Volatile hydrocarbons from hydrogen peroxide-induced lipid peroxidation of erythrocytes and their cell components

HPLC-ESI-HRMS2 Determination of N-(2-Hydroxyethyl)-l-valyl-l-leucine in Human Urine: Method Validation

Biomonitoring of human exposure to reactive electrophilic chemicals such as ethylene oxide (EO) has been commonly based on the determination of adducts with N-terminal valine in blood protein globin, but a systematic search has also been undertaken to find surrogate markers enabling non-invasive sampling. Recently, N-(2-hydroxyethyl)-L-valyl-L-leucine (HEVL) has been identified as an ultimate cleavage product of EO-adducted globin in the urine of occupationally exposed workers. Herein, full validation of the analytical procedure consisting of solid-phase extraction of HEVL from urine samples (2 mL) followed by high-performance liquid chromatography-electrospray ionization-high-resolution mass spectrometry determination using deuterium-labeled HEVL as an internal standard (IS) is described. Method limit of quantitation is 0.25 ng/mL, and its selectivity is excellent as demonstrated by the invariable ratio of the qualifier and quantifier ion intensities across diverse urine samples and synthetic standard. The linear calibration model was applicable over the whole concentration range tested (0.25-10 ng/mL). The method accuracy assessed as a recovery of HEVL using a spiking experiment was 98-100%. Within-day precision of the method ranged from 1.8% to 3.0%, while the results from consecutive analytical runs conducted within 1 week or within 10-150 weeks differed in the range of 2.2-9.7%. The stability study on urine samples (-20°C up to 3 years, freeze-and-thaw up to 10 cycles) as well as on aqueous solutions (5°C up to 4 months) indicated no relevant changes in HEVL concentration (≤4%) over the time tested. Analytical responses of both HEVL and IS correlated with urinary creatinine as an index of matrix composition, but this matrix effect was mostly eliminated using the HEVL/IS peak area ratio, attaining the IS-normalized relative matrix effect <3%. In conclusion, the method complied successfully with the bioanalytical method validation criteria, making it a reliable tool for HEVL determination in human biomonitoring.

The role of endogenous versus exogenous sources in the exposome of putative genotoxins and consequences for risk assessment

The "totality" of the human exposure is conceived to encompass life-associated endogenous and exogenous aggregate exposures. Process-related contaminants (PRCs) are not only formed in foods by heat processing, but also occur endogenously in the organism as physiological components of energy metabolism, potentially also generated by the human microbiome. To arrive at a comprehensive risk assessment, it is necessary to understand the contribution of in vivo background occurrence as compared to the ingestion from exogenous sources. Hence, this review provides an overview of the knowledge on the contribution of endogenous exposure to the overall exposure to putative genotoxic food contaminants, namely ethanol, acetaldehyde, formaldehyde, acrylamide, acrolein, α,β-unsaturated alkenals, glycation compounds, N-nitroso compounds, ethylene oxide, furans, 2- and 3-MCPD, and glycidyl esters. The evidence discussed herein allows to conclude that endogenous formation of some contaminants appears to contribute substantially to the exposome. This is of critical importance for risk assessment in the cases where endogenous exposure is suspected to outweigh the exogenous one (e.g. formaldehyde and acrolein).

The application of mass spectrometry in molecular dosimetry: ethylene oxide as an example

Mass spectrometry plays an increasingly important role in the search for and quantification of novel chemically specific biomarkers. The revolutionary advances in mass spectrometry instrumentation and technology empower scientists to specifically analyze DNA and protein adducts, considered as molecular dosimeters, derived from reactions of a carcinogen or its active metabolites with DNA or protein. Analysis of the adducted DNA bases and proteins can elucidate the chemically reactive species of carcinogens in humans and can serve as risk-associated biomarkers for early prediction of cancer risk. In this article, we review and compare the specificity, sensitivity, resolution, and ease-of-use of mass spectrometry methods developed to analyze ethylene oxide (EO)-induced DNA and protein adducts, particularly N7-(2-hydroxyethyl)guanine (N7-HEG) and N-(2-hydroxyethyl)valine (HEV), in human samples and in animal tissues. GC/ECNCI-MS analysis after HPLC cleanup is the most sensitive method for quantification of N7-HEG, but limited by the tedious sample preparation procedures. Excellent sensitivity and specificity in analysis of N7-HEG can be achieved by LC/MS/MS analysis if the mobile phase, the inlet (split or splitless), and the collision energy are properly optimized. GC/ECNCI-HRMS and GC/ECNCI-MS/MS analysis of HEV achieves the best performance as compared with GC/ECNCI-MS and GC/EI-MS. In conclusion, future improvements in high-throughput capabilities, detection sensitivity, and resolution of mass spectrometry will attract more scientists to identify and/or quantify novel molecular dosimeters or profiles of these biomarkers in toxicological and/or epidemiological studies.

Simultaneous detection of five different 2-hydroxyethyl-DNA adducts formed by ethylene oxide exposure, using a high-performance liquid chromatography/electrospray ionisation tandem mass spectrometry assay

A method has been developed for the simultaneous detection and quantitation of five different 2-hydroxyethyl-DNA (HE-DNA) adducts that could be formed as a result of exposure to ethylene oxide (EO). In addition to the major N7-HE-guanine (N7-HEG) adducts this assay can also measure the less prevalent but potentially more biologically significant N1-HE-2'-deoxyadenosine (N1-HEdA), O(6)-HE-2'-deoxyguanosine (O(6)-HEdG), N(6)-HE-2'-deoxyadenosine (N(6)-HEdA) and N3-HE-2'-deoxyuridine adducts (N3-HEdU). The method involves the isolation of HE adducts from the unmodified nucleosides by either neutral thermal hydrolysis or enzymatic digestion, followed by high-performance liquid chromatographic (HPLC) purification, before detection and quantification by liquid chromatography tandem mass spectrometry (LC/MS/MS) using selective reaction monitoring (SRM). The limits of detection were in the range 0.5-25 fmol for each individual adduct, making this one of the most sensitive assays available for the detection of N7-HEG. To illustrate the possible applications of the assay, it has been employed in the measurement of endogenous/background and EO-induced HE adducts in a variety of DNA samples.

The "Tuebingen desiccator" system, a tool to study oxidative stress in vivo and inhalation toxicokinetics

The "Tuebingen desiccator," a gas-tight all-glass closed chamber system (CCS), has been established in Herbert Remmer's Institute of Toxicology, University of Tuebingen, to investigate the mechanisms underlying the exhalation of endogenous volatile hydrocarbons in rats under oxidative stress. Remmer and associates confirmed the former view that ethane and n-pentane were derived from polyunsaturated fatty acids, and they demonstrated that propane, n-butane and isobutane were released from amino acids. Hydrocarbons exhaled following acute ethanol treatment of rats resulted predominantly from ethanol-dependent inhibition of their metabolism and partly from oxidation of proteins. Exhalation of alkanes in carbon tetrachloride exposed rats did not reflect liver damage, which was, however, directly linked to the amount of carbon tetrachloride metabolized. As has first been shown in Herbert Remmer's institute by investigating the fate of inhaled vinyl chloride in rats, the CSS proved to be also an excellent tool for studying toxicokinetics of inhaled gaseous xenobiotics by means of gas uptake experiments. Based on results gained by such studies, it was recently demonstrated that knowledge of compound-specific physicochemical and species-specific physiological parameters are often sufficient to predict important toxicokinetic properties of inhaled chemicals such as tissue burdens at steady state. By means of the CCS, not only kinetics of a parent gaseous substance but also of gaseous metabolites can be investigated in vivo, as exemplified for ethylene oxide and 1, 2-epoxy-3-butene, metabolites of ethylene and 1,3-butadiene, respectively. Gas uptake studies in closed chamber systems are now worldwide used for determining toxicokinetic parameters relevant for physiological toxicokinetic modeling.

Carcinogenicity and genotoxicity of ethylene oxide: new aspects and recent advances

Long-term inhalation studies in rodents have presented unequivocal evidence of experimental carcinogenicity of ethylene oxide, based on the formation of malignant tumors at multiple sites. However, despite a considerable body of epidemiological data only limited evidence has been obtained of its carcinogenicity in humans. Ethylene oxide is not only an important exogenous toxicant, but it is also formed from ethylene as a biological precursor. Ethylene is a normal body constituent; its endogenous formation is evidenced by exhalation in rats and in humans. Consequently, ethylene oxide must also be regarded as a physiological compound. The most abundant DNA adduct of ethylene oxide is 7-(2-hydroxyethyl)guanine (HOEtG). Open questions are the nature and role of tissue-specific factors in ethylene oxide carcinogenesis and the physiological and quantitative role of DNA repair mechanisms. The detection of remarkable individual differences in the susceptibility of humans has promoted research into genetic factors that influence the metabolism of ethylene oxide. With this background it appears that current PBPK models for trans-species extrapolation of ethylene oxide toxicity need to be refined further. For a cancer risk assessment at low levels of DNA damage, exposure-related adducts must be discussed in relation to background DNA damage as well as to inter- and intraindividual variability. In rats, subacute ethylene oxide exposures on the order of 1 ppm (1.83 mg/m3) cause DNA adduct levels (HOEtG) of the same magnitude as produced by endogenous ethylene oxide. Based on very recent studies the endogenous background levels of HOEtG in DNA of humans are comparable to those that are produced in rodents by repetitive exogenous ethylene oxide exposures of about 10 ppm (18.3 mg/m3). Experimentally, ethylene oxide has revealed only weak mutagenic effects in vivo, which are confined to higher doses. It has been concluded that long-term human occupational exposure to low airborne concentrations to ethylene oxide, at or below current occupational exposure limits of 1 ppm (1.83 mg/m3), would not produce unacceptable increased genotoxic risks. However, critical questions remain that need further discussions relating to the coherence of animal and human data of experimental data in vitro vs. in vivo and to species-specific dynamics of DNA lesions.

A physiological toxicokinetic model for exogenous and endogenous ethylene and ethylene oxide in rat, mouse, and human: formation of 2-hydroxyethyl adducts with hemoglobin and DNA

Ethylene (ET) is a gaseous olefin of considerable industrial importance. It is also ubiquitous in the environment and is produced in plants, mammals, and humans. Uptake of exogenous ET occurs via inhalation. ET is biotransformed to ethylene oxide (EO), which is also an important volatile industrial chemical. This epoxide forms hydroxyethyl adducts with macromolecules such as hemoglobin and DNA and is mutagenic in vivo and in vitro and carcinogenic in experimental animals. It is metabolically eliminated by epoxide hydrolase and glutathione S-transferase and a small fraction is exhaled unchanged. To estimate the body burden of EO in rodents and human resulting from exposures to EO and ET, we developed a physiological toxicokinetic model. It describes uptake of ET and EO following inhalation and intraperitoneal administration, endogenous production of ET, enzyme-mediated oxidation of ET to EO, bioavailability of EO, EO metabolism, and formation of 2-hydroxyethyl adducts of hemoglobin and DNA. The model includes compartments representing arterial, venous, and pulmonary blood, liver, muscle, fat, and richly perfused tissues. Partition coefficients and metabolic parameters were derived from experimental data or published values. Model simulations were compared with a series of data collected in rodents or humans. The model describes well the uptake, elimination, and endogenous production of ET in all three species. Simulations of EO concentrations in blood and exhaled air of rodents and humans exposed to EO or ET were in good agreement with measured data. Using published rate constants for the formation of 2-hydroxyethyl adducts with hemoglobin and DNA, adduct levels were predicted and compared with values reported. In humans, predicted hemoglobin adducts resulting from exposure to EO or ET are in agreement with measured values. In rodents, simulated and measured DNA adduct levels agreed generally well, but hemoglobin adducts were underpredicted by a factor of 2 to 3. Obviously, there are inconsistencies between measured DNA and hemoglobin adduct levels.

Volatile alkanes and increased concentrations of isoprene in exhaled air during hemodialysis

In this study we examined breath volatile hydrocarbon concentrations in exhaled air of hemodialysis patients. We assessed both C(2)-C(5) alkanes - among them ethane and pentane the production of which in man is essentially due to the action free radicals exert on polyunsaturated fatty acids - and isoprene, an unsaturated hydrocarbon the biosynthesis and biological effects of which are the subject of controversy and mounting interest. Twenty patients were studied. Evaluation was performed intrapatient in the breath of patients with chronic renal failure, before and after dialysis (20 patients) and, in the same cases, during hemodialytic treatment (10 patients). Breath concentrations of these volatile hydrocarbons, determined before dialysis, were not different from those of normal subjects. Dialysis did not modify the levels of the C(2)-C(5) saturated hydrocarbons ethane, propane, butane and pentane. Instead, there was a marked increase in isoprene in all patients (basal values rose by a mean of 270%). Since isoprene was not present in the fluids or filters used for dialysis and there were only traces in the ambient air, the isoprene must have been produced endogenously during hemodialysis. As no situation has previously been reported to increase endogenous production of isoprene in humans, patients in hemodialysis offer a unique opportunity to investigate in depth the medical, biological and toxicological aspects of isoprene.

The carcinogenic risk of ethene (ethylene)

On occasion of the 25th year of publication of Toxicologic Pathology, the Editor has asked for a report about recent progress in the area addressed by an article entitled “Olefinic Hydrocarbons: A First Risk Estimate,” one of the top 10 most frequently cited papers of the journal (3). Because general issues of risk assessment have very recently been addressed in this journal (6), I will focus on new specific aspects of ethene carcinogenicity.

Propylene oxide: mutagenesis, carcinogenesis and molecular dose

The results from mutagenic and carcinogenic studies of propylene oxide (PO) and the current efforts to develop molecular dosimetry methods for PO-DNA adducts are reviewed. PO has been shown to be active in several bacterial and mammalian mutagenicity tests and induces site of contact tumors in rodents after long-term administration. Quantitation of N7-(2-hydroxypropyl)guanine (7-HPG) in nasal and hepatic tissues of male F344 rats exposed to 500 ppm PO (6 h/day; 5 days/week for 4 weeks) by inhalation was performed to evaluate the potential of high concentrations of PO to produce adducts in the DNA of rodent tissues and to obtain information necessary for the design of molecular dosimetry studies. The persistence of 7-HPG in nasal and hepatic tissues was studied in rats killed three days after cessation of a 4-week exposure period. DNA samples from exposed and untreated animals were analyzed for 7-HPG by two different methods. The first method consisted of separation of the adduct from DNA by neutral thermal hydrolysis, followed by electrophoretic derivatization of the adduct and gas chromatography-high resolution mass spectrometry (GC-HRMS) analysis. The second method utilized 32P-postlabeling to quantitate the amount of this adduct in rat tissues. Adducts present in tissues from rats killed immediately after cessation of exposure were 835.4 +/- 80.1 (respiratory), 396.8 +/- 53.1 (olfactory) and 34.6 +/- 3.0 (liver) pmol adduct/mumol guanine using GC-HRMS. Lower values, 592.7 +/- 53.3, 296.5 +/- 32.6 and 23.2 +/- 0.6 pmol adduct/mumol guanine were found in respiratory, olfactory and hepatic tissues of rats killed after three days of recovery. Analysis of the tissues by 32P-postlabeling yielded the following values: 445.7 +/- 8.0 (respiratory), 301.6 +/- 49.2 (olfactory) and 20.6 +/- 1.8 (liver) pmol adduct/mumol guanine in DNA of rats killed immediately after exposure cessation and 327.1 +/- 21.7 (respiratory), 185.3 +/- 29.2 (olfactory) and 15.7 +/- 0.9 (liver) pmol adduct/mumol guanine after recovery. Current methods of quantitation did not provide evidence for the endogenous formation of this adduct in control animals. These studies demonstrated that the target tissue for carcinogenesis has much greater alkylation of DNA than liver, a tissue that did not exhibit a carcinogenic response.

Quantification of endogenous carcinogens. The ethylene oxide paradox

Although ethylene oxide is a proven genotoxic carcinogen in experimental animals, its human carcinogenicity is still being debated. Alkylations (hydroxyethylation) of DNA and proteins by ethylene oxide are well established. Ethylene oxide is metabolically formed from ethylene, which is a natural body constituent. Thus, endogenous sources of ethylene/ethylene oxide contribute to background alkylations of physiological macromolecules. There are now experimentally well established data sets on the background hydroxyethylations of the N-terminal valine of hemoglobin and of the 7-N position of guanine in DNA, in laboratory animals as well as in humans: A review of these data leads to the conclusion that these background levels display remarkable consistency between the different species studied and, as far as DNA adducts are concerned, also between different tissues. From the existing database it can be deduced that in rats a hemoglobin alkylation, equivalent to the level of normal background, would be caused by repetitive external atmospheric exposures to ethylene oxide (6 hr/day, 5 days/week for several weeks) of about 30 ppb. On the contrary, in the same species, a DNA alkylation, equivalent to the level of normal background, would be caused by similar repetitive exposures to ethylene oxide at about 1-2 ppm. This paradox is unresolved. It points, however, to the biological importance of endogenous DNA alkylations and questions current regulatory procedures of assessing the risk of minute doses of exogenous carcinogens.

Preferential formation of the hydroperoxide of linoleic acid in choline glycerophospholipids in human erythrocytes membrane during peroxidation with an azo initiator

The formation of phospholipid hydroperoxides was monitored in human red blood cell (RBC) membranes that had been peroxidized with an azo initiator. Peroxidation of RBC membranes caused a profound decrease in the amount of polyunsaturated fatty acids and concomitantly hydroperoxides, as primary products of peroxidation, appeared in the phospholipids. Hydroperoxides were predominantly generated in choline glycerophospholipid (CGP), while the extent of formation of ethanolamine glycerophospholipid (EGP) hydroperoxides was low and their presence was transient. Hydroxy and hydroperoxy moieties in CGP were identified as 9-hydroxy and 13-hydroxy octadecanoic acid, derived from linoleic acid, by gas chromatography-mass spectrometric analysis. No consistent generation of hydroperoxide from arachidonic acid was evident in CGP. The CGP-hydroperoxide accounted for approximately 76% of linoleic acid consumed during peroxidation of RBC membranes. The prominent generation of phospholipid hydroperoxides was observed in the linoleic acid-rich membranes from rabbit RBC, indicating that the level of linoleic acid in phospholipids determines, in part, the extent of formation of phospholipid hydroperoxides. Aldehydic phospholipids, as secondary products of peroxidation, were detected in oxidized membranes. EGP was the most prominent aldehydic phospholipid, while negligible amounts of aldehydic CGP were formed. This study indicates that the process of oxidation of individual phospholipids clearly differs among phospholipids and depends on the structure of each.

The potential of the hydrocarbon breath test as a measure of lipid peroxidation

The straight chain aliphatic hydrocarbons ethane and pentane have been advocated as noninvasive markers of free-radical induced lipid peroxidation in humans. In in vitro studies, the evolution of ethane and pentane as end products of n-3 and n-6 polyunsaturated fatty acids, respectively, correlates very well with other markers of lipid peroxidation and even seems to be the most sensitive test available. In laboratory animals the use of both hydrocarbons as in vivo markers of lipid peroxidation has been validated extensively. Although there are other possible sources of hydrocarbons in the body, such as protein oxidation and colonic bacterial metabolism, these apparently are of limited importance and do not interfere with the interpretation of the hydrocarbon breath test. The production of hydrocarbons relative to that of other end products of lipid peroxidation depends on variables that are difficult to control, such as the local availability of iron(II) ions and dioxygen. In addition, hydrocarbons are metabolized in the body, which especially influences the excretion of pentane. Because of the extremely low concentrations of ethane and pentane in human breath, which often are not significantly higher than those in ambient air, the hydrocarbon breath test requires a flawless technique regarding such factors as: (1) the preparation of the subject with hydrocarbon-free air to wash out ambient air hydrocarbons from the lungs, (2) the avoidance of ambient air contamination of the breath sample by using appropriate materials for sampling and storing, and (3) the procedures used to concentrate and filter the samples prior to gas chromatographic determination. For the gas chromatographic separation of hydrocarbons, open tubular capillary columns are preferred because of their high resolution capacity. Only in those settings where expired hydrocarbon levels are substantially higher than ambient air levels might washout prove to be unnecessary, at least in adults. Although many investigators have concentrated on one marker, it seems preferable to measure both ethane and pentane concurrently. The results of the hydrocarbon breath test are not influenced by prior food consumption, but both vitamin E and beta-carotene supplementation decrease hydrocarbon excretion. Nevertheless, the long-term use of a diet high in polyunsaturated fatty acids, such as in parenteral nutrition regimens, may result in increased hydrocarbon exhalation. Hydrocarbon excretion slightly increases with increasing age. Short-term increases follow physical and intellectual stress and exposure to hyperbaric dioxygen.(ABSTRACT TRUNCATED AT 400 WORDS)

New scientific arguments for regulation of ethylene oxide residues in skin-care products

Ethylene oxide (EO) occurs as a contaminant of skin-care products because current commercial preparations of polyglycol ethers may contain ethylene oxide monomer residues, up to the order of 1 ppm. Using current regulatory worst-case assumptions, the presence of EO in skin-care products might lead to a maximal human daily external ethylene oxide dose of about 2.8 micrograms, and a consecutive maximal daily absorbed dose of 0.39 microgram. Two methods of toxicokinetic analysis have been used to compare this possible EO load by use of skin-care products with the inevitable load of EO which is produced endogenously in the organism. On the basis of a previous assessment of the endogenous production of ethylene and ethylene oxide (Filser et al. 1992) it is inferred that the absorbed EO dose of 0.39 microgram is about 1/30 of the unavoidable human endogenous load by endogenous EO. Alternatively, for a second calculation molecular dosimetry data have been used which were based on experimental quantification of the hydroxyethyl adduct of EO to the N-terminal valine of hemoglobin (HOEtVal) in rats. If the worst-case assumptions for human EO absorption from skin-care products are transferred to the rat species, the associated internal EO doses are about 1/110 of the internal EO doses which were calculated from the background HOEtVal concentrations observed in untreated animals. The divergence between both lines of calculation is mainly due to differences in HOEtVal background concentrations between man and rat.(ABSTRACT TRUNCATED AT 250 WORDS)

Alkylation and cleavage of DNA by carbon-centered radical metabolites

Although carbon-centered radicals are formed during the metabolism of several genotoxic compounds, they have received little attention as DNA damaging agents. Carbon-centered radicals, however, can both cleave the DNA backbone and alkylate DNA bases, as has been demonstrated to occur in chemical and biochemical systems. Also, in vivo DNA alkylation by methyl radicals has been evidenced by isolation of C8-methylguanine in hydrolysates of DNA from rats administered 1,2-dimethylhydrazine. While most of the studies related to DNA damage by free radicals have been focused on oxyradicals, further studies on DNA alterations promoted by carbon-centered radicals may be necessary to elucidate the mechanisms of action of chemical mutagens and carcinogens.

Headspace gas chromatography of volatile lipid peroxidation products from human red blood cell membranes

An improved headspace capillary gas chromatographic (GC) method was developed to measure the oxidative susceptibility of human red blood cell (RBC) membranes. This method analyzed volatile peroxidation products of both n-6 (hexanal and pentane) and n-3 (propanal) polyunsaturated fatty acids. Oxidative susceptibility tests were standardized by incubating in a sealed 10-mL headspace bottle 0.25 or 1 mL of human RBC membrane in 40 mM phosphate buffer for 1 hr at 37 degrees C with a mixture of Fe++, ascorbic acid and H2O2. Sodium dodecyl sulfate increased significantly the amount of hexanal measured by headspace GC. By this standard headspace method, in one series of red blood cell membranes (RBCM) samples a four-fold variation in oxidative susceptibility was observed in RBCM from blood freshly drawn from six healthy subjects. In another series of RBCM samples a sixteen-fold variation in oxidative susceptibility was noted in frozen RBCM from blood freshly drawn from five healthy subjects. Correlation between hexanal formation and polyunsaturated fatty acids (PUFA) depletion provided good evidence that under these standard conditions hexanal is exclusively derived from the oxidation of arachidonic acid. Hydroperoxides of arachidonic acid are more readily formed and decomposed than those of linoleic acid in the presence of Fe++, ascorbic acid and H2O2 to produce hexanal as the main product that can be readily analyzed by headspace GC. This method may provide a useful tool to study susceptibility toward lipid peroxidative damage in human RBC membranes.

Generation of volatile hydrocarbons from amino acids and proteins by an iron/ascorbate/GSH system

Incubation of free, but not of peptide-bound methionine in an iron/ascorbate system resulted in ethylene generation, which was inhibited by glutathione. Leucine and isoleucine, however, when incubated in an iron/ascorbate/GSH system, released small amounts of propane and ethane, respectively. Peptide-bound leucine additionally yielded butane, as did bovine serum albumin or casein. Hydrocarbon generation from amino acids was inhibited by hydroxyl radical scavengers, but catalase and superoxide dismutase were more efficient. Additionally, ethane and propane generation in this system was optimal at pH 6.2 suggesting the involvement of protonated superoxide besides OH-radicals which attack the side chains of Leu and Ile and very probably produce carbon-centered radicals, which should abstract a hydrogen atom from the thiol group of GSH resulting in the formation of saturated hydrocarbons.

A highly sensitive gas chromatographic method does not detect exhalation of volatile hydrocarbons from isolated ventilated lungs under massive peroxidative stress

Lung lipid peroxidation is thought to be a basic pathophysiological phenomenon responsible for pulmonary damage in different types of oxidant attack. Measurement of volatile hydrocarbons, especially ethane and pentane, produced during peroxidative degradation of polyunsaturated fatty acids and exhaled into an animal housing chamber, has attracted increasing interest for the monitoring of in vivo lipid peroxidation. However, this approach cannot distinguish between pulmonary exhalation of hydrocarbons generated in different organs or even the intestinum and pulmonary generation of these lipid peroxidation markers. In the present study we developed a respiration and hydrocarbon trapping system for isolated, ventilated, and perfused lungs that avoided rebreathing and allowed complete sampling and gas chromatographic separation and quantification of exhaled alkanes and alkenes (C1-C5) in the absence of background levels. Using an "artificial lung," the recoveries of exogenously administered hydrocarbons ranged between 80 and 95% with good reproducibility (SD between 1.7 and 9.6%). The detection limit of the system was approximately 3 fmol of each alkane or alkene/g wet lung weight.min. However, neither under basal conditions nor during massive peroxidative stress by the application of high doses of H2O2, FeCl3/ascorbate, paraquat, or ozone was any material with a retention time similar to that of hydrocarbons exhaled from isolated rabbit lungs. We conclude that under the experimental conditions employed, there is only insignificant generation of hydrocarbons in intact lungs.

Lipid peroxidation in erythrocytes

Erythrocytes might be expected to be highly susceptible to peroxidation. Their membranes are rich in polyunsaturated fatty acids; they are continuously exposed to high concentrations of oxygen; and they contain a powerful transition metal catalyst. In fact, autoxidation is held in check in vivo by extremely efficient protective antioxidant mechanisms. These involve cellular enzymes such as superoxide dismutase and glutathione peroxidase, as well as vitamin E; but they mainly reflect effective structural compartmentalisation. This review surveys mechanisms which lead to red cell lipid autoxidation and the role of haemoglobin in these processes. The influence of haemoglobinopathies, of lipid composition and of abnormalities in antioxidant mechanisms induced by exogenous oxidant stress is also considered.

In vitro aging of red blood cells and lipid peroxidation

Incubating isolated erythrocytes in phosphate buffered saline supplied with sufficient glucose (20 mM) for several days resulted in methemoglobin formation and decrease in glycolytic and antioxidant enzyme activities. Volatile hydrocarbon gas release (ethane, ethylene, propane, butane, isobutane, pentane) and loss of the polyunsaturated fatty acids, arachidonic acid (20:4) and docosahexaenoic acid (22:6) in the erythrocyte membrane indicated possible involvement of peroxidative reactions in cellular aging processes.

Alcohol consumption and hepatic fibrosis affect the fatty acid composition of red blood cells and their susceptibility to lipid peroxidation

Erythrocytes from alcoholics with and without liver cirrhosis and from rats treated either with ethanol or thioacetamide, the latter treatment resulting in hepatic fibrosis, were analysed for their membrane fatty acid composition and their susceptibility to lipid peroxidation. Red cells containing less arachidonic acid than controls, as found in alcoholics with liver cirrhosis, were less susceptible to lipid peroxidation than controls. This observation was confirmed by experiments with rat erythrocytes obtained from animals with hepatic fibrosis. However, red cells containing less linoleic acid than controls, as found in alcoholics without liver cirrhosis, exhibited a normal degree of lipid peroxidation upon oxidant stress induced by hydrogen peroxide. The results demonstrated that in red cells only fatty acids with four double bonds seem to be involved in membrane peroxidation reactions under the condition chosen. This observation might be of relevance for in vivo aging of red cells.

Development of a method to monitor low molecular mass hydrocarbons in exhaled breath of man: preliminary evaluation of its interest for detecting a lipoperoxidation process in vivo

Low molecular mass hydrocarbons, particularly ethane and pentane, have been measured in expired air of man. The air is collected into 5-10 l polyamide bags. After removal of water vapour and CO2, the hydrocarbons are concentrated on a silicagel column kept at 0 degree C. The column desorption is carried out at 290 degrees C, and the gases are analysed by gas chromatography. A slight but statistically significant increased pentane production was detected in cirrhotic patients.

Measurement of in vivo lipid peroxidation and toxicological significance

The quantitative determination of hydrocarbons exhaled by animals as an in vivo index of extensive lipid peroxidation is described. Advantages and limitations of this method are discussed. Acetaminophen-induced hepatic lipid peroxidation in mice is an example of oxidative stress, the extent of which is determined in vivo by the turnover of endoplasmic reticulum monooxygenase and the cofactor, e.g. glutathione status of the liver. In microsomal suspensions, none of the assay methods for lipid peroxidation identifies acetaminophen as a prooxidant. Rather, it acts like an antioxidant. The obvious limitations of in vitro experiments are emphasized. Cytosolic metabolism of allyl alcohol also leads, in a dose-dependent manner, to extensive lipid peroxidation. Evidence is presented that release of iron from intracellular stores following overproduction of NADH may be the primary cause of this lesion. The term reductive stress is suggested for this metabolic initiation of iron redox cycling. In experimental hepatitis induced by galactosamine/endotoxin, a leukotriene-mediated pathomechanism, no signs of lipid peroxidation are detectable. This means that ethane or pentane formation are definitively not late consequences of membrane deterioration but rather early causal events in special cases of hepatotoxicity.

The relationship between lipid composition of red blood cells and their susceptibility to lipid peroxidation

Red blood cells from 31 healthy donors were examined for the cholesterol content, the fatty acid composition, and the susceptibility to lipid peroxidation induced by either hydrogen peroxide or phenylhydrazine. Lipid peroxidation was monitored by the release of pentane and ethane. In addition, plasma fatty acids were measured in order to find out, whether plasma and red cell fatty acids were correlated. In experiments with hydrogen peroxide, a significant positive correlation was found between the proportion of arachidonic acid (C 20:4n - 6; r = 0.57, p less than 0.01) and docosahexaenoic acid (C 22:6n - 3; r = +0.71, p less than 0.01), and the release of pentane and ethane, respectively. A significant negative correlation was found between the membrane cholesterol content and the pentane release (r -0.44, p less than 0.05). In experiments performed with phenylhydrazine, red cell membrane lipid composition did not influence the susceptibility of red cells to lipid peroxidation. A close correlation was found between plasma and red cell fatty acids (palmitic acid, r = +0.46, p less than 0.01; linoleic acid, r = +0.41, p less than 0.05; arachidonic acid, r = +0.59, p less than 0.01; docosahexaenoic acid, r = +0.67, p less than 0.01). The results demonstrated that the degree of peroxide-induced oxidation of erythrocyte lipids depends on the content of polyunsaturated fatty acids in the membrane, which on the other hand, is determined by plasma fatty acids. It is suggested that dietary variations may influence the susceptibility of red cells to lipid peroxidation.

Effect of free radical scavengers and metal ion chelators on hydrogen peroxide and phenylhydrazine induced red blood cell lipid peroxidation

Desferrioxamine a well-known iron chelator was found to decrease hydrogen peroxide and phenylhydrazine induced lipid peroxidation of red blood cell membranes assessed by hydrocarbon gas release and loss of polyunsaturated fatty acids. Hydroxyl radical scavengers like mannitol and thiourea and proteins like albumin were unable to reduce peroxidative reactions to our system. Addition of uric acid (in an unphysiological concentration of 5 mM) to the incubation medium resulted in a slight reduction in H2O2/phenylhydrazine mediated break-down of arachidonic (20:4) and docosahexaenoic acid (22:6) in the erythrocyte membrane and consequently in a decreased alkane release and haemolysis.

Lipid abnormalities in plasma and red cell membranes of chronic alcoholics

The fatty acid composition of plasma and red cell membranes was determined in alcoholics without liver dysfunction upon admission for withdrawal therapy and after a 4-week period of abstinence, and in normal subjects. Fatty acid analysis of plasma by capillary gas chromatography showed a higher proportion of saturated fatty acids in alcoholics on admission than in normal subjects. Other major differences in plasma fatty acids in alcoholics were the reduction of linoleic acid and the increase in palmitic acid. Similar abnormalities were measured in red cell membranes of alcoholics. Although there were less polyunsaturated fatty acids in red cells of alcoholics, the degree of hydrogen peroxide or phenylhydrazine-induced lipid peroxidation was the same as in controls. Membrane tolerance to ethanol was attributed to adaptive membrane alterations (increase of saturated fatty acids in membranes). However, the here reported changes in plasma and red cell lipids suggest that alterations in red cell membrane lipids reflect abnormalities in plasma lipids due to equilibrium exchanges rather than signify adaptive changes of the red cells to ethanol.

The fatty acid composition of red cells deficient in glucose-6-phosphate dehydrogenase and their susceptibility to lipid peroxidation

Oxidant damage to red cell membranes could play a part in the pathogenesis of acute and chronic haemolysis in glucose-6-phosphate dehydrogenase deficiency. Therefore, we studied the substrate for red cell membrane lipid peroxidation, i.e. the content of various polyunsaturated fatty acids in ghosts, and the susceptibility of red cells to lipid peroxidation in normal subjects and in subjects deficient in glucose-6-phosphate dehydrogenase. The fatty acid composition of red cell membranes and plasma was analysed by capillary column gas chromatography. The sensitivity of red cells to lipid peroxidation was evaluated after hydrogen-peroxide-induced oxidant stress. The degree of lipid peroxidation was monitored by measuring the release of pentane and ethane formed during the breakdown of n-6 and n-3 fatty acids. The red cell sensitivity to lipid peroxidation was found to be higher in subjects with glucose-6-phosphate dehydrogenase deficiency than in normal subjects. In the former, saturated fatty acids, in particular palmitic and stearic acid, were found to be decreased, whereas the proportion of arachidonic acid showed a clear increase. Fatty acid analysis of plasma did not reveal significant abnormalities in enzyme-deficient patients, which could explain the alteration of membrane fatty acids. Our results suggest that the increased content of substrate for lipid peroxidation, particularly arachidonic acid, in red cell membranes of subjects deficient in glucose-6-phosphate dehydrogenase, should be considered in an evaluation of an enhanced sensitivity to red cell lipid peroxidation.

Decreased susceptibility of red blood cells to lipid peroxidation in patients with alcoholic liver cirrhosis

Red blood cells from alcoholics with and without liver cirrhosis and control subjects were examined for the susceptibility to lipid peroxidation. Red blood cells of patients with liver cirrhosis were found to be less sensitive to hydrogen peroxide-induced peroxidation measured by a new, reliable and sensitive method: the release of pentane during red blood cell lipid peroxidation. Changes of sensitivity to lipid peroxidation correlated with the severity of the liver malfunction, but not with abnormalities of the lipid composition of red cell membranes which are apparent in patients with liver disease. In alcoholics without liver cirrhosis, only minor changes in the susceptibility of red cells to peroxidation were observed.

Effect of ascorbate on red blood cell lipid peroxidation

The present study investigates the effect of ascorbate on red cell lipid peroxidation. At a concentration between 0.2 mmol - 20 mmol/1 ascorbic acid reduces hydrogen peroxide-induced red blood cell lipid peroxidation resulting in a marked decrease in ethane and pentane production as well as in haemolysis. Ascorbic acid also shows an antioxidant effect on chelated iron-catalyzed hydrogen peroxide-induced peroxidation of erythrocyte membranes. At a concentration of 10 mmol/1 ascorbic acid totally inhibits oxidative break-down of polyunsaturated fatty acids by radicals originating from hydrogen peroxide. Our results indicate that ascorbate at the chosen concentration has an antioxidant effect on red blood cell lipid peroxidation.