Hydroxylation of phenol to hydroquinone catalyzed by a human myeloperoxidase-superoxide complex: possible implications in benzene-induced myelotoxicity

The many roles of myeloperoxidase: From inflammation and immunity to biomarkers, drug metabolism and drug discovery

This review provides a practical guide to myeloperoxidase (MPO) and presents to the reader the diversity of its presence in biology. The review provides a historical background, from peroxidase activity to the discovery of MPO, to its role in disease and drug development. MPO is discussed in terms of its necessity, as specific individuals lack MPO expression. An underlying theme presented throughout brings up the question of the benefit and burden of MPO activity. Enzyme structure is discussed, including accurate masses and glycosylation sites. The catalytic cycle of MPO and its corresponding pathways are presented, with a discussion of the importance of the redox couples of the different states of MPO. Cell lines expressing MPO are discussed and practically summarized for the reader, and locations of MPO (primary and secondary) are provided. Useful methods of MPO detection are discussed, and how these can be used for studying disease processes are implied through the presentation of MPO as a biomarker. The presence of MPO in neutrophil extracellular traps is presented, and the activators of the former are provided. Lastly, the transition from drug metabolism to a target for drug development is where the review concludes.

Environmental and Health Effects of Benzene Exposure among Egyptian Taxi Drivers

Objectives

To study environmental exposure to benzene as well as hematological effects of benzene exposure in taxi drivers.

Background

Exposure to benzene has been associated with adverse health effects, including haematopoietic disorders.

Methods

A retrospective cohort study was carried out from the beginning of April 2017, through the end of June 2018, on 280 taxi drivers (the exposed group) and 120 unexposed matched individuals (controls). The tools included a predesigned self-administered questionnaire which contained questions about personal data (e.g., age, educational level, and smoking) and exposure data (e.g., duration and nature of exposure). Investigations involved complete blood count. Environmental studies for light-chain aromatic hydrocarbons (BTEX components: benzene, toluene, ethylbenzene, and xylene) were done.

Results

Mean values (ppm) of ambient concentrations of benzene, toluene, ethylbenzene, and xylene (0.81 ± 0.42, 26.69 ± 0.54, 29.36 ± 2.35, and 25.11 ± 2.06, respectively) in the stations were higher than international permissible levels (P < 0.001). On studying the clinical manifestations during the last two weeks, the prevalence of pallor, dizziness, headache, waist and back pain, fatigue, dry throat, and discomfort was significantly higher in taxi drivers (27.1%, 24.3%, 21.4%, 28.6%, 45.7%, 24.3%, and 25.7%, respectively) than among the controls (6.7%, 4.2%, 6.7%, 10%, 10%, 6.7%, 6.7%, and 9.2%, respectively). For chronic diseases, hypertension was the most prevalent chronic disease among the drivers (17.1%) than the controls (5.8%) (P=0.002). Regarding self-assessment of health status, 20.0% of taxi drivers reported poor health while 31.4% reported very good health (P < 0.001). MCH (pg), TLC (×103/μl), and platelets (×103/μl) were significantly lower among taxi drivers (26.33 ± 2.31, 6.55 ± 1.38, and 189.07 ± 53.25, respectively) (P=0.005, <0.001, and <0.001 respectively).

Conclusion

Abnormal hematological findings among taxi drivers were found on exposure to benzene. Health of taxi drivers is generally affected. Setting a clinic for periodic checkup and health education for taxi drivers is highly recommended.

Benzene's toxicity: a consolidated short review of human and animal studies by HA Khan

Khan's review is a brief summary of the complex field of study revolving around bone marrow toxicity and leukemogenesis observed in people chronically exposed to benzene. These comments are intended to demonstrate the use of the Kahn review as a launching pad for an in-depth analysis of the several related areas that must be fully explored to understand benzene-related diseases. The accumulated evidence demonstrates that benzene-induced bone marrow damage results from the production of hematotoxins that are metabolic products of benzene metabolism. The metabolism of benzene is described with respect to the formation benzene metabolites with emphasis on phenol and hydroquinone, which are the major metabolites, the significance of the formation of glutathione conjugates, the activity of NAD(P)H:quinone oxidoreductase (NQO1), and the ring opening products. Results are shown suggesting that oxidative stress induced by benzene metabolites is likely to be a significant factor in damaging DNA in bone marrow cells. Although a variety of effects on bone marrow can be demonstrated it is not yet clear which metabolites are most important in either benzene-induced aplastic anemia or leukemia. Benzene metabolism alone is insufficient to fully describe benzene toxicity. The impact of benzene metabolites on bone marrow cells must be fully explored to determine how benzene exposure can result in decreased viability or genetic toxicity to cells in the bone marrow. Human & Experimental Toxicology (2007) 26, 687— 696

Benzene exposure is associated with epigenetic changes (Review)

Benzene is a volatile aromatic hydrocarbon solvent and is known as one of the predominant air pollutants in the environment. Chronic exposure to benzene is known to cause aplastic anemia and increased risk of acute myelogenous leukemia in humans. Although the mechanisms by which benzene causes toxicity remain to be fully elucidated, it is widely accepted that its metabolism is crucial to its toxicity, with involvement of one or more reactive metabolites. Novel approaches aimed at evaluating different mechanisms by which benzene can impact on human health by altering gene regulation have been developed. Among these novel approaches, epigenetics appears to be promising. The present review article summarizes the most important findings, reported from the literature, on epigenetic modifications correlated to benzene exposure. A computerized search in PubMed was performed in November 2014, using search terms, including 'benzene', 'epigenetic', 'histone modifications', 'DNA methylation' and 'microRNA'. Epidemiological and experimental studies have demonstrated the potential epigenetic effects of benzene exposure. Several of the epigenomic changes observed in response to environmental exposures may be mechanistically associated with susceptibility to diseases. However, further elucidation of the mechanisms by which benzene alters gene expression may improve prediction of the toxic potential of novel compounds introduced into the environment, and allow for more targeted and appropriate disease prevention strategies.

Characterization of changes in gene expression and biochemical pathways at low levels of benzene exposure

Benzene, a ubiquitous environmental pollutant, causes acute myeloid leukemia (AML). Recently, through transcriptome profiling of peripheral blood mononuclear cells (PBMC), we reported dose-dependent effects of benzene exposure on gene expression and biochemical pathways in 83 workers exposed across four airborne concentration ranges (from <1 ppm to >10 ppm) compared with 42 subjects with non-workplace ambient exposure levels. Here, we further characterize these dose-dependent effects with continuous benzene exposure in all 125 study subjects. We estimated air benzene exposure levels in the 42 environmentally-exposed subjects from their unmetabolized urinary benzene levels. We used a novel non-parametric, data-adaptive model selection method to estimate the change with dose in the expression of each gene. We describe non-parametric approaches to model pathway responses and used these to estimate the dose responses of the AML pathway and 4 other pathways of interest. The response patterns of majority of genes as captured by mean estimates of the first and second principal components of the dose-response for the five pathways and the profiles of 6 AML pathway response-representative genes (identified by clustering) exhibited similar apparent supra-linear responses. Responses at or below 0.1 ppm benzene were observed for altered expression of AML pathway genes and CYP2E1. Together, these data show that benzene alters disease-relevant pathways and genes in a dose-dependent manner, with effects apparent at doses as low as 100 ppb in air. Studies with extensive exposure assessment of subjects exposed in the low-dose range between 10 ppb and 1 ppm are needed to confirm these findings.

Investigation into variation of endogenous metabolites in bone marrow cells and plasma in C3H/He mice exposed to benzene

Benzene is identified as a carcinogen. Continued exposure of benzene may eventually lead to damage to the bone marrow, accompanied by pancytopenia, aplastic anemia or leukemia. This paper explores the variations of endogenous metabolites to provide possible clues for the molecular mechanism of benzene-induced hematotoxicity. Liquid chromatography coupled with time of flight-mass spectrometry (LC-TOF-MS) and principal component analysis (PCA) was applied to investigate the variation of endogenous metabolites in bone marrow cells and plasma of male C3H/He mice. The mice were injected subcutaneously with benzene (0, 300, 600 mg/day) once daily for seven days. The body weights, relative organ weights, blood parameters and bone marrow smears were also analyzed. The results indicated that benzene caused disturbances in the metabolism of oxidation of fatty acids and essential amino acids (lysine, phenylalanine and tyrosine) in bone marrow cells. Moreover, fatty acid oxidation was also disturbed in plasma and thus might be a common disturbed metabolic pathway induced by benzene in multiple organs. This study aims to investigate the underlying molecular mechanisms involved in benzene hematotoxicity, especially in bone marrow cells.

Hydroquinone: environmental pollution, toxicity, and microbial answers

Hydroquinone is a major benzene metabolite, which is a well-known haematotoxic and carcinogenic agent associated with malignancy in occupational environments. Human exposure to hydroquinone can occur by dietary, occupational, and environmental sources. In the environment, hydroquinone showed increased toxicity for aquatic organisms, being less harmful for bacteria and fungi. Recent pieces of evidence showed that hydroquinone is able to enhance carcinogenic risk by generating DNA damage and also to compromise the general immune responses which may contribute to the impaired triggering of the host immune reaction. Hydroquinone bioremediation from natural and contaminated sources can be achieved by the use of a diverse group of microorganisms, ranging from bacteria to fungi, which harbor very complex enzymatic systems able to metabolize hydroquinone either under aerobic or anaerobic conditions. Due to the recent research development on hydroquinone, this review underscores not only the mechanisms of hydroquinone biotransformation and the role of microorganisms and their enzymes in this process, but also its toxicity.

Current understanding of the mechanism of benzene-induced leukemia in humans: implications for risk assessment

Benzene causes acute myeloid leukemia and probably other hematological malignancies. As benzene also causes hematotoxicity even in workers exposed to levels below the US permissible occupational exposure limit of 1 part per million, further assessment of the health risks associated with its exposure, particularly at low levels, is needed. Here, we describe the probable mechanism by which benzene induces leukemia involving the targeting of critical genes and pathways through the induction of genetic, chromosomal or epigenetic abnormalities and genomic instability, in a hematopoietic stem cell (HSC); stromal cell dysregulation; apoptosis of HSCs and stromal cells and altered proliferation and differentiation of HSCs. These effects modulated by benzene-induced oxidative stress, aryl hydrocarbon receptor dysregulation and reduced immunosurveillance, lead to the generation of leukemic stem cells and subsequent clonal evolution to leukemia. A mode of action (MOA) approach to the risk assessment of benzene was recently proposed. This approach is limited, however, by the challenges of defining a simple stochastic MOA of benzene-induced leukemogenesis and of identifying relevant and quantifiable parameters associated with potential key events. An alternative risk assessment approach is the application of toxicogenomics and systems biology in human populations, animals and in vitro models of the HSC stem cell niche, exposed to a range of levels of benzene. These approaches will inform our understanding of the mechanisms of benzene toxicity and identify additional biomarkers of exposure, early effect and susceptibility useful for risk assessment.

Myeloperoxidase-derived oxidation: mechanisms of biological damage and its prevention

There is considerable interest in the role that mammalian heme peroxidase enzymes, primarily myeloperoxidase, eosinophil peroxidase and lactoperoxidase, may play in a wide range of human pathologies. This has been sparked by rapid developments in our understanding of the basic biochemistry of these enzymes, a greater understanding of the basic chemistry and biochemistry of the oxidants formed by these species, the development of biomarkers that can be used damage induced by these oxidants in vivo, and the recent identification of a number of compounds that show promise as inhibitors of these enzymes. Such compounds offer the possibility of modulating damage in a number of human pathologies. This reviews recent developments in our understanding of the biochemistry of myeloperoxidase, the oxidants that this enzyme generates, and the use of inhibitors to inhibit such damage.

Mammalian heme peroxidases: from molecular mechanisms to health implications

A marked increase in interest has occurred over the last few years in the role that mammalian heme peroxidase enzymes, primarily myeloperoxidase, eosinophil peroxidase, and lactoperoxidase, may play in both disease prevention and human pathologies. This increased interest has been sparked by developments in our understanding of polymorphisms that control the levels of these enzymes, a greater understanding of the basic chemistry and biochemistry of the oxidants formed by these species, the development of specific biomarkers that can be used in vivo to detect damage induced by these oxidants, the detection of active forms of these peroxidases at most, if not all, sites of inflammation, and a correlation between the levels of these enzymes and a number of major human pathologies. This article reviews recent developments in our understanding of the enzymology, chemistry, biochemistry and biologic roles of mammalian peroxidases and the oxidants that they generate, the potential role of these oxidants in human disease, and the use of the levels of these enzymes in disease prognosis.

Assessment of genotoxicity of methyl-tert-butyl ether, benzene, toluene, ethylbenzene, and xylene to human lymphocytes using comet assay

Methyl-tert-butyl ether (MTBE) is a gasoline oxygenate and antiknock additive substituting for lead alkyls currently in use worldwide. Benzene, toluene, ethylbenzene, and xylene (BTEX) are volatile monoaromatic hydrocarbons which are commonly found together in crude petroleum and petroleum products such as gasoline. The aim of this study is to evaluate the genotoxic effects of these tested chemicals in human lymphocytes. Using the alkaline comet assay, we showed that all of the tested chemicals induce DNA damage in isolated human lymphocytes. This effect could follow from the induction of DNA strands breaks. The neutral version of the test revealed that MTBE, benzene, and xylenes induce DNA double-strand breaks at 200 microM. Apart from MTBE, the spin traps, 5,5-dimethyl-pyrroline-N-oxide (DMPO) and N-tert-butyl-alpha-phenylnitrone (PBN) can decrease the level of DNA damage in BTEX at 200 microM. This indicated that DNA damage could result from the participation of free radicals in DNA-damaging effect, which was further supported by the fact that post-treatment of formamidopyrimidine-DNA glycosylase (Fpg), enzyme recognizing oxidized DNA purines, gave rise to a significant increase in the extent of DNA damage in cells treated with benzene, and xylene at 200 microM. The results obtained suggested that MTBE and BTEX could induce a variety type of DNA damage such as single-strand breaks (SSBs), double-strand breaks (DSBs), and oxidative base modification.

Genetic polymorphisms in CYP1A1, CYP2D6, UGT1A6, UGT1A7, and SULT1A1 genes and correlation with benzene exposure in a Chinese occupational population

Metabolic enzymes involved in benzene activation or detoxification, including cytochrome P-450 1A1 (CYP1A1), cytochrome P-450 2D6 (CYP2D6), UDP-glucuronosyltransferase 1A6 (UGT1A6), UDP-glucuronosyltransferase1A7 (UGT1A7), and sulfotransferase 1A1 (SULT1A1), were studied for their roles in human susceptibility to benzene poisoning. All 304 subjects were investigated with a unitary questionnaire and their DNA was isolated from blood samples by a routine phenol-chloroform extraction. The study included 152 benzene poisoning patients, and 152 control workers occupationally exposed to benzene in South China. The genotypes were determined by polymerase chain reaction-restricted fragment length polymorphism (PCR-RFLP) technique with genomic DNA. No individuals had the CYP 2D6 c.212 G>A variant alleles in this study. There is no association between the UGT1A6 c.181 T>A, UGT1A7 c.208 Trp>Arg, and SULT1A1 c.638 G>A genotypes and increased risk of benzene-induced carcinogenesis. Although most of the CYP2D6 haplotypes did not show any significant difference, the CYP2D6 haplotype CYP2D6 c.188 C/C, C/T, and c.4268 C/C was significantly overrepresented in the case group (OR 4.02, 95% CI: 2.53-6.39) compared with in controls. Overall, our data suggested that individuals with CYP1A1 c.5639 T/T, CYP2D6 c.188 C/C, C/T, and CYP2D6 c.4268 C/C genotypes tend to be more susceptible to benzene toxicity.

Exposure assessment of benzene in Thai workers, DNA-repair capacity and influence of genetic polymorphisms

Exposure to benzene can cause DNA damage and the subsequent development of cancer. In this study, study subjects were 31 laboratory workers at a petrochemical factory and 31 gasoline service attendants. Control subjects were 34 workers from a mail sorting service center. Occupational exposures to benzene were assessed using biomarkers of exposure in blood and urine. Induction of DNA-repair capacity was assessed as a biomarker of early effect. The effects of polymorphisms in a metabolizing gene (CYP2E1), in detoxification genes (NQO1 and GSTT1), and in a DNA-repair gene (XRCC1, codon 399) on biomarker levels were evaluated. The mean individual benzene exposure of laboratory workers (24.40+/-5.82 ppb) and that of gasoline service attendants (112.41+/-13.92 ppb) were significantly higher than in controls (1.39+/-0.17 ppb, p<0.001). Blood benzene levels of laboratory workers (169.12+/-30.60 ppt) and gasoline service attendants (483.46+/-59.62 ppt) were significantly higher than those of the controls (43.30+/-4.89 ppt, p<0.001). Trans,trans-muconic acid levels in post-shift urine samples collected from laboratory workers (0.14+/-0.02 mg/g creatinine) and gasoline service attendants (0.20+/-0.02 mg/g creatinine) were significantly higher than in urine samples of controls (0.04+/-0.01 mg/g creatinine, p<0.001). The level of benzene exposure was correlated with blood benzene levels (R2=0.65, p<0.01) and post-shift urinary trans,trans-muconic acid concentrations (R2=0.49, p<0.01). As a biomarker of early effect, DNA-repair capacity was assessed by use of the cytogenetic challenge assay, i.e., chromosomal aberrations in peripheral lymphocytes were assessed after challenging blood cultures with 1 Gy gamma radiation. A significantly lower DNA-repair capacity--determined as dicentrics in laboratory workers (0.17 per metaphase cell) and in gasoline service attendants (0.19 per metaphase cell) compared with controls (0.12 per metaphase cell, p<0.001)--was observed. The frequency of deletions in laboratory workers (0.22 per metaphase cell) and gasoline service attendants (0.39 per metaphase cell) were significantly higher than in control workers (0.16 per metaphase cell, p<0.01 and p<0.001, respectively). An increase in radiation-induced dicentrics and deletions indicate a lower DNA-repair capacity in benzene-exposed workers. The influence of genetic polymorphisms on the biomarkers was assessed. Benzene-exposed workers who carried CYP2E1*1/*5 or *5/*5 genotypes excreted slightly higher levels of trans,trans-muconic acid than workers who carried the CYP2E1*1/*1 genotype. In this study, NQO1 and GSTT1 genotypes did not have any effect on the levels of trans,trans-muconic acid. In the case of XRCC1, laboratory workers with 399Arg/Gln or Gln/Gln had a lower DNA-repair capacity--measured as radiation-induced frequency of dicentrics and deletions--than those with the 399Arg/Arg genotype (p<0.01). Our results show that biomarkers of internal dose and early biological effect in people occupationally exposed to benzene are influenced by genetic polymorphisms in susceptibility genes.

Superoxide-dependent Oxidation of Melatonin by Myeloperoxidase

Myeloperoxidase uses hydrogen peroxide to oxidize numerous substrates to hypohalous acids or reactive free radicals. Here we show that neutrophils oxidize melatonin to N(1)-acetyl-N(2)-formyl-5-methoxykynuramine (AFMK) in a reaction that is catalyzed by myeloperoxidase. Production of AFMK was highly dependent on superoxide but not hydrogen peroxide. It did not require hypochlorous acid, singlet oxygen, or hydroxyl radical. Purified myeloperoxidase and a superoxide-generating system oxidized melatonin to AFMK and a dimer. The dimer would result from coupling of melatonin radicals. Oxidation of melatonin was partially inhibited by catalase or superoxide dismutase. Formation of AFMK was almost completely eliminated by superoxide dismutase but weakly inhibited by catalase. In contrast, production of melatonin dimer was enhanced by superoxide dismutase and blocked by catalase. We propose that myeloperoxidase uses superoxide to oxidize melatonin by two distinct pathways. One pathway involves the classical peroxidation mechanism in which hydrogen peroxide is used to oxidize melatonin to radicals. Superoxide adds to these radicals to form an unstable peroxide that decays to AFMK. In the other pathway, myeloperoxidase uses superoxide to insert dioxygen into melatonin to form AFMK. This novel activity expands the types of oxidative reactions myeloperoxidase can catalyze. It should be relevant to the way neutrophils use superoxide to kill bacteria and how they metabolize xenobiotics.

Role of eosinophil peroxidase in the origins of protein oxidation in asthma

Eosinophils are uniquely endowed with an arsenal of enzymes that enable them to generate an array of reactive oxidants and diffusible radical species. The formidable arsenal at their disposal likely evolved because of the central role these phagocytes play in combating invading helminths and other large metazoan pathogens. Although these leukocytes constitute an essential component of the effector limb of host defenses, they also are implicated in contributing to inflammatory tissue injury. The growing prevalence and severity of asthma, a respiratory disease characterized by recruitment and activation of eosinophils in the airways of affected individuals, has focused research efforts on elaborating the many potential mechanisms through which eosinophils may contribute to tissue injury and oxidative modification of biological targets in asthma. Eosinophil activation is strongly suspected as playing a contributory role in the pathogenesis of asthma. Accordingly, an understanding of the basic chemical pathways available to the leukocytes for generating specific reactive oxidants and diffusible radical species in vivo is required. In the following review, recent progress in the elaboration of specific mechanisms through which eosinophils generate oxidants and other reactive species are discussed. The potential contributions of these intermediates to modification of biological targets during asthma are described. Particular emphasis is placed upon the secreted hemoprotein eosinophil peroxidase (EPO), a central participant in generation of reactive oxidants and diffusible radical species by the phagocytes.

Scientific update on benzene

The mechanism of benzene toxicity has been extremely difficult to fully characterize. Much progress has been made in assessing the relative potency of benzene metabolites but specific pathways to leukemia remain to be determined. Metabolite and mechanistic studies will have to focus on aplastic anemia and MDS and separate endpoints. This may serve to clarify the array of metabolite effects and consequent disparate effects. Biomarker research can contribute to the understanding of the toxicity process. The significance of understanding benzene toxicity will also lead to better clinical treatment of aplastic anemia and therapy-related MDS and AML, detection of populations particularly susceptible to benzene toxicity, screening of populations with suspected or unknown exposures, and determination of meaningful values for occupational and individual health risk while effectively monitoring ongoing exposures for early signs of toxicity.

Personal reflections on 50 years of study of benzene toxicology

The metabolism of benzene is reviewed, and the objectives of a quantitative balance study begun in 1945 are outlined; problems of toxicology and metabolism research of some 50 years ago are considered. The quantitative metabolism of 14C-benzene in the rabbit is annotated and compared with that of unlabeled benzene quantified by nonisotopic methods. The anomalies of phenylmercapturic acid and trans-trans-muconic acid as metabolites of benzene are examined in detail by isotopic and nonisotopic methods; these compounds are true but minor metabolites of benzene. Oxygen radicals are involved in both the metabolism of benzene and its toxicity; the roles of CYP2E1, the redox cycling of quinone metabolites, glutathione oxidation, and oxidative stress in the unique radiomimetic, hematopoietic toxicity of benzene are discussed. Differences between the toxicity of benzene and the halobenzenes are related to fundamental differences in their electronic structures and to the consequent pathways of metabolic activation and detoxication.

Characterization of peroxidative oxidation products of dopamine by mass spectrometry

We characterized three cytotoxic products, namely dopaminochrome (2,3-dihydro-1H-indole-5,6-dione), 2-(3,4-dihydroxyphenyl)-1-nitroethane and 2-(3,4,6-trihydroxyphenyl)-1-nitroethane. The compounds were separated from the incubation of dopamine (3,4-dihydroxyphenethylamine) with horseradish peroxidase which mimics the peroxidative activity of Prostaglandin H synthase. Incubation of 2-(3,4,6-trihydroxyphenyl)-1-nitroethane with NADPH-cytochrome c reductase led to the formation of 6-hydroxydopamine, a known neurotoxin. Several adducts were also isolated in this study. Oxidation of dopamine in the presence of N-acetylcysteine yielded a thioether conjugate namely, 5-S-(N-acetylcysteinyl)-3,4-dihydroxyphenethylamine. Reaction of the partially purified dopaminochrome with N-acetylcysteine permitted the isolation of another thioether conjugate which was tentatively identified as 7-S-(N-acetylcysteinyl)-5,6-dihydroxyindole. We also isolated the one-to-one condensation products of malonaldehyde with dopamine, norepinephrine and serotonin. The identities of these products were established by chemical synthesis and various mass spectrometric techniques.

Free hydroxyl radicals are formed on reaction between the neutrophil-derived species superoxide anion and hypochlorous acid

Superoxide anion reacts with hypochlorous acid to yield free hydroxyl radicals, as shown by the hydroxylation of benzoate. This reaction is analogous to the Haber-Weiss reaction but in the absence of metal ions is at least six orders of magnitude faster.

Protein-bound 3,4-dihydroxyphenylalanine is a major reductant formed during hydroxyl radical damage to proteins

Proteins and aromatic amino acids previously exposed to hydroxyl radicals reduced cytochrome c, free iron, and copper ions. A major product of hydroxyl radical addition to tyrosine is 3,4-dihydroxyphenylalanine (DOPA), which has these reducing properties. The reduction of nitro blue tetrazolium by radical-damaged protein was consistent with the generation of quinones in the protein. By acid hydrolysis followed by high-performance C18 reversed-phase liquid chromatography we have shown that hydroxyl radical-damaged proteins contain significant amounts of protein-bound DOPA (PB-DOPA). The authenticity of the DOPA measured was confirmed by gas chromatography-mass spectrometry. PB-DOPA was also generated enzymatically using mushroom tyrosinase, which catalyzes the hydroxylation of tyrosine residues. By comparing the levels of DOPA in radical-damaged or enzyme-treated protein with that of cytochrome c reduction, we show that PB-DOPA is a major source of the observed reducing activity. PB-DOPA may have a role in the replenishment of reduced transition metal ions involved in free radical generating systems in vivo.

Cytostatic effects of horseradish and thyroid peroxidase derived free radicals

An otherwise noncytostatic flux of H2O2 from glucose and glucose oxidase became cytostatic to cultured Chinese Hamster Ovary (CHO) cells when horseradish or thyroid peroxidase was added to the culture medium. Electron spin resonance (ESR) measurements showed that one or more factors present in the culture medium promote the one-electron oxidation of a reduced nitroxide or glutathione in an H2O2/peroxidase-dependent process. Moreover, a reduced nitroxide conferred significant protection against the cytostatic effect of H2O2/peroxidase. Cytostatic effects were not only seen in the presence of the active H2O2/peroxidase system, but also in media which had been preexposed to H2O2/peroxidase but no longer contained an active H2O2 generating system. It is suggested that peroxidases oxidize one or more factors in tissue culture media to free radicals, which react with nearby components of cells or form toxic products, causing growth inhibition. If similar free radical precursors are present in tissue fluids, some of the toxicity of H2O2 in vivo may be due to peroxidase-mediated endogenous free radical generation.

Modulation of f-met-leu-phe induced chemotactic activity and superoxide production by neutrophils during chronic ethanol intoxication

Chronic alcohol consumption has been associated with increased migration of neutrophils into liver that could contribute to the development of alcoholic liver disease. Mild endotoxemia may be at least partially responsible for this condition since endotoxemia was shown to be present in virtually all chronic alcoholics. This study examines the release of superoxide anion and chemotactic activity by Kupffer cells and sequestered hepatic as well as blood neutrophils during chronic alcohol intoxication (16 weeks) alone, and following an intravenous injection of Escherichia coli lipopolysaccharide (LPS) (1 mg/kg) 3 hr before cell isolation. Chronic ethanol consumption increased the total neutrophil yield per liver, but did not change the f-met-leu-phe induced chemotactic activity by both hepatic and blood neutrophils. However, the combined insults of ethanol and LPS increased the chemotactic activity and superoxide anion generation by these cells. Plasma from ethanol-fed rats was highly chemotactic to syngeneic normal rat neutrophils. This activity was increased 1.75-fold in the plasma obtained from chronic ethanol plus endotoxin-injected rats. The chemotactic activity of Kupffer cells was not significantly modulated during ethanol intoxication plus endotoxin treatment. The f-met-leu-phe-induced superoxide anion release by Kupffer cells was enhanced after LPS treatment. Chronic ethanol consumption did not induce any effect on this parameter. These observations suggest that functional alterations in neutrophils during chronic ethanol intoxication may contribute to hepatic injury.