was used to quantify the solvent exclusion and aliphatic character of amphiphilic pockets in the structures of VA binding proteins. VA binding sites in the protein structures were defined as the pockets with solvent exclusion and aliphatic character that exceeded minimum values observed in the VA binding sites of serum albumin, firefly luciferase, and apoferritin. We found that the structures of VA binding proteins are enriched in these pockets and that the predicted binding sites were consistent with experimental determined binding locations in several proteins. Autodock3 was used to dock the simulated molecules of 1,1,1,2,2-pentafluoroethane, difluoromethyl 1,1,1,2-tetrafluoroethyl ether, and sevoflurane and the isomers of halothane and isoflurane into these potential binding sites. We found that the binding of the various VA molecules to the amphiphilic pockets is driven primarily by VDW interactions and to a lesser extent by weak hydrogen bonding and electrostatic interactions. In addition, the trend in Delta G binding values follows the Meyer-Overton rule. These results suggest that VA potencies are related to the VDW interactions between the VA ligand and protein target. It is likely that VA bind to sites with a high degree of solvent exclusion and aliphatic character because aliphatic residues provide favorable VDW contacts and weak hydrogen bond donors. Water molecules occupying these sites maintain pocket integrity, associate with the VA ligand, and diminish the unfavorable solvation enthalpy of the VA. Water molecules displaced into the bulk by the VA ligand may provide an additional favorable enthalpic contribution to VA binding. Anesthesia is a component of many health related procedures, the outcomes of which could be improved with a better understanding of the molecular targets and mechanisms of anesthetic action.

Four-alpha-helix bundle with designed anesthetic binding pockets. Part II: halothane effects on structure and dynamics

As a model of the protein targets for volatile anesthetics, the dimeric four-alpha-helix bundle, (Aalpha(2)-L1M/L38M)(2), was designed to contain a long hydrophobic core, enclosed by four amphipathic alpha-helices, for specific anesthetic binding. The structural and dynamical analyses of (Aalpha(2)-L1M/L38M)(2) in the absence of anesthetics (another study) showed a highly dynamic antiparallel dimer with an asymmetric arrangement of the four helices and a lateral accessing pathway from the aqueous phase to the hydrophobic core. In this study, we determined the high-resolution NMR structure of (Aalpha(2)-L1M/L38M)(2) in the presence of halothane, a clinically used volatile anesthetic. The high-solution NMR structure, with a backbone root mean-square deviation of 1.72 A (2JST), and the NMR binding measurements revealed that the primary halothane binding site is located between two side-chains of W15 from each monomer, different from the initially designed anesthetic binding sites. Hydrophobic interactions with residues A44 and L18 also contribute to stabilizing the bound halothane. Whereas halothane produces minor changes in the monomer structure, the quaternary arrangement of the dimer is shifted by about half a helical turn and twists relative to each other, which leads to the closure of the lateral access pathway to the hydrophobic core. Quantitative dynamics analyses, including Modelfree analysis of the relaxation data and the Carr-Purcell-Meiboom-Gill transverse relaxation dispersion measurements, suggest that the most profound anesthetic effect is the suppression of the conformational exchange both near and remote from the binding site. Our results revealed a novel mechanism of an induced fit between anesthetic molecule and its protein target, with the direct consequence of protein dynamics changing on a global rather than a local scale. This mechanism may be universal to anesthetic action on neuronal proteins.

(19)F NMR spectroscopic characterization of the interaction of niflumic acid with human serum albumin

The interaction of a non-steroidal anti-inflammatory drug, niflumic acid (NFA), with human serum albumin (HSA) has been investigated by (19)F nuclear magnetic resonance (NMR) spectroscopy. A (19)F NMR spectrum of NFA in a buffered (pH 7.4) solution of NaCl (0.1 mol L(-1)) contained a single sharp signal of its CF(3) group 14.33 ppm from the internal reference 2,2,2-trifluoroethanol. Addition of 0.6 mmol L(-1) HSA to the NFA buffer solution caused splitting of the CF(3) signal into two broadened signals, shifted to the lower fields of 14.56 and 15.06 ppm, with an approximate intensity ratio of 1:3. Denaturation of HSA by addition of 3.0 mol L(-1) guanidine hydrochloride (GU) restored a single sharp signal of CF(3) at 14.38 ppm, indicating complete liberation of NFA from HSA as a result of its denaturation. These results suggest that the binding is reversible and occurs in at least two HSA regions. Competitive (19)F NMR experiments using warfarin, dansyl-L: -asparagine, and benzocaine (site I ligands), and L: -tryptophan and ibuprofen (site II ligands) revealed that NFA binds to site I at two different regions, Ia and Ib, in the ratio 1:3. By use of (19)F NMR with NFA as an (19)F NMR probe the nonfluorinated site I-binding drugs sulfobromophthalein and iophenoxic acid were also found to bind sites Ia and Ib, respectively. These results illustrate the usefulness and convenience of (19)F NMR for investigation of the HSA binding of both fluorinated and nonfluorinated drugs.

Kinetics of anesthetic-induced conformational transitions in a four-alpha-helix bundle protein

Inhaled anesthetics are thought to alter the conformational states of Cys-loop ligand-gated ion channels (LGICs) by binding within discrete cavities that are lined by portions of four alpha-helical transmembrane domains. Because Cys-loop LGICs are complex molecules that are notoriously difficult to express and purify, scaled-down models have been used to better understand the basic molecular mechanisms of anesthetic action. In this study, stopped-flow fluorescence spectroscopy was used to define the kinetics with which inhaled anesthetics interact with (Aalpha(2)-L1M/L38M)(2), a four-alpha-helix bundle protein that was designed to model anesthetic binding sites on Cys-loop LGICs. Stopped-flow fluorescence traces obtained upon mixing (Aalpha(2)-L1M/L38M)(2) with halothane revealed immediate, fast, and slow components of quenching. The immediate component, which occurred within the mixing time of the spectrofluorimeter, was attributed to direct quenching of tryptophan fluorescence upon halothane binding to (Aalpha(2)-L1M/L38M)(2). This was followed by a biexponential fluorescence decay containing fast and slow components, reflecting anesthetic-induced conformational transitions. Fluorescence traces obtained in studies using sevoflurane, isoflurane, and desflurane, which poorly quench tryptophan fluorescence, did not contain the immediate component. However, these anesthetics did produce the fast and slow components, indicating that they also alter the conformation of (Aalpha(2)-L1M/L38M)(2). Cyclopropane, an anesthetic that acts with unusually low potency on Cys-loop LGICs, acted with low apparent potency on (Aalpha(2)-L1M/L38M)(2). These results suggest that four-alpha-helix bundle proteins may be useful models of in vivo sites of action that allow the use of a wide range of techniques to better understand how anesthetic binding leads to changes in protein structure and function.

Anesthetic interaction with ketosteroid isomerase: insights from molecular dynamics simulations

The nature and the sites of interactions between anesthetic halothane and homodimeric Delta5-3-ketosteroid isomerase (KSI) are characterized by flexible ligand docking and confirmed by 1H-15N NMR. The dynamics consequence of halothane interaction and the implication of the dynamic changes to KSI function are studied by multiple 5-ns molecular dynamics simulations in the presence and absence of halothane. Both docking and MD simulations show that halothane prefer the amphiphilic dimeric interface to the hydrophobic active site of KSI. Halothane occupancy at the dimer interface disrupted the intersubunit hydrogen bonding formed either directly through side chains of polar residues or indirectly through the mediation of the interfacial water molecules. Moreover, in the presence of halothane, the exchange rate of the bound waters with bulk water was increased. Halothane perturbation to the dimer interface affected the overall flexibility of the active site. This action is likely to contribute to the halothane-induced reduction of the KSI activity. The allosteric halothane modulation of the dynamics-function relationship of KSI without direct competition at the enzymatic active sites may be generalized to offer a unifying explanation of anesthetic action on a diverse range of multidomain neuronal proteins that are potentially relevant to clinical general anesthesia.

19F NMR spectroscopic study on the binding of triflupromazine to bovine and human serum albumins

The 19F NMR spectrum of triflupromazine hydrochloride (TFZ) in a buffer solution (pH 6.8) showed a single sharp signal of the TFZ CF3 group at 13.5 ppm from the external trifluoroacetic acid. The addition of 1 mM HSA or BSA to the sample solution caused a split of the CF3 signal into two broadened signals shifted to slightly lower (0.2 ppm) and higher (0.7 ppm) fields, respectively, from the original position. Denaturation of the albumins by guanidine hydrochloride (3M) restored the two broadened signals to a slightly broadened single signal, indicating that TFZ has at least two binding sites on HSA and BSA, respectively. From the competitive binding 19F NMR experiments using Warfarin (Site-I ligand), l-tryptophan (Site-II ligand), NaCl, and oleate, the signal at high field was assigned to the TFZ bound to Site II. Comparison of the signal intensity revealed that the affinity of TFZ for Site II on HSA was considerably higher than that on BSA. The low-field signal could be identified as a weight-averaged signal between nonspecifically bound TFZ to HSA (BSA) and free TFZ in the water phase. In the presence of physiological concentrations of NaCl, major binding of TFZ to HSA and BSA was considered to be nonspecific. The present work indicates that 19F NMR is very useful for obtaining important detailed information regarding the binding of fluorinated drugs to serum albumins.

Binding of the volatile general anesthetics halothane and isoflurane to a mammalian beta-barrel protein

A molecular understanding of volatile anesthetic mechanisms of action will require structural descriptions of anesthetic-protein complexes. Porcine odorant binding protein is a 157 residue member of the lipocalin family that features a large beta-barrel internal cavity (515 +/- 30 angstroms(3)) lined predominantly by aromatic and aliphatic residues. Halothane binding to the beta-barrel cavity was determined using fluorescence quenching of Trp16, and a competitive binding assay with 1-aminoanthracene. In addition, the binding of halothane and isoflurane were characterized thermodynamically using isothermal titration calorimetry. Hydrogen exchange was used to evaluate the effects of bound halothane and isoflurane on global protein dynamics. Halothane bound to the cavity in the beta-barrel of porcine odorant binding protein with dissociation constants of 0.46 +/- 0.10 mM and 0.43 +/- 0.12 mM determined using fluorescence quenching and competitive binding with 1-aminoanthracene, respectively. Isothermal titration calorimetry revealed that halothane and isoflurane bound with K(d) values of 80 +/- 10 microM and 100 +/- 10 microM, respectively. Halothane and isoflurane binding resulted in an overall stabilization of the folded conformation of the protein by -0.9 +/- 0.1 kcal.mol(-1). In addition to indicating specific binding to the native protein conformation, such stabilization may represent a fundamental mechanism whereby anesthetics reversibly alter protein function. Because porcine odorant binding protein has been successfully analyzed by X-ray diffraction to 2.25 angstroms resolution [1], this represents an attractive system for atomic-level structural studies in the presence of bound anesthetic. Such studies will provide much needed insight into how volatile anesthetics interact with biological macromolecules.

Headspace gas chromatography-mass spectrometry analysis of isoflurane enantiomers in blood samples after anesthesia with the racemic mixture

Several in vivo and in vitro studies on the stereoselective potency of isoflurane enantiomers suggest beneficial effects of the (+)-(S)-enantiomer. In order to detect possible differences in the pharmacokinetics of isoflurane enantiomers, a clinical study of 41 patients undergoing general anesthesia maintained with racemic isoflurane was performed. The isoflurane enantiomers were analyzed in blood samples drawn before induction, at cessation (day 0), and up to eight days after isoflurane anesthesia (day 1-8). A multipurpose sampler (Gerstel MPS) was used for the headspace gas chromatography-mass spectrometry (GC/MS) analysis, and it was combined with a cold injection system (Gerstel CIS 3) for coldtrapping, enrichment, and focusing of the analyte. The enantiomer separation was achieved by using a capillary column coated with octakis(3-O-butanoyl-2,6-di-O-pentyl)-gamma-cyclodextrin (Lipodex E) dissolved in the polysiloxane PS 255. Detection was done in the selected ion monitoring mode with ions m/z 117 and m/z 149. An enrichment of (+)-(S)-isoflurane in all blood samples drawn after anesthesia was found. The highest enantiomer bias, up to 52-54% (+)-(S)-isoflurane as compared to 50% for the racemate, was observed on day 2 for most of the patients. Furthermore, quantification of isoflurane in blood samples of five patients was done by enantiomer labeling, employing enantiomerically pure (+)-(S)-isoflurane as internal standard. The isoflurane concentration decreased rapidly from 383 nmol/ml to 0.6 nmol/ml (mean values) eight days after anesthesia. The present study shows differences in the pharmacokinetics of isoflurane enantiomers in man. However, it is not possible to distinguish between enantioselective distribution and enantioselective metabolism, if any.

The impact of chirality of the fluorinated volatile inhalation anaesthetics on their clinical applications

This review discusses the various chromatographic enantioseparation methods on an analytical and preparative scale for fluorinated inhalation anaesthetics used clinically, namely halothane, enflurane, desflurane and isoflurane. The differences in the pharmacodynamics and pharmacokinetics between the enantiomers of those anaesthetics are presented. It can be concluded that using a single enantiomer for these fluorinated anaesthetics is advantageous over using the racemic mixture. The racemic switch to a single enantiomer for these fluorinated volatile anaesthetics offers a more effective and safe general anaesthetic.

NMR study of volatile anesthetic binding to nicotinic acetylcholine receptors

New lines of evidence suggest that volatile anesthetics interact specifically with proteins. Direct binding analysis, however, has been largely limited to soluble proteins. In this study, specific interaction was investigated between isoflurane, a clinically important volatile anesthetic, and membrane-bound nicotinic acetylcholine receptors (nAChRs) from Torpedo electroplax, using (19)F nuclear magnetic resonance spectroscopy and gas chromatography. The receptors were reconstituted into 1, 2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipid vesicles. After correcting for nonspecific partitioning into the lipid, the equilibrium dissociation constant, K(d), of isoflurane binding to nAChR at 15 degrees C was found to be 0.36 +/- 0.03 mM. This value is within the clinically relevant concentration range of the agent. Based on the receptor concentrations in the vesicle suspension assayed by the bicinchoninic acid method and the fraction of bound isoflurane, X(b), determined by gas chromatography, an estimate of an average of 9-10 specifically bound isoflurane molecules can be made for each receptor, or two for each subunit. Upon binding, the transverse relaxation time constant (T(2)) of (19)F resonance of isoflurane is decreased by nearly three orders of magnitude, indicating a dramatic reduction in the mobility of specifically bound isoflurane. Kinetic analysis reveals that the off rate of binding, k(-1), is 1.7 x 10(4) s(-1). The on rate, k(+1), can thus be calculated to be approximately 4.8 x 10(7) M(-1) s(-1), suggesting a nearly diffusion-limited association. This is in contrast to anesthetic binding to a soluble protein, bovine serum albumin (BSA), where k(+1) and k(-1) are at least an order of magnitude slower. It is concluded that the presence of lipids may be critical for the correct evaluation of binding kinetics between volatile anesthetics and neuronal receptors.

Quantitative determination of isoflurane enantiomers in blood samples during and after surgery via headspace gas chromatography-mass spectrometry

The quantitative analysis of the chiral volatile anesthetic isoflurane (1) for biomedical applications by means of enantioselective gas chromatography (mass sensitive detector, selected ion monitoring) was studied. Two methods for the quantification of the enantiomers in blood samples drawn during and after narcosis were compared. Either the isomeric enflurane (2) was selected as an internal standard or a single enantiomer of 1 was used for the standard addition method, an approach referred to as 'enantiomer labeling'. Concentrations up to 0.3 micromol/l of the single enantiomers could be differentiated two days after anesthesia. The presented data imply that the body clearance for (+)-(S)-1 and (-)-(R)-1 proceeds to a measurable degree of enantioselectivity.

Molecular dynamics simulation of a synthetic four-alpha-helix bundle that binds the anesthetic halothane

The structural features of binding sites for volatile anesthetics are examined by performing a molecular dynamics simulation study of the synthetic four-alpha-helix bundles (Aalpha2)2, which are formed by association of two 62-residue di-alpha-helical peptides. The peptide bundle (Aalpha2)2 was designed by Johansson et al. [Biochemistry 37 (1998) 1421-1429] and was shown experimentally to have a high affinity for the binding of the anesthetic halothane (CF3CBrCIH) in a hydrophobic cavity. Since (Aalpha2)2 can exhibit either the anti or syn topologies, the two distinct bundles are simulated both in the presence and in the absence of halothane. Nanosecond length molecular dynamics trajectories were generated for each system at room temperature (T = 298 K). The structural and dynamic effects of the inclusion of halothane are compared, illustrating that the structures are stable over the course of the simulation; that the (Aalpha2)2 bundles have suitable pockets that can accommodate halothane; that the halothane remains in the designed hydrophobic cavity in close proximity to the Trp residues with a preferred orientation; and that the dimensions of the peptide are perturbed by the inclusion of an anesthetic molecule.

Concentration-dependent isoflurane effects on depolarization-evoked glutamate and GABA outflows from mouse brain slices

The synaptic concentrations of glutamate and gamma-aminobutyric acid (GABA) are modulated by their release and re-uptake. The effects of general anaesthetics on these two processes remain unclear. This study evaluates the effects of isoflurane, a clinically important anaesthetic, on glutamate and GABA release and re-uptake in superfused mouse cerebrocortical slices. Experiments consisted of two 1.5-min exposures to 40 mM KCl in 30 min intervals. During the second exposure, different concentrations of isoflurane with and without 0.3 mM L-transpyrrolidine-2,4-dicarboxylic acid (PDC, a competitive inhibitor of glutamate uptake transporter) or 1 mM nipecotic acid (a competitive inhibitor of GABA uptake transporter) were introduced. The ratios of the second to first KCl-evoked increases in glutamate and GABA were used to determine the isoflurane concentration-response curves. The results can be described as a sum of two independent processes, corresponding to the inhibitions of release and re-uptake, respectively. The EC50 values for the inhibitions of release and re-uptake were 295+/-16 and 805+/-43 microM for glutamate, and 229+/-13 and 520+/-25 microM for GABA, respectively. Addition of PDC did not significantly affect glutamate release but shifted the re-uptake curve to the left (EC50= 315+/-20 microM). Nipecotic acid completely blocked GABA uptake, rendering isoflurane inhibition of GABA re-uptake undetectable. Our data suggest that isoflurane inhibits both the release and re-uptake of neurotransmitters and that the inhibitions occur at different EC50's. For GABA, both EC50's are within the clinical concentration range. The net anaesthetic effect on extracellular concentrations of neurotransmitters, particularly GABA, depends on the competition between inhibition of release and that of re-uptake.

Nitrous oxide and carbon dioxide have no effect on the blood-gas solubilities of sevoflurane and isoflurane

Nitrous oxide (N2O) has been shown to decrease the solubility (lambdaB:G) of volatile anesthetics in human blood and, consequently, affect their rate of uptake. If this is true, then carbon dioxide (CO2) may also have an effect, which is important because methods that measure the tension of volatile anesthetics in blood washout CO2 in the process. Blood samples were obtained from fasted, healthy volunteers and patients undergoing major surgery. Each sample was divided into two aliquots: one was equilibrated at 37 degrees C in a closed glass tonometer with a mixture of isoflurane 1% and sevoflurane 2% in a test gas mixture of either 50:50 N2O/O2 or 5:95 CO2/O2; the other aliquot was equilibrated with isoflurane and sevoflurane in O2 alone as a control. Using a two-stage headspace technique using gas chromatography, we measured the lambdaB:G of isoflurane and sevoflurane in the presence and absence of the test gas in each subject. There was no significant difference between the lambdaB:G of sevoflurane and isoflurane obtained from the N2O group and their controls or between the CO2 group and their controls. We conclude that neither N2O nor CO2 has an effect on the lambdaB:G of sevoflurane or isoflurane in the concentrations tested.

IMPLICATIONS

The blood solubilities of sevoflurane and isoflurane were measured with and without nitrous oxide and carbon dioxide. No differences were found. Nitrous oxide does not affect the kinetics of other anesthetics by altering their solubility. Carbon dioxide tensions need not be controlled when measuring anesthetic tensions in blood.

Comparison of anaesthetic and non-anaesthetic effects on depolarization-evoked glutamate and GABA release from mouse cerebrocortical slices

1. Investigation with substances that are similar in structure, but different in anaesthetic properties, may lead to further understanding of the mechanisms of general anaesthesia. 2. We have studied the effects of two cyclobutane derivatives, the anaesthetic, 1-chloro-1,2,2-trifluorocyclobutane (F3), and the non-anaesthetic, 1,2-dichlorohexafluorocyclobutane (F6), on K+-evoked glutamate and gamma-aminobutyric acid (GABA) release from isolated, superfused, cerebrocortical slices from mice, by use of h.p.l.c. with fluorescence detection for quantitative analysis. 3. At clinically relevant concentrations, the anaesthetic, F3, inhibited 40 mM K+-evoked glutamate and GABA release by 72% and 47%, respectively, whereas the structurally similar non-anaesthetic, F6, suppressed evoked glutamate release by 70% but had no significant effects on evoked GABA release. A second exposure to 40 mM KCl after a approximately 30 min washout of F3 or F6 showed recovery of K+-evoked release, suggesting that F3 and F6 did not cause any non-specific or irreversible changes in the brain slices. 4. Our findings suggest that suppression of excitatory neurotransmitter release may not be directly relevant to the primary action of general anaesthetics. A mechanism involving inhibitory postsynaptic action is implicated, in which a moderate suppression of depolarization-evoked GABA release by the anaesthetic may be consistent with the enhancement of postsynaptic GABAergic activities.

Binding of the volatile anesthetic chloroform to albumin demonstrated using tryptophan fluorescence quenching

The site(s) of action of the volatile general anesthetics remain(s) controversial, but evidence in favor of specific protein targets is accumulating. The techniques to measure directly volatile anesthetic binding to proteins are still under development. Further experience with the intrinsic protein fluorescence quenching approach to monitor anesthetic-protein complexation is reported using chloroform. Chloroform quenches the steady-state tryptophan fluorescence of bovine serum albumin (BSA) in a concentration-dependent, saturable manner with a Kd = 2.7 +/- 0.2 mM. Tryptophan fluorescence lifetime analysis reveals that the majority of the quenching is due to a static mechanism, indicative of anesthetic binding. The ability of chloroform to quench BSA tryptophan fluorescence was decreased markedly in the presence of 50% 2,2,2-trifluoroethanol, which causes loss of tertiary structural contacts in BSA, indicating that protein conformation is crucial for anesthetic binding. Circular dichroism spectroscopy revealed no measurable effect of chloroform on the secondary structure of BSA. The results suggest that chloroform binds to subdomains IB and IIA in BSA, each of which contains a single tryptophan. Earlier work has shown that these sites are also occupied by halothane. The present study therefore provides experimental support for the theory that structurally distinct general anesthetics may occupy the same domains on protein targets.

Minimum alveolar anesthetic concentration values for the enantiomers of isoflurane differ minimally

Results of in vivo and in vitro studies of the anesthetic potencies of the enantiomers (optical isomers) of isoflurane provide various results ranging from no difference to differences of nearly two fold. A finding of a difference in anesthetic requirement in the whole animal has particular relevance to theories of anesthetic mechanisms of action because it suggests that anesthesia may result from a specific anesthetic-receptor interaction. This led to our decision to redetermine the minimum alveolar anesthetic concentration (MAC) of (+)-S and (-)-R enantiomers of isoflurane in 12 Sprague-Dawley rats (six per group). The (+)-S enantiomer gave a MAC of 0.0144 +/- 0.0012 atm (i.e., 1.44% +/- 0.12% at 1 atm pressure; mean +/- SD) and the (-)-R enantiomer gave a MAC of 0.0169 +/- 0.0020 atm. Although the 17% greater value for the (-)-R enantiomer is qualitatively consistent with previous results the difference is not significant (P = 0.06), and the absolute difference is smaller than that found by a previous study. However, given the small sample size, our power to define a small significant difference is limited. Regardless of statistical significance, our results do not confirm the conclusion that interaction with a specific receptor is important to the mechanism of action of inhaled anesthetics.

Different distribution of fluorinated anesthetics and nonanesthetics in model membrane: a 19F NMR study

Despite their structural resemblance, a pair of cyclic halogenated compounds, 1-chloro-1,2,2-trifluorocyclobutane (F3) and 1,2-dichlorohexafluorocyclobutane (F6), exhibit completely different anesthetic properties. Whereas the former is a potent general anesthetic, the latter produces no anesthesia. Two linear compounds, isoflurane and 2,3-dichlorooctofluorobutane (F8), although not a structural pair, also show the same anesthetic discrepancy. Using 19F nuclear magnetic spectroscopy, we investigated the time-averaged submolecular distribution of these compounds in a vesicle suspension of phosphatidylcholine lipids. A two-site exchange model was used to interpret the observed changes in resonance frequencies as a function of the solubilization of these compounds in membrane and in water. At clinically relevant concentrations, the anesthetics F3 and isoflurane distributed preferentially to regions of the membrane that permit easy contact with water. The frequency changes of these two anesthetics can be well characterized by the two-site exchange model. In contrast, the nonanesthetics F6 and F8 solubilized deeply into the lipid core, and their frequency change significantly deviated from the prediction of the model. It is concluded that although anesthetics and nonanesthetics may show similar hydrophobicity in bulk solvents such as olive oil, their distributions in various regions in biomembranes, and hence their effective concentrations at different submolecular sites, may differ significantly.