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ATP-gated purinergic P2X4 receptors (P2X4Rs) are the most alcohol-sensitive P2XR subtype. We recently reported that ivermectin (IVM), an antiparasitic used in animals and humans, antagonized ethanol inhibition of P2X4Rs. Furthermore, IVM reduced ethanol intake in mice. The first molecular model of the rat P2X4R, built onto the X-ray crystal structure of zebrafish P2X4R, revealed an action pocket for both ethanol and IVM formed by Asp331, Met336 in TM2 and Trp46, and Trp50 in TM1 segments. The role of Asp331 and Met336 was experimentally confirmed. The present study tested the hypothesis that Trp46 plays a role in ethanol and IVM modulation of P2X4Rs. Trp46 was mutated to residues with different physicochemical properties and the resultant mutants tested for ethanol and IVM responses using Xenopus oocyte expression system and two-electrode voltage clamp. Nonaromatic substitutions at position 46 reduced ethanol inhibition at higher concentrations and switched IVM potentiation to inhibition. Simultaneous substitution of alanine at positions Trp46 and Met336 also resulted in similar changes in ethanol and IVM responses. Furthermore, a new molecular model based on the open pore conformation of zebrafish P2X4R suggested a role for Tyr42 that was further supported experimentally. Our previous and current findings, combined with our preliminary evidence of increased ethanol consumption in P2X4R knockout mice, suggest that the ethanol and IVM action pocket in P2X4Rs formed by positions 42, 46, 331, and 336 presents a potential target for medication development for alcohol use disorders.

We built a model of a GABAA alpha1 receptor (GABAAR) that combines the ligand binding (LBD) and the transmembrane domains (TMD). We used six steps: (1) a four-alpha helical bundle in the crystal structure of bovine cytochrome c oxidase (2OCC) was identified as a template for the TMD of a single subunit. (2) The five pore-forming alpha helices of a bacterial mechanosensitive channel (1MSL) served as a template for the pentameric ion channel. (3) Five copies of the tetrameric template from 2OCC were superimposed on 1MSL to produce a homopentamer containing 20 alpha helices arranged around a funnel-shaped central pore. (4) Five copies of the GABAAR sequence were threaded onto the alpha-helical segments of this template and inter-helical loops were generated to produce the TMD model. (5) A model of the LBD was built by threading the aligned sequence of GABAAR onto the crystal structure of the acetylcholine binding protein (1I9B). (6) The models of the LBD and the TMD were aligned along a common five-fold axis, moved together along that axis until in vdW contact, merged, and then optimized with restrained molecular dynamics. Our model corresponds closely with recently published coordinates of the acetylcholine receptor (1OED) but also explains additional features. Our model reveals structures of loops that were not visible in the cryoelectron micrograph and satisfies most labeling and mutagenesis data. It also suggests mechanisms for ligand binding transduction, ion selectivity, and anesthetic binding.

Studies using molecular modeling, genetic engineering, neurophysiology/pharmacology, and whole animals have advanced our understanding of where and how inhaled anesthetics act to produce immobility (minimum alveolar anesthetic concentration; MAC) by actions on the spinal cord. Numerous ligand- and voltage-gated channels might plausibly mediate MAC, and specific amino acid sites in certain receptors present likely candidates for mediation. However, in vivo studies to date suggest that several channels or receptors may not be mediators (e.g., gamma-aminobutyric acid A, acetylcholine, potassium, 5-hydroxytryptamine-3, opioids, and alpha(2)-adrenergic), whereas other receptors/channels (e.g., glycine, N-methyl-D-aspartate, and sodium) remain credible candidates.

There has been rapid progress in molecular modelling in recent years. The convergence of improved software for molecular mechanics and dynamics, techniques for chimeric substitution and site-directed mutations, and the first x-ray structures of transmembrane ion channels have made it possible to build and test models of anaesthetic binding sites. These models have served as guides for site-directed mutagenesis and as starting points for understanding the molecular dynamics of anaesthetic-site interactions. Ligand-gated ion channels are targets for inhaled anaesthetics and alcohols in the central nervous system. The inhibitory strychnine-sensitive glycine and gamma-aminobutyric acid type A receptors are positively modulated by anaesthetics and alcohols; site-directed mutagenesis techniques have identified amino acid residues important for the action of volatile anaesthetics and alcohols in these receptors. Key questions are whether these amino acid mutations form part of alcohol- or anaesthetic-binding sites or if they alter protein stability in a way that allows anaesthetic molecules to act remotely by non-specific mechanisms. It is likely that molecular modelling will play a major role in answering these questions.

Recent mutational analyses of ligand-gated ion channels (LGICs) have demonstrated a plausible site of anesthetic action within their transmembrane domains. Although there is a consensus that the transmembrane domain is formed from four membrane-spanning segments, the secondary structure of these segments is not known. We utilized 10 state-of-the-art bioinformatics techniques to predict the transmembrane topology of the tetrameric regions within six members of the LGIC family that are relevant to anesthetic action. They are the human forms of the GABA alpha 1 receptor, the glycine alpha 1 receptor, the 5HT3 serotonin receptor, the nicotinic AChR alpha 4 and alpha 7 receptors and the Torpedo nAChR alpha 1 receptor. The algorithms utilized were HMMTOP, TMHMM, TMPred, PHDhtm, DAS, TMFinder, SOSUI, TMAP, MEMSAT and TOPPred2. The resulting predictions were superimposed on to a multiple sequence alignment of the six amino acid sequences created using the CLUSTAL W algorithm. There was a clear statistical consensus for the presence of four alpha helices in those regions experimentally thought to span the membrane. The consensus of 10 topology prediction techniques supports the hypothesis that the transmembrane subunits of the LGICs are tetrameric bundles of alpha helices.

Unlabelled: The in vivo potencies of anesthetics correlate with their capacity to suppress the reaction of luciferin with luciferase. In addition, luciferin has structural resemblances to etomidate. These observations raise the issues of whether luciferin, itself, might affect anesthetic requirement, and whether luciferase resembles the site of anesthetic action. Because the polar luciferin is unlikely to cross the blood-brain barrier (we found that the olive oil/water partition coefficient was 100 +/- 36 x 10(-7)), we studied these issues in rats by measuring the effect of infusion of luciferin in artificial cerebrospinal fluid into the lumbar subarachnoidal space and into the cerebral intraventricular space on the MAC (the minimum alveolar anesthetic concentration required to eliminate movement in response to a noxious stimulus in 50% of tested subjects) of isoflurane. MAC in rats given lumbar intrathecal doses of luciferin estimated to greatly exceed anesthetizing doses of etomidate, did not differ significantly from MAC in rats receiving only artificial cerebrospinal fluid into the lumbar intrathecal space. MAC slightly decreased when doses of luciferin estimated to greatly exceed anesthetizing doses of etomidate were infused intraventricularly (P < 0.05). In contrast to the absent or minimal effects of luciferin, intrathecal or intraventricular infusion of etomidate at similar or smaller doses significantly decreased isoflurane MAC. Luciferin did not affect +-aminobutyric acid type A or acetylcholine receptors expressed in Xenopus oocytes. These results suggest that luciferin has minimal or no anesthetic effects. It also suggests that luciferin/luciferase may not provide a good surrogate for the site at which anesthetics act, if this site is on the surface of neuronal cells., Implications: In proportion to their potencies, anesthetics inhibit luciferin's action on luciferase, and luciferin structurally resembles the anesthetic etomidate. However, in contrast to etomidate, luciferin given intrathecally or into the third cerebral ventricle does not have anesthetic actions, and it does not affect +-aminobutyric acid or acetylcholine receptors in vitro. Luciferase may not provide a good surrogate for the site at which anesthetics act.

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.

Unlabelled: Meyer and Overton suggested that anesthetic potency correlates inversely with lipophilicity. Thus, MAC times the olive oil/gas partition coefficient equals an approximately constant value of 1.82 +/- 0.56 atm (mean +/- SD). MAC is the minimum alveolar concentration of anesthetic required to eliminate movement in response to a noxious stimulus in 50% of subjects. Although MAC times the olive oil/gas partition coefficient also equals an approximately constant value for normal alkanols from methanol through octanol, the value (0.156 +/- 0.072 atm) is 1/10th that found for conventional anesthetics. We hypothesized that substitution of sulfur for the oxygen in n-alkanols would decrease their saline/gas partition coefficients (i.e., decrease polarity) while sustaining lipid/gas partition coefficients. Further, we hypothesized that these changes would produce products of MAC times olive oil partition coefficients that approximate those of conventional anesthetics. To test these predictions, we measured MAC in rats, and saline and olive oil solubilities for the series H(CH(2))(n)SH, comparing the results with the series H(CH(2))(n)OH for compounds having three to six carbon atoms. As hypothesized, the alkanethiols had similar oil/gas partition coefficients, 1000-fold smaller saline gas partition coefficients, and MAC values 30 times greater than for comparable alkanols. Such findings are consistent with the notion that the greater potency of many alkanols (greater than would be predicted from conventional inhaled anesthetics and the Meyer-Overton hypothesis) results from their greater polarity., Implications: The in vivo anesthetic potency of alkanols and alkanethiols correlates with their lipophilicity and hydrophilicity.

1. Each residue in the second transmembrane segment (TM2) of the human GABA(A) receptor alpha(2) subunit was individually mutated to tryptophan. The wild-type or mutant alpha(2) subunits were expressed with the wild-type human GABA(A) receptor beta(2) subunit in Xenopus oocytes, and the effects of these mutations were investigated using two-electrode voltage-clamp recording. 2. Four mutations (V257W, T262W, T265W and S270W) produced receptors which were active in the absence of agonist, and this spontaneous open channel activity was blocked by both picrotoxin and bicuculline, except in the alpha(2)(V257W)beta(2) mutant receptor, which was not sensitive to picrotoxin. 3. Six mutations (V257W, V260W, T262W, T267W, S270W and A273W) enhanced the agonist sensitivity of the receptor, by 10 - 100 times compared with the wild-type alpha(2)beta(2) receptor. Other mutations (T261W, V263W, L269W, I271W and S272W) had little or no effect on the apparent affinity of the receptor to GABA. Eight of the tryptophan mutations (R255, T256, F258, G259, L264, T265, M266 or T268) resulted in undetectable GABA-induced currents. 4. The S270W mutation eliminated potentiation of GABA by ethanol, whereas T261W markedly increased the action of ethanol. The T262W mutation produced direct activation (10% of maximal GABA response) by ethanol in the absence of GABA, while other mutations did not alter the action of ethanol significantly. 5. These results are consistent with a unique role for S270 in the action of ethanol within the TM2 region, and with models of GABA(A) receptor channel function, in which specific residues within TM2 are critical for the regulation of channel gating (S270, L264), while other residues (L269, I271 and S272) have little effect on these functions and may be non-critical structural residues.

Ligand-gated ion channels are a target for inhaled anesthetics and alcohols in the central nervous system. The inhibitory strychnine-sensitive glycine and gamma-aminobutyric acid type A receptors are positively modulated by anesthetics and alcohols, and site-directed mutagenesis techniques have identified amino acid residues important for the action of volatile anesthetics and alcohols in these receptors. A key question is whether these amino acids are part of an alcohol/anesthetic-binding site. In the present study, we used an alkanethiol anesthetic to covalently label its binding site by mutating selected amino acids to cysteine. We demonstrated that the anesthetic propanethiol, or alternatively, propyl methanethiosulfonate, covalently binds to cysteine residues introduced into a specific second transmembrane site in glycine receptor and gamma-aminobutyric acid type A receptor subunits and irreversibly enhances receptor function. Moreover, upon permanent occupation of the site by propyl disulfide, the usual ability of octanol, enflurane, and isoflurane to potentiate the function of the ion channels was lost. This approach provides strong evidence that the actions of anesthetics in these receptors are due to binding at a single site.

Unlabelled: All series of volatile and gaseous compounds contain members that can produce anesthesia, as defined by the minimum alveolar anesthetic concentration (MAC) required to produce immobility in response to a noxious stimulus. For unhalogenated n-alkanes, cycloalkanes, aromatic compounds, and n-alkanols, potency (1 MAC) increases by two-to threefold with each carbon addition in the series (e.g., ethanol is twice as potent as methanol). Total fluorination (perfluorination) of n-alkanes essentially eliminates anesthetic potency: only CF4 is anesthetic (MAC = 66.5 atm), which indicates that fluorine atoms do not directly influence sites of anesthetic action. Fluorine may enhance the anesthetic action of other moieties, such as the hydrogen atom in CHF3 (MAC = 1.60 atm), but, consistent with the notion that the fluorine atoms do not directly influence sites of anesthetic action, adding -(CF2)n moieties does not further increase potency (e.g., CHF2-CF3 MAC = 1.51 atm). Similarly, adding -(CF2)n moieties to perfluorinated alkanols (CH2OH-[CF2]nF) does not increase potency. However, adding a second terminal hydrogen atom (e.g., CHF2-CHF2 or CH2OH-CHF2) produces series in which the addition of each -CF2- "spacer" in the middle of the molecule increases potency two- to threefold, as in each unhalogenated series. This parallel stops at four or five carbon atom chain lengths. Further increases in chain length (i.e., to CHF2[CF2]4CHF2 or CHF2[CF2]5CH2OH) decrease or abolish potency (i.e., a discontinuity arises). This leads to our hypothesis that the anesthetic moieties (-CHF2 and -CH2OH) interact with two distinct, spatially separate, sites. Both sites must be influenced concurrently to produce a maximal anesthetic (immobility) effect. We propose that the maximal potency (i.e., for CHF2[CF2]2CHF2 and CHF2[CF2]3CH2OH) results when the spacing between the anesthetic moieties most closely matches the distance between the two sites of action. This reasoning suggests that a distance equivalent to a four or five carbon atom chain, approximately 5 A, separates the two sites., Implications: Volatile anesthetics may produce immobility by a concurrent action on two sites five carbon atom lengths apart.

Unlabelled: Previous work demonstrates that various anesthetics enhance the effect of gamma-aminobutyric acid (GABA), and this enhancement has been proposed as an explanation for how anesthetics cause anesthesia. This explanation extends to both fluorinated and unfluorinated alkanols. In the present study, we tested the capacity of fluorinated alkanols to enhance the function of the GABA(A) receptors expressed in Xenopus oocytes. CF3CH2OH, CF3(CF2)2CH2OH and CF3(CF2)4CH2OH potentiated GABA(A) receptor function, but CF3(CF2)5CH2OH did not. The degree of potentiation decreased in proportion to the chain length of the alkanols. These findings were not specific for receptors expressed in oocytes, as similar results were obtained with muscimol-stimulated 36Cl- uptake using mouse brain membrane vesicles. Although CF3(CF2)5CH2OH has been reported to enhance the capacity of desflurane to produce immobility in vivo, in our in vitro studies, this compound reduced potentiation of GABA-gated response by anesthetics such as isoflurane, enflurane, and pentobarbital. CHF2(CF2)5CH2OH, which has in vivo anesthetic effects, also failed to potentiate GABA(A) receptor function. These results indicate that the GABA(A) receptor is not the only receptor affected by fluorinated alkanols and that other receptors contribute to the capacity of alkanols to produce immobility. In particular, CF3(CF2)5CH2OH and CF3CH2OH inhibited N-methyl-D-aspartate receptor-mediated responses, which raises the possibility that this receptor is important for actions of fluorinated alkanols., Implications: We find a consistent parallel between the immobilization produced by fluorinated alkanols and their actions on N-methyl-D-aspartate receptors but do not find a consistent parallel between immobilization and effects on gamma-aminobutyric acid type A receptors. Thus, we suggest that N-methyl-D-aspartate, but not gamma-aminobutyric acid type A, receptors may mediate the capacity of anesthetics to produce immobilization.

Unlabelled: The Meyer-Overton hypothesis predicts that the potency of conventional inhaled anesthetics correlates inversely with lipophilicity: minimum alveolar anesthetic concentration (MAC) x the olive oil/gas partition coefficient equals a constant of approximately 1.82 +/- 0.56 atm (mean +/- SD), whereas MAC x the octanol/gas partition coefficient equals a constant of approximately 2.55 +/- 0.65 atm. MAC is the minimum alveolar concentration of anesthetic required to eliminate movement in response to a noxious stimulus in 50% of subjects. Although MAC x the olive oil/gas partition coefficient also equals a constant for normal alkanols from methanol through octanol, the constant (0.156 +/- 0.072 atm) is one-tenth that found for conventional anesthetics, whereas the product for MAC x the octanol/gas partition coefficient (1.72 +/- 1.19) is similar to that for conventional anesthetics. These normal alkanols also have much greater affinities for water (saline/gas partition coefficients equaling 708 [octanol] to 3780 [methanol]) than do conventional anesthetics. In the present study, we examined whether fluorination lowers alkanol saline/gas partition coefficients (i.e., decreases polarity) while sustaining or increasing lipid/gas partition coefficients, and whether alkanols with lower saline/gas partition coefficients had products of MAC x olive oil or octanol/gas partition coefficients that approached or exceeded those of conventional anesthetics. Fluorination decreased saline/gas partition coefficients to as low as 0.60 +/- 0.08 (CF3[CF2]6CH2OH) and, as hypothesized, increased the product of MAC x the olive oil or octanol/gas partition coefficients to values equaling or exceeding those found for conventional anesthetics. We conclude that the greater potency of many alkanols (greater than would be predicted from conventional inhaled anesthetics and the Meyer-Overton hypothesis) is associated with their greater polarity., Implications: Inhaled anesthetic potency correlates with lipophilicity, but potency of common alkanols is greater than their lipophilicity indicates, in part because alkanols have a greater hydrophilicity--i.e., a greater polarity.

Unlabelled: How some noble and diatomic gases produce anesthesia remains unknown. Although these gases have apparently minimal capacities to interact with a putative anesthetic site, xenon is a clinical anesthetic, and argon, krypton, and nitrogen produce anesthesia at hyperbaric pressures. In contrast, neon, helium, and hydrogen do not cause anesthesia at partial pressures up to their convulsant thresholds. We propose that anesthetic sites influenced by noble or diatomic gases produce binding energies composed of London dispersion and charge-induced dipole energies that are sufficient to overcome the concurrent unfavorable decrease in entropy that occurs when a gas molecule occupies the site. To test this hypothesis, we used the x-ray diffraction model of the binding site for Xe in metmyoglobin. This site offers a positively charged moiety of histidine 93 that is 3.8 A from Xe. We simulated placement of He, Ne, Ar, Kr, Xe, H2, and N2 sequentially at this binding site and calculated the binding energies, as well as the repulsive entropy contribution. We used free energies obtained from tonometry experiments to validate the calculated binding energies. We used partial pressures of gases that prevent response to a noxious stimulus (minimum alveolar anesthetic concentration [MAC]) as the anesthetic endpoint. The calculated binding energies correlated with binding energies derived from the in vivo (ln) data (RTln[MAC], where R is the gas constant and T is absolute temperature) with a slope near 1.0, indicating a parallel between the Xe binding site in metmyoglobin and the anesthetic site of action of noble and diatomic gases. Nonimmobilizing gases (Ne, He, and H2) could be distinguished by an unfavorable balance between binding energies and the repulsive entropy contribution. These gases also differed in their inability to displace water from the cavity., Implications: The Xe binding site in metmyoglobin is a good model for the anesthetic sites of action of noble and diatomic gases. The additional binding energy provided by induction of a dipole in the gas by a charge at the binding site enhanced binding.

Unlabelled: We assessed the anesthetic properties of helium and neon at hyperbaric pressures by testing their capacity to decrease anesthetic requirement for desflurane using electrical stimulation of the tail as the anesthetic endpoint (i.e., the minimum alveolar anesthetic concentration [MAC]) in rats. Partial pressures of helium or neon near those predicted to produce anesthesia by the Meyer-Overton hypothesis (approximately 80-90 atm), tended to increase desflurane MAC, and these partial pressures of helium and neon produced convulsions when administered alone. In contrast, the noble gases argon, krypton, and xenon were anesthetic with mean MAC values of (+/- SD) of 27.0 +/- 2.6, 7.31 +/- 0.54, and 1.61 +/- 0.17 atm, respectively. Because the lethal partial pressures of nitrogen and sulfur hexafluoride overlapped their anesthetic partial pressures, MAC values were determined for these gases by additivity studies with desflurane. Nitrogen and sulfur hexafluoride MAC values were estimated to be 110 and 14.6 atm, respectively. Of the gases with anesthetic properties, nitrogen deviated the most from the Meyer-Overton hypothesis., Implications: It has been thought that the high pressures of helium and neon that might be needed to produce anesthesia antagonize their anesthetic properties (pressure reversal of anesthesia). We propose an alternative explanation: like other compounds with a low affinity to water, helium and neon are intrinsically without anesthetic effect.

Alcohols in the homologous series of n-alcohols increase in central nervous system depressant potency with increasing chain length until a "cutoff" is reached, after which further increases in molecular size no longer increase alcohol potency. A similar phenomenon has been observed in the regulation of ligand-gated ion channels by alcohols. Different ligand-gated ion channels exhibit radically different cutoff points, suggesting the existence of discrete alcohol binding pockets of variable size on these membrane proteins. The identification of amino acid residues that determine the alcohol cutoff may, therefore, provide information about the location of alcohol binding sites. Alcohol regulation of the glycine receptor is critically dependent on specific amino acid residues in transmembrane domains 2 and 3 of the alpha subunit. We now demonstrate that these residues in the glycine alpha1 and the gamma-aminobutyric acid rho1 receptors also control alcohol cutoff. By mutation of Ser-267 to Gln, it was possible to decrease the cutoff in the glycine alpha1 receptor, whereas mutation of Ile-307 and/or Trp-328 in the gamma-aminobutyric acid rho1 receptor to smaller residues increased the cutoff. These results support the existence of alcohol binding pockets in these membrane proteins and suggest that the amino acid residues present at these positions can control the size of the alcohol binding cavity.

Oil/saline partition coefficients for inhaled compounds often are defined by the ratio of the separately determined oil/gas and saline/gas partition coefficients. This approach assumes that the concurrent presence of oil with saline has no effect on the characteristics of either solvent. To test this assumption, we measured the oil/gas and saline/gas partition coefficients for CF3(CCIF)2CF3 and 1,2-dichloroperfluorocyclobutane (C4Cl2F6) separately and with the two phases mixed in a common container. We chose these compounds because they have radically different oil/gas and saline/gas partition coefficients and thus would provide a severe test of the assumption. For CF3(CCIF)2CF3, olive oil/saline partition coefficients were, respectively, 13,200 and 13,300 when measured separately and in mixed phases, and the octanol/saline partition coefficients were 19,200 and 18,100. Similarly, olive oil/saline partition coefficients for 1,2-dichloroperfluorocyclobutane were 3660 and 3500 when measured separately and in mixed phases, respectively, and the octanol/saline partition coefficients were 5140 and 4560. We conclude that differences between separate and mixed-phase determinations of ratios are small or nonexistent.

The authors are studying the molecular details of the process that begins with hepatic metabolism of halogenated inhalation anesthetics and ends with hepatic necrosis. In previous studies they have shown that the halothane-free radical produced by UV-irradiation is identical to that produced during reductive metabolism of halothane by hepatic cytochrome P-450. In the present study, the authors have examined a mechanism by which free radicals may propagate damage in the endoplasmic reticulum of liver cells. The 1-chloro-2,2,2-trifluoroethyl free radical produced by UV-irradiation of halothane can abstract a hydrogen radical from arachidonic acid to yield 2-chloro-1,1,1-trifluoroethane and an arachidonic acid-free radical. The arachidonic acid-free radical reacts with molecular oxygen to form 5- and 15-hydroperoxyeicosatetraenoic acid. There is considerable evidence that the peroxidation process that we studied in the model system will be similar when the arachidonic acid is an acyl chain on a membrane phospholipid and the free radicals are generated metabolically. The authors suggest that these hydroperoxides may be toxic by acting as intermediates in the pathway of leukotriene production as well as by direct oxidation of membrane components.

Nitrous oxide labeled with a stable heavy nitrogen isotope was used for in-vitro studies of nitrous oxide metabolism in man and rat. At 5 per cent oxygen tension, which is comparable to normal oxygen tension in the intestine in vivo, each gram of intestinal contents during a 16-hr in-vitro incubation produced 47 +/- 13 nmol of molecular nitrogen for the rat and 103 +/- 17 nmol for man. Active reductive metabolism of nitrous oxide by intestinal contents was significantly inhibited by antibiotics and by 20 per cent oxygen tension. It is suggested that the reduction of nitrous oxide to nitrogen may proceed through a single-electron transfer process with formation of free radicals. Under these circumstances, metabolism of nitrous oxide could produce toxic intermediates, even thought the end-metabolite is inert.

The molecular motion and distribution of the inhalation anesthetic halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) in a phospholipid bilayer model nerve membrane preparation was studied using fluorine nuclear magnetic resonance. Bilayers containing stable free radicals at known depths were studied to measure possible localization of halothane within certain areas of the bilayer. Bilayer suspensions containing manganese ions in the aqueous phase were used to test the partition of halothane between the aqueous and lipid phases. It was found that halothane rapidly achieves complete exchange throughout the bilayer and the surrounding aqueous phase. The results provide experimental evidence against the formation of anesthetic clathrates hypothesized by Pauling and Miller in their theories of anesthesia.

Inasmuch as it is known that the toxicity of anesthetic agents is potentiated by hypoxia and that the reductive metabolism of these agents results in the formation of lipid hydroperoxides, we investigated the toxicity of hydroperoxides under low-oxygen concentrations. We found that hypoxia exacerbates the toxicity of t-butyl hydroperoxide, shifting the dose-response curve of t-butyl hydroperoxide vs. lysis of hepatocytes approximately an order of magnitude to the left. Furthermore, although at the end of a 4-h exposure to 0.5% O2 hepatocyte monolayers seemed normal by three indices (release of 51Cr and serum glutamate transaminase or exclusion of trypan blue), they were completely lysed after an additional 20 h reoxygenation at 20% O2. In contrast, monolayers exposed to 2% O2 for 4 h seemed normal after 20 h reoxygenation. However, cells exposed to both a subtoxic dose of hydroperoxide and 4 h of 2% O2, although seeming healthy at the end of the hypoxic period, were completely lysed within 20 h after reoxygenation.

Binding of halothane metabolites to rat liver histones was investigated after in vivo administration of 14C-halothane. Animals were injected with either a mixture of triiodothyronine, glucagon and heparin (TGH) to stimulate liver growth or with saline as a control. Twenty-four hours later, animals were administered 14C-halothane and maintained at 8--10 per cent O2 for 6 hours. Detergent washed nuclei from liver homogenates were subfractionated to allow quantitative measurements of 14C-halothane binding to histones. Although our studies suggest that much of the previously reported binding of halothane metabolites to major cell fractions was a result of redistribution of endoplasmic reticulum components during isolation procedures, carefully controlled experiments demonstrated that the radioactivity associated with histones could not be due to residual microsomal lipid. Of the initial 132 mumol of 14C-halothane administered, 1.1 mumol remained as nonvolatile metabolites in the liver homogenate and 25 pmol were associated with purified histones. This corresponds to approximately one halothane moiety per 15,000 histone molecules. No significant binding to liver cell RNA or DNA was observed. With this low level of histone modification and lack of convincing evidence of halothane metabolite binding to hepatic DNA or RNA, it is unlikely that significant alteration of the genome occurs after exposure to halothane.