Showing papers in "Chemical Research in Toxicology in 1998"
1,043 citations
TL;DR: A new colorimetric assay of lipid peroxidation was developed to clarify complex patterns of tissue-specific MDA production in vivo, following hydrolysis of Schiff bases, in rodents treated with doxorubicin.
Abstract: Under acidic and mild-temperature conditions, 1-methyl-2-phenylindole was found to react with malondialdehyde (MDA) and 4-hydroxyalkenals to yield a stable chromophore with intense maximal absorbance at 586 nm. The use of methanesulfonic acid results in optimal yields of chromophore produced from MDA as well as from 4-hydroxynonenal. By contrast, the use of hydrochloric acid results in an optimal yield of chromophore produced from MDA and a negligible reaction of 4-hydroxynonenal. Taking advantage of such chromogenic reactions, we developed a new colorimetric assay of lipid peroxidation. Using a methanesulfonic acid-based medium, MDA and 4-hydroxyalkenals can be measured at the 586 nm wavelength. However, the presence of endogenous inhibitors of the reaction with 4-hydroxyalkenals is common, and this means that the latter may be underestimated in some biological samples. The assay performed in a hydrochloric acid-based medium enables the specific measurement of MDA in the presence of 4-hydroxyalkenals. Up...
409 citations
TL;DR: The selective toxicity of fipronil and fipronsil-derived residues is due in part to the higher potency of the parent compound at the insect versus the mammalian GABA receptor but is also dependent on the relative rates of conversion to the more persistent and less selective sulfone metabolite and desulfinyl photoproduct.
Abstract: Fipronil, an N-phenylpyrazole with a trifluoromethylsulfinyl substituent, initiated the second generation of insecticides acting at the γ-aminobutyric acid (GABA) receptor to block the chloride channel. The first generation includes the polychlorocycloalkanes α-endosulfan and lindane. In this study, we examine the mechanisms for selective toxicity of the sulfoxide fipronil and its sulfone metabolite and desulfinyl photoproduct relative to their target site interactions in vitro and ex vivo and the importance in fipronil action of biooxidation to the sulfone. Differences in GABA receptor sensitivity, assayed by displacement of 4‘-ethynyl-4-n-[2,3-3H2]propylbicycloorthobenzoate ([3H]EBOB) from the noncompetitive blocker site, appear to be a major factor in fipronil being much more toxic to the insects (housefly and fruit fly) than to the vertebrates (humans, dogs, mice, chickens, quail, and salmon) examined; in insects, the IC50s range from 3 to 12 nM for fipronil and its sulfone and desulfinyl derivatives,...
336 citations
TL;DR: Results indicate that BG, the primary furanocoumarin extracted from grapefruit juice, is a mechanism-based inactivator of P450 3A4.
Abstract: Grapefruit juice has been found to significantly increase oral bioavailability of several drugs metabolized by cytochrome P450 3A4 (P450 3A4) through inhibiting the enzymatic activity and decreasing the content of intestinal P450 3A4. HPLC/MS/MS and HPLC/UV analyses of ethyl acetate extracts from grapefruit juice revealed the presence of several furanocoumarins of which bergamottin (BG) is the major one. BG was shown to inactivate P450 3A4 in a reconstituted system consisting of purified P450 3A4, NADPH-cytochrome P450 reductase, cytochrome b5, and phospholipids. Inactivation was time- and concentration-dependent and required metabolism of BG. The loss of catalytic activity exhibited pseudo-first-order kinetics. The values of kinactivation and KI calculated from the inactivation studies were 0.3 min-1 and 7.7 μM, respectively. While approximately 70% of the erythromycin N-demethylation activity was lost during incubation with BG in the reconstituted system, P450 3A4 retained more than 90% of the heme as d...
311 citations
TL;DR: The fractionation of ACT solutions prepared from the tar from five cigarettes supports the hypothesis that the tar radical system is an equilibrium mixture of semiquinones, hydroquinones, and quinones.
Abstract: Previously, we have shown that aqueous cigarette tar (ACT) extracts contain a long-lived tar radical that associates with DNA in isolated rat alveolar macrophages and causes DNA damage in isolated ...
283 citations
274 citations
TL;DR: The combination of urinary metabolites which were significantly altered at various time points allowed for differentiation between biliary and parenchymal injury and assisting in the discovery of novel biomarkers of hepatotoxicity.
Abstract: 1H NMR spectroscopy of urine combined with pattern recognition (PR) methods of data analysis has been used to investigate the time-related biochemical changes induced in Sprague-Dawley rats by three model hepatotoxins: alpha-naphthyl isothiocyanate (ANIT), d-(+)-galactosamine (GalN), and butylated hydroxytoluene (BHT). The development of hepatic lesions was monitored by conventional plasma analysis and liver histopathology. Urine was collected continuously postdosing up to 144 h and analyzed by 600-MHz 1H NMR spectroscopy. NMR spectra of the urine samples showed a number of time-dependent perturbations of endogenous metabolite levels that were characteristic for each hepatotoxin. Biochemical changes common to all three hepatotoxins included a reduction in the urinary excretion of citrate and 2-oxoglutarate and an increased excretion of taurine and creatine. Increased urinary excretion of betaine, urocanic acid, tyrosine, threonine, and glutamate was characteristic of GalN toxicity. Both GalN and ANIT caused increased urinary excretion of bile acids, while glycosuria was evident in BHT- and ANIT-treated rats. Data reduction of the NMR spectra into 256 integrated regions was used to further analyze the data. Mean values of each integrated region were analyzed by principal components analysis (PCA). Each toxin gave a unique time-related metabolic trajectory that could be visualized in two-dimensional PCA maps and in which the maximum distance from the control point corresponded to the time of greatest cellular injury (confirmed by conventional toxicological tests). Thereafter, the metabolic trajectories changed direction and moved back toward the control region of the PR map during the postdose recovery phase. The combination of urinary metabolites which were significantly altered at various time points allowed for differentiation between biliary and parenchymal injury. This NMR-PR approach to the noninvasive detection of liver lesions will be of value in furthering the understanding of hepatotoxic mechanisms and assisting in the discovery of novel biomarkers of hepatotoxicity.
267 citations
TL;DR: It is concluded that the nature of the acid plays a crucial role in the oxidative fragmentation of intermediates into chromophores, allowing the selective assay of MDA in the presence of 4-HNE, using HCl acidic conditions.
Abstract: Under specific acidic conditions, both malondialdehyde (1, MDA) and 4-hydroxynonenal (2, 4-HNE) react with N-methyl-2-phenylindole (3) to give the same chromophoric cyanine 4 with maximal absorbance at 586 nm. Under such conditions, the reaction of 3 with 4-HNE (2) as well as with alkanals yields a second chromophoric cyanine 10 with maximal absorbance at 505 nm. The influence of different acids, iron(III), and oxygen on the reaction of 3 with such aldehydes was studied in detail. Under anaerobic conditions, the acid-induced reaction of 4-HNE with 3 afforded three rapidly interconverting intermediates, 5−7. Their subsequent fragmentation to 4 and hexanal in the presence of iron(III) and oxygen is consistent with the tandem β-fragmentation of an indolyl radical cation. 1-Indolylalkenes were identified as essential intermediates in the acid-induced reaction of 3 with alkanals. A very mild iron(III)-catalyzed fragmentation of these intermediates afforded the corresponding 3-formylindole 11 as the direct prec...
229 citations
216 citations
TL;DR: Although at toxic doses of APAP P 450 2E1 is the more efficient catalyst for the formation of the toxic metabolite NAPQI, P450 2A6 also can contribute significantly to NAPZI production.
Abstract: Acetaminophen (APAP), a widely used analgesic and antipyretic agent, is bioactivated by cytochromes P450 to cause severe hepatotoxicity. APAP is oxidized by two pathways to form a toxic intermediate, N-acetyl-p-benzoquinone imine (NAPQI), and a nontoxic catechol metabolite, 3-hydroxy-APAP (3-OH-APAP). We investigated the role of P450 2E1 and 2A6 in APAP oxidation by using baculovirus-expressed and highly purified forms of human P450 2E1 and 2A6. An electrochemical HPLC assay was developed to quantify both oxidative metabolites simultaneously. For the first time, it was demonstrated that human P450 2E1 selectively oxidized APAP to NAPQI (assayed as its glutathione conjugate, GS-APAP), whereas human P450 2A6 selectively oxidized APAP to 3-OH-APAP. At 1 mM APAP, the relative ratio for the formation of GS-APAP vs 3-OH-APAP with human P450 2E1 was approximately 6:1, whereas the ratio with human P450 2A6 was 1:3. Apparent Km and Vmax values for the formation of GS-APAP by human P450 2E1 were 1.3 mM and 6.9 nmol/min/nmol of P450, respectively, whereas they were 4.6 mM and 7.9 nmol/min/nmol of P450 for P450 2A6. Apparent Km and Vmax values for the formation of 3-OH-APAP by human P450 2E1 were 4.0 mM and 2.5 nmol/min/nmol of P450, respectively, whereas they were 2.2 mM and 14.2 nmol/min/nmol of P450, respectively, for P450 2A6. Thus, although at toxic doses of APAP P450 2E1 is the more efficient catalyst for the formation of the toxic metabolite NAPQI, P450 2A6 also can contribute significantly to NAPQI production.
216 citations
TL;DR: Treatment of mice with a toxic dose of acetaminophen significantly increased hepatotoxicity at 4 h, as evidenced by histological necrosis in the centrilobular areas of the liver, and increased serum levels of alanine aminotransferase (ALT).
Abstract: Treatment of mice with a toxic dose of acetaminophen (300 mg/kg, ip) significantly increased hepatotoxicity at 4 h, as evidenced by histological necrosis in the centrilobular areas of the liver, and increased serum levels of alanine aminotransferase (ALT) (from 8 ± 1 IU/L in saline-treated mice to 3226 ± 892 IU/L in the acetaminophen-treated mice). Serum levels of nitrate plus nitrite (a marker of nitric oxide synthesis) were also increased from 62 ± 8 μM in saline-treated mice to 110 ± 14 μM in acetaminophen-treated mice (P < 0.05). Regression analysis of serum ALT levels to serum nitrate plus nitrite levels in individual mice revealed a positive, linear relationship between serum ALT levels and serum nitrate plus nitrite levels with a correlation coefficient of 0.9 (P < 0.05). The y intercept value (nitrate plus nitrite level) was 63 ± 15 μM. Immunohistochemical analysis of liver sections from acetaminophen-intoxicated mice using an anti-3-nitrotyrosine antibody indicated tyrosine nitration in the prote...
TL;DR: Evidence is provided that several of the flavo-estrogens possess estrogenic properties comparable in activity to the well-established isoflavonoid estrogens.
Abstract: A newly developed recombinant yeast strain, in which the human estrogen receptor has been stably integrated into the genome of the yeast, was used to gain information on the estrogenic activity of a large series of dietary flavonoids. Among 23 flavonoids investigated, 8 were found to markedly stimulate the transcriptional activity of the human estrogen receptor in the yeast assay increasing transcriptional activity 5−13-fold above background level, corresponding to EC50 values between 0.1 and 25 μM. Five compounds increased the transcriptional activity 2−5-fold over the control, with EC50 values ranging from 84 to 102 μM, whereas the remaining flavonoids were devoid of activity. The most potent flavonoid estrogens tested were naringenin, apigenin, kaempferol, phloretin, and the four isoflavonoids equol, genistein, daidzein, and biochanin A. With the exception of biochanin A, the main feature required to confer estrogenicity was the presence of a single hydroxyl group in the 4‘-position of the B-ring of th...
TL;DR: Polycyclic hydrocarbons devoid of acetylenic groups were also inhibitory with respect to all three P450s, and several compounds were inhibitory at low nanomolar concentrations.
Abstract: Human cytochrome P450s 1A1, 1A2, and 1B1 are known to have overlapping substrate specificities. All are regulated in part by the Ah locus; P450 1A2 is expressed essentially only in liver, but P450s 1A1 and 1B1 are both expressed in many extrahepatic tissues. Twenty-five polycyclic hydrocarbons, many containing acetylenic side chains, were examined as inhibitors of the three enzymes using 7-ethoxyresorufin O-deethylation as the enzyme assay in all cases. Several compounds were inhibitory at low nanomolar concentrations. 1-(1-Propynyl)pyrene and 2-(1-propynyl)phenanthrene nearly completely inhibited P450 1A1 at concentrations at which no P450 1B1 inhibition was observed. 2-Ethynylpyrene and alpha-naphthoflavone (7, 8-benzoflavone) nearly completely inhibited P450 1B1 at concentrations at which no P450 1A1 inhibition was noted. All four of the above compounds also inhibited P450 1A2. Several polycyclic hydrocarbons devoid of acetylenic groups were also inhibitory with respect to all three P450s. Some of the acetylenic compounds examined showed enhanced inhibition following preincubation with the P450s in the presence of cofactors NADPH and O2. However, of seven compounds (five acetylenes) tested with P450 1B1, only two [2-ethynylpyrene and 4-(1-propynyl)biphenyl] showed such evidence for mechanism-based inactivation. We conclude that (i) several polycyclic hydrocarbons and their oxidation products are very inhibitory with respect to human P450s 1A1, 1A2, and 1B1; (ii) of these inhibitors only some are mechanism-based inactivators; and (iii) some of the inhibitors are potentially useful for distinguishing between human P450s 1A1 and 1B1.
TL;DR: Both P450s 1A2 and 3A4 have major roles in oxidations of estradiol and estrone in human liver and that the contents of these two P450 forms in liver microsomes determine which P450 enzymes are most important in hepatic estrogen hydroxylation by individual humans.
Abstract: Of seven cDNA-expressed human cytochrome P450 (P450) enzymes (P450s 1A2, 2B6, 2C9, 2C19, 2D6, 2E1, and 3A4) examined, P450 1A2 was the most active in catalyzing 2- and 4-hydroxylations of estradiol...
TL;DR: Hydroxylated PHAHs are potent inhibitors of thyroid hormone sulfation, known to accumulate in fetal tissues after maternal exposure to PCBs, which may have implications for fetal thyroid hormone homeostasis and development.
Abstract: Earlier studies in our laboratory showed that hydroxylated metabolites of polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs), and dibenzofurans (PCDFs) competitively inhibit thyroxine (T4) binding to transthyretin (TTR) and type I deiodinase (D1) activity. In this study, we investigated the possible inhibitory effects of hydroxylated metabolites of polyhalogenated aromatic hydrocarbons (PHAHs) on iodothyronine sulfotransferase activity. Rat liver cytosol was used as a source of sulfotransferase enzyme in an in vitro assay with 125I-labeled 3,3‘-diiodothyronine (T2) as a model substrate. Increasing amounts of hydroxylated PCBs, PCDDs, or PCDFs or extracts from incubation mixtures of PHAHs and induced liver microsomes were added as potential inhibitors of T2 sulfotransferase activity. Hydroxylated metabolites of PCBs, PCDDs, and PCDFs were found to be potent inhibitors of T2 sulfotransferase activity in vitro with IC50 values in the low micromolar range (0.2−3.8 μM). The most potent inhibitor of T2...
TL;DR: Cytochrome P450 1A2 expression in rodent and human liver tissue varies greatly and there are considerable differences between the enzymes in the two species in the activation of some heterocyclic aromatic amines, which must be considered when assessing human health risk.
Abstract: The dietary mutagens 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) are activated to genotoxins by rat and human liver cytochrome P450 (P450) 1A1- and 1A2-mediated N-oxidation. Immunoquantitation of 51 human liver samples revealed a wide range in P450 1A2 expression (10−250 pmol/mg of microsomal protein, median 71 pmol/mg), with 39% of the livers containing >100 pmol/mg of protein. There was no evidence for expression of P450 1A1 ( 1000-fold induction of P450 1A2 and 1A1, respectively, and a 220-fold increase in N-oxidation activity. Approximately 20% of th...
TL;DR: Formation of linear DNA, under conditions that generated relatively few single-strand breaks, suggests that these four transition-metal ions partake in Fenton reactions to generate true double-Strand breaks.
Abstract: The formation of 8-hydroxydeoxyguanosine (8-OHdG) and both single- and double-strand breaks in DNA by Fenton-type reactions has been investigated. Salmon sperm DNA was exposed to hydrogen peroxide (50 mM) and one of nine different transition-metal ions (25 microM-1 mM). Modified DNA was isolated and subjected to analysis by liquid chromatography coupled to an electrochemical detection system (LC-ECD), to evaluate the formation of 8-OHdG. The highest yield of 8-OHdG was obtained following treatment of DNA with the chromium(III) Fenton reaction (a maximum of 19 400/10(6) nucleotides), followed by iron(II) (13 600), vanadium(III) (5800), and copper(II) (5200). The chromium(VI) Fenton reaction generated a moderate yield of 8-OHdG (3600/10(6) nucleotides), while the yield obtained in DNA treated with cobalt(II), nickel(II), cadmium(II), and zinc Fenton reactions was not significantly higher than in control incubations of DNA with hydrogen peroxide alone. Similar treatment of the double-stranded plasmid pBluescript K+ with hydrogen peroxide (1 mM) and each transition-metal ion (1-100 microM) followed by quantitative agarose gel electrophoresis demonstrated that open-circle DNA, resulting from single-strand breaks, was generated in Fenton reactions involving all nine metal ions. In contrast, linear DNA was only formed in Fenton reactions involving chromium(III), copper(II), iron(II), and vanadium(III) ions. Formation of linear DNA, under conditions that generated relatively few single-strand breaks, suggests that these four transition-metal ions partake in Fenton reactions to generate true double-strand breaks. Furthermore, the generation of 8-OHdG exhibits a good correlation with the formation of double-strand breaks, suggesting that they arise by a similar mechanism.
TL;DR: In an effort to improve predictability, two-parameter QSAR, or response surface, analyses were performed to develop quantitative structure-activity relationships (QSARs), and no high-quality relationship was observed between toxicity and hydrophobicity, or Elumo, individually.
Abstract: Toxicity data for the 50% growth inhibitory concentration against Tetrahymena pyriformis (log (IGC50-1)) for 42 alkyl- and halogen-substituted nitro- and dinitrobenzenes were obtained experimentally. Log (IGC50-1) along with the hydrophobicity, the logarithm of the 1-octanol/water partition coefficient (log Kow), and the molecular orbital properties, the lowest unoccupied molecular orbital energy (Elumo) and maximum acceptor superdelocalizability (Amax), were used to develop quantitative structure−activity relationships (QSARs). All the nitroaromatic compounds tested had toxicity in excess of baseline, nonpolar narcosis. The nitrobenzenes were thought to elicit their toxic response through multiple (and mixed) mechanisms. No high-quality relationship was observed between toxicity and hydrophobicity, or Elumo, individually. However, a strong relationship {log (IGC50-1) = 16.4(Amax) − 4.64; n = 42, r2 = 0.847, s = 0.279, F = 229} was obtained. In an effort to improve predictability, two-parameter QSAR, or r...
TL;DR: In this article, the chiral inversion and hydrolysis of thalidomide and the catalysis by bases and human serum albumin were investigated by using a stereoselective HPLC assay.
Abstract: The chiral inversion and hydrolysis of thalidomide and the catalysis by bases and human serum albumin were investigated by using a stereoselective HPLC assay. Chiral inversion was catalyzed by albumin, hydroxyl ions, phosphate, and amino acids. Basic amino acids (Arg and Lys) had a superior potency in catalyzing chiral inversion compared to acid and neutral ones. The chiral inversion of thalidomide is thus subject to specific and general base catalysis, and it is suggested that the ability of HSA to catalyze the reaction is due to the basic groups of the amino acids Arg and Lys and not to a single catalytic site on the macromolecule. The hydrolysis of thalidomide was also base-catalyzed. However, albumin had no effect on hydrolysis, and there was no difference between the catalytic potencies of acidic, neutral, and basic amino acids. This may be explained by different reaction mechanisms of the chiral inversion and hydrolysis of thalidomide. Chiral inversion is deduced to occur by electrophilic substituti...
TL;DR: Results are consistent with a reaction mechanism that involves displacement of chloride from DCA by glutathione to afford S-(alpha-chlorocarboxymethyl)glutathione, which may undergo hydrolysis to give the hemithioacetal S-( alpha-hydroxycarboxymethy)glUTathione.
Abstract: Dichloroacetic acid (DCA) is a common drinking-water contaminant, is hepatocarcinogenic in rats and mice, and is a therapeutic agent used clinically in the management of lactic acidosis. Recent studies show that glutathione transferase Zeta (GSTZ) catalyzes the oxygenation of DCA to glyoxylic acid [Tong et al. (1998) Biochem. J. 331, 371-374]. In the present studies, the substrate selectivity of GSTZ, the kinetics of DCA metabolism, and the fate of DCA and glutathione were investigated. The results showed that GSTZ catalyzed the oxygenation of bromochloro-, bromofluoro-, chlorofluoro-, dibromo-, and dichloroacetic acid, but not difluoroacetic acid, to glyoxylic acid. GSTZ also catalyzed the biotransformation of fluoroacetic acid to S-(carboxymethyl)glutathione, and of (R,S)-2-bromopropionic acid, (R)-, (S)-, and (R,S)-2-chloropropionic acid, and (R, S)-2-iodopropionic acid, but not (R,S)-2-fluoropropionic acid, to S-(alpha-methylcarboxymethyl)glutathione; and of 2, 2-dichloropropionic acid to pyruvate. No biotransformation of 3, 3-dichloropropionic acid was detected, and no GSTZ-catalyzed fluoride release from ethyl fluoroacetate and fluoroacetamide was observed. The relative rates of DCA biotransformation by hepatic cytosol were mouse > rat > human. Immunoblotting showed the presence of GSTZ in mouse, rat, and human liver cytosol. 13C NMR spectroscopic studies showed that [2-13C]glyoxylic acid was the only observable, stable metabolite of [2-13C]DCA. Also, glutathione was required, but was neither consumed nor oxidized to glutathione disulfide, during the oxygenation of DCA to glyoxylic acid. These results are consistent with a reaction mechanism that involves displacement of chloride from DCA by glutathione to afford S-(alpha-chlorocarboxymethyl)glutathione, which may undergo hydrolysis to give the hemithioacetal S-(alpha-hydroxycarboxymethyl)glutathione. Elimination of glutathione from the hemithioacetal would give glyoxylic acid.
TL;DR: It is proposed that the neuroprotective effects of idebenone can be attributed, at least in part, to its ability to function as an antioxidant, involving redox cycling between hydroquinone and quinone.
Abstract: Idebenone [2,3-dimethoxy-5-methyl-6-(10-hydroxydecyl)-1,4-benzoquinone] is a synthetic analogue of coenzyme Q that is currently employed in the treatment of vascular and degenerative diseases of the central nervous system. There is some evidence to suggest that idebenone might function as an antioxidant; however, it has not been demonstrated whether this function pertains to the quinone or hydroquinone form of idebenone. Here we demonstrate that idebenone can scavenge a variety of free radical species, including organic radicals such as 2,2‘-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) and diphenylpicrylhydrazyl, peroxyl and tyrosyl radicals, and peroxynitrite. Idebenone can also redox couple with hypervalent species of Mb or Hb, thus preventing lipid peroxidation promoted by these species. Likewise, idebenone inhibits microsomal lipid peroxidation induced by ADP−iron complexes or organic hydroperoxides. In so doing, idebenone prevents the destruction of cytochrome P450, which otherwise would accompan...
TL;DR: It is found that DB[a,l]P is stereoselectively metabolized in human mammary carcinoma MCF-7 cells to the (-)-anti- and (+)-syn-DB[a-l]PDE which both bind extensively to cellular DNA.
Abstract: Chinese hamster V79 cell lines were constructed for stable expression of human cytochrome P450 1B1 (P450 1B1) in order to study its role in the metabolic activation of chemicals and toxicological consequences. The new V79 cell lines were applied to studies on DNA adduct formation of the polycyclic aromatic hydrocarbon (PAH) dibenzo[a,l]pyrene (DB[a,l]P). This compound has been found to be an environmental pollutant, and in rodent bioassays it is the most carcinogenic PAH yet discovered. Activation of DB[a,l]P in various metabolizing systems occurs via fjord region DB[a,l]P-11,12-dihydrodiol 13,14-epoxides (DB[a,l]PDE): we found that DB[a,l]P is stereoselectively metabolized in human mammary carcinoma MCF-7 cells to the (−)-anti- and (+)-syn-DB[a,l]PDE which both bind extensively to cellular DNA. To follow up this study and to relate specific DNA adducts to activation by individual P450 isoforms, the newly established V79 cells stably expressing human P450 1B1 were compared with those expressing human P45...
TL;DR: Iron, manganese, and alterations in glutathione and CySH metabolism have been implicated in the selective degeneration of nigrostriatal dopaminergic neurons in idiopathic and chemically induced Parkinson's disease.
Abstract: Iron(II/III) and manganese(II) both catalyze the autoxidation of the neurotransmitter dopamine (DA) in the presence of L-cysteine (CySH) in buffered aqueous solution at pH 7.4. Fe2+/Fe3+ and CySH together generate the hydroxyl (HO.) and cysteinyl thiyl (CyS.) radicals. DA is oxidized by HO. to DA semiquinone radical species that either react with CyS. to give 5-S-cysteinyldopamine (5-S-CyS-DA), 2-S-CyS-DA, and 6-S-CyS-DA or disproportionate to DA-o-quinone that reacts with CySH to give the same cysteinyl conjugates of DA. The major product of this initial reaction is 5-S-CyS-DA. However, 5-S-CyS-DA can be further oxidized by HO. to an o-quinone (2) that undergoes intramolecular cyclization to an o-quinone imine (3). The latter intermediate is the precursor of the dihydrobenzothiazine (DHBT) 7-(2-aminoethyl)-3,4-dihydro-5-hydroxy-2H-1, 4-benzothiazine-3-carboxylic acid (DHBT-1) and several other cyclized products. However, cysteinyl conjugates of DA can also be oxidized by HO. in a one-electron abstraction reaction that leads to DA thiyl radicals. Reactions of these radicals with CyS. or DA semiquinone radicals lead to some novel DA disulfides and thioethers, respectively. The Mn(II)-catalyzed oxidation of DA generates DA-o-quinone that is scavenged by CySH to give 5-S-CyS-DA (major initial product) with lower yields of other cysteinyldopamines. Subsequent Mn(II)-catalyzed oxidation of 5-S-CyS-DA gives o-quinone 2 and thence o-quinone imine 3 that serve as the precursors of DHBT-1 and several other DHBTs. Organic or oxygen radicals do not play significant roles in the Mn(II)-catalyzed oxidation of DA in the presence of CySH. Recent studies have demonstrated that DHBT-1 can be accumulated by brain mitochondria and evoke irreversible inhibition of NADH-coenzyme Q reductase (complex I). Furthermore, iron, manganese, and alterations in glutathione and CySH metabolism have been implicated in the selective degeneration of nigrostriatal dopaminergic neurons in idiopathic and chemically induced Parkinson's disease (PD). Because DHBT-1 is formed in both the iron- and manganese-catalyzed oxidation of DA in the presence of CySH and a defect in mitochondrial complex I respiration contributes to dopaminergic neuronal cell death in PD, the results of this investigation are discussed in terms of their possible implications to an understanding of the neuropathological processes in idiopathic and chemically induced parkinsonism.
TL;DR: It is found that alpha-tocopherol can act as a potent DNA-damaging agent in the presence of copper(II) ions, using a simplified, in vitro model, and may be of relevance to the tumorigenic properties of the vitamin reported in the literature.
Abstract: There is currently much interest in the possibility that dietary antioxidants may confer protection from certain diseases, such as atherosclerosis and cancer. The importance of α-tocopherol (vitami...
TL;DR: The data support the conclusion that GPx can exhibit a biological function by acting as a peroxynitrite reductase, and the rate constant for reduced GPx is similar to that obtained previously for ebselen.
Abstract: Glutathione peroxidases and their mimics, e.g., ebselen or diaryl tellurides, efficiently reduce peroxynitrite/peroxynitrous acid (ONOO-/ONOOH) to nitrite and protect against oxidation and nitration reactions. Here, we report the second-order rate constant for the reaction of the reduced form of glutathione peroxidase (GPx) with peroxynitrite as (8.0 ± 0.8) × 106 M-1 s-1 (per GPx tetramer) at pH 7.4 and 25 °C. The rate constant for oxidized GPx is about 10 times lower, (0.7 ± 0.2) × 106 M-1 s-1. On a selenium basis, the rate constant for reduced GPx is similar to that obtained previously for ebselen. The data support the conclusion that GPx can exhibit a biological function by acting as a peroxynitrite reductase.
TL;DR: It is concluded that peroxynitrous acid homolyzes to yield nitrogen dioxide and hydroxyl (OH) free radicals and derive delta f G o(ONOOH) = 7.7 kcal/mol.
Abstract: The rate constant of homolysis of peroxynitrite, ONOO-, into O2- and NO was determined to be 0.017 s-1 at 20 degrees C. In combination with other experimental data taken from the literature, this value yields the Gibbs free energy of formation of ONOO-, delta f G o(ONOO-) = 16.6 kcal/mol. On the basis of this result, we conclude that peroxynitrous acid homolyzes to yield nitrogen dioxide (NO2) and hydroxyl (OH) free radicals and derive delta f G o(ONOOH) = 7.7 kcal/mol. The rate constant of the reaction between NO and ONOO- was found to be 5 x 10(-)2 M-1 s-1 at most. The implications of the two homolysis reactions are discussed.