Spectroscopic analysis of halothane binding to the plasma membrane Ca2+-ATPase

The intrinsic tryptophan (Trp) fluorescence of the plasma membrane Ca2+-ATPase (PMCA) is significantly quenched by halothane, a volatile anesthetic common in clinical practice. It has been proposed that halothane inhibition of the Ca2+-ATPase activity results from conformational changes following anesthetic binding in the enzyme. We have investigated whether the observed quenching reflects halothane binding to PMCA. We have shown that the quenching is dose dependent and saturable and can be fitted to a binding curve with an equilibrium constant K(Hal) = 2.1 mM, a concentration at which the anesthetic approximately half-maximally inhibits the Ca2+-ATPase activity. The relatively low sensitivity of halothane quenching of Trp fluorescence to the concentration of phosphatidylcholine and detergent in the PMCA preparation concurs with the quenching resulting from anesthetic binding in the PMCA molecule. Analysis of the Trp fluorescence quenching by acrylamide indicates that the Trp residues are not considerably exposed to the solvent (Stern-Volmer quenching constant of 2.9 M(-1)) and do not differ significantly in their accessibility to halothane. Other volatile anesthetics, diethyl ether and diisopropyl ether, reduce the quenching caused by halothane in a dose-dependent manner, suggesting halothane displacement from its binding site(s). These observations indicate that halothane quenching of intrinsic Trp fluorescence of PMCA results from anesthetic binding to the protein. The analysis, used as a complementary approach, provides new information to the still rudimentary understanding of the process of anesthetic interaction with membrane proteins.

Effects of i.v. anaesthetic agents and halothane on [3H]tetracaine binding to rat cerebrocortical membranes

In an attempt to determine if there is any overlap in local and general anaesthetic binding sites, we have examined the effects of thiopental, phenobarbital, pentobarbital, propofol, ketamine (racemic and R(+)/S(-)), alfaxalone, etomidate and halothane on [3H]tetracaine binding to rat cerebrocortical membranes. Membranes were prepared in Tris HCI 50 mmol litre-1, pH 7.4, by homogenization and centrifugation. Binding assays were performed in 1-ml volumes of Tris HCI buffer at room temperature or 37 degrees C for halothane. Binding of [3H]tetracaine was displaced dose-dependently by unlabelled tetracaine with a mean pIC50 value of 6.91 (SEM 0.07) (123 nmol litre-1). With the exception of propofol (at high concentrations), all i.v. anaesthetic agents failed to displace the binding of [3H]tetracaine. In contrast, halothane produced a dose-dependent and statistically significant reduction in total [3H]tetracaine binding at clinically achievable concentrations (0.289, 0.885 and 1.484 mmol litre-1 equivalent to 1.0, 3.1 and 5.1 rat MAC) without markedly affecting the pIC50. Collectively these data may suggest some overlap in the binding sites for [3H]tetracaine and volatile but not i.v. general anaesthetic agents.

Cytochrome P-4502E1-dependent formation of trifluoroacetyl adducts from halothane by transduced HepG2 cells

E-9 cells, a HepG2 cell line that has the alcohol-inducible human cytochrome P-4502E1 (CYP2E1) cloned into its genome, was tested for its ability to produce trifluoroacetyl (TFA) adducts from the metabolism of halothane. The metabolism of halothane results in the production of TFA halide that can readily react with cellular proteins to form TFA adducts, which can be detected by antibodies directed against them. E-9 cells formed TFA adducts when incubated with halothane. The major consistent bands that were detected were of molecular weights of approximately 50, 78, and 100 kDa. The formation of these adducts was dependent on halothane concentration and time of incubation with halothane. MV-5 cells, the control cell line that has the viral vector without the P-450, did not produce any adducts. Inhibitors of CYP2E1 function, such as 4-methylpyrazole, inhibited adduct formation. Furthermore, phorbol esters, which have been shown to increase the CYP2E1 level in this cell line, increased TFA adduct formation. This HepG2 cell line may be of value in studying the metabolism and toxicity of halothane in a human cell culture model.

Effect of high volume epidural morphine, ketamine and butorphanol on halothane minimum alveolar concentration in ponies

This study determined the effects of epidurally administered morphine, ketamine and butorphanol on halothane minimum alveolar concentration (MAC) in ponies. Seven ponies were anaesthetised with thiopentone and succinylcholine, intubated and anaesthesia maintained with halothane. Ventilation was controlled and blood pressure was maintained within normal limits. Following the determination of baseline halothane MAC for the pelvic and thoracic limbs the ponies were given morphine (0.1 mg/kg bwt), ketamine (0.8 or 1.2 mg/kg bwt), butorphanol (0.05 mg/kg bwt) or saline, epidurally, to a final volume of 0.15 ml/kg bwt. The halothane MAC for the pelvic and thoracic limbs was redetermined following each treatment. The baseline halothane MAC for the control group was mean +/- s.e. 0.85 +/- 0.02% and no significant change occurred after saline administration. Morphine significantly (P = 0.002) decreased MAC from, mean +/- s.e. 0.90 +/- 0.05% to 0.77 +/- 0.06% in the pelvic limb. Ketamine significantly decreased MAC in the pelvic limb from mean +/- s.e. 0.86 +/- 0.06% to 0.71 +/- 0.04%, and 0.82 +/- 0.03% to 0.71 +/- 0.02%, for the low (P = 0.008) and high dose (P = 0.001), respectively. No significant change in MAC occurred following butorphanol. No treatment reduced halothane MAC for the thoracic limb.

Effect of acepromazine and butorphanol on halothane minimum alveolar concentration in ponies

The effect of i.v. acepromazine (0.05 mg/kg bwt), butorphanol (0.05 mg/kg bwt) and a combination of acepromazine and butorphanol on halothane minimum alveolar concentration (MAC) was determined in 7 mixed-breed ponies. Ventilation was controlled, and blood pressure and temperature were maintained within normal limits. Following the determination of baseline MAC, treatments were administered to each pony in a random manner. The control treatment was normal saline. The baseline halothane dMAC for the control group was 0.91 +/- 0.04%, and no significant change occurred after saline administration. Acepromazine decreased (P = 0.0001) the halothane MAC from mean +/- s.e. 0.92 +/- 0.02% to 0.58 +/- 0.04%, and the combination of acepromazine and butorphanol, decreased (P = 0.003) halothane MAC, from mean +/- s.e. 0.95 +/- 0.04% to 0.59 +/- 0.06%. This represents a decrease of 36.9 and 37.8%, respectively. However, the difference between these 2 treatments was not significant. Butorphanol did not significantly change the mean group value for MAC; nevertheless, 3 ponies had an increase, one a decrease, while the MAC did not change in the remaining 3 ponies following butorphanol treatment.

Entropy-driven interactions of anesthetics with membrane proteins

Thermodynamic analysis of anesthetic effects on Ca2+-ATPase activity was performed to evaluate the feasibility of anesthetic binding and gain insight into the molecular events underlying the anesthetic-enzyme interactions. The Ca2+-ATPases, integral membrane proteins vital in cellular Ca2+ regulation, are suitable models for investigation of the mechanism of anesthetic action on membrane proteins that are targeted by the anesthetics. Ca2+-ATPase of plasma membrane, PMCA, and SERCA1 in the intracellular sarcoplasmic reticulum membrane were used to study two general anesthetics: halothane, a halogenated two-carbon alkane; and propofol, an intravenous, strongly lipophilic-substituted phenol. Interactions of both anesthetics result in a negative Gibbs free energy change, which in both enzymes is more favorable for the more lipophilic propofol than halothane. Temperature dependence (more negative change in Gibbs free energy at increased temperature) is in agreement with predominantly nonpolar interactions. The interactions are entropy-driven, characterized by positive enthalpy which is overcompensated by positive entropy changes. This is in contrast to the reported in literature enthalpy-driven anesthetic binding to soluble proteins. The possible contributions to the observed positive entropy change are discussed including displacement of ordered water molecules by anesthetic binding in nonpolar cavities in the membrane proteins and subtle structural rearrangements.

Focal heat stimulation for the determination of the minimum alveolar concentration of halothane in the rabbit

A focal heat stimulus of 54.37 +/- 0.07 (SD) degrees C was applied for 30 s to the inner aspect of the pinna of the ear for the determination of the minimum alveolar concentration (MAC) of halothane in New Zealand White rabbits. The latency before head movement was measured electromanometrically. The MAC value was 1.05 +/- 0.09 (SD)%. Other physiological responses occurred inconsistently and could not be used as reliable end points for the determination of the MAC in the rabbit.

Cytochrome P450 2E1 is the principal catalyst of human oxidative halothane metabolism in vitro

The volatile anesthetic halothane undergoes substantial biotransformation generating metabolites that mediate hepatotoxicity. Aerobically, halothane undergoes cytochrome P450-catalyzed oxidation to trifluoroacetic acid (TFA), bromide and a reactive intermediate that can acetylate liver proteins. These protein neo-antigens stimulate an immune reaction that mediates severe hepatic necrosis ("halothane hepatitis"). This investigation identified the human P450 isoform(s) that catalyze oxidative halothane metabolism. Halothane oxidation by human liver microsomes was assessed by TFA and bromide formation. Eadie-Hofstee plots of TFA and bromide formation were both nonlinear, suggesting the participation of multiple P450s. Microsomal TFA and bromide formation were inhibited 45 to 66% and 21 to 26%, respectively, by the P450 2A6 inhibitors 8-methoxypsoralen and coumarin, 84 to 90% by the P450 2E1 inhibitor 4-methylpyrazole and 55% by diethyldithiocarbamate, an inhibitor of both P450 2A6 and 2E1. Selective inhibitors of P450s 1A, 2B6, 2C9/10, 2D6 and 3A4 did not affect halothane oxidation. At saturating halothane concentrations (2.4 vol%) only cDNA-expressed P450 2A6 and 2B6 catalyzed significant rates of TFA and bromide formation, and P450 2E1 catalyzed comparatively minimal oxidation. Conversely, at subsaturating halothane concentrations (0.30 vol%), metabolism by P450 2E1 exceeded that by P450 2A6. Among a panel of human liver microsomes, there were significant linear correlations between halothane oxidation and P450 2A6 activity and protein content at saturating halothane concentrations (2.4 vol%), and a significant correlation between metabolite formation and P450 2E1 activity (but not P450 2A6 activity) at subsaturating concentrations (0.12 vol%). These experiments suggested P450 2A6 and 2E1 as the predominant catalysts at saturating and subsaturating halothane concentrations, respectively. Further kinetic analysis using cDNA-expressed P450 and liver microsomes clearly demonstrated that P450 2E1 is the high affinity/low capacity isoform (Km = 0.030-0.053 vol%) and P450 2A6 is the low affinity/high capacity isoform (Km = 0.77-1.2 vol%). Evidence was also obtained for substrate inhibition of P450 2E1. The in vitro clearance estimates (Vmax/Km) for microsomal P450 2E1 (4.3-5.7 ml/min/g) were substantially greater than those for microsomal P450 2A6 (0.12-0.21). These clearances, as well as rates of apparent halothane oxidation predicted from kinetic parameters in conjunction with plasma halothane concentrations measured during clinical anesthesia in humans, demonstrated that both P450 2E1 and P450 2A6 participate in human halothane metabolism, and that P450 2E1 is the predominant catalytic isoform.

Selective protein covalent binding and target organ toxicity

Protein covalent binding by xenobiotic metabolites has long been associated with target organ toxicity but mechanistic involvement of such binding has not been widely demonstrated. Modern biochemical, molecular, and immunochemical approaches have facilitated identification of specific protein targets of xenobiotic covalent binding. Such studies have revealed that protein covalent binding is not random, but rather selective with respect to the proteins targeted. Selective binding to specific cellular target proteins may better correlate with toxicity than total protein covalent binding. Current research is directed at characterizing and identifying the targeted proteins and clarifying the effect of such binding on their structure, function, and potential roles in target organ toxicity. The approaches employed to detect and identify the tartgeted proteins are described. Metabolites of acetaminophen, halothane, and 2,5-hexanedione form covalently bound adducts to recently identified protein targets. The selective binding may influence homeostatic or other cellular responses which in turn contribute to drug toxicity, hypersensitivity, or autoimmunity.

Heterogeneous halothane binding in the SR Ca2+-ATPase

The activity of various Ca2+-ATPases is affected by volatile anesthetics, such as halothane, commonly used in clinical practice. The effect on the enzyme in skeletal muscle sarcoplasmic reticulum (SR) is biphasic, including stimulation at clinical anesthetic concentrations and subsequent inhibition at higher concentrations. We have previously proposed that the action of a volatile anesthetic on Ca2+-ATPases results from its binding in the interior of the enzyme molecule [Lopez, M.M. and Kosk-Kosicka, D. (1995) J. Biol. Chem. 270, 28239-28245]. Presently, we investigated whether the anesthetic interacts directly with the skeletal muscle SR Ca2+-ATPase (SERCA1) as evidenced by binding. Photoaffinity labeling with [14C]halothane demonstrated that the anesthetic binds saturably to SR membranes, and that approximately 80% of the binding is specific, with a KI of 0.6 mM. The KI value agrees well with the concentration at which halothane half-maximally activates SERCA1. SDS gel electrophoresis of labeled membranes indicates that 38-56% of [14C]halothane incorporates into SERCA1, and 38-53% in lipids. Distribution of label among the three fragments produced by controlled tryptic digestion of SERCA1 suggests heterogeneous halothane binding presumably in discrete sites in the enzyme. The results provide the first direct evidence that halothane binds to SERCA1. Potentially this binding could be related to anesthetic effect on enzyme's function.

Physiologically based pharmacokinetic analysis of the concentration-dependent metabolism of halothane

1. Previous studies with the halothane analogue and chlorofluorocarbon replacement 2,2-dichloro-1,1,1-trifluoroethane (HCFC-123) have shown that there are concentration-dependent, sex-specific differences in the rate of uptake during inhalation exposure in rat. Since it is well established that there are sex-specific differences in the control of enzyme activity in drug metabolism, male and female rats were exposed by inhalation to halothane concentrations ranging from 500 to 4000 ppm. 2. A physiologically based pharmacokinetic model describing the concentration-dependent reduction in uptake and metabolism of halothane in male and female rats was developed. The in vivo metabolic rate constants obtained were: for male rats, Km = 0.4 mg litre-1 (2.03 mumol litre-1) and Vmaxc = 9.2 mg kg1 h-1 (46.6 mumol kg1 h-1); for female rats, Km = 0.4 mg litre-1 (2.03 mumol litre-1) and Vmaxc = 10.2 mg kg-1 h-1 (51.7 mumol kg-1 h-1). 3. An equation describing the concentration-dependent decrease of hepatic metabolism of halothane successfully simulated the gas-uptake data. Simulation of cumulative urinary excretion of the major metabolite, trifluoroacetic acid, required introduction of a proportionality constant to limit the extent of reduction of halothane metabolism to 20% of the amount of enzyme activity. Good simulation of urinary excretion data was achieved, which was interpreted to indicate that, when only 20% of the enzyme is inactivated, the rate of enzyme resynthesis was adequate to replenish enzyme activity within 24 h. 4. A rapidly reversible, non-biological inactivation mechanism called "physical toxicity' is discussed as a possible explanation of concentration-dependent gas uptake.

Sevoflurane inhibits human platelet aggregation and thromboxane A2 formation, possibly by suppression of cyclooxygenase activity

BACKGROUND

Halothane increases bleeding time and suppresses platelet aggregation in vivo and in vitro. A previous study by the authors suggests that halothane inhibits platelet aggregation by reducing thromboxane (TX) A2 receptor-binding affinity. However, no studies of the effects of sevoflurane on platelet aggregation have been published.

METHODS

The effects of sevoflurane, halothane, and isoflurane were examined at doses of 0.13-1.4 mM. Human platelet aggregation was induced by adenosine diphosphate, epinephrine, arachidonic acid, prostaglandin G2, and a TXA2 agonist ([+]-9, 11-epithia-11, 12-methano-TXA2, STA2) and measured by aggregometry. Platelet TXB2 levels were measured by radioimmunoassay, and the ligand-binding characteristics of the TXA2 receptors were examined by Scatchard analysis using a [3H]-labeled TXA2 receptor antagonist (5Z-7-(3-endo-([ring-4-[3H] phenyl) sulphonylamino-[2.2.1.] bicyclohept-2-exo-yl) heptenoic acid, [3H]S145).

RESULTS

Isoflurane (0.28-0.84 mM) did not significantly affect platelet aggregation induced by adenosine diphosphate and epinephrine. Sevoflurane (0.13-0.91 mM) and halothane (0.49-1.25 mM) inhibited secondary platelet aggregation induced by adenosine diphosphate (1-10 microM) and epinephrine (1-10 microM) without altering primary aggregation. Sevoflurane (0.13 mM) also inhibited arachidonic acid-induced aggregation, but not that induced by prostaglandin G2 or STA2, although halothane (0.49 mM) inhibited the latter. Sevoflurane (3 mM) did not affect the binding of [3H]S145 to platelets, whereas halothane (3.3 mM) suppressed it strongly. Sevoflurane (0.26 mM) and halothane (0.98 mM) strongly suppressed TXB2 formation by arachidonic acid-stimulated platelets.

CONCLUSIONS

The findings that sevoflurane suppressed the effects of arachidonic acid, but not those of prostaglandin G2 and STA2, suggest strongly that sevoflurane inhibited TXA2 formation by suppressing cyclooxygenase activity. Halothane appeared to suppress both TXA2 formation and binding to its receptors. Sevoflurane has strong antiaggregatory effects at subanesthetic concentrations (greater than 0.13 mM; i.e., approximately 0.5 vol/%), whereas halothane has similar effects at somewhat greater anesthetic concentrations (0.49 mM; i.e., approximately 0.54 vol/%). Isoflurane at clinical concentration (0.84 mM; i.e., approximately 1.82 vol/%) does not affect platelet aggregation significantly.

Characterization of the NADPH-dependent covalent binding of [14C]halothane to human liver microsomes: a role for cytochrome P4502E1 at low substrate concentrations

Activation of halothane to trifluoroacetyl halide, followed by covalent binding to proteins (neoantigen formation) has been proposed to be the mechanism by which halothane causes immune hepatitis. The aim of this study was to identify the cytochrome P450 (CYP) enzyme primarily responsible for the NADPH-dependent covalent binding of [14C]halothane to human liver microsomes. Human liver microsomes were incubated in the absence and presence of NADPH with various concentrations of halothane (from 4.6 to 3,300 microM) to examine the effects of substrate concentration on the nonspecific and specific (NADPH-dependent) binding of [14C]halothane to microsomal protein. As a function of substrate concentration, the specific binding of [14C]halothane to human liver microsomes was biphasic, suggesting that the activation of halothane is catalyzed by a high-affinity enzyme(s) at low substrate concentrations (<150 microM) and by a low-affinity enzyme(s) at high substrate concentrations (>150 microM). For the high-affinity enzyme, the apparent KM for the covalent binding of [14C]halothane was approximately 10 microM, and Vmax was approximately 32 pmol equivalents of halothane bound/mg protein/min under conditions where covalent binding was directly proportional to incubation time and protein concentration. Ten individual samples of human liver microsomes were incubated with a low concentration of halothane (35 microM) to determine the sample-to-sample variation in the specific binding of [14C]halothane to microsomal protein. Covalent binding ranged from 10 to 40 pmol equivalents of halothane bound/mg protein/min and was highly correlated (r2 = 0.93) with the sample-to-sample variation in chlorzoxazone 6-hydroxylase activity, which reflects the levels of CYP2E1. These results suggest that CYP2E1 is the high-affinity enzyme in human liver microsomes responsible for activating halothane to a reactive metabolite. This is supported by the observation that 4-methylpyrazole, a CYP2E1 inhibitor, inhibited the NADPH-dependent binding of [14C]halothane to microsomal protein. The sample-to-sample variation in the covalent binding of [14C]halothane at high substrate concentrations did not correlate with any known CYP enzyme activity. This suggests that several enzymes catalyze the oxidation of halothane at higher substrate concentrations. In conclusion, at pharmacologically relevant concentrations, the covalent binding of halothane to human liver microsomes is primarily catalyzed by CYP2E1.

Halothane acts as a partial agonist of the alpha6 beta2 gamma2S GABA(A)receptor

Whole-cell patch clamp recording was performed on human embryonic kidney 293 cells stably transfected with rat cDNAs for the alpha6, beta2, and gamma2S subunits of the GABA(A) receptor. The volatile anesthetic halothane directly activated a current in the absence of the ligand gamma-aminobutyric acid (GABA). Both the current amplitude and the rate of desensitization increased in a dose-dependent manner with an EC50 of 1.0+/-0.2 mM and a Hill coefficient (nh) of 1.5+/-0.1. The EC50 and nh for GABA to activate the receptor were 1.0+/-0.3 microM and 1.4+/-0.2, respectively. The peak amplitude of the halothane-activated current was about 4% of the maximal GABA response, which was not changed when the concentration of Ca2+ in the external solution was decreased from 2 mM to 0.2 mM. The reversal potential of both halothane- and GABA-activated currents changed with the external Cl- concentration as predicted by the Nernst equation for chloride ions. The halothane- and GABA-activated currents were blocked by both the noncompetitive GABA(A) receptor antagonist picrotoxin and the competitive GABA(A) receptor antagonist bicuculline. Schild plots revealed that the K(i)s for bicuculline to competitively antagonize the currents activated by halothane and GABA are similar (0.69 and 0.72 microM, respectively). These results indicate that halothane activates the alpha6 beta2 gamma2S GABA(A) receptor to induce a current similar to the GABA-induced current.

Binding of the volatile anesthetic halothane to the hydrophobic core of a tetra-alpha-helix-bundle protein

Although volatile general anesthetics interact with several proteins, little is known about the location or characteristics of the binding sites at the molecular level. A detailed structural description of how anesthetics associate with macromolecules is necessary for understanding anesthetic mechanisms of action. The recent introduction of designed synthetic proteins provides new opportunities for obtaining structural and functional information on anesthetic-protein interactions. A synthetic tetra-alpha-helix-bundle protein was used to examine the interaction of halothane with a designed protein interior. The tetra-alpha-helix-bundle comprises 124 residues in the form of two identical 62-residue di-alpha-helical peptides, held together in an all-parallel bundle by hydrophobic forces. Steady-state and time-resolved tryptophan fluorescence and circular dichroism spectroscopy were used to study the anesthetic-protein interaction. Halothane quenches bundle tryptophan fluorescence with a dissociation constant of 2.3 +/- 0.4 mM and a Hill number of 0.9 +/- 0.1. Tryptophan fluorescence decay analysis indicates that halothane quenches the protein fluorescence by a static mechanism. Circular dichroism spectroscopy revealed no change in protein secondary structure on exposure to halothane. Dissociation of the tetra-alpha-helix-bundle into 62-residue di-alpha-helical peptides by trifluoroethanol eliminated the halothane-protein interaction. The results suggest that halothane binds to the hydrophobic interior of the tetra-alpha-helix-bundle, close to the tryptophan residues. The protein tertiary and quaternary structures are required for anesthetic binding. This study demonstrates the feasibility of using synthetic tetra-alpha-helix-bundles as model anesthetic-binding proteins. The use of de novo designed bundle proteins should allow structural, energetic and functional descriptions of anesthetic-protein interactions.

Human reductive halothane metabolism in vitro is catalyzed by cytochrome P450 2A6 and 3A4

The anesthetic halothane undergoes extensive oxidative and reductive biotransformation, resulting in metabolites that cause hepatotoxicity. Halothane is reduced anaerobically by cytochrome P450 (P450) to the volatile metabolites 2-chloro-1,1-difluoroethene (CDE) and 2-chloro-1,1,1-trifluoroethane (CTE). The purpose of this investigation was to identify the human P450 isoform(s) responsible for reductive halothane metabolism. CDE and CTE formation from halothane metabolism by human liver microsomes was determined by GC/MS analysis. Halothane metabolism to CDE and CTE under reductive conditions was completely inhibited by carbon monoxide, which implicates exclusively P450 in this reaction. Eadie-Hofstee plots of both CDE and CTE formation were nonlinear, suggesting multiple P450 isoform involvement. Microsomal CDE and CTE formation were each inhibited 40-50% by P450 2A6-selective inhibitors (coumarin and 8-methoxypsoralen) and 55-60% by P450 3A4-selective inhibitors (ketoconazole and troleandomycin). P450 1A-, 2B6-, 2C9/10-, and 2D6-selective inhibitors (7,8-benzoflavone, furafylline, orphenadrine, sulfaphenazole, and quinidine) had no significant effect on reductive halothane metabolism. Measurement of product formation catalyzed by a panel of cDNA-expressed P450 isoforms revealed that maximal rates of CDE formation occurred with P450 2A6, followed by P450 3A4. P450 3A4 was the most effective catalyst of CTE formation. Among a panel of 11 different human livers, there were significant linear correlations between the rate of CDE formation and both 2A6 activity (r = 0.64, p < 0.04) and 3A4 activity (r = 0.64, p < 0.03). Similarly, there were significant linear correlations between CTE formation and both 2A6 activity (r = 0.55, p < 0.08) and 3A4 activity (r = 0.77, p < 0.005). The P450 2E1 inhibitors 4-methylpyrazole and diethyldithiocarbamate inhibited CDE and CTE formation by 20-45% and 40-50%, respectively; however, cDNA-expressed P450 2E1 did not catalyze significant amounts of CDE or CTE production, and microsomal metabolite formation was not correlated with P450 2E1 activity. This investigation demonstrated that human liver microsomal reductive halothane metabolism is catalyzed predominantly by P450 2A6 and 3A4. This isoform selectivity for anaerobic halothane metabolism contrasts with that for oxidative human halothane metabolism, which is catalyzed predominantly by P450 2E1.

Reductive metabolism of halothane by cytochrome P450 isoforms in rats and humans

Cytochrome P450 (P450)-halothane complex formation, an index of reductive metabolism of halothane, was investigated by using rat hepatic microsomes and purified rat P450s under anaerobic conditions. P450-halothane complex formation was produced by the hepatic microsomes from phenobarbital and dexamethasone treated rats. Anti-P450 3A2 and 2B1/2 antibodies extensively inhibited complex formation in hepatic microsomes from dexamethasone and phenobarbital treated rats, respectively. In reconstituted systems using purified rat P450s, P450 3A2 and 2B1 complexed halothane efficiently. Complex formation was also recognized in human hepatic microsomes under anaerobic conditions.

Amino acid resolution of halothane binding sites in serum albumin

Saturable binding of various inhaled anesthetics to serum albumin has been shown with a variety of approaches. In order to determine the location of halothane binding sites in serum albumin, both human and bovine serum albumins (HSA and BSA) were photolabeled with [14C]halothane, and subjected to proteolysis and microsequencing. BSA was found to have a higher affinity for halothane than HSA, and it contained two specifically labeled sites. One site was characterized by diffuse labeling from Trp212-Leu217, and the other by a more discrete and higher affinity labeling at Trp134-Gly135. HSA contained only a single labeled site, and although lower affinity, was determined to be analogous to BSA Trp212. The position 130-140 region of HSA, having a leucine instead of tryptophan at position 134, was not labeled. These results demonstrate specific and discrete binding of an inhaled anesthetic to a mammalian-soluble protein, and further suggest the importance of aromatic residues as one feature of inhaled anesthetic binding sites.

Identification of the enzyme responsible for oxidative halothane metabolism: implications for prevention of halothane hepatitis

BACKGROUND

Fulminant hepatic necrosis ("halothane hepatitis") is an unusual and often fatal complication of halothane anaesthesia. It is mediated by immune sensitisation in susceptible individuals to trifluoroacetylated liver protein neoantigens, formed by oxidative halothane metabolism. The seminal event in halothane hepatitis is hepatic metabolism, yet the enzyme responsible for oxidative halothane metabolism and trifluoroacetylated neoantigen formation remains unidentified. This investigation tested the hypothesis that cytochrome P450 2E1 (CYP2E1) is responsible for human halothane metabolism in vivo.

METHODS

20 elective surgical patients received either disulfiram (500 mg orally, n = 10) or nothing (controls, n = 10) the night before surgery. Disulfiram, converted in vivo to an effective inhibitor of P450 2E1, was used as a metabolic probe for P450 2E1. All patients received standard halothane anaesthesia (1.0% end-tidal, 3 h). Blood halothane and plasma and urine trifluoroacetic acid, bromide, and fluoride concentrations were measured for up to 96 h postoperatively.

FINDINGS

Total halothane dose, measured by cumulative end-tidal (3.8 SE 0.1 minimum alveolar concentration hours) and blood halothane concentrations, was similar in the two groups. Plasma concentrations and urinary excretion of trifluoroacetic acid and bromide, indicative of oxidative and total (oxidative and reductive) halothane metabolism, respectively, were significantly diminished in disulfiram-treated patients. In control and disulfiram-treated patients cumulative 96 h postoperative trifluoroacetic acid excretion was 12,900 (SE 1700) and 2010 (440) mumol, respectively (p < 0.001) while that of bromide was 1720 (290) and 160 (70) mumol (p < 0.001).

INTERPRETATION

The substantial attenuation of trifluoroacetic acid production by disulfiram after halothane anaesthesia suggests that P450 2E1 is a predominant enzyme responsible for human oxidative halothane metabolism. Inhibition of P450 2E1 by a single preoperative oral disulfiram dose greatly diminished production of the halothane metabolite responsible for the neoantigen formation that initiates halothane hepatitis. Single-dose disulfiram may provide effective prophylaxis against halothane hepatitis.

An inhalational anesthetic binding domain in the nicotinic acetylcholine receptor

To determine inhalational anesthetic binding domains on a ligand-gated ion channel, I used halothane direct photoaffinity labeling of the nicotinic acetylcholine receptor (nAChR) in native Torpedo membranes. [14C]Halothane photoaffinity labeling of both the native Torpedo membranes and the isolated nAChR was saturable, with Kd values within the clinically relevant range. All phospholipids were labeled, with greater than 95% of the label in the acyl chain region. Electrophoresis of labeled nAChR demonstrated no significant subunit selectivity for halothane incorporation. Within the alpha-subunit, greater than 90% of label was found in the endoprotease Glu-C digestion fragments which contain the four transmembrane regions, and the pattern was different from that reported for photoactivatable phospholipid binding to the nAChR. Unlabeled halothane reduced labeling more than did isoflurane, suggesting differences in the binding domains for inhalational anesthetics in the nAChR. These data suggest multiple similar binding domains for halothane in the transmembrane region of the nAChR.

Isoflurane acts as an inhibitor of oxidative dehalogenation while acting as an accelerator of reductive dehalogenation of halothane in guinea pig liver microsomes

The effects of isoflurane, 1-chloro-2,2,2-trifluoroethyl difluoromethyl ether, on the oxidative metabolism of halothane to produce trifluoroacetic acid (TFA) and on the reductive metabolism of halothane to produce chlorodifluoroethylene (CDE) and chlorotrifluoroethane (CTE) in liver microsomes of guinea pig were examined. Isoflurane enhanced the production of CDE and CTE and inhibited the production of TFA. Isoflurane enhanced cytochrome P450 reduction and formation of an intermediate complex with cytochrome P450 without enhancement of NADPH-cytochrome P450 reductase (EC 1.6.2.4) activity. We conclude that isoflurane interacts with cytochrome P450 to prevent the formation of the halothane-cytochrome P450 complex, causing inhibition of the oxidative dehalogenation. This interaction of isoflurane enhances the reduction of cytochrome P450 and the formation of a reductive intermediate-cytochrome P450 complex under anaerobic conditions causing reductive dehalogenation of halothane.

Halogenated anesthetics form liver adducts and antigens that cross-react with halothane-induced antibodies

Two halogenated anesthetics, enflurane and isoflurane, have been associated with an allergic-type hepatic injury both alone and following previous exposure to halothane. Halothane hepatitis appears to involve an aberrant immune response. An antibody response to a protein-bound biotransformation product (trifluoroacetyl adduct) has been detected on halothane hepatitis patients. This study was performed to determine cross-reactivity between enflurane and isoflurane with the hypersensitivity induced by halothane. The subcellular and lobular production of hepatic neoantigens recognized by halothane-induced antibodies following enflurane and isoflurane, and the biochemical nature of these neoantigens was investigated in two animal models. Enflurane administration resulted in neoantigens detected in both the microsomal and cytosolic fraction of liver homogenates and in the centrilobular region of the liver. In the same liver, biochemical analysis detected fluorinated liver adducts that were up to 20-fold greater in guinea pigs than in rats. This supports and extends previous evidence for a mechanism by which enflurane and/or isoflurane could produce a hypersensitivity condition similar to that of halothane hepatitis either alone or subsequent to halothane administration. The guinea pig would appear to be a useful model for further investigations of the immunological response to these antigens.

Binding of halothane to serum albumin demonstrated using tryptophan fluorescence

BACKGROUND

The site of action of general anesthesia remains controversial, but evidence in favor of specific protein target(s) is accumulating. Saturable binding of halothane to bovine serum albumin (BSA) has recently been reported using photoaffinity labeling and fluorine 19 nuclear magnetic resonance spectroscopy. We report a new approach to study anesthetic binding to soluble proteins, based on native tryptophan fluorescence.

METHODS

Thymol-free halothane and fatty acid-free BSA were equilibrated in gas-tight Hamilton syringes and dispensed into stoppered quartz cuvettes at predetermined dilutions. Steady-state fluorescence spectroscopy was used to study their interaction.

RESULTS

Halothane quenched the tryptophan fluorescence of BSA in a concentration-dependent, saturable manner with a dissociation constant = 1.8 +/- 0.2 mM and a Hill number = 1.0 +/- 0.1. The two optical isomers of halothane bound to BSA with equal affinity. The ability of halothane to quench BSA tryptophan fluorescence was markedly decreased at pH 3.0 (which causes full uncoiling of BSA), with loss of saturable binding. Diethyl ether displaced a portion of halothane from its binding sites. Circular dichroism spectroscopy revealed no significant effect of halothane or diethyl ether on the secondary structure of BSA.

CONCLUSIONS

The results suggest that halothane binds in hydrophobic domains containing tryptophan in BSA. This approach may prove useful for studying the interaction of volatile anesthetics and proteins and has the advantage that the location of halothane in the protein is identified.

Molecular mimicry in halothane hepatitis: biochemical and structural characterization of lipoylated autoantigens

Exposure of human individuals to halothane causes, in about 20% of all cases, a mild transient form of hepatotoxicity. A small subset of exposed individuals, however, develops a potentially severe and life-threatening form of hepatic damage, coined halothane hepatitis. Halothane hepatitis is thought to have an immunological basis. Sera of afflicted individuals contain a wide variety of autoantibodies against hepatic proteins, in both trifluoroacetylated form (CF3CO-proteins) and, at least in part, in native form. CF3CO-proteins are elicited in the course of oxidative biotransformation of halothane, and include the trifluoroacetylated forms of protein disulfide isomerase, microsomal carboxylesterase, calreticulin, ERp72, GRP 78, and ERp99. Current evidence suggests that CF3CO-proteins arise in all halothane-exposed individuals; however, the vast majority of individuals appear to immunochemically tolerate CF3CO-proteins. The lack of immunological responsiveness of these individuals towards CF3CO-proteins might be due to tolerance, induced through the occurrence of structures in the repertoire of self-determinants, which immunochemically and structurally mimic CF3CO-proteins very closely. In fact, lipoic acid, the prosthetic group of the constitutively expressed E2 subunits of the family of mammalian 2-oxoacid dehydrogenase complexes and of protein X, was shown by immunochemical and molecular modelling analysis to be a perfect structural mimic of N6-trifluoroacetyl-L-lysine (CF3 CO-Lys), the major haptenic group of CF3CO-proteins. As a consequence of molecular mimicry, autoantibodies in patients' sera not only recognize CF3CO-proteins, but also the E2 subunit proteins of the 2-oxoacid dehydrogenase complexes and protein X, as autoantigens associated with halothane hepatitis. Furthermore, a fraction of patients with halothane hepatitis exhibit irregularities in the hepatic expression levels of these native, not trifluoroacetylated autoantigens. Collectively, these data suggest that molecular mimicry of CF3CO-Lys by lipoic acid, or the impairment thereof, might play a role in the susceptibility of individuals for the development of halothane hepatitis.

Metabolism of 1-fluoro-1,1,2-trichloroethane, 1,2-dichloro-1,1-difluoroethane, and 1,1,1-trifluoro-2-chloroethane

1-Fluoro-1,1,2-trichloroethane (HCFC-131a), 1,2-dichloro-1,1-difluoroethane (HCFC-132b), and 1,1,1-trifluoro-2-chloroethane (HCFC-133a) were chosen as models for comparative metabolism studies on 1,1,1,2-tetrahaloethanes, which are under consideration as replacements for ozone-depleting chlorofluorocarbons (CFCs). Male Fischer 344 rats were given 10 mmol/kg ip HCFC-131a or HCFC-132b or exposed by inhalation to 1% HCFC-133a for 2 h. Urine collected in the first 24 h after exposure was analyzed by 19F NMR and GC/MS and with a fluoride-selective ion electrode for the formation of fluorine-containing metabolites. Metabolites of HCFC-131a included 2,2-dichloro-2-fluoroethyl glucuronide, 2,2-dichloro-2-fluoroethyl sulfate, dichlorofluoroacetic acid, and inorganic fluoride. Metabolites of HCFC-132b were characterized as 2-chloro-2,2-difluoroethyl glucuronide, 2-chloro-2,2-difluoroethyl sulfate, chlorodifluoroacetic acid, chlorodifluoroacetaldehyde hydrate, chlorodifluoroacetaldehyde-urea adduct, and inorganic fluoride. HCFC-133a was metabolized to 2,2,2-trifluoroethyl glucuronide, trifluoroacetic acid, trifluoroacetaldehyde hydrate, trifluoroacetaldehyde-urea adduct, inorganic fluoride, and a minor, unidentified metabolite. With HCFC-131a and HCFC-132b, glucuronide conjugates of 2,2,2-trihaloethanols were the major urinary metabolites, whereas with HCFC-133a, a trifluoroacetaldehyde-urea adduct was the major urinary metabolite. Analysis of metabolite distribution in vivo indicated that aldehydic metabolites increased as fluorine substitution increased in the order HCFC-131a < HCFC-132b < HCFC-133a. With NADPH-fortified rat liver microsomes, HCFC-133a and HCFC-132b were biotransformed to trifluoroacetaldehyde and chlorodifluoroacetaldehyde, respectively, whereas HCFC-131a was converted to dichlorofluoroacetic acid. No covalently bound metabolites were detected by 19F NMR spectroscopy.(ABSTRACT TRUNCATED AT 250 WORDS)

Trifluoroacetylation potentiates the humoral immune response to halothane in the guinea pig

Halothane hepatitis appears to result from an inappropriate immune response to the products of halothane metabolism. Attempts to produce an animal model for halothane hepatitis have been largely unsuccessful. Although guinea pigs produce neoantigens following treatment with halothane, the subsequent antibody response is weak, possibly accounting for the failure to produce halothane hepatitis in these animals. In order to increase the antibody response to halothane neoantigens, three methods for trifluoroacetylating proteins were used. Guinea pigs were either treated with S-ethylthiotrifluoroacetate, autologous lymphocytes trifluoroacetylated ex vivo, or immunized with trifluoroacetylated mycobacterial protein, followed by exposure to halothane, and examined for anti-halothane metabolite antibodies (anti-TFA antibodies). Animals treated with S-ethylthiotrifluoroacetate developed anti-TFA antibodies, and following exposure to halothane exhibited an enhanced antibody response. Treatment with trifluoroacetylated lymphocytes also resulted in an enhanced anti-TFA antibody response following halothane exposure. Immunization with trifluoroacetylated mycobacterial proteins resulted in very high anti-TFA antibody titers. However, subsequent exposure to halothane had no observable effect on specific antibody titers. Exposure to halothane, regardless of treatment, resulted in the production of anti-microsomal protein antibodies. Signs of halothane hepatitis were not observed, indicating that enhancement of the humoral immune response does not appear to be sufficient for production of halothane hepatitis.

Reductive activation of halothane by human haemoglobin results in the modification of the prosthetic haem

The metabolic activation of halothane by human haemoglobin (Hb) under reducing conditions in vitro is reported. Absolute spectra of sodium dithionite-reduced Hb, recorded during its anaerobic incubation in the presence of the substrate, showed decreasing concentrations of reduced Hb (Hb2+) with time. The loss of Hb2+ was accompanied, although only to some extent, by a concurrent oxidation to methaemoglobin (Hb3+), suggesting that electron transfer from Hb to the substrate had occurred. Reductive halothane metabolism was observed under these conditions as indicated by a dose-dependent inorganic fluoride (F-) production, which was, however, lower than that observed with heated Hb or a water soluble haem preparation (methaemalbumin). A rapid, partial loss of Hb was found upon addition of the substrate to the incubation mixture, as indicated by a decrease of the typical peak at 418 nm in the absolute spectra recorded in the presence of carbon monoxide (CO). This effect was associated with a loss of the Hb prosthetic group, haem, as shown by a decrease of the pyridine-haemochromogen reaction. Both effects were time and dose dependent. The inhibition of the Hb inactivation reaction by adding exogenous CO or the spin trapping agent N-t-butyl-alpha-phenylnitrone (PBN) to the incubation mixture beforehand indicated that (a) a reduced and free haem iron is required by Hb to activate halothane, and (b) the formation of free radical reactive metabolites of halothane is likely to be responsible for Hb inactivation. The mechanism of the reaction may involve the attack of these metabolites on the haem group of Hb, as indicated by the detection, with a reverse-phase ion-pairing HPLC system, of two Hb-derived products showing a typical haem-like absorption spectrum. The present results resemble those obtained recently with carbon tetrachloride (Ferrara et al., Alternatives to Laboratory Animals 21: 57-64, 1993) and suggest a common mechanism of activation of the two polyhalogenated alkanes by Hb.

Inhibitory effects of corticoids on reductive halothane dehalogenation

The effects of corticoids, hydrocortisone, methylprednisolone, betamethasone and dexamethasone, on the reductive metabolism of halothane to produce chloro-difluoroethylene (CDE) and chlorotrifluoroethane (CTE) in the liver microsomes of guinea-pig were examined. The substrate differential spectrum for methylprednisolone showed a peak at 412 nm and trough at 395 nm, typical modified type II. The other corticoids showed a similar spectral change. The corticoids had no effect on NADPH-cytochrome P450 reductase. All of these corticoids inhibited the reductive dehalogenation of halothane. The concentrations of hydrocortisone, methylprednisolone, betamethasone and dexamethasone causing 50% inhibition of CDE formation from halothane were 5.1 +/- 0.7 mg/ml, 3.1 +/- 1.2 mg/ml, 2.3 +/- 0.4 mg/ml and 2.4 +/- 0.6 mg/ml, respectively, with no significant differences except for hydrocortisone. The concentrations of hydrocortisone, methylprednisolone, betamethasone and dexamethasone causing 50% inhibition of CTE formation from halothane were 4.9 +/- 0.5 mg/ml, 3.1 +/- 1.0 mg/ml, 2.4 +/- 0.3 mg/ml and 28.0 +/- 9.1 mg/ml, respectively, with no significant differences except for dexamethasone. Our results showed that corticoids inhibit halothane metabolism.

Binding of anti-trifluoroacetyl antibodies to isolated hepatocytes observed by digital fluorescence microscopy

These experiments were designed to observe specific binding of fluorescein-conjugated FAB'2 secondary antibodies to epitopes on the surface of isolated hepatocytes. The hepatocytes were attached as monolayers on microscope cover slips and an antigenic adduct known to be formed during metabolism of halothane, -trifluoroacetyl-1,2-dioleoyl-sn-glycero-3-phosphoethanolamine, was exchanged into their surface. Then the monolayers of hepatocytes were incubated with primary rabbit antibodies specific for the trifluoroacetyl group. Each coverslip was mounted in a perfusion chamber on a fluorescence microscope and a set of digital fluorescence images was made. Then fluorescein-conjugated goat-anti-rabbit FAB'2 secondary antibodies were flowed over the monolayer, the perfusion chamber was washed with buffer, and a second set of digital fluorescence images was made. The difference of these two sets of images demonstrated intense fluorescence superimposed on the outline of the cells. This intense fluorescence was not observed in control experiments in which the primary antibodies were omitted.

Glutathione-S-transferase is a target for covalent modification by a halothane reactive intermediate in the guinea pig liver

The anesthetic halothane is bioactivated by the liver cytochrome P450 system to the reactive intermediate, trifluoroacetyl chloride, which can acylate liver protein. Cytosolic glutathione-S-transferase (GST) was identified as a major target for protein adduct formation in guinea pig liver slices exposed to halothane. To determine if GST is also a target in vivo, male Hartley guinea pigs were exposed to 1% halothane in 40% O2 for 4 h. At 10 h post exposure, livers were removed and microsomal and cytosolic fractions prepared. Past studies have shown these conditions resulted in maximal covalent binding of halothane intermediates to hepatic protein. Protein was isolated by ethanol precipitation and washed with trichloroacetic acid to remove unbound metabolites. Cytosolic GST was isolated by gel filtration and S-hexyl-glutathione affinity chromatography to electrophoretic purity. Protein adducts were quantified using a covalently bound fluorine assay. Covalent binding of a halothane intermediate to cytosolic and microsomal protein was determined as 2.0 +/- 0.4 and 13.2 +/- 2.3 nmol F/mg protein, respectively. Liver glutathione depletion by buthionine sulfoximine pretreatment produced an increase in covalent binding only to cytosolic proteins (3.3 +/- 0.4 nmol F/mg protein). Adduct formation to cytosolic GST was determined to be 4.7 +/- 1.6 nmol F/mg protein. Glutathione-S-transferase is a target for covalent modification in the liver following an inhalation exposure to halothane.

Sensitization of adenylate cyclase by halothane in human myocardium and S49 lymphoma wild-type and cyc- cells: evidence for inactivation of the inhibitory G protein Gi alpha

Halothane has been reported to sensitize the myocardium towards the effects of exogenous catecholamines in patients and laboratory animals. This study was aimed at investigating the catecholamine-sensitizing effects of halothane as well as the underlying subcellular mechanisms in human myocardium. Halothane augmented the positive inotropic effect of isoprenaline but not of Ca2+. The increase of the effect of isoprenaline by halothane was more pronounced in failing myocardium, with increased Gi, than in nonfailing donor hearts. Halothane (1%) increased basal as well as isoprenaline-, NaF-, cholera toxin-, and guanylylimidodiphosphate [Gpp(NH)p]-stimulated adenylate cyclase in human myocardial membranes (p < 0.05). Treatment of membranes with pertussis toxin increased adenylate cyclase by 40% and abolished the effect of halothane. Halothane had no effect on forskolin-stimulated adenylate cyclase. The same results, i.e., a pertussis toxin-sensitive increase of adenylate cyclase stimulation by halothane, were obtained in S49 cyc-, wild-type, or recombinant Gs alpha-reconstituted cyc- cell membranes. Carbachol-stimulated guanosine-5'-O-(3-[35S]thio)triphosphate binding was not influenced by halothane, but halothane attenuated the inhibition of adenylate cyclase by Gpp(NH)p in S49 cyc- cells. These data show that halothane stimulates adenylate cyclase and sensitizes adenylate cyclase after stimulation by beta-adrenoceptor agonists and guanine nucleotides due to an impairment of Gi alpha function. This mechanism may play a role in the halothane sensitization of myocardial adenylate cyclase towards catecholamines.

Metabolism of the chlorofluorocarbon substitute 1,1-dichloro-2,2,2-trifluoroethane by rat and human liver microsomes: the role of cytochrome P450 2E1

1,1-Dichloro-2,2,2-trifluoroethane (HCFC-123) has been developed as a substitute for ozone-depleting chlorofluorocarbons. The atmospheric lifetime of HCFC-123 is expected to be much shorter than those of chlorofluorocarbons; however, due to its lower stability and the presence of carbon-hydrogen bonds, metabolism of HCFC-123 in mammals and metabolism-dependent toxicity is likely. We compared the metabolism of HCFC-123 and its analog halothane in rat and human liver microsomes. 19F-NMR studies showed that trifluoroacetic acid is a major metabolite of HCFC-123. Besides trifluoroacetic acid, chlorodifluoroacetic acid and inorganic fluoride were identified as products of the enzymatic oxidation of HCFC-123 in rat and human liver microsomes by 19F-NMR and mass spectrometry. The metabolites were not detected in incubations with halothane. HCFC-123 and halothane were transformed by liver microsomes from untreated rats at low rates. Microsomes from ethanol-and pyridine-treated rats metabolized both HCFC-123 and halothane at much higher rates. These microsomes also exhibited high rates of p-nitrophenol oxidation. p-Nitrophenol is a model substrate mainly oxidized by P450 2E1 to p-nitrocatechol. Samples of human liver microsomes showed considerable differences in the extent of HCFC-123, p-nitrophenol oxidation, and chlorzoxazone hydroxylation. In human liver microsomes, rabbit anti-rat P450 2E1 IgG recognized a single protein band corresponding in apparent molecular weight to human P450 2E1. Immunoblot analysis revealed considerable heterogenity in the P450 2E1 protein content of the human liver samples. Trifluoroacetic acid formation from HCFC-123 and halothane and p-nitrocatechol formation from p-nitrophenol were significantly reduced by the P450 2E1 inhibitor diethyldithiocarbamate.(ABSTRACT TRUNCATED AT 250 WORDS)

Leukotriene B4 formation upon halothane-induced lipid peroxidation in liver membrane fractions under low O2 concentrations in vitro

Lipid peroxidation was induced in rat liver membrane fractions in vitro upon NADPH-dependent metabolic activation of the anesthetic agent halothane at low O2 concentrations. Halothane-induced lipid peroxidation was dependent on time, concentration of halothane, and the calculated O2 concentrations present in the system. Lipid peroxidation was inducible at increasing O2 concentrations up to 12 microM, decreased at higher O2 concentrations up to 48 microM, and was not detectable at normoxic conditions. Leukotriene B4 (LTB4) was identified as a product arising upon lipid peroxidation by reverse-phase high-pressure liquid chromatography combined with a radioimmunoassay. LTB4 formation was maximal under conditions of maximal lipid peroxidation at a calculated O2 concentration of 12 microM. Even at high concentrations, the 5-lipoxygenase inhibitors MK886 (10 microM), ZD2138 (20 microM), and ZM230487 (20 microM) were not inhibitory in halothane-induced lipid peroxidation nor in the associated formation of LTB4. Synthetic LTB4 was transformed into its 20-hydroxy derivative by omega-oxidation in an O2-concentration-dependent manner, being considerably reduced at the low O2 concentrations that maximally promoted lipid peroxidation. The collective evidence of these data raises the possibility that exposure to halothane might lead to peroxidation-associated net synthesis of LTB4 through 5-lipoxygenase-independent escape routes in liver tissue under physiologically or pathophysiologically low O2 concentrations.

Halothane-induced cytotoxicity to rat centrilobular hepatocytes in primary culture is not increased under low oxygen concentration

BACKGROUND

Halothane can be metabolized by both oxidative and reductive pathways in the liver. This anesthetic can induce direct liver injury preferentially localized in centrilobular areas, probably in relation with lower oxygen tension. The reductive pathway has been related to liver damage; however, a correlation between lower oxygen concentration in centrilobular areas, the extent of reductive metabolism of halothane, and the degree of liver injury has not yet been demonstrated. This study was designed to better evaluate the toxicity of the reduced metabolites by using centrilobular and periportal rat hepatocyte subpopulations.

METHODS

Adult rat hepatocytes, either as whole cell preparations or after separation in centrilobular and periportal cell subpopulations, were placed in primary culture and exposed to either 2% or 4% halothane under various oxygen concentrations. The enriched centrilobular hepatocyte subpopulations isolated by the digitonin-collagenase method were characterized by immunolocalization of glutamine synthetase. Three oxygen concentrations were tested: 5%, 20%, and 95%, and the main parameters measured were cell viability and fluoride ion formation.

RESULTS

Viability of centrilobular hepatocytes was similar under 5% and 20% O2, but the unpurified hepatocyte population was more susceptible to 5% O2 (P < 0.01). Significantly higher cytochrome P-450 content was found in whole hepatocyte populations under 5% versus 20% oxygen, indicating that centrilobular hepatocytes that contained higher cytochrome P-450 monooxygenase activities were less sensitive to low oxygen concentrations. Halothane toxicity to centrilobular hepatocytes was enhanced under 95% versus 20% O2 (P < 0.05). By contrast, no significant difference was observed when the cells were maintained under 5% O2, although fluoride ions, indicative of reductive metabolism of halothane, were found in much higher amounts in the culture medium. Moreover, under 20% O2, halothane toxicity was significantly greater in centrilobular versus unpurified hepatocytes (P < 0.05).

CONCLUSIONS

Isolated centrilobular hepatocytes appear to be more sensitive to halothane than their periportal counterparts in vitro. However, the authors' results support the conclusion that increased reductive metabolism of halothane induced by decreasing oxygen concentration is not a critical parameter for the occurrence of liver damage in these cells.

Inhibitory effect of gomisi on reductive metabolism of halothane

The effect of Gomisi (dried ripe fruit of schizandra chinensis) on chlorodifluoroethylene (CDE) and chlorotrifluoroethane (CTE) formation was investigated. The incubation mixtures for the measurement of reductive metabolites of halothane consisted of liver microsomal suspensions, 3 mM NADPH, extract solution of Gomisi and halothane in 0.1 M potassium phosphate buffer (pH 7.4). The production of CDE and CTE was inhibited by Gomisi in a dose-dependent way. The production were reduced to half in the presence of 0.5% Gomisi extract in the reaction mixture. The results suggest that Gomisi can inhibit the reductive metabolism of halothane in vitro; thus it may protect against halothane-induced hepatitis.

Autoantibodies to hepatic microsomal carboxylesterase in halothane hepatitis

Halothane hepatitis can be life-threatening, and this severe adverse reaction may arise via an immune process. We have detected autoantibodies to purified human liver microsomal carboxylesterase in sera of 17 out of 20 patients with halothane hepatitis (85%) but not in 9 halothane-exposed controls and in only 2 (at low levels) of 33 patients with liver disease due to other causes. Immunohistochemical studies localised the carboxylesterase predominantly to the centrilobular region of liver sections, which is consistent with the area affected by halothane hepatitis. Human hepatic microsomal carboxylesterase is a target antigen in halothane hepatitis, and an immune response to this protein may be involved in the liver damage observed.

Halothane hepatitis patients have serum antibodies that react with protein disulfide isomerase

Clinical and laboratory evidence suggests that the fulminant liver failure sometimes associated with the inhalation anesthetic halothane may be an immune-mediated toxicity. Most importantly, the vast majority of patients with a clinical diagnosis of halothane hepatitis have serum antibodies, which react with one or more specific liver microsomal proteins that have been covalently altered by the trifluoroacetyl chloride metabolite of halothane. The serum antibodies are specific to halothane hepatitis patients and are not seen in sera of patients with other types of liver pathology. In this study, a 57-kD trifluoroacetylated liver microsomal neoantigen associated with halothane hepatitis and native 57-kD protein were purified from liver microsomes of halothane-treated and -untreated rats, respectively. When the purified trifluoroacetylated 57-kD and native 57-kD proteins were used as test antigens in an enzyme-linked immunosorbent assay, serum antibodies from halothane hepatitis patients (n = 40) reacted with both of these proteins to a significantly greater extent than did serum antibodies from control patients (n = 32). On the basis of its apparent monomeric molecular mass, isoelectric point and NH2-terminal amino acid and tryptic peptide sequences, the 57-kD protein has been identified as rat liver protein disulfide isomerase. Antibodies raised against rat liver protein disulfide isomerase also reacted with a protein of approximately 58-kD in human liver microsomes. The results of this investigation suggest that trifluoroacetylated protein disulfide isomerase is one of the immunogens associated with halothane hepatitis. In certain patients it might lead either to specific antibodies or, possibly, to specific T cells, which could be responsible for halothane hepatitis.

[Halothane hepatitis]

The greatest disadvantage of the halothane, widely used in the anaesthesiology is its ability to cause liver damage. After halothane anaesthesia mild liver enzyme elevation in the one fifth of the patients was detected. The incidence of fatal halothane hepatitis is rare. It depends on several risk factors: on the genetic predisposition, repeated halothane anaesthetics, female sex, age of patient, obesity, intrahepatic hypoxia and enzyme induction. In the pathophysiology of liver toxicity the metabolism of halothane and immune functions play an important role. In this review the last results of researches concerning to the hepatotoxicity of halothane are summarised and the authors call the attention on the opportunity of its effective prevention.

Drug metabolism and genetic polymorphism in subjects with previous halothane hepatitis

To test the hypothesis that halothane hepatitis is caused by a combination of altered drug metabolism and an immunoallergic disposition, the metabolism of antipyrine, metronidazole, sparteine, phenytoin, and racemic R- and S-mephenytoin was investigated in seven subjects with previous halothane hepatitis. The HLA tissue types and the complement C3 phenotypes were also determined. The metabolism of antipyrine and metronidazole was within normal range in all subjects, and they were all fast or extensive metabolizers of sparteine, mephenytoin, and phenytoin. HLA tissue types were unremarkable. Five of the seven subjects had complement C3 phenotypes F or FS. In the general population phenotype S is the most common, but the difference in complement C3 phenotypes is not statistically significant (p = 0.07). We conclude, although in a limited number of patients, that subjects with previous halothane hepatitis do not appear to be different from controls with regard to drug metabolism and HLA tissue type. The possibility of a higher frequency of complement C3 phenotype F and FS needs further investigation.

Volatile anesthetics compete for common binding sites on bovine serum albumin: a 19F-NMR study

There is controversy as to the molecular nature of volatile anesthetic target sites. One proposal is that volatile anesthetics bind directly to hydrophobic binding sites on certain sensitive target proteins. Consistent with this hypothesis, we have previously shown that a fluorinated volatile anesthetic, isoflurane, binds saturably [Kd (dissociation constant) = 1.4 +/- 0.2 mM, Bmax = 4.2 +/- 0.3 sites] to fatty acid-displaceable domains on serum albumin. In the current study, we used 19F-NMR T2 relaxation to examine whether other volatile anesthetics bind to the same sites on albumin and, if so, whether they vary in their affinity for these sites. We show that three other fluorinated volatile anesthetics bind with varying affinity to fatty acid-displaceable domains on serum albumin: halothane, Kd = 1.3 +/- 0.2 mM; methoxyflurane, Kd = 2.6 +/- 0.3 mM; and sevoflurane, Kd = 4.5 +/- 0.6 mM. These three anesthetics inhibit isoflurane binding in a competitive manner: halothane, K(i) (inhibition constant) = 1.3 +/- 0.2 mM; methoxyflurane, K(i) = 2.5 +/- 0.4 mM; and sevoflurane, K(i) = 5.4 +/- 0.7 mM--similar to each anesthetic's respective Kd of binding to fatty acid displaceable sites. These results illustrate that a variety of volatile anesthetics can compete for binding to specific sites on a protein.

Halothane binding to soluble proteins determined by photoaffinity labeling

BACKGROUND

Recently, halothane and isoflurane have been shown to bind in a saturable manner to serum albumin using NMR and gas chromatography methods. To validate a novel direct photoaffinity labeling method developed in our laboratory, the authors also determined the binding characteristics of halothane to serum albumin, and then extended this approach to other soluble proteins in an initial attempt to understand the interaction of volatile anesthetics and proteins.

METHODS

Serum albumin (BSA), bacterial luciferase (BL), poly-(L-lysine)(PLL), and poly-(L-glutamate)(PLG) were dissolved in 0.154 M NaCl containing 14C-halothane with or without other volatile anesthetics or ligands, and exposed to 254 nm UV light for 10 s. Covalently bound label was quantitated by scintillation counting after precipitation, filtration, and washing. Binding parameters were calculated by nonlinear least-squares fitting of rectangular hyperbolas or logistic equations.

RESULTS

Serum albumin bound halothane in a saturable manner at an apparent KD between 0.3 and 0.5 mM. Other volatile anesthetics inhibited binding (KI, in mM): halothane (0.36), chloroform (1.26), methoxyflurane (2.66), isoflurane (1.47), diethyl ether (45.5), and ethanol (1,040). Oleate and BSA conformational changes (low pH) also inhibited label incorporation. Binding to BL and PLL at pH 7 was nonsaturable and not displaced by unlabeled halothane or the BL substrate decanal. Conversion of PLL to an alpha-helical conformation (pH > 10) increased binding and created a saturable component with an apparent KD of 0.55 mM. Alkaline conditions decreased binding to PLG consistent with the loss of alpha-helical domains.

CONCLUSIONS

Photoaffinity labeling produced results in close agreement with more conventional methods for studying halothane binding, and should be a useful tool for the study of volatile anesthetic binding sites. Halothane binding to soluble proteins depended on their type and conformation, and, in some cases, was saturable within the clinical concentration range, increasing the tenability of discrete proteinaceous sites of action for the inhalational anesthetics.

Enhancement of gamma-aminobutyric acidA receptor function and binding by the volatile anesthetic halothane

The volatile general anesthetics halothane and enflurane increased muscimol-stimulated 36Cl- efflux via gamma-aminobutyric acid (GABA)A receptors in rat brain cortical slices and also increased basal 36Cl- efflux in the absence of GABA agonist. The effects occurred in the clinical range of anesthetic concentrations (0.56-1.7 mM halothane and 0.46-1.4 mM enflurane). Both anesthetics induced a slow onset increase in basal 36Cl- efflux rate when added alone with no exogenous GABA agonist. This direct effect of halothane had a biphasic dependence on anesthetic concentrations, with a maximal effect in the range 1.1 to 1.7 mM. Replacing extracellular calcium with magnesium or blocking voltage-gated calcium entry with cobalt (200 microM) altered the direct halothane effect, shifting the concentration-dependence curve to the right. Halothane direct potentiation of chloride flux in the absence of GABA agonist was blocked by the GABAA chloride channel antagonist picrotoxin but not by the GABAA receptor antagonist bicuculline. The halothane potentiation of the muscimol response was detectable at concentrations of 0.56 mM halothane in the assay buffer, and was linear with concentration up to 2.8 mM. The effect was more pronounced at low GABA agonist concentrations, apparently due to an increase in GABA affinity. Lowering the extracellular calcium concentration to micromolar levels did not affect halothane potentiation of muscimol responses. Halothane at similar concentrations increased the high-affinity binding of [3H]muscimol to GABAA receptor sites in rat brain cortical membranes in a calcium-independent manner.(ABSTRACT TRUNCATED AT 250 WORDS)