Reactive products formed by peroxidase catalyzed oxidation of p-phenetidine

A New Pyrimidine Schiff Base with Selective Activities against Enterococcus faecalis and Gastric Adenocarcinoma

Enterococcus faecalis is known as a significant nosocomial pathogen due to its natural resistance to many antibacterial drugs. Moreover, it was found that E. faecalis infection causes inflammation, production of reactive oxygen species, and DNA damage to human gastric cancer cells, which can induce cancer. In this study, we synthesized and tested the biological activity of a new Schiff base, 5-[(4-ethoxyphenyl)imino]methyl-N-(4-fluorophenyl)-6-methyl-2-phenylpyrimidin-4-amine (3), and compared its properties with an analogous amine (2). In the biological investigation, 3 was found to have antibacterial activity against E. faecalis 29212 and far better anticancer properties, especially against gastric adenocarcinoma (human Caucasian gastric adenocarcinoma), than 2. In addition, both derivatives were non-toxic to normal cells. It is worth mentioning that 3 could potentially inhibit cancer cell growth by inducing cell apoptosis. The results suggest that the presence of the –C=N– bond in the molecule of 3 increases its activity, indicating that 5-iminomethylpyrimidine could be a potent core for further drug discovery research.

Inactivation of creatine kinase during the interaction of mefenamic acid with horseradish peroxidase and hydrogen peroxide: participation by the mefenamic acid radical

Creatine kinase (CK) was used as a marker molecule to examine the side effects of damage to tissues by mefenamic acid, an effective drug to treat rheumatic and arthritic diseases, with horseradish peroxidase and hydrogen peroxide (HRP-H(2)O(2)). Mefenamic acid inactivated CK during its interaction with HRP-H(2)O(2). Also, diphenylamine and flufenamic acid caused a loss of CK activity, indicating the imino group, not substituent groups, in the phenyl rings have a crucial role in CK inactivation. Rapid change in mefenamic acid spectra was detected, suggesting that mefenamic acid is efficiently oxidized by HRP-H(2)O(2). Peroxidases oxidize xenobiotics to free radicals by a one-electron transfer. However, direct detection of mefenamic acid radicals by electron spin resonance (ESR) was unsuccessful. Reduced glutathione and 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) in the reaction mixture containing mefenamic acid with HRP-H(2)O(2) produced ESR signals consistent with a DMPO-glutathionyl radical adduct. These results suggest that inactivation of CK is probably caused through formation of mefenamic acid radicals. Sulfhydryl groups and tryptophan residues of CK were diminished by mefenamic acid with HRP-H(2)O(2). Other SH enzymes, including alcohol dehydrogenase and glyceraldehyde-3-phosphate dehydrogenase, were very sensitive to mefenamic acid with HRP-H(2)O(2). Inactivation of SH enzymes may explain some deleterious actions of mefenamic acid.

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.

Activation of carcinogens by peroxidase. Horseradish peroxidase-mediated formation of benzenediazonium ion from a non-aminoazo dye, 1-phenylazo-2-hydroxynaphthalene (Sudan I) and its binding to DNA

Horseradish peroxidase in the presence of hydrogen peroxide (HRP/H2O2) oxidizes a carcinogenic non-aminoazo dye, 1-phenylazo-2-hydroxynaphthalene (Sudan I) to the ultimate carcinogen, which binds to calf thymus DNA. The principal product of Sudan I oxidation by the HRP/H2O2 system is the benzenediazonium ion. Minor products are hydroxy derivatives of Sudan I, in which the aromatic rings are hydroxylated. The principal oxidative product (the benzenediazonium ion) is responsible for the carcinogenicity of Sudan I, because this ion, formed from this azo dye, binds to DNA.

Drug-protein conjugates--XIV. Mechanisms of formation of protein-arylating intermediates from amodiaquine, a myelotoxin and hepatotoxin in man

The enzymic and non-enzymic formation of protein-arylating intermediates from amodiaquine (AQ,7-chloro-4-(3'-diethylamino-4'-hydroxyanilino) quinoline), an anti-malarial associated with agranulocytosis and liver damage in man, was studied in vitro. [14C]AQ in phosphate buffer, pH 7.4, under air was autoxidized to a reactive derivative(s) which possessed characteristics indicative of a semiquinone/quinone imine: reduction by NADPH and ascorbic acid, conjugation with thiols and irreversible binding to microsomal and soluble proteins. Cysteinyl SH groups were major sites of arylation. Radiolabelled material irreversibly bound to HSA after 24 hr and to human liver microsomes after 4 hr represented 26.5 +/- 1.8% and 31.4 +/- 0.6% (means +/- SD, N = 3) of incubated [14C]AQ (10 microM), respectively. The quinone imine of AQ(AQQI) was synthesized, and displayed the same oxidative and electrophilic reactions as the product(s) of AQ's autoxidation. A water-soluble product formed in buffered solutions of AQ and N-acetylcysteine was identified as an AQ mercapturate by comparison with an adduct prepared from synthetic AQQI. Irreversible binding of [14C]AQ was inhibited by a radical scavenger; this indicated that the semiquinone imine contributed to the binding. Although AQ was extensively de-ethylated by human liver microsomes, oxidation by cytochrome P-450 did not appear to be principally responsible for its activation and irreversible binding in microsomal incubations. AQ was oxidized to protein-arylating intermediates by horseradish peroxidase. It also formed reactive derivatives, possibly N-chloro compounds, in chlorine solutions. These findings indicated that AQ can give rise to chemically reactive species by at least three distinct mechanisms, viz. autoxidation in neutral solution under air, peroxidase-catalyzed oxidation and N-chlorination. Formation of such species in liver and myeloid cells might be responsible for the adverse reactions associated with AQ.

Formation of 4-ethoxy-4'-nitrosodiphenylamine in the reaction of the phenacetin metabolite 4-nitrosophenetol with glutathione

Phenacetin, a constituent of several analgesic and antipyretic formulations has been made responsible for a variety of toxic and carcinogenic actions. 4-Nitrosophenetol, the N-oxydation product of intermediate 4-phenetidine, forms methemoglobin and binds covalently to sulfhydryl groups of proteins and glutathione. In the reaction of 4-nitrosophenetol with glutathione and other thiols an intermediate so-called "semimercaptal" is formed from which N-(thiol-S-yl)-4-phenetidine S-oxide, N-(thiol-S-yl)-4-phenetidine and 4-phenetidine derive. Besides thiol adducts, a yellow compound is formed which was isolated as a pure crystalline product (elemental analysis) and identified by FAB-MS, EI-MS, 13C-, 1H-NMR, and UV-VIS spectroscopy as 4-ethoxy-4'-nitrosodiphenylamine. This nitrosoarene is formed by an unknown mechanism from 4-nitrosophenetol and 4-phenetidine under liberation of ethanol. In human erythrocytes this compound is easily reduced to 4-amino-4'-ethoxydiphenylamine (FAB-MS, EI-MS, 13C-NMR). During the reaction of 4-nitrosophenetol with red cells only traces of 4-ethoxy-4'-nitrosodiphenylamine were formed, whereas up to 10% appeared as the reduction product 4-amino-4'-ethoxydiphenylamine. This latter compound is unstable in red cells and is metabolized further to unidentified products.

Oxygen activation during drug metabolism

The peroxidase-H2O2 catalyzed oxidation of certain drugs in the presence of GSH resulted in extensive oxidation to thiyl radicals and GSSG. NADH or arachidonate in place of GSH was also readily oxidized. Extensive oxygen uptake ensued resulting in the formation of superoxide radicals and H2O2. Only catalytic amounts of drugs and low peroxide levels were required, indicating a radox cycling mechanism. Active drugs included morphine, phenothiazines, aminopyrine, p-phenetidine, acetaminophen and 4-N,N-(CH3)2-aminophenol. Other drugs, including dopamine and methyl-alpha-dopa, did not catalyze oxygen uptake, nor was GSH oxidized to GSSG. Instead, GSH was depleted by GSH conjugate formation. Drugs of the former group, e.g. acetaminophen, aminopyrine or N,N-(CH3)2-aniline, have also been found by other investigators to form GSSG and hydrogen peroxide when added to hepatocytes or when perfused through an isolated liver. Although cytochrome P-450 normally catalyzes a two-electron oxidation of drugs, serious consideration should be given to some one-electron oxidation occurring as well and resulting in radical formation, oxygen activation and GSSG formation.

Glutathione oxidation during peroxidase catalysed drug metabolism

The peroxidase catalyzed oxidation of certain drugs in the presence of glutathione (GSH) resulted in extensive oxidation to oxidized glutathione (GSSG). Extensive oxygen uptake ensued and thiyl radicals could be trapped. Only catalytic amounts of drugs were required indicating a redox cycling mechanism. Active drugs included phenothiazines, aminopyrine, p-phenetidine, acetaminophen and 4-N,N-(CH3)2-aminophenol. Other drugs, including dopamine and alpha-methyl dopa, did not catalyse oxygen uptake, nor were GSSG or thiyl radicals formed. Instead, GSH was depleted by GSH conjugate formation. Drugs of the former group, e.g. acetaminophen, aminopyrine or N,N-(CH3)2-aniline have also been found by other investigators to form GSSG and hydrogen peroxide when added to hepatocytes or when perfused through an isolated liver. Although cytochrome P-450 normally catalyses a two-electron oxidation of drugs, serious consideration should be given for some one-electron oxidation resulting in radical formation, oxygen activation and GSSG formation.

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

The interaction of N-(4-ethoxyphenyl)p-benzoquinone imine (NEPBQI), a metabolite formed during peroxidase catalyzed metabolism of p-phenetidine, with GSH and its effects in isolated rat hepatocytes were investigated. When reacted with GSH NEPBQI formed both a mono- and a diglutathione conjugate as well as GSSG. Formation of glutathione conjugates and GSSG also occurred when NEPBQI was added to isolated hepatocytes. The formation of GSSG was, however, only detectable if the hepatocytes had been pretreated with the GSSG reductase inhibitor BCNU (1,3-bis-(2-chloroethyl-1-nitrosourea). Similarly, NEPBQI caused a rapid decrease in cellular free protein thiols when added to hepatocytes, however this was expressed at higher concentrations than for effects on GSH. The protein thiol decrease was correlated with protein binding of NEPBQI. NEPBQI was also shown to be toxic to isolated hepatocytes. At a concentration of 400 microM extensive bleb formation was followed by loss of cell membrane integrity and cell death. To assess further the subcellular metabolism of NEPBQI microsomes and cytosol was used. NEPBQI was found to be preferentially reduced by cytochrome P-450 reductase in the microsomes whereas DT-diaphorase catalyzed its reduction in cytosol. NEPBQI did not undergo significant redox cycling since no formation of O2 was observed. Thus, the cytotoxic effect of NEPBQI appears to be due to protein arylation rather than redox cycling.

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

The oxidation of the phenacetin metabolites p-phenetidine and acetaminophen by peroxidases was investigated. Free radical intermediates from both metabolites were detected using fast-flow ESR spectroscopy. Oxidation of acetaminophen with either lactoperoxidase and hydrogen peroxide or horseradish peroxidase and hydrogen peroxide resulted in the formation of the N-acetyl-4-aminophenoxyl free radical. Totally resolved spectra were obtained and completely analyzed. The radical concentration was dependent on the square root of the enzyme concentration, indicating second-order decay of the radical, as is consistent with its dimerization or disproportionation. The horseradish peroxidase/hydrogen peroxide-catalyzed oxidation of p-phenetidine (4-ethoxyaniline) at pH 7.5-8.5 resulted in the one-electron oxidation products, the 4-ethoxyaniline cation free radical. The ESR spectra were well resolved and could be unambiguously assigned. Again, the enzyme dependence of the radical concentration indicated a second-order decay. The ESR spectrum of the conjugate base of the 4-ethoxyaniline cation radical, the neutral 4-ethoxyphenazyl free radical, was obtained at pH 11-12 by the oxidation of p-phenetidine with potassium permanganate.

The biochemistry of nitrates, nitrites, nitrosamines and other potential carcinogens in human saliva

Human whole saliva protects the oral environment in many different ways from invading pathogenic microorganisms. Human saliva is also capable of inactivating mutagenic and carcinogenic agents by various mechanisms. The peroxidation of these agents is likely to be one of the degrading reactions. However, under certain circumstances some potentially carcinogenic compounds, such as N-nitrosamines, may be generated in whole saliva or--even more likely--in the saliva-gastric juice mixture after swallowing. The formation of N-nitroso compounds requires relatively high intake of nitrate e.g. from vegetable juices. Nitrate is partly reduced to nitrite by oral bacterial enzymes. The nitrosation of various secondary amines is favoured by high salivary (or gastric) concentration of thiocyanate and by low pH. The endogenous generation of N-nitroso compounds may be causally related to the development of oral or gastric cancer.