Cellular effects of N(4-ethoxyphenyl)p-benzoquinone imine, a p-phenetidine metabolite formed during peroxidase reactions

Mechanism of peroxidase-mediated oxidation of carcinogenic o-anisidine and its binding to DNA

2-Methoxyaniline (o-anisidine) is a urinary bladder carcinogen in both mice and rats. Since the urinary bladder contains substantial peroxidase activity, we investigated the metabolism of this carcinogen by prostaglandin H synthase (PHS), a prominent enzyme in the urinary bladder, and lactoperoxidase as model mammalian peroxidases. Horseradish peroxidase (HRP)-mediated oxidation of o-anisidine was also determined and compared with the reactions catalyzed by mammalian peroxidases. All three peroxidases oxidized o-anisidine via a radical mechanism. Using HPLC combined with electrospray tandem mass spectrometry, we determined that peroxidases oxidized o-anisidine to a diimine metabolite, which subsequently hydrolyzed to form a quinone imine. Two additional metabolites were identified as a dimer linked by an azo bond and another metabolite consisting of three methoxybenzene rings, which exact structure has not been identified as yet. Using [14C]-labeled o-anisidine, we observed substantial peroxidase-dependent covalent binding of o-anisidine to DNA, tRNA and polydeoxynucleotides [poly(dX)]. The 32P-postlabeling assay (a standard procedure and enrichment of adducts by digestion with nuclease P1 or by extraction into 1-butanol prior to 32P-labeling) was employed as the second method to detect and quantitate binding of o-anisidine to DNA. Using these versions of the 32P-postlabeling technique we did not observe any DNA adducts derived from o-anisidine. The o-anisidine-DNA adducts became detectable only when DNA modified by o-anisidine was digested using three times higher concentrations of micrococcal nuclease and spleen phosphodiesterase (MN/SPD). We found deoxyguanosine to be the target for o-anisidine binding in DNA using poly(dX) and deoxyguanosine 3'-monophosphate (dGp). A diimine metabolite of o-anisidine is the reactive species forming adducts in dGp. The results strongly indicate that peroxidases play an important role in o-anisidine metabolism to reactive species, which might be responsible for its genotoxicity, and its carcinogenicity to the urinary bladder in rodents. The limitation of the 32P-postlabeling technique to analyze DNA adducts derived from o-anisidine as a means to estimate its genotoxicity is discussed.

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.

Molecular mechanisms of quinone cytotoxicity

Quinones are probably found in all respiring animal and plant cells. They are widely used as anticancer, antibacterial or antimalarial drugs and as fungicides. Toxicity can arise as a result of their use as well as by the metabolism of other drugs and various environmental toxins or dietary constituents. In rapidly dividing cells such as tumor cells, cytotoxicity has been attributed to DNA modification. However the molecular basis for the initiation of quinone cytotoxicity in resting or non-dividing cells has been attributed to the alkylation of essential protein thiol or amine groups and/or the oxidation of essential protein thiols by activated oxygen species and/or GSSG. Oxidative stress arises when the quinone is reduced by reductases to a semiquinone radical which reduces oxygen to superoxide radicals and reforms the quinone. This futile redox cycling and oxygen activation forms cytotoxic levels of hydrogen peroxide and GSSG is retained by the cell and causes cytotoxic mixed protein disulfide formation. Most quinones form GSH conjugates which also undergo futile redox cycling and oxygen activation. Prior depletion of cell GSH markedly increases the cell's susceptibility to alkylating quinones but can protect the cell against certain redox cycling quinones. Cytotoxicity induced by hydroquinones in isolated hepatocytes can be attributed to quinones formed by autoxidation. The higher redox potential benzoquinones and naphthoquinones are the most cytotoxic presumably because of their higher electrophilicty and thiol reactivity and/or because the quinones or GSH conjugates are more readily reduced to semiquinones which activate oxygen.

Structure-activity relationships in the myotoxicity of ring-methylated p-phenylenediamines in rats and correlation with autoxidation rates in vitro

A number of N-methylated p-phenylenediamines are known to cause necrosis of skeletal and cardiac muscle in rats. The severity of the muscle damage induced by these compounds in vivo was found to be directly proportional to their autoxidation rates in vitro, suggesting that reactive species formed during oxidation may be involved in the initiation of this toxic effect. In the present study, the in vitro oxidation rates and in vivo toxicities of a number of ring-methylated p-phenylenediamines have been evaluated. 2,3,5,6-Tetramethyl p-phenylenediamine readily autoxidized at neutral pH. Hydrogen peroxide was formed in this reaction, while oxidation in the presence of glutathione or reduced pyridine nucleotides led to the production of both hydrogen peroxide and superoxide radical. Less highly methylated derivatives oxidized more slowly, with rates decreasing in the order 2,3,5,6-tetramethyl greater than 2,5-dimethyl greater than 2,6-dimethyl greater than 2-methyl. All these compounds were myotoxic in rats, with damage being largely confined to skeletal muscle. Toxicity was again proportional to oxidation rate. Myotoxicity appears to be a general property of certain substituted p-phenylenediamines and the structure-activity relationships identified may permit an estimate to be made of the potential toxicity of other compounds of this type.

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.