Direct electron spin resonance detection of free radical intermediates during the peroxidase catalyzed oxidation of phenacetin metabolites

A comprehensive weight of evidence assessment of published acetaminophen genotoxicity data: Implications for its carcinogenic hazard potential

In 2019, the California Office of Environmental Health Hazard Assessment initiated a review of the carcinogenic hazard potential of acetaminophen, including an assessment of its genotoxicity. The objective of this analysis was to inform this review process with a weight-of-evidence assessment of more than 65 acetaminophen genetic toxicology studies that are of widely varying quality and conformance to accepted standards and relevance to humans. In these studies, acetaminophen showed no evidence of induction of point or gene mutations in bacterial and mammalian cell systems or in in vivo studies. In reliable, well-controlled test systems, clastogenic effects were only observed in unstable, p53-deficient cell systems or at toxic and/or excessively high concentrations that adversely affect cellular processes (e.g., mitochondrial respiration) and cause cytotoxicity. Across the studies, there was no clear evidence that acetaminophen causes DNA damage in the absence of toxicity. In well-controlled clinical studies, there was no meaningful evidence of chromosomal damage. Based on this weight-of-evidence assessment, acetaminophen overwhelmingly produces negative results (i.e., is not a genotoxic hazard) in reliable, robust high-weight studies. Its mode of action produces cytotoxic effects before it can induce the stable, genetic damage that would be indicative of a genotoxic or carcinogenic hazard.

Antifouling Activities of Immobilized Ferrocenyl Glucose on a Glassy Carbon Surface

Polar coatings are used to protect surfaces from marine fouling based on the formation of a hydrated surface layer, which acts as a barrier to marine microorganisms. In this context, we have developed a material with glucose bound to ferrocene to prevent surface absorption. Glucose brings the polarity while ferrocene has the role of varying the state of charge of the surface. We therefore describe the synthesis of 6-deoxy-6-(4-ferrocenyl-1H-1,2,3-triazol-1-yl)-1-(4-aminophenyl)-β-D-glucopyranose 6 and its immobilization on the surface of a C electrode and develop a methodology used for antibacterial testing. We were able to demonstrate that the immobilization of glucose 6 could be done in an artificial seawater environment by oxidation of an amine. The use of a 96-microwell platform equipped with electrodes for cyclic voltammetry, linked to a potentiostat, allowed the electrical solicitation of the coating in the presence of marine bacteria with a greater number of biological replicates. We have shown that the coating has an antibacterial effect, and this effect is accentuated when the coating is electrically stressed, resulting in the appearance and disappearance of charge on ferrocene, a phenomenon that seems to be conducive to colonization by bacteria.

Enzymatic oligomerization and polymerization of arylamines: state of the art and perspectives

The literature concerning the oxidative oligomerization and polymerization of various arylamines, e.g., aniline, substituted anilines, aminonaphthalene and its derivatives, catalyzed by oxidoreductases, such as laccases and peroxidases, in aqueous, organic, and mixed aqueous organic monophasic or biphasic media, is reviewed. An overview of template-free as well as template-assisted enzymatic syntheses of oligomers and polymers of arylamines is given. Special attention is paid to mechanistic aspects of these biocatalytic processes. Because of the nontoxicity of oxidoreductases and their high catalytic efficiency, as well as high selectivity of enzymatic oligomerizations/polymerizations under mild conditions-using mainly water as a solvent and often resulting in minimal byproduct formation-enzymatic oligomerizations and polymerizations of arylamines are environmentally friendly and significantly contribute to a "green" chemistry of conducting and redox-active oligomers and polymers. Current and potential future applications of enzymatic polymerization processes and enzymatically synthesized oligo/polyarylamines are discussed.

Nitrones as therapeutics

Nitrones have the general chemical formula X-CH=NO-Y. They were first used to trap free radicals in chemical systems and then subsequently in biochemical systems. More recently several nitrones, including alpha-phenyl-tert-butylnitrone (PBN), have been shown to have potent biological activity in many experimental animal models. Many diseases of aging, including stroke, cancer development, Parkinson disease, and Alzheimer disease, are known to have enhanced levels of free radicals and oxidative stress. Some derivatives of PBN are significantly more potent than PBN and have undergone extensive commercial development for stroke. Recent research has shown that PBN-related nitrones also have anti-cancer activity in several experimental cancer models and have potential as therapeutics in some cancers. Also, in recent observations nitrones have been shown to act synergistically in combination with antioxidants in the prevention of acute acoustic-noise-induced hearing loss. The mechanistic basis of the potent biological activity of PBN-related nitrones is not known. Even though PBN-related nitrones do decrease oxidative stress and oxidative damage, their potent biological anti-inflammatory activity and their ability to alter cellular signaling processes cannot readily be explained by conventional notions of free radical trapping biochemistry. This review is focused on our studies and others in which the use of selected nitrones as novel therapeutics has been evaluated in experimental models in the context of free radical biochemical and cellular processes considered important in pathologic conditions and age-related diseases.

L-tryptophan radical cation electron spin resonance studies: connecting solution-derived hyperfine coupling constants with protein spectral interpretations

Fast-flow electron spin resonance (ESR) spectroscopy has been used to detect a free radical formed from the reaction of l-tryptophan with Ce (4+) in an acidic aqueous environment. Computer simulations of the ESR spectra from l-tryptophan and several isotopically modified forms strongly support the conclusion that the l-tryptophan radical cation has been detected by ESR for the first time. The hyperfine coupling constants (HFCs) determined from the well-resolved isotropic ESR spectra support experimental and computational efforts to understand l-tryptophan's role in protein catalysis of oxidation-reduction processes. l-Tryptophan HFCs facilitated the simulation of fast-flow ESR spectra of free radicals from two related compounds, tryptamine and 3-methylindole. Analysis of these three compounds' beta-methylene hydrogen HFC data along with equivalent l-tyrosine data has led to a new computational method that can distinguish between these two amino acid free radicals in proteins without dependence on isotope labeling, electron-nuclear double resonance, or high-field ESR. This approach also produces geometric parameters (dihedral angles for the beta-methylene hydrogens) that should facilitate protein site assignment of observed l-tryptophan radicals as has been done for l-tyrosine radicals.

Acetaminophen stimulates the peroxidative metabolism of anthracyclines

Acetaminophen, a common analgesic and antipyretic drug, is frequently administered to individuals undergoing anthracycline chemotherapy. Here, the effect of acetaminophen on the metabolism of daunorubicin and doxorubicin by isolated enzymes lactoperoxidase and myeloperoxidase, and by myeloperoxidase-containing human leukemia HL-60 cells was investigated using spectrophotometric and EPR techniques. We report that at pharmacological concentrations acetaminophen strongly stimulates oxidation of the anthracyclines by lactoperoxidase and myeloperoxidase systems, which results in irreversibly altered (colorless) products. The initial rate and efficacy of daunorubicin oxidation depends on pH. While at pH approximately 7 the oxidation is rapid and extensive, almost no oxidation occurs at pH approximately 5. In the absence of daunorubicin, oxidation of acetaminophen by lactoperoxidase/hydrogen peroxide is only weakly dependent on pH, however, at pH 7.4 it strongly depends on [daunorubicin]. Ascorbate and reduced glutathione strongly inhibited oxidation of anthracyclines by lactoperoxidase and HL-60 systems. Using EPR, a daunorubicin-derived radical was detected in a daunorubicin/acetaminophen/peroxidase/hydrogen peroxide system as a narrow single line (0.175 mT) with g = 2.0047. When daunorubicin was omitted, only an acetaminophen-melanin EPR signal was detected (g = 2.0043, line width approximately 0.5 mT). Similar results were obtained with doxorubicin. We suggest that the stimulation by acetaminophen is primarily due to its preferential oxidation by peroxidases to the corresponding phenoxyl radical, which subsequently reacts with daunorubicin (doxorubicin). Because biological properties of oxidatively transformed anthracyclines will certainly be different from those of their parent compounds, the possible acetaminophen-enhanced degradation of the anthracyclines in vivo is likely to interfere with anticancer and/or cardiotoxic activities of these agents.

Immobilized Enzyme Electron Spin Resonance: A Method for Detecting Enzymatically Generated Transient Radicals

The study of enzymatically generated, transient radicals provides valuable information about radical reactivity as well as enzyme function. ESR methods to detect transient radicals are generally based on continuous flow and have the potential to consume large quantities of enzyme, substrate, and buffer. Experimental approaches have been pursued to minimize sample volumes, although none have made the continuous-flow ESR approach generally applicable for enzymes and substrates available in limited quantities. We have developed an alternative approach to the traditional continuous-flow ESR method that provides the same high-resolution ESR spectra, but does not consume large quantities of enzyme, substrate, or buffer. The method utilizes enzyme immobilized onto an inert substrate packed directly into an ESR flat cell. Flowing substrate solution over the immobilized enzyme generates in situ, transient radicals, which can then be observed on the submillisecond time scale. We have termed this method "immobilized enzyme ESR," abbreviated IE-ESR. In this paper, we have described the details of the IE-ESR technique and have presented data collected using the IE-ESR technique for transient radicals from limited quantity enzymes, limited quantity substrates, and D2O buffers. An extension of this technique to ESR spin trapping has also been discussed.

Clinical significance of a highly sensitive enzyme immunoassay of hepatitis B surface antigen using a novel electron spin resonance technique

We developed a highly sensitive enzyme immunoassay (EIA), the p-AP/HHTIO method, that detects serum hepatitis B surface antigen (HBsAg) by measuring stabilized nitroxide radicals using a novel electron spin resonance technique [Matsuo et al. (1998) Free Radic Biol Med 25:929-935]. To demonstrate the clinical significance of this method and to reveal occult hepatitis B virus (HBV) infection in patients, we used the method to analyze serum samples of 30 patients with acute or fulminant hepatitis who were negative for HBsAg by standard EIA, and those of seven chronic HBV carriers who became negative for HBsAg during a follow-up period by standard EIA. We also examined serum HBV DNA by amplification of the HBV S gene, using the polymerase chain reaction (PCR) technique. The p-AP/HHTIO method showed that 9 of 20 (45%) patients with acute hepatitis and 2 of 10 (20%) with fulminant hepatitis were positive for HBsAg; PCR detected HBV DNA in these HBsAg-positive patients. Antibody against hepatitis B core antigen was detected in one patient with fulminant hepatitis. The p-AP/HHTIO method demonstrated prolonged seropositivity of HBsAg even after standard EIA showed a loss of HBsAg in all seven HBV carriers. Our p-AP/HHTIO method is useful for screening and diagnosing HBV infection in patients with liver diseases who are negative for conventional HBV-related serological markers.

Oxidation sensitivity may be a useful tool for the detection of the hematotoxic potential of newly developed molecules: application to antipsychotic drugs

Some antipsychotic agents have been found to produce agranulocytosis and aplastic anemia. The oxidation phenomena and/or the formation of free radicals has been suggested to be causally related to various hematological disorders, e.g., agranulocytosis. Using five experimental conditions, we tested the oxidative potential of compounds with and without a history of hematological side effects, e.g., agranulocytosis and aplastic anemia. A statistical analysis was undertaken for each experimental condition and a multivariate analysis combining all results was performed. Two peroxidase-induced free radical models did not successfully discriminate between drugs with and without a history of causing hematologic problems (<70%). The lipid peroxidation system provided even less satisfactory discrimination, with only 56.25% correct classification. However, an 87.5% correct classification was obtained when using the oxidation potentials of these drugs determined at pH 4.7 and at pH 7.4. A multivariate analysis taking into account the five variables provided 87.5% success in classification. The two clusters were better discriminated in terms of a "distance coefficient." In a second analysis, the putative antipsychotic pyridobenzodiazepine analogues (JL5, JL8, JL18, and JL25) were classified in the cluster of toxic compounds, while the oxa- and thiazepine analogues (JL2, JL3, and JL13) were classified as nontoxic compounds. On the other hand, a few metabolites of clozapine and fluperlapine were classified in the toxic compound group. The procedure described herein is, to our knowledge, the first which classifies molecules of different structures as well as different pharmacological profiles according to their hematotoxic potential. Such a procedure could be used to predict drug-induced hematological side effects.

Highly sensitive hepatitis B surface antigen detection by measuring stable nitroxide radical formation with ESR spectroscopy

In areas where hepatitis B virus (HBV) is prevalent, HBV carriers negative for hepatitis B surface antigen (HbsAg) by enzyme-linked immunosorbent assay (ELISA) have been reported. Moreover, even after screening donor blood for HbsAg and hepatitis B core antibody (HBcAb), post-transfusion hepatitis B continues to occur, though with a decreasing frequency. Therefore, screening tests far more sensitive for detecting HBsAg than those currently available are needed. We developed a highly sensitive method for HBsAg detection. It is based on the recognition of peroxidase activity through measuring the formation of stable nitroxide radical with electron spin resonance (ESR) spectroscopy in the presence of hydrogen peroxide, p-acetamidophenol (p-AP), and 4-hydrazonomethyl-1-hydroxy-2,2,5,5,-tetramethyl-3-imidazoline-3-o xide (HHTIO). A cut-off value was established by testing of 186 healthy adults and 50 HBsAg-positive individuals. The signal to noise (S/N) ratio of less than 1.488 obtained by ESR spectroscopy was considered to be negative and more than 2.181, positive. The p-AP/HHTIO method was found to be 10 times more sensitive than the standard ELISA and reproducibility was excellent. Additional investigations were made on the HBsAg levels in the serum from 26 healthy subjects, in whom cut-off index levels on ELISA were negative but relatively high (range: 0.6 to 1.0); and on 15 patients with non B non C hepatitis. Three of 26 cases and 3 of 15 with non B non C hepatitis were judged to be HBsAg positive. Of these, 5 were found to be positive for HBV DNA by polymerase chain reaction (PCR). It was shown in this study that the p-AP/HHTIO method is practical and useful in screening HBV carriers because of the sensitivity in HBsAg detection, which is comparable to PCR analysis.

Hydrogen atom abstraction of 3,5-disubstituted analogues of paracetamol by horseradish peroxidase and cytochrome P450

1. The formation of free radicals during enzyme catalysed oxidation of eight 3,5-disubstituted analogues of paracetamol (PAR) has been studied. A simple peroxidase system as well as cytochrome P450-containing systems were used. Radicals were detected by electron spin resonance (ESR) on incubation of PAR and 3,5-diCH3-, 3,5-diC2H5-, 3,5-ditC4H9-, 3,5-diOCH3-, 3,5-diSCH3-, 3,5-diF-, 3,5-diCl- and 3,5-diBr-substituted analogues of PAR with horseradish peroxidase in the presence of hydrogen peroxide (H2O2). Initial analysis of the observed ESR spectra revealed all radical species to be phenoxy radicals, based on the absence of dominant nitrogen hyperfine splittings. No radicals were detected in rat liver cytochrome P450-containing microsomal or reconstituted systems. 2. To rationalize the observed ESR spectra, hydrogen atom abstraction of PAR and four of the 3,5-disubstituted analogues (3,5-diCH3-, 3,5-diOCH3-, 3,5-diF- and 3,5-diCl-PAR) was calculated using ab initio calculations, and a singlet oxygen atom was used as the oxidizing species. The calculations indicated that for all compounds studied an initial hydrogen atom abstraction from the phenolic hydroxyl group is favoured by approximately 125 kJ/mol over an initial hydrogen atom abstraction from the acetylamino nitrogen atom, and that after hydrogen abstraction from the phenolic hydroxyl group, the unpaired electron remains predominantly localised at the phenoxy oxygen atom (+/-85%). 3. The experimental finding of phenoxy radicals in horseradish peroxidase/H2O2 incubations paralleled these theoretical findings. The failure to detect experimentally phenoxy radicals in cytochrome P450-catalysed oxidation of any of the eight 3,5-disubstituted PAR analogues is more likely due to the reducing effects that agents like NADPH and protein thiol groups have on phenoxy radicals rather than on the physical instability of the respective substrate radicals.

Series: current issues in mutagenesis and carcinogenesis, No. 65. The genotoxicity and carcinogenicity of paracetamol: a regulatory (re)view

The publication of several studies reporting genotoxic effects of paracetamol, one of the world's most popular over-the-counter drugs, has raised the question of regulatory action. Paracetamol does not cause gene mutations, either in bacteria or in mammalian cells. There are, however, published data giving clear evidence that paracetamol causes chromosomal damage in vitro in mammalian cells at high concentrations and indicating that similar effects occur in vivo at high dosages. Available data point to three possible mechanisms of paracetamol-induced genotoxicity: (1) inhibition of ribonucleotide reductase; (2) increase in cytosolic and intranuclear Ca2+ levels; (3) DNA damage caused by NAPQI after glutathione depletion. All mechanisms involve dose thresholds. Studies of the relationship between genotoxicity and toxic effects in the rat (induction of micronuclei in rat bone marrow including dose-response relationship, biotransformation of paracetamol at different dosages, concomitant toxicity and biochemical markers) have recently been completed. These studies, which employed doses ranging from the dose resulting in human therapeutic peak plasma levels to highly toxic doses, give convincing evidence that genotoxic effects of paracetamol appear only at dosages inducing pronounced liver and bone marrow toxicity and that the threshold level for genotoxicity is not reached at therapeutic dosage. Reliable studies on the ability of paracetamol to affect germ cell DNA are not available. However, based on the amount of drug likely to reach germ cells and the evidence of thresholds, paracetamol is not expected to cause heritable damage in man. Various old and poorly designed long-term studies of paracetamol in the mouse and rat have given equivocal results. A few of these studies showed increased incidence of liver and bladder tumours at hepatotoxic doses. National Toxicology Program (U.S.A.) feeding studies have shown that paracetamol is non-carcinogenic when given at non-hepatotoxic doses up to 300 mg/kg/d to the rat and up to 1000 mg/kg/d to the mouse. Taking into account the knowledge of the hepatotoxicity and metabolism of paracetamol and the existence of thresholds for its genotoxicity, the animal studies do not indicate a carcinogenic potential at non-hepatotoxic dose levels. Based on this updated assessment of the genotoxicity and carcinogenicity of paracetamol, it is concluded that there is no need for regulatory action.

International Commission for Protection against Environmental Mutagens and Carcinogens. An evaluation of the genetic toxicity of paracetamol

During the last years, several reports have indicated genotoxic effects of paracetamol, a widely used non-prescription analgesic and antipyretic drug. Thus, a careful evaluation of a possible genotoxic effect related to paracetamol use is warranted. Studies in vitro and in vivo indicate that the reactive metabolite of paracetamol can bind irreversibly to DNA and cause DNA strand breaks. Paracetamol inhibits both replicative DNA synthesis and DNA repair synthesis in vitro and in experimental animals. Paracetamol does not cause gene mutations, either in bacteria or in mammalian cells. On the other hand, a co-mutagenic effect of paracetamol has been reported. Furthermore, paracetamol increases the frequency of chromosomal damage in mammalian cell lines, isolated human lymphocytes and experimental animals. Two independent studies have shown an increase in chromosomal damage in lymphocytes of human volunteers after intake of therapeutic doses of paracetamol, whereas a third study was negative. Paracetamol-induced chromosomal damage appears to be caused by an inhibition of ribonucleotide reductase. This indicates that a threshold level for the paracetamol-induced chromosomal damage may exist. Genotoxic effects of paracetamol have, however, been demonstrated both in vitro and in vivo at or near therapeutic concentrations. The data indicate that the use of paracetamol may contribute to an increase in the total burden of genotoxic damage in man. Thus, there may be a need to evaluate the therapeutic benefit of paracetamol, taking into consideration not only its potential to induce acute and chronic organ damage, but also genotoxic effects.

The covalent binding of acetaminophen to cellular nucleic acids as the result of the respiratory burst of neutrophils derived from the HL-60 cell line

After being induced to differentiate into a neutrophilic type, cultures of the leukemic cell line HL-60 were able to cause the bioactivation and nucleic acid binding of acetaminophen upon stimulation of the respiratory burst. This phenomenon was found to simulate the same process as that previously shown with normal human granulocytes. Binding to both DNA and RNA of the cells was determined quantitatively by use of 14C-labeled acetaminophen congeners. Protein binding occurred to about the same extent as did RNA binding. Simultaneous labeling experiments with [ring-14C]- and [14C = O]acetaminophen further showed that the acetaminophen molecule was bound to DNA in an intact manner, while binding to RNA showed about a 50% excess binding of the acetaminophen ring relative to the carbonyl group. Experiments with certain inhibitors showed that catalase and azide ion strongly inhibited DNA binding, while superoxide dismutase had a slight stimulatory effect on binding. These results suggest a significant role for myeloperoxidase in the bioactivation process, which contrasts with the proposed bioactivation mechanism of certain arylamine compounds. A mechanism was proposed for acetaminophen binding to nucleic acids that requires the 1 e- oxidation of this substrate to its phenoxyl radical, although the production of the N-acetyl-p-benzoquinoneimine metabolite, which has been proposed to account for the extensive protein binding known to occur for acetaminophen, might also contribute to such binding. The potential genotoxicity of acetaminophen was considered in view of what might be a unique pathway which can metabolize this chemical to a nucleic acid-binding species.

Cellular effects of some metabolic oxidation products pertinent to 4-ethoxyaniline

The toxicity of some metabolic products pertinent to 4-ethoxyaniline in isolated hepatocytes were investigated. The compounds investigated were 4-ethoxynitrosobenzene (1), 4-ethoxy-4'-nitrosodiphenylamine (2), 3,6-bis(4-ethoxy-phenylimino)-4-ethoxy-1,4-cyclohexadienylamine (3), 4-(4-ethoxyphenylimino)-2,3-dimethyl-2,5-cyclohexadiene-1-one (4) and 4-(4-ethoxyphenylimino)-2,6-dimethyl-2,5-cyclohexadiene-1-one (5). Of these, 1, 2 and 3 are oxidation products of 4-ethoxyaniline. Compounds 4 and 5 are dimethyl analogues of previously investigated oxidation product 4-(4-ethoxyphenylimino(-2,5-cyclohexadiene-1-one (NEPBQI). Among the investigated compounds, 1 and 2 were the most toxic towards isolated hepatocytes. In hepatocytes treated with compounds 1, 2 and 4, loss of cell viability was also accompanied by surface bleb formation. All compounds except 3 reacted with GSH resulting in depletion of cellular GSH. No formation of GSSG was observed, however. Thus, the GSH depletion was apparently due to conjugate formation rather than oxidation. No superoxide dismutase inhibitable reduction of acetylated cytochrome c was observed, thus none of the compounds undergoes measurable redox cycling.

Muscle necrosis by N-methylated p-phenylenediamines in rats: structure-activity relationships and correlation with free-radical production in vitro

Certain derivatives of p-phenylenediamine have been shown to cause necrosis of cardiac and skeletal muscle in rats; in vitro, such compounds are known to autoxidize to the corresponding radical cations, with concomitant formation of oxygen free-radicals. In the present study, the autoxidation rates of p-phenylenediamine and its N-methyl, dimethyl, trimethyl and tetramethyl derivatives have been determined and compared with the severity of the muscle necrosis induced by each of these compounds in rats. A close correlation was observed between autoxidation rate in vitro and toxicity in vivo, suggesting that free-radical species may be involved in the initiation of the muscle damage caused by these substances.