Reduction of halocarbons to hydrocarbons by NADH models and NADH

The reduction of halocarbons by NADH models and NADH under ambient conditions is reported as a new type of reactivity pointing towards a hitherto unknown disruptive pathway for NADH/NADPH-dependent processes. The reaction was studied with the omnipresent pesticide DDT, the inhalation anesthetic halothane, and several simple halocarbons. The halide-hydride exchange represents a biochemical equivalent for the reduction of halocarbons by traditional synthetic reagents like silanes (R₃Si-H) and stannanes (R₃Sn-H). High precision thermochemical calculations (CBS-QB3) reveal the carbon-hydrogen bond dissociation energy of NADH (70.8 kcal·mol^-1) to be lower than that of stannane (SnH₄: 78.1 kcal·mol^-1), approaching that of the elusive plumbane (PbH₄: 68.9 kcal·mol^-1). The ready synthetic accessibility of NADH models, their low carbon-hydrogen bond dissociation energy, and their dehalogenation activity in the presence of air and moisture recommend these compounds as substitutes for the air-sensitive or toxic metal hydrides currently employed in synthesis.

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.

Computational predictions of volatile anesthetic interactions with the microtubule cytoskeleton: implications for side effects of general anesthesia

The cytoskeleton is essential to cell morphology, cargo trafficking, and cell division. As the neuronal cytoskeleton is extremely complex, it is no wonder that a startling number of neurodegenerative disorders (including but not limited to Alzheimer's disease, Parkinson's disease and Huntington's disease) share the common feature of a dysfunctional neuronal cytoskeleton. Recently, concern has been raised about a possible link between anesthesia, post-operative cognitive dysfunction, and the exacerbation of neurodegenerative disorders. Experimental investigations suggest that anesthetics bind to and affect cytoskeletal microtubules, and that anesthesia-related cognitive dysfunction involves microtubule instability, hyper-phosphorylation of the microtubule-associated protein tau, and tau separation from microtubules. However, exact mechanisms are yet to be identified. In this paper the interaction of anesthetics with the microtubule subunit protein tubulin is investigated using computer-modeling methods. Homology modeling, molecular dynamics simulations and surface geometry techniques were used to determine putative binding sites for volatile anesthetics on tubulin. This was followed by free energy based docking calculations for halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) on the tubulin body, and C-terminal regions for specific tubulin isotypes. Locations of the putative binding sites, halothane binding energies and the relation to cytoskeleton function are reported in this paper.

Formation/fate of reactive metabolites from general anesthetics and a comparison of toxic and non-toxic analogues: a DFT study

Chloroform and Halothane are well known hepatotoxic anesthetics for which toxicity is attributed to their reactive metabolites. The molecular level details of reactions leading to the formation of reactive metabolites from chloroform and halothane have not been explored. Potential energy surface (PES) for the formation of phosgene (a toxic intermediate) from Chloroform has been studied using quantum chemical methods. The HOOH mediated reaction of chloroform to give phosgene has been found to be exothermic by 81.24 kcal/mol with a barrier of ~ 3 kcal/mol through the water catalyzed transition sate. The quantum chemical studies on the reactivity profile of phosgene indicated that urea derivatives need to be considered on the mechanism leading to toxicity. Similarly, metabolic pathways of Halothane oxidation have been studied. The C-H bond dissociation energies (BDE) and radical stabilization energies (RSE) for Chloroform and Halothane (< 95 kcal/mol and > 10 kcal/mol) were found to be significantly different for these toxic anesthetics in comparison to their safer analogues (> 100 kcal/mol and < 5 kcal/mol) respectively; thus these parameters can be employed to distinguish toxic and non-toxic general anesthetics. Enthalpy for the Cpd I, a widely used model for CYP450 enzymes, mediated reactions also agreed well with these results.

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.

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.

Halothane binding proteome in human brain cortex

Inhaled anesthetics bind specifically to a wide variety of proteins in the brain. This set of proteins must include those that contribute to the physiological and behavioral phenotypes of anesthesia and the related side effects. To identify the anesthetic-binding targets and functional pathways associated with these targets in human brain, halothane photolabeling and two-dimensional (2D) gel electrophoresis were used. Both membrane and soluble proteins from human temporal cortex were prepared. More than 300 membrane and 400 soluble protein spots were detected on the stained blots, of which 23 membrane and 34 soluble proteins were labeled by halothane and identified by mass spectroscopy. Their functional classification reveals five groups, including carbohydrate metabolism, protein folding, oxidative phosphorylation, nucleoside triphosphatase, and dimer/kinase activity with different correlative stringency. When network analysis of the interaction between these protein molecules is used, the weighted interaction accentuates the cellular protein components important in cell growth and proliferation, cell cycle and cell death, and cell-cell signaling and interactions, although no pathway was specific. This study provides evidence for multiple anesthetic binding targets and suggests potential pathways involved in their actions.

Effects of breed, sex, and halothane genotype on fatty acid composition of pork longissimus muscle1

The objective of this study was to estimate the effects of breed, sex, and halothane genotype on fatty acid composition and several fatty acid indices of lipid extracted from porcine LM. Purebred Yorkshire (n = 436), Duroc (n = 353), Hampshire (n = 218), Spotted (n = 187), Chester White (n = 173), Poland China (n = 124), Berkshire (n = 256), and Landrace (n = 187) pigs (n = 1,934; 1,128 barrows and 806 gilts) from 1991, 1992, 1994, and 2001 National Barrow Show Sire Progeny Tests were used. Pigs were classified as the HAL-1843 normal (NN) genotype (n = 1,718) or the HAL-1843 carrier (Nn) genotype (n = 216). For statistical analysis, a mixed model was used that included fixed effects of breed, sex, halothane genotype, test, slaughter date, interaction of breed x sex, and random effects of sire and dam within breed. Breed significantly affected the concentration of individual fatty acids, total lipid content, and the values of several fatty acid indices of LM. Duroc pigs had the greatest (P < 0.01) content of total SFA. Total MUFA concentration in Poland China pigs was greater (P < 0.05) than in all other breeds except the Spotted (P > 0.05). The concentrations of total PUFA were greater (P < 0.01) in Hampshire, Landrace, and Yorkshire pigs compared with those of other breeds. Significant sex differences for individual fatty acids were detected. Compared with gilts, barrows had greater (P < 0.01) concentrations of SFA and MUFA but lower (P < 0.01) total PUFA. Halothane genotype was a significant source of variation for the percentages of some fatty acids. Pigs with the carrier (Nn) genotype had lower concentrations of SFA (P < 0.05) and MUFA (P < 0.01) but a greater concentration of PUFA (P < 0.01) compared with NN pigs. There were significant negative correlations between total lipid content and individual PUFA and significant positive correlations between lipid concentration and most individual SFA and MUFA. In conclusion, the results suggest that breed and sex are important sources of variation for fatty acid composition of LM.

Role of neutrophils in a mouse model of halothane-induced liver injury

Drug-induced liver injury (DILI) is a major safety concern in drug development. Its prediction and prevention have been hindered by limited knowledge of the underlying mechanisms, in part the result of a lack of animal models. We developed a mouse model of halothane-induced liver injury and characterized the mechanisms accounting for tissue damage. Female and male Balb/c, DBA/1, and C57BL/6J mice were injected intraperitoneally with halothane. Serum levels of alanine aminotransferase and histology were evaluated to determine liver injury. Balb/c mice were found to be the most susceptible strain, followed by DBA/1, with no significant hepatotoxicity observed in C57BL/6J mice. Female Balb/c and DBA/1 mice developed more severe liver damage compared with their male counterparts. Bioactivation of halothane occurred similarly in all three strains based on detection of liver proteins adducted by the reactive metabolite. Mechanistic investigations revealed that hepatic message levels of tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta); IL-6, and IL-8 were significantly higher in halothane-treated Balb/c mice compared to DBA/1 and C57BL/6J mice. Moreover, a higher number of neutrophils were recruited into the liver of Balb/c mice upon halothane treatment compared with DBA/1, with no obvious neutrophil infiltration detected in C57BL/6J mice. Neutrophil depletion experiments demonstrated a crucial role for these cells in the development of halothane-induced liver injury. The halothane-initiated hepatotoxicity and innate immune response-mediated escalation of tissue damage are consistent with events that occur in many cases of DILI. In conclusion, our model provides a platform for elucidating strain-based and gender-based susceptibility factors in DILI development.

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.

Monolayers of a model anesthetic-binding membrane protein: formation, characterization, and halothane-binding affinity

hbAP0 is a model membrane protein designed to possess an anesthetic-binding cavity in its hydrophilic domain and a cation channel in its hydrophobic domain. Grazing incidence x-ray diffraction shows that hbAP0 forms four-helix bundles that are vectorially oriented within Langmuir monolayers at the air-water interface. Single monolayers of hbAP0 on alkylated solid substrates would provide an optimal system for detailed structural and dynamical studies of anesthetic-peptide interaction via x-ray and neutron scattering and polarized spectroscopic techniques. Langmuir-Blodgett and Langmuir-Schaeffer deposition and self-assembly techniques were used to form single monolayer films of the vectorially oriented peptide hbAP0 via both chemisorption and physisorption onto suitably alkylated solid substrates. The films were characterized by ultraviolet absorption, ellipsometry, circular dichroism, and polarized Fourier transform infrared spectroscopy. The alpha-helical secondary structure of the peptide was retained in the films. Under certain conditions, the average orientation of the helical axis was inclined relative to the plane of the substrate, approaching perpendicular in some cases. The halothane-binding affinity of the vectorially oriented hbAP0 peptide in the single monolayers, with the volatile anesthetic introduced into the moist vapor environment of the monolayer, was found to be similar to that for the detergent-solubilized peptide.

Effects of tail fat on halothane biotransformation in fat-tailed sheep

1. The aim of the present study was to evaluate the effects of tail fat on halothane biotransformation following similar anaesthetic exposure in intact sheep and sheep with a ligated median sacral artery. 2. A prospective randomized experimental study was performed using 12 healthy, 10-12-month-old female sheep. 3. Sheep were randomly divided into two groups of six animals each and were anaesthetized twice at 2 weekly intervals. After mask induction with halothane in 100% oxygen, sheep were intubated and anaesthesia was maintained for 3 h using a rebreathing system. Serum fluoride concentration (SFC) was measured at 0, 1, 3, 6, 12, 24, 48 and 72 h following the induction of anaesthesia. Serum biochemistry was also evaluated at baseline and 72 h after anaesthesia. Induction and extubation times and time to sternal recumbency were also recorded during anaesthetic induction and recovery. Prior to the second anaesthesia (2 weeks later), the median sacral artery (MSA) was ligated under epidural anaesthesia in the experimental group. Sheep in the control group underwent sham operation. All sheep were anaesthetized as before. 4. Following the first halothane anaesthesia, SFC was significantly increased from 3 to 48 h compared with baseline. In the second stage of the experiment, the increases in SFC in the control group were similar to those seen in the first stage of the experiment. However, in MSA-ligated sheep, the increases in SFC were only significant between 3 and 12 h compared with baseline. The SFC was significantly higher in intact sheep from 3 to 72 h compared with the MSA-ligated group. Extubation and sternal recumbency times were significantly longer in intact sheep. 5. Ligation of the MSA in fat-tailed sheep induced a significant reduction in SFC, suggesting that the presence of tail fat substantially affects halothane metabolism during the peri-anaesthetic period in sheep. The greater extent of halothane biotransformation may be clinically important in, otherwise normal, obese patients.

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.

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.

Expression and Characterization of a Four-α-Helix Bundle Protein That Binds the Volatile General Anesthetic Halothane

The structural features of volatile anesthetic binding sites on proteins are being investigated with the use of a defined model system consisting of a four-alpha-helix bundle scaffold with a hydrophobic core. The current study describes the bacterial expression, purification, and initial characterization of the four-alpha-helix bundle (Aalpha(2)-L1M/L38M)(2). The alpha-helical content and stability of the expressed protein are comparable to that of the chemically synthesized four-alpha-helix bundle (Aalpha(2)-L38M)(2) reported earlier. The affinity for binding halothane is somewhat improved with a K(d) = 120 +/- 20 microM as determined by W15 fluorescence quenching, attributed to the L1M substitution. Near-UV circular dichroism spectroscopy demonstrated that halothane binding changes the orientation of the aromatic residues in the four-alpha-helix bundle. Nuclear magnetic resonance experiments reveal that halothane binding results in narrowing of the peaks in the amide region of the one-dimensional proton spectrum, indicating that bound anesthetic limits protein dynamics. This expressed protein should prove to be amenable to nuclear magnetic resonance structural studies on the anesthetic complexes, because of its relatively small size (124 residues) and the high affinities for binding volatile anesthetics. Such studies will provide much needed insight into how volatile anesthetics interact with biological macromolecules and will provide guidelines regarding the general architecture of binding sites on central nervous system proteins.

Towards a three-alpha-helix bundle protein that binds volatile general anesthetics

The general anesthetics halothane and chloroform are capable of binding to synthetic water-soluble four-alpha-helix bundles, which model the putative in vivo receptors. In this study, we investigate the binding of these anesthetics to synthetic water-soluble three-alpha-helix bundles. A series of variants containing up to four X-to-Ala and up to four X-to-Met substitutions was made; and the effect of these substitutions on structure, stability and anesthetic binding affinity was examined. Generally, the amount of alpha-helix and the stability of the three-alpha-helix bundles decreased as the number of X-to-Ala substitutions increased. A concomitant red-shift in tryptophan fluorescence lambdamax was seen, suggesting an increased flexibility of the native structure. Up to four X-to-Met substitutions had little effect on the amount of alpha-helix, but an increase in tryptophan lambdamax was seen for the variants with three and four methionine substitutions. The exceptions were a) a variant with a clustering of alanine and methionine residues at one end of the three-alpha-helix bundle, suggesting a gate structure that can admit ligand molecules; and b) a variant with a single Leu35Ala substitution, suggesting that at select positions, the size of the side chain is important for defining anesthetic binding affinity.

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.

Comparative binding character of two general anaesthetics for sites on human serum albumin

Propofol and halothane are clinically used general anaesthetics, which are transported primarily by HSA (human serum albumin) in the blood. Binding characteristics are therefore of interest for both the pharmacokinetics and pharmacodynamics of these drugs. We characterized anaesthetic-HSA interactions in solution using elution chromatography, ITC (isothermal titration calorimetry), hydrogen-exchange experiments and geometric analyses of high-resolution structures. Binding affinity of propofol to HSA was determined to have a K(d) of 65 microM and a stoichiometry of approx. 2, whereas the binding of halothane to HSA showed a K(d) of 1.6 mM and a stoichiometry of approx. 7. Anaesthetic-HSA interactions are exothermic, with propofol having a larger negative enthalpy change relative to halothane. Hydrogen-exchange studies in isolated recombinant domains of HSA showed that propofol-binding sites are primarily found in domain III, whereas halothane sites are more widely distributed. Both location and stoichiometry from these solution studies agree with data derived from X-ray crystal-structure studies, and further analyses of the architecture of sites from these structures suggested that greater hydrophobic contacts, van der Waals interactions and hydrogen-bond formation account for the stronger binding of propofol as compared with the less potent anaesthetic, halothane.

Influence of morphine sulfate on the halothane sparing effect of xylazine hydrochloride in horses

OBJECTIVE

To quantitate the dose and time-related effects of morphine sulfate on the anesthetic sparing effect of xylazine hydrochloride in halothane-anesthetized horses and determine the associated plasma xylazine and morphine concentration-time profiles.

ANIMALS

6 healthy adult horses.

PROCEDURE

Horses were anesthetized 3 times to determine the minimum alveolar concentration (MAC) of halothane in O2 and characterize the anesthetic sparing effect (ie, decrease in MAC of halothane) by xylazine (0.5 mg/kg, i.v.) administration followed immediately by i.v. administration of saline (0.9% NaCI) solution, low-dose morphine (0.1 mg/kg), or high-dose morphine (0.2 mg/kg). Selected parameters of cardiopulmonary function were also determined over time to verify consistency of conditions.

RESULTS

Mean (+/- SEM) MAC of halothane was 1.05 +/- 0.02% and was decreased by 20.1 +/- 6.6% at 49 +/- 2 minutes following xylazine administration. The amount of MAC reduction in response to xylazine was time dependent. Addition of morphine to xylazine administration did not contribute further to the xylazine-induced decrease in MAC (reductions of 21.9 +/- 1.2 and 20.7 +/- 1.5% at 43 +/- 4 and 40 +/- 4 minutes following xylazine-morphine treatments for low- and high-dose morphine, respectively). Overall, cardiovascular and respiratory values varied little among treatments. Kinetic parameters describing plasma concentration-time curves for xylazine were not altered by the concurrent administration of morphine.

CONCLUSIONS AND CLINICAL RELEVANCE

Administration of xylazine decreases the anesthetic requirement for halothane in horses. Concurrent morphine administration to anesthetized horses does not alter the anesthetic sparing effect of xylazine or its plasma concentration-time profile.

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.

Increased sensitivity of the ryanodine receptor to halothane-induced oligomerization in malignant hyperthermia-susceptible human skeletal muscle

Mutations in the skeletal muscle RyR1 isoform of the ryanodine receptor (RyR) Ca2+-release channel confer susceptibility to malignant hyperthermia, which may be triggered by inhalational anesthetics such as halothane. Using immunoblotting, we show here that the ryanodine receptor, calmodulin, junctin, calsequestrin, sarcalumenin, calreticulin, annexin-VI, sarco(endo)plasmic reticulum Ca2+-ATPase, and the dihydropyridine receptor exhibit no major changes in their expression level between normal human skeletal muscle and biopsies from individuals susceptible to malignant hyperthermia. In contrast, protein gel-shift studies with halothane-treated sarcoplasmic reticulum vesicles from normal and susceptible specimens showed a clear difference. Although the alpha2-dihydropyridine receptor and calsequestrin were not affected, clustering of the Ca2+-ATPase was induced at comparable halothane concentrations. In the concentration range of 0.014-0.35 mM halothane, anesthetic-induced oligomerization of the RyR1 complex was observed at a lower threshold concentration in the sarcoplasmic reticulum from patients with malignant hyperthermia. Thus the previously described decreased Ca2+-loading ability of the sarcoplasmic reticulum from susceptible muscle fibers is probably not due to a modified expression of Ca2+-handling elements, but more likely a feature of altered quaternary receptor structure or modified functional dynamics within the Ca2+-regulatory apparatus. Possibly increased RyR1 complex formation, in conjunction with decreased Ca2+ uptake, is of central importance to the development of a metabolic crisis in malignant hyperthermia.

Reductive activation of HCFC-123 by methaemalbumin

Hydrochlorofluorocarbon 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123), a close structural analogue of the hepatotoxic anaesthetic halotane and a replacement for some ozone-depleting chlorofluorocarbons, is metabolized by liver cytochrome P450 (P450), both in vitro and in vivo. P450 activates HCFC-123, both oxidatively and reductively, to reactive species which attack P450 itself and also damage other targets leading to hepatotoxicity. Previous work in our laboratory has shown that some haloalkanes, including halomethanes CCl4, CCl3Br, CHCl4 and CH2Cl2 as well as halothane, are activated by different haemoproteins to reactive metabolites resulting in the protein's suicidal inactivation. Among these is methaemalbumin (MHA), a synthetic complex of haem with human albumin often used as a model for various natural haemoproteins, such as P450. The aim of this study was to use MHA as a model to investigate the mechanism of P450 inactivation by HCFC-123. We found that MHA can reductively activate HCFC-123 to reactive species resulting in the loss of its haem group. During anaerobic incubation of MHA with 10 mM HCFC-123, a typical reduced difference spectrum was observed with a 470-nm peak that increased with time, indicating an interaction between HCFC-123 or HCFC-123 metabolites and haem. In similar anaerobic incubations, a significant loss of haem was measured using both the pyridine-haemochromogen technique and an ion-pairing reverse-phase HPLC method (37 and 30%, respectively). The loss of haem was time-, but not dose-dependent. No statistically significant loss of protoporphyrin IX, as measured by a fluorescence technique, or of the absolute haem spectrum produced in presence of CO (CO-haem complex) was observed up to 10 mM HCFC-123. Finally, a small but statistically significant inorganic fluoride production was measured in the presence of 20 mM HCFC-123 using an F(-)-specific electrode. Taken together, these results indicate that incubation of the non-enzymatic P450 model MHA with HCFC-123 under anaerobic conditions leads to reductive activation of the substrate, resulting in the modification of haem, as was previously shown to occur for halothane. The haem modification is due to interaction of the prosthetic haem group of MHA with HCFC-123 metabolites. These data confirm the results of previous work with rat liver microsomal P450 and confirm suicidal destruction of haem to be the mechanism responsible for the HCFC-123-dependent loss of the enzyme's content and catalytic function.

Effect of Four-α-Helix Bundle Cavity Size on Volatile Anesthetic Binding Energetics

Currently, it is thought that inhalational anesthetics cause anesthesia by binding to ligand-gated ion channels. This is being investigated using four-alpha-helix bundles, small water-soluble analogues of the transmembrane domains of the "natural" receptor proteins. The study presented here specifically investigates how multiple alanine-to-valine substitutions (which each decrease the volume of the internal binding cavity by 38 A(3)) affect structure, stability, and anesthetic binding affinity of the four-alpha-helix bundles. Structure remains essentially unchanged when up to four alanine residues are changed to valine. However, stability increases as the number of these substitutions is increased. Anesthetic binding affinities are also affected. Halothane binds to the four-alpha-helix bundle variants with 0, 1, and 2 substitutions with equivalent affinities but binds to the variants with 3 and 4 more tightly. The same order of binding affinities was observed for chloroform, although for a particular variant, chloroform was bound less tightly. The observed differences in binding affinities may be explained in terms of a modulation of van der Waals and hydrophobic interactions between ligand and receptor. These, in turn, could result from increased four-alpha-helix bundle binding cavity hydrophobicity, a decrease in cavity size, or improved ligand/receptor shape complementarity.

Concordance between trifluoroacetic acid and hepatic protein trifluoroacetylation after disulfiram inhibition of halothane metabolism in rats

BACKGROUND

Cytochrome P4502E1(CYP2E1)-mediated oxidation of halothane to a reactive intermediate (trifluoroacyl chloride) that covalently binds to hepatic proteins forming trifluoroacetylated neoantigens is believed to be the initiating event in a complex immunologic cascade culminating in antibody formation and severe hepatic necrosis ('halothane hepatitis') in susceptible patients. Trifluoroacyl chloride may also hydrolyze to the stable metabolite trifluoroacetic acid (TFA). CYP2E1 inactivation by disulfiram or its primary metabolite, diethyldithiocarbamate, inhibits human halothane oxidation to TFA in vitro and in vivo. Nevertheless, disulfiram effects on hepatic protein trifluoroacetylation by halothane in vivo are unknown. This investigation tested the hypotheses that disulfiram prevents halothane-dependent protein trifluoroacetylation in vivo, and that TFA represents a biomarker for hepatic protein trifluoroacetylation.

METHODS

Rats were pretreated with isoniazid (CYP2E1 induction), isoniazid followed by disulfiram (CYP2E1 inhibition), or nothing (controls), then anesthetized with halothane or nothing (controls). Plasma and urine TFA were quantified by ion HPLC; hepatic microsomal TFA-proteins were analyzed by Western blot.

RESULTS

CYP2E1 induction increased both TFA and TFA-protein formation compared with uninduced halothane-treated rats. Disulfiram, even after CYP2E1 induction, nearly abolished both TFA and TFA-protein formation. Pretreatments similarly affected both TFA and TFA-protein formation across all groups.

CONCLUSIONS

Disulfiram inhibition of CYP2E1-mediated halothane oxidation prevents hepatic protein trifluoroacetylation. Based on the concordance between TFA and TFA-protein formation, TFA appears to be a valid biomarker for TFA-protein formation. Disulfiram inhibition of human halothane oxidation in vivo, previously assessed by diminished TFA formation, probably also confers inhibition of hepatic TFA-protein formation.

A Caenorhabditis elegans pheromone antagonizes volatile anesthetic action through a go-coupled pathway

Volatile anesthetics (VAs) disrupt nervous system function by an ill-defined mechanism with no known specific antagonists. During the course of characterizing the response of the nematode C. elegans to VAs, we discovered that a C. elegans pheromone antagonizes the VA halothane. Acute exposure to pheromone rendered wild-type C. elegans resistant to clinical concentrations of halothane, increasing the EC(50) from 0.43 +/- 0.03 to 0.90 +/- 0.02. C. elegans mutants that disrupt the function of sensory neurons required for the action of the previously characterized dauer pheromone blocked pheromone-induced resistance (Pir) to halothane. Pheromone preparations from loss-of-function mutants of daf-22, a gene required for dauer pheromone production, lacked the halothane-resistance activity, suggesting that dauer and Pir pheromone are identical. However, the pathways for pheromone's effects on dauer formation and VA action were not identical. Not all mutations that alter dauer formation affected the Pir phenotype. Further, mutations in genes not known to be involved in dauer formation completely blocked Pir, including those altering signaling through the G proteins Goalpha and Gqalpha. A model in which sensory neurons transduce the pheromone activity through antagonistic Go and Gq pathways, modulating VA action against neurotransmitter release machinery, is proposed.

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.

Low-affinity analytical chromatography for measuring inhaled anesthetic binding to isolated proteins

The direct measure of volatile anesthetic binding to protein is complicated by weak affinity and therefore rapid kinetics. Consequently, several puted targets for these clinically important drugs have only functional data to support a direct mode of action. While several methods for measuring some aspects of binding are available, all have significant limitations. We introduce the use of analytical chromatography for the purpose of directly measuring volatile anesthetic binding to protein, and show that it can provide estimates of both affinity and stoichiometry for proteins that can be obtained in fairly high purity and mass. Using this approach we characterize halothane binding to serum albumin as low affinity and multisite, and to myoglobin or cytochrome C as strictly nonspecific. This approach will be useful in directly characterizing equilibrium, solution binding to isolated proteins in preparation for more time-consuming methods with structural resolution.

The effect of magnesium deficiency on volatile anaesthetic requirement in the rat: the role of central noradrenergic neuronal activity

Volatile anaesthetic minimum alveolar concentration (MAC, a measure of anaesthetic requirement) increased in a time-dependent manner in rats fed a Mg2+-deficient diet. MAC values in hypomagnesemic rats were 22-30 per cent greater than those in age-matched controls at 12 and 17 days after starting the diet (p < 0.01). Noradrenergic neuronal activity, as assessed from the ratio of the concentration of 3,4-dihydroxyphenylethylene-glycol (DHPG) to that of norepinephrine (NE), decreased in the brain stem and cerebrum-cerebellum in hypomagnesemic rats owing to an increase in NE concentration in both regions of the brain (p < 0.025). We conclude that prolonged hypomagnesemia (> or = 12 days) increases volatile anaesthetic MAC in the rat. The concomitant decrease in the ratio of DHPG/NE suggests that this increase in MAC cannot be attributed to an increase in noradrenergic neuronal activity in brain.

Bioactivation and cytotoxicity of 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123) in isolated rat hepatocytes

The bioactivation and cytotoxicity of 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123), a replacement for some ozone-depleting chlorofluorocarbons, were investigated using freshly isolated hepatocytes from non-induced male rats. A time- and concentration-dependent increase in the leakage of lactate dehydrogenase and a concentration-dependent loss of total cellular glutathione were observed in cells incubated with 1, 5 and 10 mM HCFC-123 under normoxic or hypoxic (about 4% O2) conditions. Lactate dehydrogenase leakage was completely prevented by pretreating the cell suspension with the free radical trapper N-t-butyl-alpha-phenylnitrone. The aspecific cytochrome P450 (P450) inhibitor, metyrapone, totally prevented the lactate dehydrogenase leakage from hepatocytes, while two isoform-specific P450 inhibitors, 4-methylpyrazole and troleandomycin (a P450 2E1 and a P450 3A inhibitor, respectively), provided a partial protection against HCFC-123 cytotoxicity. Interestingly, pretreatment of cells with glutathione depletors, such as phorone and diethylmaleate, did not enhance the HCFC-123-dependent lactate dehydrogenase leakage. Two stable metabolites of HCFC-123, 1-chloro-2,2,2-trifluoroethane and 1-chloro-2,2-difluoroethene, were detected by gas chromatography/mass spectrometry analysis of the head space of the hepatocyte incubations carried out under hypoxic and, although at a lower level, also normoxic conditions, indicating that reductive metabolism of HCFC-123 by hepatocytes had occurred. The results overall indicate that HCFC-123 is cytotoxic to rat hepatocytes under both normoxic and hypoxic conditions, due to its bioactivation to reactive metabolites, probably free radicals, and that P450 2E1 and, to a lower extent, P450 3A, are involved in the process.

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.

Cooperative binding of inhaled anesthetics and ATP to firefly luciferase

Firefly luciferase is considered a reasonable model of in vivo anesthetic targets despite being destabilized by anesthetics, as reflected by differential scanning calorimetry (DSC). We examined the interaction between two inhaled anesthetics, ATP, luciferase, and temperature, using amide hydrogen exchange, tryptophan fluorescence, and photolabeling in an attempt to examine this apparent discrepancy. In the absence of ATP/Mg2+, halothane and bromoform cause destabilization, as measured by hydrogen exchange, suggesting nonspecific interactions. In the presence of ATP/Mg2+ and at room temperature, the anesthetics produce considerable stabilization with a negative DeltaH, indicating population of a conformer with a specific anesthetic binding site. Stabilizing interactions are lost, however, at unfolding temperatures. We suggest that preferential binding to aggregated forms of luciferase explain the higher temperature destabilization detected with DSC. Our results demonstrate a cooperative binding equilibrium between native ligands and anesthetics, suggesting that similar interactions could underlie actions at biologically relevant targets.

Effects of halothane on excitatory neurotransmission to medullary expiratory neurons in a decerebrate dog model

BACKGROUND

The activity of canine expiratory (E) neurons in the caudal ventral respiratory group is primarily dependent on N-methyl-D-aspartic acid (NMDA) receptor-mediated excitatory chemodrive inputs and modulated by an inhibitory mechanism mediated via gamma-aminobutyric acidA (GABA(A)) receptors. In an intact canine preparation, halothane depressed the activity of these neurons mainly by reduction in overall glutamatergic excitation. A new decerebrate preparation allows comparison of the effects of halothane on these synaptic mechanisms with an anesthetic-free baseline state.

METHODS

Two separate studies were performed in decerebrate, vagotomized, paralyzed, mechanically ventilated dogs during hypercapnic hyperoxia. In study 1, the effect of 1 minimum alveolar concentration (MAC) halothane on extracellularly recorded E neuronal activity was studied before and during complete GABA(A) receptor blockade by localized pressure ejection of bicuculline. Complete blockade of the inhibitory mechanism allowed differentiation between the effects of halothane on overall GABA(A)-mediated inhibition and on overall NMDA receptor-mediated excitation. In study 2, the effect of 1 MAC halothane on the dose response of neurons to localized picoejection of the glutamate agonist NMDA was used to estimate halothane effect on postsynaptic glutamatergic excitatory neurotransmission.

RESULTS

In study 1, the spontaneous activity of 14 E neurons was depressed 38.6 +/- 20.6% (mean +/- SD) by 1 MAC halothane. Overall excitation was depressed 31.5 +/- 15.5%. The GABAergic inhibition showed a 11.7 +/- 18.3% enhancement during halothane. In study 2, the spontaneous activity of 13 E neurons was again significantly depressed by 1 MAC halothane (27.9 +/- 10.6%), but the postsynaptic response of the neurons to exogenous NMDA was not significantly depressed by halothane (3.3 +/- 38.4%).

CONCLUSIONS

Together these results suggest that in our E neuron paradigm, halothane exerted its depressive effect mainly via reduction of glutamatergic presynaptic mechanisms.

The effects of chronic exercise on anesthesia induced hepatotoxicity

A hypoxic rat model of halothane-induced hepatotoxicity, which is known to produce liver damage, was used to determine the effects of chronic exercise on halothane-induced hepatotoxicity and on reduced hepatic glutathione (GSH) levels. Metabolism of volatile anesthetics may generate metabolites that can cause mild and transient hepatotoxicity.

METHODS

Six male Sprague-Dawley rats completed a 10-wk (5 d x wk(-1)) treadmill running protocol. Twelve age-matched animals were used as sedentary controls. After the completion of exercise training, rats were exposed for 2 h to 1% halothane in 14% O2. Twenty-four hours later, animals were anesthetized with sodium pentobarbital and sacrificed. Livers were excised, stained, and evaluated for hepatotoxicity using a histopathological 0 (normal) to 5 (severe damage) point categorical scale and for the determination of GSH levels.

RESULTS

Median histopathologic scores revealed significantly lower indications of hepatotoxicity in exercise animals as compared with control animals (score = 0.25 vs 1.50; P < 0.05). Liver damages scores between 1 and 5 were observed in 75% (9 of 12) of the control animals, whereas only 1 of 6 exercise animals had a score greater than 1 (P < 0.05). No significant difference was observed in reduced GSH levels.

CONCLUSIONS

Chronic exercise improves the detoxicant ability of the liver for halothane anesthesia as noted by the ameliorated liver damage and reduced incidence of halothane-induced hepatotoxicity in the exercise animals.

Inhaled anesthetic binding sites in human serum albumin

Previous evidence suggests multiple anesthetic binding sites on human serum albumin, but to date, we have only identified Trp-214 in an interdomain cleft as contributing to a binding site. We used a combination of site-directed mutagenesis, photoaffinity labeling, amide hydrogen exchange, and tryptophan fluorescence spectroscopy to evaluate the importance to binding of a large domain III cavity and compare it to binding character of the 214 interdomain cleft. The data show anesthetic binding in this domain III cavity of similar character to the interdomain cleft, but selectivity for different classes of anesthetics exists. Occupancy of these sites stabilizes the native conformation of human serum albumin. The features necessary for binding in the cleft appear to be fairly degenerate, but in addition to hydrophobicity, there is evidence for the importance of polarity. Finally, myristate isosterically competes with anesthetic binding in the domain III cavity and allosterically enhances anesthetic binding in the interdomain cleft.

Molecular dynamics simulation of four-alpha-helix bundles that bind the anesthetic halothane

The mutation of a single leucine residue (L38) to methionine (M) is known experimentally to significantly increase the affinity of the synthetic four-alpha-helix bundle (Aalpha(2))(2) for the anesthetic halothane. We present a molecular dynamics study of the mutant (Aalpha(2)-L38M)(2) peptide, which consists of a dimer of 62-residue U-shaped di-alpha-helical monomers assembled in an anti topology. A comparison between the simulation results and those obtained for the native (Aalpha(2))(2) peptide indicates that the overall secondary structure of the bundle is not affected by the mutation, but that the side chains within the monomers are better packed in the mutant structure. Unlike the native peptide, binding of a single halothane molecule to the hydrophobic core of (Aalpha(2)-L38M)(2) deforms the helical nature of one monomer in a region close to the mutation site. Increased exposure of the cysteine side chain to the hydrophobic core in the mutant structure leads to the enhancement of the attractive interaction between halothane and this specific residue. Since the mutated residues are located outside the hydrophobic core the observed increased affinity for halothane appears to be an indirect effect of the mutation.

Halothane binding to a G protein coupled receptor in retinal membranes by photoaffinity labeling

General anesthetics have been reported to alter the functions of G protein coupled receptor (GPCR) signaling systems. To determine whether these effects might be mediated by direct binding interactions with the GPCR or its associated G protein, we studied the binding character of halothane on mammalian rhodopsin, structurally the best understood GPCR, by using direct photoaffinity labeling with [(14)C]halothane. In the bleached bovine rod disk membranes (RDM), opsin and membrane lipids were dominantly photolabeled with [(14)C]halothane, but none of the three G protein subunits were labeled. In opsin itself, halothane labeling was inhibited by unlabeled halothane with an IC(50) of 0.9 mM and a Hill coefficient of -0.8. The stoichiometry was 1.1:1.0 (halothane:opsin molar ratio). The IC(50) values of isoflurane and 1-chloro-1,2, 2-trifluorocyclobutane were 5.0 and 15 mM, respectively. Ethanol had no effect on opsin labeling by halothane. A nonimmobilizer, 1, 2-dichlorohexafluorocyclobutane, inhibited halothane labeling by 50% at 0.05 mM. The present results demonstrate that halothane binds specifically and selectively to GPCRs in the RDM. The absence of halothane binding to any of the G protein subunits strongly suggests that the functional effects of halothane on GPCR signaling systems are mediated by direct interactions with receptor proteins.

General anesthetic binding to gramicidin A: the structural requirements

There is a distinct possibility that general anesthetics exert their action on the postsynaptic receptor channels. The structural requirements for anesthetic binding in transmembrane channels, however, are largely unknown. High-resolution (1)H nuclear magnetic resonance and direct photoaffinity labeling were used in this study to characterize the volatile anesthetic binding sites in gramicidin A (gA) incorporated into sodium dodecyl sulfate (SDS) micelles and into dimyristoylphosphatidylcholine (DMPC) bilayers, respectively. To confirm that the structural arrangement of the peptide side chains can affect anesthetic binding, gA in nonchannel forms in methanol was also analyzed. The addition of volatile anesthetic halothane to gA in SDS with a channel conformation caused a concentration-dependent change in resonant frequencies of the indole amide protons of W9, W11, W13, and W15, with the most profound changes in W9. These frequency changes were observed only for gA carefully prepared to ensure a channel conformation and were absent for gA in methanol. For gA in DMPC bilayers, direct [(14)C]halothane photolabeling and microsequencing demonstrated dominant labeling of W9, less labeling of W11 and W13, and no significant labeling of W15. In methanol, gA showed much less labeling of any residues. Inspection of the 3-D structure of gA suggests that the spatial arrangements of the tryptophan residues in the channel form of gA, combined with the amphiphilic regions of lipid, create a favorable anesthetic binding motif.

Human halothane metabolism, lipid peroxidation, and cytochromes P(450)2A6 and P(450)3A4

OBJECTIVE

Halothane undergoes both oxidative and reductive metabolism by cytochrome P450 (CYP), respectively causing rare immune-mediated hepatic necrosis and common, mild subclinical hepatic toxicity. Halothane also causes lipid peroxidation in rodents in vitro and in vivo, but in vivo effects in humans are unknown. In vitro investigations have identified a role for human CYPs 2E1 and 2A6 in oxidation and CYPs 2A6 and 3A4 in reduction. The mechanism-based CYP2E1 inhibitor disulfiram diminished human halothane oxidation in vivo. This investigation tested the hypotheses that halothane causes lipid peroxidation in humans in vivo, and that CYP2A6 or CYP3A4 inhibition can diminish halothane metabolism.

METHODS

Patients (n = 9 each group) received single doses of the mechanism-based inhibitors troleandomycin (CYP3A4), methoxsalen (CYP2A6) or nothing (controls) before a standard halothane anaesthetic. Reductive halothane metabolites chlorotrifluoroethane and chlorodifluoroethylene in exhaled breath, fluoride in urine, and oxidative metabolites trifluoroacetic acid and bromide in urine were measured for 48 h postoperatively. Lipid peroxidation was assessed by plasma F2-isoprostane concentrations.

RESULTS

The halothane dose was similar in all groups. Methoxsalen decreased 0- to 8-h trifluoroacetic acid (23 +/- 20 micromol vs 116 +/- 78 micromol) and bromide (17 +/- 11 micromol vs 53 +/- 49 micromol) excretion (P < 0.05), but not thereafter. Plasma F2-isoprostanes in controls were increased from 8.5 +/- 4.5 pg/ml to 12.5 +/- 5.0 pg/ml postoperatively (P < 0.05). Neither methoxsalen nor troleandomycin diminished reductive halothane metabolite or F2-isoprostane concentrations.

CONCLUSIONS

These results provide the first evidence for halothane-dependent lipid peroxidation in humans. Methoxsalen effects on halothane oxidation confirm in vitro results and suggest limited CYP2A6 participation in vivo. CYP2A6-mediated, like CYP2E1-mediated human halothane oxidation, can be inhibited in vivo by mechanism-based CYP inhibitors. In contrast, clinical halothane reduction and lipid peroxidation were not amenable to suppression by CYP inhibitors.

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.

Effects of barbiturates on facilitative glucose transporters are pharmacologically specific and isoform selective

Barbiturates inhibit GLUT-1-mediated glucose transport across the blood-brain barrier, in cultured mammalian cells, and in human erythrocytes. Barbiturates also interact directly with GLUT-1. The hypotheses that this inhibition of glucose transport is (i) selective, preferring barbiturates over halogenated hydrocarbon inhalation anesthetics, and (ii) specific, favoring some GLUT-# isoforms over others were tested. Several oxy- and thio-barbiturates inhibited [3H]-2-deoxyglucose uptake by GLUT-1 expressing murine fibroblasts with IC50s of 0.2-2.9 mm. Inhibition of GLUT-1 by barbiturates correlates with their overall lipid solubility and pharmacology, and requires hydrophobic side chains on the core barbiturate structure. In contrast, several halogenated hydrocarbons and ethanol (all </=10 mm) do not significantly inhibit glucose transport. The interaction of these three classes of anesthetics with purified GLUT-1 was evaluated by quenching of intrinsic protein fluorescence and displayed similar specificities and characteristics. The ability of barbiturates to inhibit other facilitative glucose transporters was determined in cell types expressing predominantly one isoform. Pentobarbital inhibits [3H]-2-deoxyglucose and [14C]-3-O-methyl-glucose uptake in cells expressing GLUT-1, GLUT-2, and GLUT-3 with IC50s of approximately 1 mm. In contrast, GLUT-4 expressed in insulin-stimulated rat adipocytes was much less sensitive than the other isoforms to inhibition by pentobarbital (IC50 of >10 mm). Thus, barbiturates selectively inhibit glucose transport by some, but not all, facilitative glucose transporter isoforms.

Mutation screening of the RYR1 gene and identification of two novel mutations in Italian malignant hyperthermia families

Point mutations in the ryanodine receptor (RYR1) gene are associated with malignant hyperthermia, an autosomal dominant disorder triggered in susceptible people (MHS) by volatile anaesthetics and depolarising skeletal muscle relaxants. To date, 17 missense point mutations have been identified in the human RYR1 gene by screening of the cDNA obtained from muscle biopsies. Here we report single strand conformation polymorphism (SSCP) screening for nine of the most frequent RYR1 mutations using genomic DNA isolated from MHS patients. In addition, the Argl63Cys mutation was analysed by restriction enzyme digestion. We analysed 57 unrelated patients and detected seven of the known RYR1 point mutations. Furthermore, we found a new mutation, Arg2454His, segregating with the MHS phenotype in a large pedigree and a novel amino acid substitution at position 2436 in another patient, indicating a 15.8% frequency of these mutations in Italian patients. A new polymorphic site in intron 16 that causes the substitution of a G at position -7 with a C residue was identified.

Bound volatile general anesthetics alter both local protein dynamics and global protein stability

BACKGROUND

Recent studies have demonstrated that volatile general anesthetic agents such as halothane and isoflurane may bind to discrete sites on protein targets. In the case of bovine serum albumin, the sites of halothane and chloroform binding have been identified as being located in the IB and IIA subdomains. This structural information provides a foundation for more detailed studies into the potential mechanisms of anesthetic action.

METHODS

The effect of halothane and isoflurane and the nonimmobilizer 1,2-dichlorohexafluorocyclobutane on the mobility of the indole ring in the tryptophan residues of albumin was investigated using measurements of fluorescence anisotropy. Myoglobin served as a negative control. In addition, the effect of bound anesthetic agents on global protein stability was determined by thermal denaturation experiments using near-ultraviolet circular dichroism spectroscopy.

RESULTS

The fluorescence anisotropy measurements showed that halothane and isoflurane decreased the mobility of the indole rings in a concentration-dependent manner. The calculated dissociation constants were 1.6+/-0.4 and 1.3+/-0.3 mM for isoflurane and halothane, respectively. In contrast, both agents failed to increase the fluorescence anisotropy of the tryptophan residues in myoglobin, compatible with lack of binding. The nonimmobilizer 1,2-dichlorohexafluorocyclobutane caused no change in the fluorescence anisotropy of albumin. Binding of the anesthetic agents stabilized the native folded form of albumin to thermal denaturation. Analysis of the thermal denaturation data yielded dissociation constant values of 0.98+/-0.10 mM for isoflurane and 1.0+/-0.1 mM for halothane.

CONCLUSIONS

Attenuation of local side-chain dynamics and stabilization of folded protein conformations may represent fundamental modes of action of volatile general anesthetic agents. Because protein activity is crucially dependent on inherent flexibility, anesthetic-induced stabilization of certain protein conformations may explain how these important clinical agents change protein function.

Differential halothane binding and effects on serum albumin and myoglobin

To understand further the weak molecular interactions between inhaled anesthetics and proteins, we studied the character and dynamic consequences of halothane binding to bovine serum albumin (BSA) and myoglobin using photoaffinity labeling and hydrogen-tritium exchange (HX). We find that halothane binds saturably and with submillimolar affinity to BSA, but either nonspecifically or with considerably lower affinity to myoglobin. Titration of halothane binding with guanidine hydrochloride suggested more protection of binding sites from solvent in BSA as compared with myoglobin. Protection factors for slowly exchanging albumin hydrogens are increased in a concentration-dependent manner by up to 27-fold with 10 mM halothane, whereas more rapidly exchanging groups of albumin hydrogens have either unaltered or decreased protection factors. Protection factors for slowly exchanging hydrogens in myoglobin are decreased by halothane, suggesting destabilization through binding to an intermediate or completely unfolded conformer. These results demonstrate the conformation dependence of halothane binding and clear dynamic consequences that correlate with the character of binding in these model proteins. Preferential binding and stabilization of different conformational states may underlie anesthetic-induced protein dysfunction, as well as provide an explanation for heterogeneity of action.

Intracellular calcium homeostasis in human primary muscle cells from malignant hyperthermia-susceptible and normal individuals. Effect Of overexpression of recombinant wild-type and Arg163Cys mutated ryanodine receptors

Malignant hyperthermia (MH) is a hypermetabolic disease triggered by volatile anesthetics and succinylcholine in genetically predisposed individuals. Nine point mutations in the skeletal muscle ryanodine receptor (RYR) gene have so far been identified and shown to correlate with the MH-susceptible phenotype, yet direct evidence linking abnormal Ca2+ homeostasis to mutations in the RYR1 cDNA has been obtained for few mutations. In this report, we show for the first time that cultured human skeletal muscle cells derived from MH-susceptible individuals exhibit a half-maximal halothane concentration causing an increase in intracellular Ca2+ concentration which is twofold lower than that of cells derived from MH-negative individuals. We also present evidence demonstrating that overexpression of wild-type RYR1 in cells obtained from MH-susceptible individuals does not restore the MH-negative phenotype, as far as Ca2+ transients elicited by halothane are concerned; on the other hand, overexpression of a mutated RYR1 Arg163Cys Ca2+ channel in muscle cells obtained from MH-negative individuals conveys hypersensitivity to halothane. Finally, our results show that the resting Ca2+ concentration of cultured skeletal muscle cells from MH-negative and MH-susceptible individuals is not significantly different.

Immunochemical detection and identification of protein adducts of diclofenac in the small intestine of rats: possible role in allergic reactions

Idiosyncratic adverse drug reactions are unpredictable, target multiple organ systems, and often become life-threatening events. Although the causes of idiosyncratic adverse drug reactions are not known in most cases, evidence suggests that they may be mediated through immunological mechanisms. It is generally thought that for a drug to lead to an immune response, it must first become covalently bound to a carrier protein. Since most drugs are unreactive, it is usually a reactive metabolite that is expected to form covalent adducts. However, it is not clear why more people do not develop immune reactions against drug-protein adducts. One possible explanation is that orally administered drugs may lead to oral tolerance in most individuals through mechanisms similar to that found with orally administered antigens. However, very little is known regarding the interaction of drugs with gut-associated lymphoid tissue of the small intestine, where oral tolerance can develop. As an initial step to test this hypothesis, we have investigated whether diclofenac, a commonly used nonsteroidal antiinflammatory drug, can lead to protein adducts in rat small intestine. Diclofenac was administered to rats by gastric gavage. Immunoblot analysis of small intestine homogenates and isolated enterocyte subcellular fractions with drug-specific antiserum revealed 142-, 130-, 110-, and 55-kDa protein adducts of diclofenac. The 142- and 130-kDa adducts of diclofenac were identified as aminopeptidase N (CD13) and sucrase-isomaltase, respectively, by amino acid sequence analyses and by their reactions with protein-specific antibodies. The adducts were localized by immunohistochemistry and found primarily in the mid-villus and villus-tip enterocytes and also in the dome overlying Peyer's patches. Similar adducts were detected immunochemically in villus-tip enterocytes of animals treated with halothane or acetaminophen. These results show that intestinal protein adducts of drugs can be formed in gut-associated lymphoid tissue where they may lead to the down-regulation of drug-induced allergic reactions in many individuals.

A designed cavity in the hydrophobic core of a four-alpha-helix bundle improves volatile anesthetic binding affinity

The structural features of protein binding sites for volatile anesthetics are being explored using a defined model system consisting of a four-alpha-helix bundle scaffold with a hydrophobic core. Earlier work has demonstrated that a prototype hydrophobic core is capable of binding the volatile anesthetic halothane. Exploratory work on the design of an improved affinity anesthetic binding site is presented, based upon the introduction of a simple cavity into a prototype (alpha 2)2 four-alpha-helix bundle by replacing six core leucines with smaller alanines. The presence of such a cavity increases the affinity (Kd = 0.71 +/- 0.04 mM) of volatile anesthetic binding to the designed bundle core by a factor of 4.4 as compared to an analogous bundle core lacking such a cavity (Kd = 3.1 +/- 0.4 mM). This suggests that such packing defects present on natural proteins are likely to be occupied by volatile general anesthetics in vivo. Replacing six hydrophobic core leucine residues with alanines results in a destabilization of the folded bundle by 1.7-2.7 kcal/mol alanine, although the alanine-substituted bundle still exhibits a high degree of thermodynamic stability with an overall folded conformational delta GH2O = 14.3 +/- 0.8 kcal/mol. Covalent attachment of the spin label MTSSL to cysteine residues in the alanine-substituted four-alpha-helix bundle indicates that the di-alpha-helical peptides dimerize in an anti orientation. The rotational correlation time of the four-alpha-helix bundle is 8.1 +/- 0.5 ns, in line with earlier work on similar peptides. Fluorescence, far-UV circular dichroism, and Fourier transform infrared spectroscopies verified the hydrophobic core location of the tryptophan and cysteine residues, showing good agreement between experiment and design. These small synthetic proteins may prove useful for the study of the structural features of small molecule binding sites.