How to Personalize General Anesthesia-A Prospective Theoretical Approach to Conformational Changes of Halogenated Anesthetics in Fire Smoke Poisoning

Smoke intoxication is a central event in mass burn incidents, and toxic smoke acts at different levels of the body, blocking breathing and oxygenation. The majority of these patients require early induction of anesthesia to preserve vital functions. We studied the influence of hemoglobin (HMG) and myoglobin (MGB) blockade by hydrochloric acid (HCl) in an interaction model with gaseous anesthetics using molecular docking techniques. In the next part of the study, molecular dynamics (MD) simulations were performed on the top-scoring ligand-receptor complexes to investigate the stability of the ligand-receptor complexes and the interactions between ligands and receptors in more detail. Through docking analysis, we observed that hemoglobin creates more stable complexes with anesthetic gases than myoglobin. Intoxication with gaseous hydrochloric acid produces conformational and binding energy changes of anesthetic gases to the substrate (both the pathway and the binding site), the most significant being recorded in the case of desflurane and sevoflurane, while for halothane and isoflurane, they remain unchanged. According to our theoretical model, the selection of anesthetic agents for patients affected by fire smoke containing hydrochloric acid is critical to ensure optimal anesthetic effects. In this regard, our model suggests that halothane and isoflurane are the most suitable choices for predicting the anesthetic effects in such patients when compared to sevoflurane and desflurane.

Molecular dynamics simulation study of the effect of halothane on mixed DPPC/DPPE phospholipid membranes

We report results of a molecular dynamics simulation study of the effect of one general anesthetic, halothane, on some properties of mixed DPPC/DPPE phospholipid membranes. This is a suitable model for the study of simple, two-phospholipid membrane systems. From the simulation runs, we determined several membrane properties for five different molecular proportions of DPPC/DPPE. The effect of halothane on the studied membrane properties (area per lipid molecule, density of membrane, order parameter, etc.) was rather small. The distribution of halothane is not uniform through the bilayer thickness. Instead, there is a maximum of anesthetic concentration around 1.2 nm from the center of the membrane. The anesthetic molecule is located close to the phospholipid headgroups. The position of the halothane density maximum depends slightly on the DPPC/DPPE molar proportion. Snapshots taken over the plane of the membrane, as well as calculated two-dimensional radial distribution functions show that the anesthetic has no preference for either phospholipid (DPPC or DPPE). Our results indicate that this anesthetic molecule has only small effects on DPPC/DPPE mixed membranes. In addition, halothane displays no preferential location around DPPC or DPPE. This is probably due to the hydrophobic nature of halothane and to the fact that the chosen phospholipids have the same hydrophobic tails.

Hydrostatic Pressure Promotes Domain Formation in Model Lipid Raft Membranes

Neutron diffraction measurements demonstrate that hydrostatic pressure promotes liquid-ordered (Lo) domain formation in lipid membranes prepared as both oriented multilayers and unilamellar vesicles made of a canonical ternary lipid mixture for which demixing transitions have been extensively studied. The results demonstrate an unusually large dependence of the mixing transition on hydrostatic pressure. Additionally, data at 28 °C show that the magnitude of increase in Lo caused by 10 MPa pressure is much the same as the decrease in Lo produced by twice minimum alveolar concentrations (MAC) of general anesthetics such as halothane, nitrous oxide, and xenon. Therefore, the results may provide a plausible explanation for the reversal of general anesthesia by hydrostatic pressure.

Emulsified halothane produces long-term epidural anesthetic effect: a study in rabbits

Previous studies have demonstrated that volatile anesthetics could produce local anesthesia. Emulsified isoflurane at 8% has been reported to produce epidural anesthetic effect in rabbits. This study was designed to investigate the long-term epidural anesthetic effect of emulsified halothane in rabbits. In this study, 40 healthy adult rabbits (weighting 2.0-2.5 kg) with an epidural catheter were randomly divided into 4 groups (n=10/group), receiving epidural administration of 1% lidocaine (lido group), 8% emulsified isoflurane 1ml (8% E-iso group), 8% emulsified halothane (8% E-Halo group) and 12% emulsified halothane (12% E-Halo group). After administration, sensory and motor functions as well as consciousness state were assessed until 60 minutes after sensory and motor function returned to its baseline or at least for 180 min. After epidural anesthesia, all the rabbits were continuously observed for 7 days and sacrificed for pathological evaluations. As a result, all the four study solutions produced typical epidural anesthesia. Onset times of sensory and motor function blockade were similar among the four groups (P>0.05). Duration of sensory blockade in 12% E-Halo group (83±13 min) was significantly longer than other groups: 51±12 min in 8% E-Halo group (P<0.01), 57±8 min in 8% E-iso group (P<0.01) and 47±9 min in lido group (P<0.01). Duration of sensory blockade in 8% E-iso group is longer than lido group (P<0.05). Duration of motor blockade in 12% E-Halo group (81±12 min) was also significantly longer than other groups: 40±8 min in 8% E-Halo group (P<0.01), 37±3 min in 8% E-iso group (P<0.01), 37±6 min in lido group (P<0.01). Normal consciousness was found in the rabbits from 8% E-Halo, 8% E-iso and lido groups while there were four rabbits in 12% E-Halo group (4/10) showed a light sedation. For all the rabbits, no pathological injury was found. The present study demonstrates that emulsified halothane produces reversible concentration-dependent epidural anesthesia and at 12% (v/v), emulsified halothane could produce long-term anesthesia without pathological injury.

NMR structures of the human α7 nAChR transmembrane domain and associated anesthetic binding sites

The α7 nicotinic acetylcholine receptor (nAChR), assembled as homomeric pentameric ligand-gated ion channels, is one of the most abundant nAChR subtypes in the brain. Despite its importance in memory, learning and cognition, no structure has been determined for the α7 nAChR TM domain, a target for allosteric modulators. Using solution state NMR, we determined the structure of the human α7 nAChR TM domain (PDB ID: 2MAW) and demonstrated that the α7 TM domain formed functional channels in Xenopus oocytes. We identified the associated binding sites for the anesthetics halothane and ketamine; the former cannot sensitively inhibit α7 function, but the latter can. The α7 TM domain folds into the expected four-helical bundle motif, but the intra-subunit cavity at the extracellular end of the α7 TM domain is smaller than the equivalent cavity in the α4β2 nAChRs (PDB IDs: 2LLY; 2LM2). Neither drug binds to the extracellular end of the α7 TM domain, but two halothane molecules or one ketamine molecule binds to the intracellular end of the α7 TM domain. Halothane and ketamine binding sites are partially overlapped. Ketamine, but not halothane, perturbed the α7 channel-gate residue L9'. Furthermore, halothane did not induce profound dynamics changes in the α7 channel as observed in α4β2. The study offers a novel high-resolution structure for the human α7 nAChR TM domain that is invaluable for developing α7-specific therapeutics. It also provides evidence to support the hypothesis: only when anesthetic binding perturbs the channel pore or alters the channel motion, can binding generate functional consequences.

Effects of isoflurane, halothane, and chloroform on the interactions and lateral organization of lipids in the liquid-ordered phase

The first quantitative insight has been obtained into the effects that volatile anesthetics have on the interactions and lateral organization of lipids in model membranes that mimic "lipid rafts". Specifically, nearest-neighbor recogntion measurements, in combination with Monte Carlo simulations, have been used to investigate the action of isoflurane, halothane, and chloroform on the compactness and lateral organization of cholesterol-rich bilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in the liquid-ordered (l(o)) phase. All three anesthetics induce a similar weakening of sterol-phospholipid association, corresponding to ca. 30 cal/mol of lipid at clinically relevant concentrations. Monte Carlo lattice simulations show that the lateral organization of the l(o) phase, under such conditions, remains virtually unchanged. In sharp contrast to their action on the l(o) phase, these anesthetics have been found to have a similar strengthening effect on sterol-phospholipid association in the liquid-disordered (l(d)) phase. The possibility of discrete complexes being formed between DPPC and these anesthetics and the biological relevance of these findings are discussed.

Molecular dynamics and brownian dynamics investigation of ion permeation and anesthetic halothane effects on a proton-gated ion channel

Bacterial Gloeobacter violaceus pentameric ligand-gated ion channel (GLIC) is activated to cation permeation upon lowering the solution pH. Its function can be modulated by anesthetic halothane. In the present work, we integrate molecular dynamics (MD) and Brownian dynamics (BD) simulations to elucidate the ion conduction, charge selectivity, and halothane modulation mechanisms in GLIC, based on recently resolved X-ray crystal structures of the open-channel GLIC. MD calculations of the potential of mean force (PMF) for a Na(+) revealed two energy barriers in the extracellular domain (R109 and K38) and at the hydrophobic gate of transmembrane domain (I233), respectively. An energy well for Na(+) was near the intracellular entrance: the depth of this energy well was modulated strongly by the protonation state of E222. The energy barrier for Cl(-) was found to be 3-4 times higher than that for Na(+). Ion permeation characteristics were determined through BD simulations using a hybrid MD/continuum electrostatics approach to evaluate the energy profiles governing the ion movement. The resultant channel conductance and a near-zero permeability ratio (P(Cl)/P(Na)) were comparable to experimental data. On the basis of these calculations, we suggest that a ring of five E222 residues may act as an electrostatic gate. In addition, the hydrophobic gate region may play a role in charge selectivity due to a higher dehydration energy barrier for Cl(-) ions. The effect of halothane on the Na(+) PMF was also evaluated. Halothane was found to perturb salt bridges in GLIC that may be crucial for channel gating and open-channel stability, but had no significant impact on the single ion PMF profiles.

Anesthetic binding in a pentameric ligand-gated ion channel: GLIC

Cys-loop receptors are molecular targets of general anesthetics, but the knowledge of anesthetic binding to these proteins remains limited. Here we investigate anesthetic binding to the bacterial Gloeobacter violaceus pentameric ligand-gated ion channel (GLIC), a structural homolog of cys-loop receptors, using an experimental and computational hybrid approach. Tryptophan fluorescence quenching experiments showed halothane and thiopental binding at three tryptophan-associated sites in the extracellular (EC) domain, transmembrane (TM) domain, and EC-TM interface of GLIC. An additional binding site at the EC-TM interface was predicted by docking analysis and validated by quenching experiments on the N200W GLIC mutant. The binding affinities (K(D)) of 2.3 ± 0.1 mM and 0.10 ± 0.01 mM were derived from the fluorescence quenching data of halothane and thiopental, respectively. Docking these anesthetics to the original GLIC crystal structure and the structures relaxed by molecular dynamics simulations revealed intrasubunit sites for most halothane binding and intersubunit sites for thiopental binding. Tryptophans were within reach of both intra- and intersubunit binding sites. Multiple molecular dynamics simulations on GLIC in the presence of halothane at different sites suggested that anesthetic binding at the EC-TM interface disrupted the critical interactions for channel gating, altered motion of the TM23 linker, and destabilized the open-channel conformation that can lead to inhibition of GLIC channel current. The study has not only provided insights into anesthetic binding in GLIC, but also demonstrated a successful fusion of experiments and computations for understanding anesthetic actions in complex proteins.

Truncated human serum albumin retains general anaesthetic binding activity

Multiple binding sites for anaesthetics in HSA (human serum albumin) make solution studies difficult to interpret. In the present study, we expressed the wild-type HSA domain 3 (wtHSAd3), a peptide with two known anaesthetic binding sites in a yeast expression system. We also expressed a site-directed mutant of domain 3 (Y411Wd3). The stability and secondary structure of the constructed fragments were determined by HX (hydrogen-tritium exchange) and CD spectroscopy. The binding of two general anaesthetics, 2-bromo-2-chloro-1,1,1-trifluoroethane and propofol, to wtHSAd3 and Y411Wd3 was determined using isothermal titration calorimetry, HX and intrinsic tryptophan fluorescence quenching. Although the expressed fragments are less stable than intact wtHSA as indicated by both CD and HX, they retain the secondary structure and anaesthetic-binding characteristics of an intact HSA molecule, but with fewer binding sites. Y411Wd3 had decreased affinity for propofol but not for 2-bromo-2-chloro-1,1,1-trifluoroethane, consistent with steric hindrance. Retention of structural features and anaesthetic binding properties with fewer binding sites in this truncated protein provide feasibility for using scaled-down models of otherwise intractable systems to gain an understanding of anaesthetic binding requirements and binding-stability relationships.

Mechanism of interaction between the general anesthetic halothane and a model ion channel protein, III: Molecular dynamics simulation incorporating a cyanophenylalanine spectroscopic probe

A nitrile-derived amino acid, Phe(CN), has been used as an internal spectroscopic probe to study the binding of an inhalational anesthetic to a model membrane protein. The infrared spectra from experiment showed a blue-shift of the nitrile vibrational frequency in the presence of the anesthetic halothane. To interpret the infrared results and explore the nature of the interaction between halothane and the model protein, all-atom molecular dynamics (MD) simulations have been used to probe the structural and dynamic properties of the protein in the presence and absence of one halothane molecule. The frequency shift analyzed from MD simulations agrees well with the experimental infrared results. Decomposition of the forces acting on the nitrile probes demonstrates an indirect impact on the probes from halothane, namely a change of the protein's electrostatic local environment around the probes induced by halothane. Although the halothane remains localized within the designed hydrophobic binding cavity, it undergoes a significant amount of translational and rotational motion, modulated by the interaction of the trifluorine end of halothane with backbone hydrogens of the residues forming the cavity. This dominant interaction between halothane and backbone hydrogens outweighs the direct interaction between halothane and the nitrile groups, making it a good "spectator" probe of the halothane-protein interaction. These MD simulations provide insight into action of anesthetic molecules on the model membrane protein, and also support the further development of nitrile-labeled amino acids as spectroscopic probes within the designed binding cavity.

Intermolecular Interactions between Halothane and Dimethyl Ether: A Cryosolution Infrared and Ab Initio Study

The complex of halothane (CHClBrCF(3)) and dimethyl ether has been investigated experimentally in solutions of liquid krypton using infrared spectroscopy and theoretically using ab initio calculations at the MP2/6-311++G(d,p) level. The formation of a 1:1 complex was experimentally detected. The most stable ab initio geometry found is the one in which the C--H bond of halothane interacts with the oxygen atom of dimethyl ether. The complexes in which the chlorine or the bromine atom of halothane interacts with the oxygen atom of the ether were found to be local energy minima and were less stable by 14.5 and 9.3 kJ mol(-1), respectively, than the global minimum. The formation of a single complex species was observed in the infrared spectra; the standard complexation enthalpy of this complex was determined to be -12.3(8) kJ mol(-1). Analysis of the observed complexation shifts supports the identification of the complex as the hydrogen-bonded species. The C--H stretching vibration of halothane was found to show a redshift upon complexation of 19(2) cm(-1). The infrared intensity ratios epsilon(complex)/epsilon(monomer) for the fundamental and its first overtone were measured to be 6.5(1) and 0.31(1). The frequency shift was analyzed using Morokuma-type analysis, and the infrared intensity ratios were rationalized using a model including the mechanical and electric anharmonicity of the C--H stretching fundamental.

Halothane, isoflurane, and sevoflurane increase the kinetics of Ca2+-induced conformational change of recombinant human cardiac troponin C

BACKGROUND

Halothane, isoflurane, and sevoflurane exert negative inotropic side effects, generally mediated via a reduced availability of intracellular calcium. Other possible mechanisms include modified intracellular calcium handling, impaired actomyosin cross-bridge cycling, and/or alteration of calcium-induced conformational changes of the regulatory troponin complex.

METHODS

We investigated the effect of halothane, isoflurane, and sevoflurane on calcium-dependent kinetics of isolated human recombinant cardiac troponin C labeled with IAANS (HrcTnC(IAANS)) using stopped-flow and calcium titration techniques.

RESULTS

Calcium concentration at half-maximal fluorescence intensity (K(d)) in the control group was 2.1 +/- 0.1 mM. Volatile anesthetics increased calcium sensitivity in a concentration-dependent fashion sevoflurane (K(d) 1.5-1.7 mM, P = 0.001) > halothane (K(d) 1.7-1.9 mM, P < 0.01) > isoflurane (K(d) 1.8-1.9 mM, P < 0.05). The rate constant of conformational changes after rapid dissociation of calcium from HrcTnC(IAANS) (k(off(c))) was moderately prolonged at 4 degrees C by halothane and isoflurane > sevoflurane.

CONCLUSION

These mechanisms may counteract the effects of lower calcium availability, and can be responsible for abbreviated, and possibly incomplete, relaxation of cardiac muscle fibers in the presence of volatile anesthetics.

A guest molecule–host cavity fitting algorithm to mine PDB for small molecule targets

Inhaled anesthetic molecule occupancy of a protein internal cavity depends in part on the volumes of the guest molecule and the host site. Current algorithms to determine volume and surface area of cavities in proteins whose structures have been determined and cataloged make no allowance for shape or small degrees of shape adjustment to accommodate a guest. We developed an algorithm to determine spheroid dimensions matching cavity volume and surface area and applied it to screen the cavities of 6,658 nonredundant structures stored in the Protein Data Bank (PDB) for potential targets of halothane (2-bromo-2-chloro-1,1,1-trifluoroethane). Our algorithm determined sizes of prolate and oblate spheroids matching dimensions of each cavity found. If those spheroids could accommodate halothane (radius 2.91 A) as a guest, we determined the packing coefficient. 394,766 total cavities were identified. Of 58,681 cavities satisfying the fit criteria for halothane, 11,902 cavities had packing coefficients in the range of 0.46-0.64. This represents 20.3% of cavities large enough to hold halothane, 3.0% of all cavities processed, and found in 2,432 protein structures. Our algorithm incorporates shape dependence to screen guest-host relationships for potential small molecule occupancy of protein cavities. Proteins with large numbers of such cavities are more likely to be functionally altered by halothane.

Prediction of volatile anesthetic binding sites in proteins

Computational methods designed to predict and visualize ligand protein binding interactions were used to characterize volatile anesthetic (VA) binding sites and unoccupied pockets within the known structures of VAs bound to serum albumin, luciferase, and apoferritin. We found that both the number of protein atoms and methyl hydrogen, which are within approximately 8 A of a potential ligand binding site, are significantly greater in protein pockets where VAs bind. This computational approach was applied to structures of calmodulin (CaM), which have not been determined in complex with a VA. It predicted that VAs bind to [Ca(2+)](4)-CaM, but not to apo-CaM, which we confirmed with isothermal titration calorimetry. The VA binding sites predicted for the structures of [Ca(2+)](4)-CaM are located in hydrophobic pockets that form when the Ca(2+) binding sites in CaM are saturated. The binding of VAs to these hydrophobic pockets is supported by evidence that halothane predominantly makes contact with aliphatic resonances in [Ca(2+)](4)-CaM (nuclear Overhauser effect) and increases the Ca(2+) affinity of CaM (fluorescence spectroscopy). Our computational analysis and experiments indicate that binding of VA to proteins is consistent with the hydrophobic effect and the Meyer-Overton rule.

Partitioning of anesthetics into a lipid bilayer and their interaction with membrane-bound peptide bundles

Molecular dynamics simulations have been performed to investigate the partitioning of the volatile anesthetic halothane from an aqueous phase into a coexisting hydrated bilayer, composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipids, with embedded alpha-helical peptide bundles based on the membrane-bound portions of the alpha- and delta-subunits, respectively, of nicotinic acetylcholine receptor. In the molecular dynamics simulations halothane molecules spontaneously partitioned into the DOPC bilayer and then preferentially occupied regions close to lipid headgroups. A single halothane molecule was observed to bind to tyrosine (Tyr-277) residue in the alpha-subunit, an experimentally identified specific binding site. The binding of halothane attenuated the local loop dynamics of alpha-subunit and significantly influenced global concerted motions suggesting anesthetic action in modulating protein function. Steered molecular dynamics calculations on a single halothane molecule partitioned into a DOPC lipid bilayer were performed to probe the free energy profile of halothane across the lipid-water interface and rationalize the observed spontaneous partitioning. Partitioned halothane molecules affect the hydrocarbon chains of the DOPC lipid, by lowering of the hydrocarbon tilt angles. The anesthetic molecules also caused a decrease in the number of peptide-lipid contacts. The observed local and global effects of anesthetic binding on protein motions demonstrated in this study may underlie the mechanism of action of anesthetics at a molecular level.

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.

AFM study of interaction forces in supported planar DPPC bilayers in the presence of general anesthetic halothane

In spite of numerous investigations, the molecular mechanism of general anesthetics action is still not well understood. It has been shown that the anesthetic potency is related to the ability of an anesthetic to partition into the membrane. We have investigated changes in structure, dynamics and forces of interaction in supported dipalmitoylphosphatidylcholine (DPPC) bilayers in the presence of the general anesthetic halothane. In the present study, we measured the forces of interaction between the probe and the bilayer using an atomic force microscope. The changes in force curves as a function of anesthetic incorporation were analyzed. Force measurements were in good agreement with AFM imaging data, and provided valuable information on bilayer thickness, structural transitions, and halothane-induced changes in electrostatic and adhesive properties.

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.

Structural basis for high‐affinity volatile anesthetic binding in a natural 4‐helix bundle protein

Physiologic sites for inhaled anesthetics are presumed to be cavities within transmembrane 4-alpha-helix bundles of neurotransmitter receptors, but confirmation of binding and structural detail of such sites remains elusive. To provide such detail, we screened soluble proteins containing this structural motif, and found only one that exhibited evidence of strong anesthetic binding. Ferritin is a 24-mer of 4-alpha-helix bundles; both halothane and isoflurane bind with K(A) values of approximately 10(5) M(-1), higher than any previously reported inhaled anesthetic-protein interaction. The crystal structures of the halothane/apoferritin and isoflurane/apoferritin complexes were determined at 1.75 A resolution, revealing a common anesthetic binding pocket within an interhelical dimerization interface. The high affinity is explained by several weak polar contacts and an optimal host/guest packing relationship. Neither the acidic protons nor ether oxygen of the anesthetics contribute to the binding interaction. Compared with unliganded apoferritin, the anesthetic produced no detectable alteration of structure or B factors. The remarkably high affinity of the anesthetic/apoferritin complex implies greater selectivity of protein sites than previously thought, and suggests that direct protein actions may underlie effects at lower than surgical levels of anesthetic, including loss of awareness.

Positive and negative ion chemistry of the anesthetic halothane (1-bromo-1-chloro-2,2,2-trifluoroethane) in air plasma at atmospheric pressure

The ion chemistry of 1-bromo-1-chloro-2,2,2-trifluoroethane (the common anesthetic halothane) in air plasma at atmospheric pressure was investigated by atmospheric pressure chemical ionization mass spectrometry (APCI-MS). The major positive ion observed at low declustering (API interface) energies is the ionized dimer, M(+.)M, an unexpectedly abundant species which possibly is stabilized by two H-bonding interactions. At higher energies [M--HF](+.) and [M--Br](+) prevail; the former, corresponding to ionized olefin [ClBrC=CF(2)](+.), appears to originate from M(+.)M and is quite stable towards fragmentation. The latter fragment ion ([M--Br](+)) and its analogue, [M--Cl](+), which is also observed though at much lower abundance, are originally ethyl cations (+)CHX--CF(3) (X = Br, Cl) which, upon collisional activation, rearrange and fragment to CHFX(+) via elimination of CF(2). All of the above described ions are also observed in humid air: in addition, the oxygenated ion [ClBrC=CFOH](+.) also forms in humid air via water addition to [ClBrC=CF(2)](+.) and HF elimination, as observed earlier for ionized trichloroethene. In contrast with similar chloro- and fluoro-substituted ethanes, halothane does not react with H(3)O(+) in the APCI plasma, a result confirmed by selected ion APCI triple-quadrupole (TQ) experiments. Major negative ions formed from halothane in the air plasma are Br(-) and, to a lesser extent, Cl(-), and their complexes with neutral halothane. APCI-TQ experiments indicated that Br(-) and Cl(-) are formed via reaction of halothane with O(2) (-.), O(2) (-.)(H(2)O) and O(3) (-.), possibly via dissociative electron transfer or nucleophilic substitution. Competing proton transfer was also observed in the reaction with O(2) (-.) and, at high halothane pressure, also with O(2) (-.)(H(2)O); at lower pressures the molecular anion M(-.) was observed instead. The other minor anions of the air plasma, NO(2) (-), N(2)O(2) (-.) and NO(3) (-), were found to be unreactive towards halothane.

Concentration effects of volatile anesthetics on the properties of model membranes: a coarse-grain approach

To gain insights into the molecular level mechanism of drug action at the membrane site, we have carried out extensive molecular dynamics simulations of a model membrane in the presence of a volatile anesthetic using a coarse-grain model. Six different anesthetic (halothane)/lipid (dimyristoylphosphatidylcholine) ratios have been investigated, going beyond the low doses typical of medical applications. The volatile anesthetics were introduced into a preassembled fully hydrated 512-molecule lipid bilayer and each of the molecular dynamics simulations were carried out at ambient conditions, using the NPT ensemble. The area per lipid increases monotonically with the halothane concentration and the lamellar spacing decreases, whereas the lipid bilayer thickness shows no appreciable differences and only a slight increase upon addition of halothane. The density profiles of the anesthetic molecules display a bimodal distribution along the membrane normal with maxima located close to the lipid-water interface region. We have studied how halothane molecules fluctuate between the two maxima of the bimodal distribution and we observed a different mechanism at low and high anesthetic concentrations. Through the investigation of the reorientational motions of the lipid tails, we found that the anesthetic molecules increase the segmental order of the lipids close to the membrane surface.

Volatile anesthetic modulation of oligomerization equilibria in a hexameric model peptide

To determine if occupancy of interfacial pockets in oligomeric proteins by volatile anesthetic molecules can allosterically regulate oligomerization equilibria, variants of a three-helix bundle peptide able to form higher oligomers were studied with analytical ultracentrifugation, hydrogen exchange and modeling. Halothane shifted the oligomerization equilibria towards the oligomer only in a mutation predicted to create sufficient volume in the hexameric pocket. Other mutations at this residue, predicted to create a too small or too polar pocket, were unaffected by halothane. Inhaled anesthetic modulation of oligomerization interactions is a novel and potentially generalizable biophysical basis for some anesthetic actions.

Binding of volatile anesthetics to serum albumin: measurements of enthalpy and solvent contributions

This study directly examines the enthalpic contributions to binding in aqueous solution of closely related anesthetic haloethers (desflurane, isoflurane, enflurane, and sevoflurane), a haloalkane (halothane), and an intravenous anesthetic (propofol) to bovine and human serum albumin (BSA and HSA) using isothermal titration calorimetry. Binding to serum albumin is exothermic, yielding enthalpies (DeltaH(obs)) of -3 to -6 kcal/mol for BSA with a rank order of apparent equilibrium association constants (K(a) values): desflurane > isoflurane approximately enflurane > halothane >or= sevoflurane, with the differences being largely ascribed to entropic contributions. Competition experiments indicate that volatile anesthetics, at low concentrations, share the same sites in albumin previously identified in crystallographic and photo-cross-linking studies. The magnitude of the observed DeltaH increased linearly with increased reaction temperature, reflecting negative changes in heat capacities (DeltaC(p)). These -DeltaC(p) values significantly exceed those calculated for burial of each anesthetic in a hydrophobic pocket. The enhanced stabilities of the albumin/anesthetic complexes and -DeltaC(p) are consistent with favorable solvent rearrangements that promote binding. This idea is supported by substitution of D(2)O for H(2)O that significantly reduces the favorable binding enthalpy observed for desflurane and isoflurane, with an opposing increase of DeltaS(obs). From these results, we infer that solvent restructuring, resulting from release of water weakly bound to anesthetic and anesthetic-binding sites, is a dominant and favorable contributor to the enthalpy and entropy of binding to proteins.

A model membrane protein for binding volatile anesthetics

Earlier work demonstrated that a water-soluble four-helix bundle protein designed with a cavity in its nonpolar core is capable of binding the volatile anesthetic halothane with near-physiological affinity (0.7 mM Kd). To create a more relevant, model membrane protein receptor for studying the physicochemical specificity of anesthetic binding, we have synthesized a new protein that builds on the anesthetic-binding, hydrophilic four-helix bundle and incorporates a hydrophobic domain capable of ion-channel activity, resulting in an amphiphilic four-helix bundle that forms stable monolayers at the air/water interface. The affinity of the cavity within the core of the bundle for volatile anesthetic binding is decreased by a factor of 4-3.1 mM Kd as compared to its water-soluble counterpart. Nevertheless, the absence of the cavity within the otherwise identical amphiphilic peptide significantly decreases its affinity for halothane similar to its water-soluble counterpart. Specular x-ray reflectivity shows that the amphiphilic protein orients vectorially in Langmuir monolayers at higher surface pressure with its long axis perpendicular to the interface, and that it possesses a length consistent with its design. This provides a successful starting template for probing the nature of the anesthetic-peptide interaction, as well as a potential model system in structure/function correlation for understanding the anesthetic binding mechanism.

An isothermal titration calorimetry study on the binding of four volatile general anesthetics to the hydrophobic core of a four-alpha-helix bundle protein

A molecular understanding of volatile anesthetic mechanisms of action will require structural descriptions of anesthetic-protein complexes. Previous work has demonstrated that the halogenated alkane volatile anesthetics halothane and chloroform bind to the hydrophobic core of the four-alpha-helix bundle (Aalpha(2)-L38M)(2) (Johansson et al., 2000, 2003). This study shows that the halogenated ether anesthetics isoflurane, sevoflurane, and enflurane are also bound to the hydrophobic core of the four-alpha-helix bundle, using isothermal titration calorimetry. Isoflurane and sevoflurane both bound to the four-alpha-helix bundle with K(d) values of 140 +/- 10 micro M, whereas enflurane bound with a K(d) value of 240 +/- 10 micro M. The DeltaH degrees values associated with isoflurane, sevoflurane, and enflurane binding were -7.7 +/- 0.1 kcal/mol, -8.2 +/- 0.2 kcal/mol, and -7.2 +/- 0.1 kcal/mol, respectively. The DeltaS degrees values accompanying isoflurane, sevoflurane, and enflurane binding were -8.5 cal/mol K, -10.4 cal/mol K, and -8.0 cal/mol K, respectively. The results indicate that the hydrophobic core of (Aalpha(2)-L38M)(2) is able to accommodate three modern ether anesthetics with K(d) values that approximate their clinical EC(50) values. The DeltaH degrees values point to the importance of polar interactions for volatile general anesthetic binding, and suggest that hydrogen bonding to the ether oxygens may be operative.

Theoretical Scales of Hydrogen Bond Acidity and Basicity for Application in QSAR/QSPR Studies and Drug Design. Partitioning of Aliphatic Compounds

Phenomenological analysis of existing hydrogen bond (HB) donor and acceptor scales and apparent physical considerations have enabled the establishment of new quantitative scales of hydrogen bond basicity and acidity. Chemical structures represented by molecular graphs and the orbital electronegativities of Hinze and Jaffe are utilized as an input data. The scales obtained correlate well with several experimental solvent polarity scales such as and, pK(HB), and E(T)(30). To demonstrate the applicability of the new quantities, we have applied them to seven equilibrium partitioning data sets: octanol-water, hexadecane-water, chloroform-water, gas-water, gas-octanol, gas-hexadecane, and gas-chloroform partition coefficients. The hydrogen bond descriptors when supplemented by a cavity-forming term and a dipolarity term show high performance in correlations of the partition coefficients of aliphatic compounds. These new HB descriptors can be used in studying hydrogen bonding and fluid phase equilibria as well as scoring functions in ligand docking and descriptors in ADME evaluations.

Inhalational anesthetic-binding proteins in rat neuronal membranes

Molecular targets of inhaled anesthetics must be represented in the group that specifically bind these drugs, but the paucity of direct binding data has limited the number of candidates for further evaluation. To find candidate targets, we used a combination of photolabeling, two-dimensional gel electrophoresis, and mass spectrometry to identify halothane-binding targets in rat neuronal membranes. Of the 265 spots detected on the two-dimensional gels, 90 were labeled by [(14)C]halothane, and 34 were identified. Mitochondrial proteins, especially respiratory complex and voltage-dependent anion channels, dominated the labeled group, and there were several examples of subunit- and state-dependent binding. A significant correlation was found between internal protein cavities and binding in a group of proteins with high resolution structures. Therefore, in addition to identifying novel neuronal targets, these data suggest a general molecular feature, the buried cavity, as a dominant attribute of volatile anesthetic-binding sites found in a limited number of neuronal membrane proteins.

In vitro evaluation of the dissolving effect of solvents on root canal sealers

The purpose of this in vitro study was to evaluate two commonly used gutta-percha solvents for their effectiveness in dissolving several types of root canal sealers. Seven different sealers (AH26, AH Plus, Diaket, Roekoseal, Sankin Apatite Root Sealer, Sealapex, and Sultan U/P) were used in this study. After mixing according to the manufacturers' directions, each material was syringed into 30 glass capillary tubes, and a total of 210 tubes were placed in a humidifier at 37 degrees C for one week to allow the materials to set completely. Each group of 30 tubes, obturated with one type of sealer, was then randomly divided into three subgroups, including 10 tubes each. Chloroform was used in the first ten tubes from each sealer group. Halothane was used for the second group. In the last group, the sealer was removed with files, without using any solvent. The time necessary to pass a file through to the end of the tube was recorded for each sample in seconds. Results were analyzed using one-way analysis of variance. Sealapex did not set at all unless in contact with air. Roekoseal did not adhere to the glass capillary tubes, and was therefore easily removed from the tube in all samples. AH26 and AH Plus root canal sealers tightly adhered to the tube walls, so none of the techniques were effective in removing them from the tubes within 30 min. Diaket root canal sealer was easily removed using solvents (P < 0.05). There was no advantage in using solvents to remove Sankin Apatite Root Sealer (P > 0.05). Solvents were found to be very effective in dissolving the Sultan U/P root canal sealer (P < 0.05).

Identification of nicotinic acetylcholine receptor amino acids photolabeled by the volatile anesthetic halothane

To identify inhalational anesthetic binding domains in a ligand-gated ion channel, we photolabeled nicotinic acetylcholine receptor (nAChR)-rich membranes from Torpedo electric organ with [(14)C]halothane and determined by Edman degradation some of the photolabeled amino acids in nAChR subunit fragments isolated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and high-performance liquid chromatography. Irradiation at 254 nm for 60 s in the presence of 1 mM [(14)C]halothane resulted in incorporation of approximately 0.5 mol of (14)C/mol of subunit, with photolabeling distributed within the nAChR extracellular and transmembrane domains, primarily at tyrosines. GammaTyr-111 in ACh binding site segment E was labeled, while alphaTyr-93 in segment A was not. Within the transmembrane domain, alphaTyr-213 within alphaM1 and deltaTyr-228 within deltaM1 were photolabeled, while no labeled amino acids were identified within the deltaM2 ion channel domain. Although the efficiency of photolabeling at the subunit level was unaffected by agonist, competitive antagonist, or isoflurane, state-dependent photolabeling was seen in a delta subunit fragment beginning at deltaPhe-206. Labeling of deltaTyr-212 in the extracellular domain was inhibited >90% by d-tubocurarine, whereas addition of either carbamylcholine or isoflurane had no effect. Within M1, the level of photolabeling of deltaTyr-228 with [(14)C]halothane was increased by carbamylcholine (90%) or d-tubocurarine (50%), but it was inhibited by isoflurane (40%). Within the structure of the nAChR transmembrane domain, deltaTyr-228 projects into an extracellular, water accessible pocket formed by amino acids from the deltaM1-deltaM3 alpha-helices. Halothane photolabeling of deltaTyr-228 provides initial evidence that halothane and isoflurane bind within this pocket with occupancy or access increased in the nAChR desensitized state compared to the closed channel state. Halothane binding at this site may contribute to the functional inhibition of nAChRs.

Application of the 19F NMR Technique to Observe Binding of the General Anesthetic Halothane to Human Serum Albumin

19F NMR techniques were employed to characterize the binding property of the widely used general anesthetic halothane with human serum albumin (HSA). It was found that 19F(1H) NOE and 2D 1H-19F HOESY experiments detected intermolecular NOEs between halothane 19F and HSA protons. Measurements of the diffusion coefficients for halothane were also carried out by 1H and 19F NMR, indicating the interaction of halothane with HSA. The present results indicate that these techniques are very suitable to identify a fluorine-containing ligand binding with a protein receptor in the drug-discovery process.

Biological activity of bacterial lectins and their molecular complexes with heterocyclic bis-adducts

A new convenient method for the preparation of heterocyclic bis-adducts: of imidazole, benzimidazole, uracile with 1,1,1-trifluoro-2-bromo-2-chloroethane is described. The reactions are catalysed by the 18-crown-6-complex. The critical toxicity and antitumour activity of saprophytic strains Bacillus genus (B. subtilis 668 IMV and B. polymyxa 102 KSU) extracellular lectins were studies. It was discovered that these substances apply to a few toxic preparations and have a expression antitumour action on the tumours: Walker carcinosarcoma 256, Pliss' lymphosarcoma and Sarcoma 45. The new molecular complexes were created with bacterial lectins and the same heterocyclic-bis-adducts of unsubstituted benzimidazole and 6-methyluracile. A strongly antitumour effect of these complexes has been discovered: of growth relaxation of Pliss' lymphosarcoma tumour mass was 62.5-82.01%.

Equilibrium titration of protein-ligand binding affinity in the presence of volatile reagents--a semiautomated approach

An existing method of equilibrium titration was significantly improved for investigating the effects of volatile anesthetics on Ca(2+) binding characteristics of human recombinant cardiac troponin C in in vitro conditions. The modified method increases stability of volatile compound concentrations in solution and allows for faster and more accurate data acquisition. The time to complete a titration series could be reduced from 28.3 +/- 6.2 min to 9.3 +/- 2.1 min, whereas the dispersion for pK(d) was decreased from 2.16 +/- 0.27 to 0.63 +/- 0.27. The method utilizes a semiautomatic approach to continuously monitor stability of fluorescence signals in a sealed chamber with greatly reduced air space.

Large-scale molecular dynamics simulations of general anesthetic effects on the ion channel in the fully hydrated membrane: the implication of molecular mechanisms of general anesthesia

Interactions of volatile anesthetics with the central nervous system are characterized by low yet specific binding affinities. Although neurotransmitter-gated ion channels are considered the primary anesthetic targets, the mechanism of action at the molecular level remains elusive. We consider here the theoretical implications of channel dynamics on anesthetic action in a simplified membrane-channel system. Large-scale 2.2-ns all-atom molecular dynamics simulations were performed to study the effects of halothane, a clinical anesthetic, on a gramicidin A (gA) channel in a fully hydrated dimyristoyl phosphatidylcholine membrane. In agreement with experimental results, anesthetics preferentially target the anchoring residues at the channel-lipid-water interface. Although the anesthetic effect on channel structure is minimal, the presence of halothane profoundly affects channel dynamics. For 2.2-ns simulation, the rms fluctuation of gA backbone in the lipid core increases from approximately equal 1 A in the absence of anesthetics to approximately equal 1.5 A in the presence of halothane. Autocorrelation analysis reveals that halothane (i) has no effect on the subpicosecond librational motion, (ii) prolongs the backbone autocorrelation time in the 10- to 100-ps time scale, and (iii) significantly decreases the asymptotic values of generalized order parameter and correlation time of nanosecond motions for the inner but not the outer residues. The simulation results discount the viewpoint of a structure-function paradigm that overrates the importance of structural fitting between general anesthetics and yet-unidentified hydrophobic protein pockets. Instead, the results underscore the global, as opposed to local, effects of anesthetics on protein dynamics as the underlying mechanisms for the action of general anesthetics and possibly of other low-affinity drugs.

Halogenated diazirines as photolabel mimics of the inhaled haloalkane anesthetics

The inhaled anesthetics are low affinity volatile compounds whose mechanism of action remains unclear, in part due to the difficulty of determining their binding targets. Photolabeling may help resolve this difficulty, and thus we have synthesized six compounds (four previously unreported) with structural and physical similarity to halothane (1-bromo-1-chloro-2,2,2-trifluoroethane), a commonly used clinical anesthetic. These compounds incorporate either a diazo, diazirine, or azido group to provide photolability in the long-UV range and to provide a highly reactive photolysis product. While several of the compounds have immobilizing activity in tadpoles, it is complicated by either toxicity or very low potency. One compound however, a halogenated three-carbon diazirine 4, is a potent anesthetic, is apparently nontoxic, potentiates GABA(A) Cl(-) currents, and stabilizes serum albumin, all of which are features of halothane. When tagged with radioactivity, this compound should serve as a reasonable probe of haloalkane anesthetic binding targets and sites.

Selected ion flow tube studies of the reactions of H3O+, NO+ and O2+ with the anaesthetic gases halothane, isoflurane and sevoflurane

We have carried out a study of the reactions of H(3)O(+), NO(+) and O(2) (+), the commonly used precursor ions for selected ion flow tube mass spectrometry (SIFT-MS), with three anaesthetic gases, halothane, isoflurane and sevoflurane. The motivation for this study was to provide the necessary kinetic data that would allow the quantification of these anaesthetic gases in operating theatre air and in the breath of theatre staff and post-operative patients. A clear negative result from these experiments is that NO(+), although undergoing the simplest chemistry, is unsuitable for this SIFT-MS application. However, although the ion chemistry of H(3)O(+) and O(2) (+) with these compounds is very complex, there being several product ions in each reaction, many of which react rapidly with water molecules, monitor ions have been identified for all three anaesthetic gases when using H(3)O(+) and O(2) (+) as precursor ions. The detailed ion chemistry is discussed and the specific monitor ions are indicated. Hence, the feasibility of on-line breath monitoring is demonstrated by simple examples. These studies have opened the way to measurements in the clinical environment.

Carbon monoxide production from desflurane, enflurane, halothane, isoflurane, and sevoflurane with dry soda lime

BACKGROUND

Previous studies in which volatile anesthetics were exposed to small amounts of dry soda lime, generally controlled at or close to ambient temperatures, have demonstrated a large carbon monoxide (CO) production from desflurane and enflurane, less from isoflurane, and none from halothane and sevoflurane. However, there is a report of increased CO hemoglobin in children who had been induced with sevoflurane that had passed through dry soda lime. Because this clinical report appears to be inconsistent with existing laboratory work, the authors investigated CO production from volatile anesthetics more realistically simulating conditions in clinical absorbers.

METHODS

Each agent, 2.5 or 5% in 2 l/min oxygen, were passed for 2 h through a Dräger absorber canister (bottom to top) filled with dried soda lime (Drägersorb 800). CO concentrations were continuously measured at the absorber outlet. CO production was calculated. Experiments were performed in ambient air (19-20 degrees C). The absorbent temperature was not controlled.

RESULTS

Carbon monoxide production peaked initially and was highest with desflurane (507 +/- 70, 656 +/- 59 ml CO), followed by enflurane (460 +/- 41, 475 +/- 99 ml CO), isoflurane (176 +/- 2.8, 227 +/- 21 ml CO), sevoflurane (34 +/- 1, 104 +/- 4 ml CO), and halothane (22 +/- 3, 20 +/- 1 ml CO) (mean +/- SD at 2.5 and 5%, respectively).

CONCLUSIONS

The absorbent temperature increased with all anesthetics but was highest for sevoflurane. The reported magnitude of CO formation from desflurane, enflurane, and isoflurane was confirmed. In contrast, a smaller but significant CO formation from sevoflurane was found, which may account for the CO hemoglobin concentrations reported in infants. With all agents, CO formation appears to be self-limited.

Viscosity and density of common anaesthetic gases: implications for flow measurements

Although viscosity (mu) is a crucial factor in measurements of flow with a pneumotachograph, and density (rho) also plays a role in the presence of turbulent flow, these material constants are not available for the volatile anaesthetic agents commonly administered in clinical practice. Thus, we determined experimentally mu and rho of pure volatile anaesthetic agents. Input impedance of a rigid-wall polyethylene tube (Zt) was measured when the tube was filled with various mixtures of carrier gases (air, 100% oxygen, 50% oxygen+50% nitrogen) to which different concentrations of volatile anaesthetic inhalation agents (halothane, isoflurane, sevoflurane, and desflurane) had been added. Mu and rho were calculated from real and imaginary portions of Zt, respectively, using the appropriate physical equations. Multiple linear regression was applied to estimate mu and rho of pure volatile agents. Viscosity values of pure volatile agents were markedly lower than those for oxygen or nitrogen. Clinically applied concentrations, however, did not markedly affect the viscosity of the gas mixture (maximum of 3.5% decrease in mu for 2 MAC desflurane). In contrast, all of the volatile agents significantly affected rho even at routinely used concentrations. Our results suggest that the composition of the carrier gas has a greater impact on viscosity than the amount and nature of the volatile anaesthetic agent whereas density is more influenced by volatile agent concentrations. Thus, the need for a correction factor in flow measurements with a pneumotachograph depends far more on the carrier gas than the concentration of volatile agent administered, although the latter may play a role in particular experimental or clinical settings.

Dynamic changes in blood solubility of desflurane, isoflurane, and halothane during cardiac surgery

OBJECTIVE

To determine an estimate of blood/gas partition coefficients of volatile anesthetics during cardiac surgery.

DESIGN

Descriptive

SETTING

University hospital

PARTICIPANTS

Six adult patients undergoing valvular replacement with hypothermic cardiopulmonary bypass.

MEASUREMENTS AND MAIN RESULTS

Blood samples were obtained from patients at 6 time points: before induction, at skin incision, at aortic cannulation, at rewarming during bypass, at weaning off bypass, and at skin suture. Measured blood/gas partition coefficients were plotted against corresponding solubilities estimated according to the combined effects of hypothermia and hemodilution. Significant differences were found in blood/gas partition coefficients of the 3 anesthetics at different times during surgery (p < 0.05). Blood/gas partition coefficients at weaning off bypass were the lowest, about 75% of that before anesthetic induction. A direct linear relationship for estimated solubility against measured solubility was found (r2 = 0.94; p < 0.05).

CONCLUSION

Dynamic changes in blood/gas partition coefficients of volatile anesthetics were found during cardiac surgery. They could be estimated by using multiple linear regression equations reflecting the combined effects of hypothermia and hemodilution.

Anesthetic stabilization of protein intermediates: myoglobin and halothane

Halothane, an inhaled anesthetic, destabilizes the folded structure of myoglobin. To determine whether this is due to preferential interactions with less stable folded conformers of myoglobin versus the completely unfolded state, we used photoaffinity labeling, hydrogen exchange, fluorescence spectroscopy, and circular dichroism spectroscopy. Apomyoglobin was used as a model of a less stable conformer of myoglobin. Halothane destabilizes myoglobin and binds with low affinity and stoichiometry but stabilizes and binds with higher affinity to apomyoglobin. The same halothane concentration has no effect on cytochrome c stability. The apomyoglobin/halothane complex is favored at pH 6.5 as compared to pH 4.5 or pH 2.5. Halothane photoincorporates into several sites in apomyoglobin, some allosteric to the heme pocket. Guanidinium unfolding of myoglobin, monitored by CD spectroscopy, shows destabilization at less than 1.3 M Gdm but stabilization at greater than 1.3 M Gdm, consistent with the hypothesis that less stable conformers of myoglobin bind halothane preferentially. We suggest the structural feature underlying preferential binding to less stable conformers is an enlarged cavity volume distribution, since myoglobin has several intermediate-sized cavities, while cytochrome c is more well packed and has no cavities detected by GRASP. Specific binding to less stable intermediates may underlie anesthetic potentiation of protein activity.

Evidence for a common binding cavity for three general anesthetics within the GABAA receptor

The GABA(A) receptor is an important target for a variety of general anesthetics (Franks and Lieb, 1994) and for benzodiazepines such as diazepam. Specific point mutations in the GABA(A) receptor selectively abolish regulation by benzodiazepines (Rudolph et al., 1999; McKernan et al., 2000) and by anesthetic ethers (Mihic et al., 1997; Krasowski et al., 1998; Koltchine et al., 1999), suggesting the existence of discrete binding sites on the GABA(A) receptor for these drugs. Using anesthetics of different molecular size (isoflurane > halothane > chloroform) together with complementary mutagenesis of specific amino acid side chains, we estimate the volume of a proposed anesthetic binding site as between 250 and 370 A(3). The results of the "cutoff" analysis suggest a common site of action for the anesthetics isoflurane, halothane, and chloroform on the GABA(A) receptor. Moreover, the data support a crucial role for Leu232, Ser270, and Ala291 in the alpha subunit in defining the boundaries of an amphipathic cavity, which can accommodate a variety of small general anesthetic molecules.

Prediction of volatile anaesthetic solubility in blood and priming fluids for extracorporeal circulation

Volatile anaesthetics are often used during cardiopulmonary bypass (CPB). To understand the kinetics of inhaled anaesthetics during CPB, anaesthetists should understand changes in blood solubility caused by fluid use. We set out to predict the solubility of three volatile anaesthetics, desflurane, isoflurane and halothane, during CPB by determining: (i) their solubility in fresh whole blood and eight CPB priming fluids at 37 degrees C; (ii) the effect of temperature on the solubility of these anaesthetics in lactated Ringer's, gelofusin, banked blood and plasma; (iii) their solubility in different mixtures of these four priming fluids at different temperatures; and (iv) their estimated and actual solubility in blood during hypothermic CPB. We calculated solubility using a concept of volume fraction partition coefficient and compared estimated and measured solubilities. For the three anaesthetics tested, solubilities are in the order: fresh whole blood approximately = plasma > banked blood > normal saline approximately = lactated Ringer's approximately = gelofusin approximately = Haemaccel approximately = hydroxyethyl starch > mannitol. The solubilities of the anaesthetics in all priming fluids increased logarithmically at lower temperatures (P<0.05). The volume-fraction estimates of the partition coefficients were within approximately +/-20% of the measured values for all values of solubility. The corresponding estimates of solubility for CPB blood samples were between -36% and +24% of the measured values. During normothermic CPB, blood solubility of volatile anaesthetics would be unchanged when using plasma, slightly reduced when using banked blood and markedly reduced when using crystalloids and colloids.

Cytotoxicity of halothane on human gingival fibroblast cultures in vitro

Recently halothane has been reported to be the most suitable alternative to chloroform in dissolving gutta-percha. Periapical tissue toxicity of halothane is not completely known. In this study gutta-percha dissolved by halothane was evaluated with the almar blue dye assay using human gingival fibroblast cultures. The cytotoxic effects of halothane on human gingival fibroblasts depended on the exposure dose, frequency, and duration. A reduced concentration and smaller amount of gutta-percha solvents may minimize the cytotoxic effects on host tissues.

Binding of the general anesthetics propofol and halothane to human serum albumin. High resolution crystal structures

Human serum albumin (HSA) is one of the most abundant proteins in the circulatory system and plays a key role in the transport of fatty acids, metabolites, and drugs. For many drugs, binding to serum albumin is a critical determinant of their distribution and pharmacokinetics; however, there have as yet been no high resolution crystal structures published of drug-albumin complexes. Here we describe high resolution crystal structures of HSA with two of the most widely used general anesthetics, propofol and halothane. In addition, we describe a crystal structure of HSA complexed with both halothane and the fatty acid, myristate. We show that the intravenous anesthetic propofol binds at two discrete sites on HSA in preformed pockets that have been shown to accommodate fatty acids. Similarly we show that the inhalational agent halothane binds (at concentrations in the pharmacologically relevant range) at three sites that are also fatty acid binding loci. At much higher halothane concentrations, we have identified additional sites that are occupied. All of the higher affinity anesthetic binding sites are amphiphilic in nature, with both polar and apolar parts, and anesthetic binding causes only minor changes in local structure.

Water vapour in a closed anaesthesia circuit reduces degradation/adsorption of halothane by dried soda lime

Dry lime causes a loss of volatile anaesthetics by degrading and adsorbing them. Degradation produces toxic substances and heat. Rehydration of lime stops degradation. If humidified breathing gases rehydrate lime, closed anaesthesia-circuits may reduce the loss of anaesthetics. To test this hypothesis we ventilated a reservoir bag with PhysioFlex-devices using fresh (F) and dried (D) soda lime both in the presence (+H) and absence (-H) of halothane. We measured halothane delivery, humidity, temperature, and lime weight. Halothane was lost for 13 min in D + H. Humidity increased steeper with fresh lime, whereas absorbent weight increased more with dried lime; halothane increased both variables (F + H: 99%, 8 g; F - H: 93%, 6 g; D + H: 58%, 17 g; D - H: 24%, 15 g). Surprisingly, temperature remained constant, probably because of the high gas flow (70 litres min-1) generated inside the Physioflex. These findings indicate rehydration of dried lime by humid gases and a rapid cessation of the loss of halothane in the PhysioFlex.