The oxidation of tetrachloro-1,4-hydroquinone by microsomes and purified cytochrome P-450b. Implications for covalent binding to protein and involvement of reactive oxygen species

Assessment of Pentachlorophenol Exposure in Humans using the Clearance Concept

1 Pentachlorophenol (PeCP), a widely-used wood preservative, is a ubiquitous compound which has been found to be carcinogenic in mice. The objective of this study is to assess the average net daily intake of PeCP in cohorts of individuals who are: (1) not specifically exposed to PeCP, (2) residents of homes made of PeCP-treated logs and (3) occupationally exposed to PeCP.

2 The average net daily intake was calculated using a basic pharmacokinetic principle, the clearance (CL) concept: net daily intake equals CL (in 1 d-1) times the average steady-state concentration of PeCP in plasma ( Css). Css values reported in the literature were used for the calculations.

3 Because the two definitive studies on PeCP toxicokinetics in humans have given conflicting results, kinetic information from human exposure to PeCP was reviewed. Plasma clearance was estimated from retrospective analysis of urine and plasma concentrations measured in people after long-term exposure to PeCP. An overall clearance of 0.425 1 d-1 was obtained.

4 In groups of individuals who are not specifically exposed to PeCP, net daily intake estimated in eight countries varied from 5 μg (Nigeria) to 37 μg (The Netherlands). Net intake was between 51 μg d-1 and 157 μg d-1 in residents of homes made of PeCP-treated logs. In individuals occupationally exposed to PeCP, net daily intake varied widely (from 35 μg to about 24 000 μg) depending on the type of work.

Genome shuffling improves degradation of the anthropogenic pesticide pentachlorophenol by Sphingobium chlorophenolicum ATCC 39723

Pentachlorophenol (PCP), a highly toxic anthropogenic pesticide, can be mineralized by Sphingobium chlorophenolicum, a gram-negative bacterium isolated from PCP-contaminated soil. However, degradation of PCP is slow and S. chlorophenolicum cannot tolerate high levels of PCP. We have used genome shuffling to improve the degradation of PCP by S. chlorophenolicum. We have obtained several strains that degrade PCP faster and tolerate higher levels of PCP than the wild-type strain. Several strains obtained after the third round of shuffling can grow on one-quarter-strength tryptic soy broth plates containing 6 to 8 mM PCP, while the original strain cannot grow in the presence of PCP at concentrations higher than 0.6 mM. Some of the mutants are able to completely degrade 3 mM PCP in one-quarter-strength tryptic soy broth, whereas no degradation can be achieved by the wild-type strain. Analysis of several improved strains suggests that the improved phenotypes are due to various combinations of mutations leading to an enhanced growth rate, constitutive expression of the PCP degradation genes, and enhanced resistance to the toxicity of PCP and its metabolites.

A previously unrecognized step in pentachlorophenol degradation in Sphingobium chlorophenolicum is catalyzed by tetrachlorobenzoquinone reductase (PcpD)

The first step in the pentachlorophenol (PCP) degradation pathway in Sphingobium chlorophenolicum has been believed for more than a decade to be conversion of PCP to tetrachlorohydroquinone. We show here that PCP is actually converted to tetrachlorobenzoquinone, which is subsequently reduced to tetrachlorohydroquinone by PcpD, a protein that had previously been suggested to be a PCP hydroxylase reductase. pcpD is immediately downstream of pcpB, the gene encoding PCP hydroxylase (PCP monooxygenase). Expression of PcpD is induced in the presence of PCP. A mutant strain lacking functional PcpD has an impaired ability to remove PCP from the medium. In contrast, the mutant strain removes tetrachlorophenol from the medium at the same rate as does the wild-type strain. These data suggest that PcpD catalyzes a step necessary for degradation of PCP, but not for degradation of tetrachlorophenol. Based upon the known mechanisms of flavin monooxygenases such as PCP hydroxylase, hydroxylation of PCP should produce tetrachlorobenzoquinone, while hydroxylation of tetrachlorophenol should produce tetrachlorohydroquinone. Thus, we proposed and verified experimentally that PcpD is a tetrachlorobenzoquinone reductase that catalyzes the NADPH-dependent reduction of tetrachlorobenzoquinone to tetrachlorohydroquinone.

Characterization of metabolic activation of pentachlorophenol to quinones and semiquinones in rodent liver

Pentachlorophenol (PCP), a widely used biocide, induces liver tumors in mice but not in rats. Metabolic activation of PCP to chlorinated quinones and semiquinones in liver cytosol from Sprague-Dawley rats and B6C3F1 mice was investigated in vitro (1) with microsomes in the presence of either beta-nicotinamide adenine dinucleotide phosphate (NADPH) or cumene hydroperoxide (CHP), (2) with CHP in the absence of microsomes, and (3) with horseradish peroxidase (HRP) and H2O2. Mono-S- and multi-S-substituted adducts of tetrachloro-1,4-benzoquinone (Cl4-1,4-BQ) and Cl4-1,2-BQ and their corresponding semiquinones [i.e. tetrachloro-1,4-benzosemiquinone (Cl4-1,4-SQ) and tetrachloro-1,2-benzosemiquinone (Cl4-1,2-SQ)] were measured by gas chromatography-mass spectrometry (GC-MS). Qualitatively, the metabolites of PCP were the same in both rats and mice for all activation systems. Induction of PCP metabolism by either 3MC or PB-treated microsomes was observed in NADPH- but not in CHP-supported systems. In rats, the amount of induction was comparable with either 3MC or PB. 3MC was a stronger inducer than PB in mice and also induced a greater amount of metabolism than in rats. This suggests that induction of specific P450 isozymes may play a role in the toxicity of PCP to mice. Both HRP/H2O2 and CHP led to production of the full spectrum of chlorinated quinones and semiquinones, confirming the direct oxidation of PCP. CHP (with or without microsomes) converted PCP into much greater quantities of quinones and semiquinones than did microsomal P450/NADPH or HRP/H2O2 in both species. This implies that, under conditions of oxidative stress, endogenous lipid hydroperoxides may increase PCP metabolism sufficiently to enhance the toxicity and carcinogenicity of PCP.

Activation of the hsp70 promoter by environmental inorganic and organic chemicals: relationships with cytotoxicity and lipophilicity

Stress proteins (heat shock proteins, HSPs) have been proposed as general markers of cellular aggression and their use for environmental monitoring is often suggested. The aim of this work was to study the potency of various environmentally relevant organic and inorganic chemicals to induce the expression of the HSP70 marker. For this purpose, we used an established HeLa cell line containing the chloramphenicol acetyl transferase (CAT) gene under the control of the hsp70 promoter. The screening of three metallic and 15 organic chemicals revealed differences in their capacities to induce the hsp70 promoter. The three metals tested (cadmium, zinc and mercury) were able to induce a stress response. Some organochlorine compounds (chlorophenol derivatives, tetrachlorohydroquinone, 3, 4-dichloroaniline, ethyl parathion and 1-chloro-2,4-dinitrobenzene) induced a response, whereas other common halogenated pesticides or aromatic hydrocarbons (e.g. benzo(a)pyrene, 2, 4-dichlorophenoxyacetic acid, endosulfan, diuron, 4-nonylphenol) did not. The potency to induce hsp70 was significantly correlated to the octanol-water partition coefficient (log K(ow)) of the inducing chemicals, except for 1-chloro-2,4-dinitrobenzene and ethyl parathion. Cytotoxicity assays run in parallel to the induction measurements revealed that the three metals were effective at non cytotoxic doses whereas all organic compounds, except tetrachlorohydroquinone and 1-chloro-2,4-dinitrobenzene, induced the promoter at cytotoxic doses. These results suggest that hsp70 is induced by different mechanisms of toxicity. We propose that this model can be used in mechanistic studies for the detection of toxic effects of certain pollutants.

Dosimetry of chlorinated quinone metabolites of pentachlorophenol in the livers of rats and mice based upon measurement of protein adducts

The dosimetry of chlorinated quinones arising from metabolism of pentachlorophenol (PCP), in the livers of male Sprague-Dawley rats and B6C3F1 mice was investigated via measurements of cysteinyl protein adducts and estimates of the second-order reaction rate constants between the quinones and the proteins. We had previously shown that adducts of tetrachloro-1,4-benzoquinone (Cl4-1,4-BQ) and tetrachloro-1,2-benzosemiquinone (Cl4-1,2-SQ) were observed at the highest levels in the livers of Sprague-Dawley rats to which PCP had been administered by gavage (5-40 mg/kg body wt) (Biomarkers 1, 232-243, 1996). In the current study we observed that adducts of Cl4-1,4-BQ and tetrachloro-1,2-benzoquinone (CL4-1,2-BQ) were predominant in the livers of B6C3F1 mice receiving 20 mg PCP/kg body wt. The second-order rate constants, representing in vitro reactions between Cl4-1,2-BQ and Cl4-1,4-BQ and various cysteine residues of hepatic proteins of liver cytosol and liver nuclei, were estimated to be 0.012-1.96 L(g protein)(-1) hr(-1) in rats and 0.082-1.67 L(g protein)(-1) hr(-1) in mice. The estimated tissue doses of the quinones to liver cytosol decreased in the order rat Cl4-1,4-BQ > mouse Cl4-1,4-BQ > mouse Cl4-1,2-BQ and to liver nuclei in the order mouse Cl4-1,2-BQ > mouse Cl4-1,4-BQ > rat Cl4-1,4-BQ. The corresponding doses of Cl4-1,2-SQ could not be inferred due to our inability to estimate the second-order rate constants. After aggregating the estimated contributions of all quinone species, mice had a fourfold greater dose to liver nuclei than rats, whereas rats had a threefold greater dose to liver cytosol. The increased nuclear dose to mouse liver compared to that of the rat suggests that the mouse is at greater risk to hepatic DNA damage from PCP-derived quinones. Investigation of the time course of levels of unconjugated tetrachlorohydroquinone (Cl4HQ) in the livers indicated that about 0.4% of Cl4HQ was oxidized to Cl4-1,4-BQ in both rats and mice.

Influence of substituents in fluorobenzene derivatives on the cytochrome P450-catalyzed hydroxylation at the adjacent ortho aromatic carbon center

In a previous study, the in vivo cytochrome P450-catalyzed regioselectivity of aromatic ring hydroxylation for a series of (poly)fluorobenzenes could be quantitatively predicted by the calculated frontier orbital density distribution in the aromatic ring [Rietjens et al. (1993) Biochemistry 32, 4801-4812]. However, the relative small fluorine, its size almost comparable to a hydrogen, is not expected to influence the regioselectivity of aromatic hydroxylation due to steric hindrance. The aim of the present study was to investigate the influence of substituents larger than a hydrogen or fluorine on the possibilities for hydroxylation at the adjacent carbon center. First, the in vivo regioselectivity of aromatic ring hydroxylation of a series of C4-substituted fluorobenzenes was investigated. The results obtained demonstrate that a chlorine and cyano C4 substituent do not hamper hydroxylation at the positions ortho to the C4 carbon atom. For 4-chloro- and 4-cyanofluorobenzene, the observed regioselectivity of aromatic hydroxylation correlated with the regioselectivity predicted on the basis of the frontier orbital density distribution. In contrast, a bromine and iodine substituent affected the hydroxylation at the adjacent ortho carbon centers, reducing it to respectively 40 and 6% of the calculated intrinsic reactivity of the carbon centers. Additional experiments were performed to investigate whether the regioselectivity of the aromatic hydroxylation of the C4-substituted fluorobenzene model compounds was influenced by changes in the cytochrome P450 enzyme pattern. Results obtained demonstrate that for these relatively small substrates the regioselectivity of their hydroxylation was not significantly influenced by several cytochrome P450 inducers. This suggests that the active sites of the cytochromes P450 catalyzing the aromatic hydroxylation do not impose a stereoselective orientation of the aromatic rings with respect to the iron-oxo porphyrin reaction center. Thus, the working hypothesis for additional experiments was that the deviations for the regioselectivity of aromatic hydroxylation observed for 4-bromo- and 4-iodofluorobenzene may be ascribed to steric hindrance by the bromine and iodine substituents hampering the attack of the cytochrome P450 iron-oxo species on the adjacent carbon centers in the benzene derivative. This working hypothesis was further tested by investigating whether useful steric correction factors could be derived from the results obtained with the series C4-substituted fluorobenzenes. These correction factors should make it possible to correct calculated relative reactivities of carbon sites for steric hindrance by substituents positioned ortho with respect to the carbon to be hydroxylated. This will make it possible to better explain and predict the regioselectivities for other chlorine-, bromine-, iodine-, and cyano-containing fluorobenzenes. The in vivo regioselectivity of aromatic ring hydroxylation of a series of five chlorine-, bromine-, iodine-, or cyano-containing fluorobenzenes did not correlate with the noncorrected calculated reactivities (r = 0.49). However, upon correction of the calculated reactivity values by using the steric correction factors, a correlation between the observed and calculated regioselectivity for the substrates of the present study was obtained (r = 0.91). Together these results strongly indicate that for the fluorobenzenes studied the main factors directing the regioselectivity of their aromatic hydroxylation are (i) the nucleophilic chemical reactivity of the site to be hydroxylated and (ii) the steric influence of the substituent ortho with respect to the site of hydroxylation. This latter effect appears to be negligible for a fluorine, chlorine, and cyano substituent but significant for a bromine and iodine substituent.

NADPH-cytochrome reductase catalysed redox cycling of 1,4-benzoquinone; hampered at physiological conditions, initiated at increased pH values

In the present study the inability of 1,4-benzoquinone to support NADPH-cytochrome reductase catalysed redox cycling was investigated. The results obtained demonstrate that NADPH-cytochrome reductase is able to initiate a rapid two-electron reduction of 1,4-benzoquinone resulting in formation of the hydroquinone. The intermediate one-electron reduced semiquinone form does not pass its electron on to molecular oxygen, i.e. giving rise to redox cycling, but is reduced by a second electron, either by NADPH-cytochrome reductase upon protonation of the semiquinone or through disproportionation, both giving rise to the two-electron reduced hydroquinone. At pH values below the pK alpha of the hydroquinone, the electrons of the hydroquinone are also not passed on to molecular oxygen due to efficient protonation. However, at pH values around or above the pK alpha (9.85) of the two-electron reduced hydroquinone form, significant redox cycling activity is observed in a 1,4-benzoquinone containing incubation. Further experiments demonstrate a similarity in both the concentration and pH dependence of 1,4-benzoquinone or 1,4-hydroquinone supported NADPH-cytochrome reductase catalysed redox cycling. From these observations it is concluded that 1,4-benzoquinone is able to redox cycle from its deprotonated two-electron reduced hydroquinone form, but only at relatively high pH values. Together the data provide an insight into why the NADPH-cytochrome reductase catalysed redox cycling of 1,4-benzoquinone is inhibited at physiological conditions, but initiated at increased pH values.

The glutathione conjugates of tert-butyl hydroquinone as potent redox cycling agents and possible reactive agents underlying the toxicity of butylated hydroxyanisole

The glutathione conjugates of tert-butyl hydroquinone, a metabolite of butylated hydroxyanisole (BHA), possess redox potentials which are much higher as compared to the non-conjugated hydroquinone (0.36 V for the hydroquinone and 1.2-1.4 V for the conjugates). As a result, the redox cycling activity of the conjugates, as measured by oxygen consumption in the presence of a reducing agent, is increased tenfold as compared to the non-conjugated hydroquinone. Since evidence for both oxidative damage and the involvement for glutathione in the toxicity of butylated hydroxyanisole is available, this mechanism may be involved in the toxic action of this compound.

Pentachlorophenol toxicokinetics after intravenous and oral administration to rat

1. The toxicokinetics of pentachlorophenol (PCP) were studied in rats. Doses of 2.5 mg/kg were given i.v. (bolus, five rats) and orally (gastric intubation, five rats). Concentrations in plasma, urine and faeces were measured by capillary g.l.c. with electron-capture detection. 2. After i.v. administration, the clearance and volume of distribution at steady state were 0.026 +/- 0.003 l/h per kg and 0.25 +/- 0.02 l/kg, respectively. These two parameters exhibit low inter-rat variability (coefficients of variation less than 15%). The half-life of the initial decline of PCP plasma concn. was less than 1.3 h, while the second phase half-life was 7.11 +/- 0.87 h. 3. After oral administration the peak plasma concn. (7.3 +/- 2.8 micrograms/ml) occurred between 1.5 and 2 h and absorption was complete (bioavailability = 0.91-0.97). No distinct distribution phase was observed and the elimination half-life was 7.54 +/- 0.44 h. 4. PCP clearance is essentially metabolic since only 5.3 +/- 0.2% dose is eliminated unchanged by the kidney. About 60% dose was recovered in urine, mainly as conjugated PCP and conjugated tetrachlorohydroquinone (TCHQ). 5. For both routes of administration, about 10% dose was recovered in faeces as PCP and/or metabolites, which indicates that biliary excretion contributes to total elimination.

Irreversible inhibition of rat glutathione S-transferase 1-1 by quinones and their glutathione conjugates. Structure-activity relationship and mechanism

The irreversible inhibition of the rat glutathione S-transferase (GST) isoenzyme 1-1 by a series of halogenated 1,4-benzoquinones and their GSH conjugates was studied quantitatively by analysing the time course of enzyme inactivation. With increasing numbers of chlorine substituents, the rate of inhibition greatly increased. Incorporation of a GSH moiety in all cases increased the rate of inactivation compared with the non-substituted compound, and this was due to the increased affinity of the inhibitor for the active site. The ratio between the rates of inhibition for a given quinone with and without GSH substituent was largest for the three dichlorobenzoquinones, with the 2,6-isomer showing a 41-fold increase in rate of inhibition upon conjugation with GSH. The time courses of inhibition could be fitted either to a bi-exponential function (for the GSH conjugates and the higher chlorinated quinones) or to a mono-exponential function (all other quinones). It is concluded that the second component describes the affinity part of the reaction. GST 1-1 possesses two cysteine residues, with modification of one of these, probably located in the vicinity of the active site, having a major impact on the enzyme activity. Compounds with affinity towards the active site preferentially react with this residue. Non-specific quinones react equally with both cysteine residues. This was confirmed by the observation that complete inactivation of GST 1-1 by 2,5-dichlorobenzoquinone was achieved only after modification of two residues, whereas the corresponding GSH conjugate already completely inhibited the enzyme after modification of one residue.

Irreversible inhibition of human glutathione S-transferase isoenzymes by tetrachloro-1,4-benzoquinone and its glutathione conjugate

The quinones tetrachloro-1,4-benzoquinone (1,4-TCBQ) and its glutathione conjugate (GS-1,4-TCBQ) are potent irreversible inhibitors of most human glutathione S-transferase (GST) isoenzymes. Human pi, psi, and mu are almost completely inhibited at a molar ratio 1,4-TCBQ/GST = 2/1. The isoenzyme B1B1 was inhibited up to 75%, and higher concentrations (1,4-TCBQ/GST = 6/1) were needed to reach this maximum effect. For these isoenzymes 75-85% of the maximal amount of inhibition was already reached on incubation of equimolar ratios of 1,4-TCBQ and subunit GST, while approximately 1 nmol (0.82-0.95) 1,4-[U-14C]TCBQ per nmol subunit GST could be covalently bound. These results suggest that these GST isoenzymes possess only one cysteine in or near the active site of GST, which is completely responsible for the inhibition. In agreement, human isoenzyme B2B2 which possesses no cysteine, was not inhibited and no 1,4-TCBQ was bound to it. The rate of inhibition was studied at 0 degrees: 1,4-TCBQ, trichloro-1,4-benzoquinone and GS-1,4-TCBQ all inhibit GST very fast. Especially for B1B1, the inhibition by the glutathione conjugate is significantly faster than inhibition by 1,4-TCBQ: the glutathione moiety seems to target the quinone to the enzyme. For the other isoenzymes only minor differences are observed between 1,4-TCBQ and its glutathione conjugate under the conditions used.

Metabolic activation of 1,2,4-trichlorobenzene and pentachlorobenzene by rat liver microsomes: a major role for quinone metabolites

Microsomal metabolism of 1,2,4-[14C]trichlorobenzene (1,2,4-TrCB) and [14C]pentachlorobenzene (PeCB) was studied with special emphasis on the conversion-dependent covalent binding to protein and DNA. 1,2,4-TrCB was metabolized to 2,3,6- and 2,4,5-trichlorophenol, and to a lesser extent to 2,4,6- and 2,3,5-trichlorophenol, and trichlorohydroquinone. About 10% of all metabolites became covalently bound to protein in a rather nonselective way. For 1,2,4-TrCB and PeCB a strong correlation between secondary metabolism to hydroquinones and covalent binding was established. Protein binding was completely inhibited by the addition of ascorbic acid, indicating quinone metabolites as the sole reactive species formed. Both 1,2,4-TrCB and PeCB alkylated DNA, although to a much lesser extent than protein (0.5 and 0.3% of all metabolites, respectively). Nonquinone intermediates, presumably epoxides, were responsible for a minor portion of the observed DNA binding, since complete inhibition by ascorbic acid was not reached. The differential role of cytochrome P450 both in primary and in secondary metabolism was demonstrated by the use of microsomes from rats pretreated with different inducers. Dexamethasone (DEX) microsomes (cytochrome P450IIIA1) showed the highest activity toward these chlorinated benzenes (14 nmol/mg/5 min for 1,2,4-TrCB and 36 nmol/mg/10 min for PeCB, both with regard to the formation of phenols and to the formation of protein-bound metabolites. In addition, DEX microsomes preferentially formed 2,3,6-trichlorophenol, whereas other microsomal suspensions formed 2,4,5-trichlorophenol as the major isomer. The present study clearly demonstrates the high alkylating potency of secondary quinone metabolites derived from chlorinated benzenes and poses a need for reevaluation of the role of epoxides in the observed toxicity of these compounds.

The toxicity of menadione (2-methyl-1,4-naphthoquinone) and two thioether conjugates studied with isolated renal epithelial cells

Menadione (2-methyl-1,4-naphthoquinone) was used as a model compound to test the hypothesis that thioether conjugates of quinones can be toxic to tissues associated with their elimination through a mechanism involving oxidative stress. Unlike menadione, the glutathione (2-methyl-3-(glutathion-S-yl)-1,4-naphthoquinone; MGNQ) and N-acetyl-L-cysteine (2-methyl-3-(N-acetylcysteine-S-yl)-1,4-naphthoquinone; M(NAC)NQ) thioether conjugates were not able to arylate protein thiols but were still able to redox cycle with cytochrome c reductase/NADH and rat kidney microsomes and mitochondria. Interestingly, menadione and M(NAC)NQ were equally toxic to isolated rat renal epithelial cells (IREC) while MGNQ was nontoxic. The toxicity of both menadione and M(NAC)NQ was preceded by a rapid depletion of soluble thiols and was associated with a depletion of soluble thiols and was associated with a depletion of protein thiols. Treatment of IREC with the glutathione reductase inhibitor, 1,3-bis(2-chloroethyl)-1-nitrosourea, potentiated the thiol depletion and toxicity observed with menadione and M(NAC)NQ indicating the involvement of oxidative stress in this model of renal cell toxicity. The lack of MGNQ toxicity can be attributed to an intramolecular cyclization reaction which destroys the quinone nucleus and therefore eliminates its ability to redox cycle. These findings have important implications with regard to our understanding of the toxic potential of quinone thioether conjugates and of quinone toxicity in general.

Metabolic activation of the herbicide dichlobenil in the olfactory mucosa of mice and rats

The metabolic activation of the herbicide dichlobenil (2,6-dichloro[ring-14C]benzonitrile) in the olfactory mucosa of C57BL mice and Sprague-Dawley rats was examined. In homogenates of the olfactory mucosa (mouse 1000 x g supernatants; rat microsomes), dichlobenil was metabolized and covalently bound to protein. The apparent Km, Vmax and V/K values showed that the olfactory mucosa had both a higher affinity for dichlobenil and a higher capacity/mg protein to activate dichlobenil in comparison to the liver. The covalent binding was dependent on NADPH and was inhibited by the addition of dithionite, metyrapone and glutathione indicating an oxidative cytochrome P-450 dependent activation of dichlobenil into an electrophilic intermediate. The covalent binding was also inhibited by the addition of superoxide dismutase whereas catalase, mannitol or dimethylsulfoxide had no effect indicating the involvement of O2- but not of H2O2 or OH. in the activation. In explants of the olfactory mucosa incubated with [14C]dichlobenil a preferential covalent binding was observed in the Bowman's glands suggesting an activation of dichlobenil in these structures. The highly efficient metabolic activation of dichlobenil to reactive intermediates in the olfactory mucosa is suggested to be of importance for the potent dichlobenil-induced toxicity in this tissue.

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.

Butylated hydroxyanisole in perspective

Butylated hydroxyanisole (BHA) is a synthetic food antioxidant used to prevent oils, fats and shortenings from oxidative deterioration and rancidity. This review depicts the current knowledge on BHA. The physical and chemical characteristics of BHA are summarized and its function as a food antioxidant is made clear. The toxicological characteristics of BHA and its metabolic fate in man and animal are briefly reviewed. Special emphasis is laid on the carcinogenicity of BHA in the forestomach of rodents and to related events in the forestomach and other tissues in experimental animals. At present there is sufficient evidence for carcinogenicity of BHA, but there is hardly any indication that BHA is genotoxic. Therefore risk assessment for this epigenetic carcinogen is based on non-stochastic principles. However, the mechanism underlying the tumorigenicity of BHA is not known. In the last part of this review an attempt is made to unravel the unknown mechanism of carcinogenicity. It is hypothesized that BHA gives rise to tumor formation in rodent forestomach by inducing heritable changes in DNA. Evidence is being provided that reactive oxygen species, in particular hydroxylradicals, may play a crucial role. The key question with respect to risk assessment for BHA is whether or not the underlying mechanism is thresholded, which is important for the choice of the appropriate model to assess the risk, if any, for man and to manage any potential risk.

Reaction pathways for biodehalogenation of fluorinated anilines

Pathways for biodehalogenation of fluorinated aniline derivatives were investigated. Microsomal NADPH-dependent dehalogenation of fluoroanilines was shown to proceed by three different reaction pathways. The first route appeared to result in monooxygenation at a fluorinated position and release of the fluorine atom as a fluoride anion. The primary additional reaction product formed is the reactive quinoneimine, not the 4-hydroxyaniline. In NADPH-containing microsomal systems with 4-fluoro-substituted anilines, formation of the 4-hydroxyaniline derivative is observed because NADPH chemically reduces this quinoneimine metabolite. A second pathway for dehalogenation proceeds by protein binding of a fluoro-containing (semi)quinoneimine metabolite, the formation of which may result from the mono-oxygenase reaction (pathway 1) and/or from (re)oxidation of a hydroxyaniline metabolite by superoxide anion radicals produced by the microsomal system. This latter reaction pathway becomes more important with increasing number of fluoro-substituents in the fluoroaniline derivative. The higher ratio of fluoride anion formed to 4-hydroxyaniline derivative detected in incubations with liver microsomes from dexamethasone-treated rats, as compared to incubations with liver microsomes from control rats, can in part be explained by the higher production of superoxide anion radicals observed in the dexamethasone systems. The third mechanism was shown to proceed by formation of a hydroxylated metabolite that loses fluoride anion upon exposure to oxygen. The reactive intermediate formed upon oxygen exposure might be the semiquinoneimine which loses its fluorine atom as a fluoride anion upon dimerization or polymerization and/or protein binding. The fluorohydroxyanilines, in which the hydroxyl group is ortho or para with respect to the fluoro substituent, appear especially to be highly unstable and lose fluoride anion in the presence of oxygen. Finally, it is concluded that all three pathways for dehalogenation of fluorinated aniline derivatives are bioactivation pathways. The reactivity of the (semi)quinoneimines formed in these reactions is dependent on their substitution pattern and increases with increasing number of fluoro-substituents. Therefore, bioactivation for a series of fluorinated aniline derivatives, can be expected to vary with the substitution pattern and to increase with increasing number of halogen substituents.

Simultaneous assay of pentachlorophenol and its metabolite, tetrachlorohydroquinone, by gas chromatography without derivatization

A sensitive capillary gas chromatographic method was developed for the simultaneous determination of pentachlorophenol and its major metabolite, tetrachlorohydroquinone, in plasma, urine and feces. The method involved a simple one-step liquid-liquid extraction with diethyl ether and electron-capture detection gas chromatography on a fused-silica capillary column coated with 50% methylsilicone-50% trifluoropropylsilicone. The detection limit of both compounds was 50 ng/ml in plasma (from an initial volume of 0.1 ml), 100 ng/ml in urine and 100 ng/g in feces. Optimal conditions for both chemical and enzymatic hydrolysis were defined to measure conjugates of both pentachlorophenol and tetrachlorohydroquinone in urine. Tetrachlorohydroquinone was found to be unstable in plasma and urine; means to prevent its degradation during sample collection and storage by addition of ascorbic acid and ethylenediaminetetracetic acid are presented. This chromatographic method was shown to be precise, accurate and specific. It was successfully applied to toxicokinetic studies in rat.

The relation between the oxidative biotransformation of hexachlorobenzene and its porphyrinogenic activity

The relation between the major toxic effect of hexachlorobenzene, hepatic porphyria, and its oxidative biotransformation was studied in vivo, by observing the effect of modulating its biotransformation on the expression of porphyria. This modulation was achieved by selective in vivo inhibition of the major cytochrome P450 isoenzyme involved in both the hydroxylation of hexachlorobenzene and its primary oxidative metabolite, pentachlorophenol. The involvement of this isoenzyme, cytochrome P450p, was established by in vitro biotransformation studies using microsomes derived from rats treated with various inducers of cytochrome P450 isoenzymes and selective in vitro inactivation of cytochrome P450p by triacetyloleandomycin (TAO), resulting in a strong inhibition of the microsomal conversion of hexachlorobenzene and pentachlorophenol. In vivo inactivation of cytochrome P450p was achieved by coadministration of hexachlorobenzene and TAO. Female rats which were treated with this diet for 10 weeks showed a strongly diminished urinary excretion of the major oxidative metabolites, pentachlorophenol and tetrachloro-1,4-hydroquinone, as compared to rats treated with hexachlorobenzene alone. The TAO coadministration was found to result in complexation of 70% of the total amount of hepatic microsomal cytochrome P450. The group treated with hexachlorobenzene alone displayed a 600-fold increase in the amount of hepatic porphyrins, whereas an almost complete absence of hepatic porphyrins was observed after administration of hexachlorobenzene together with TAO. The urinary excretion of porphyrins was also significantly lowered by cotreatment with TAO. A strong correlation was found to exist between the amount of porphyrins excreted and the amount of oxidative metabolites excreted, as a function of exposure time. Glucuronidation of pentachlorophenol was observed to an average extent of 30%. This percentage was not influenced by either TAO or phenobarbital. These results suggest that oxidative biotransformation, and thus the formation of the very reactive tetrachloro-1,4-benzoquinone, is directly related to the porphyrinogenic action of hexachlorobenzene.

Studies on the active site of rat glutathione S-transferase isoenzyme 4-4. Chemical modification by tetrachloro-1,4-benzoquinone and its glutathione conjugate

The active site of glutathione S-transferase isoenzyme 4-4, purified from rat liver, was studied by chemical modification. Tetrachloro-1,4-benzoquinone, a compound previously shown to inactivate glutathione S-transferases very efficiently by covalent binding in or close to the active site, completely prevented the alkylation of the enzyme by iodoacetamide, indicating that the reaction had taken place with cysteine residues. Both from radioactive labeling and spectral quantification experiments, evidence was obtained for the covalent binding of three benzoquinone molecules per subunit, i.e. equivalent to the number of cysteine residues present. This threefold binding was achieved with a fourfold molar excess of the benzoquinone, illustrating the high reactivity of this compound. Comparison of the number of amino acid residues modified by tetrachloro-1,4-benzoquinone with the decrease of catalytic activity revealed an almost complete inhibition after modification of one cysteine residue. Chemical modification studies with diethylpyrocarbonate indicated that all four histidine residues of the subunit are ethoxyformylated in an at least partially sequential manner. Modification of the second histidine residue resulted in complete loss of catalytic activity. Preincubation of the transferase with the glutathione conjugate of tetrachloro-1,4-benzoquinone resulted in 78% protection against this modification. However, glutathione itself hardly protected against the reaction with diethylpyrocarbonate. The intrinsic fluorescence properties of the enzyme were affected by covalent binding of tetrachloro-1,4-benzoquinone. The concentration dependency of the fluorescence quenching is strongly correlated with the inactivation of the enzyme, indicating that covalent binding of the benzoquinone occurs in the vicinity of at least one tryptophan residue. Finally, the binding of bilirubin, as measured by means of circular dichroism, was inhibited by preincubation of the enzyme with tetrachloro-1,4-benzoquinone in a manner which strongly correlated with the loss of enzymatic activity, the protection against inactivation by diethylpyrocarbonate, and the fluorescence quenching. All processes showed a 70-80% decrease after incubation of the enzyme with an equimolar amount of the benzoquinone. Thus, evidence is presented for the presence of a cysteine, a histidine and a tryptophan residue in, or in the vicinity of, the active site of the glutathione S-transferase 4 subunit.

Possible reactive intermediates in the oxidative biotransformation of hexachlorobenzene

In this review the biotransformation of hexachlorobenzene is discussed, with special reference to the possible generation of reactive metabolites or intermediates during this process. Evidence is presented for the direct involvement of certain cytochrome P-450 isoenzymes in the major toxic effect of hexachlorobenzene, hepatic porphyria. The in vivo biotransformation is discussed and compared with in vitro experiments (microsomal and cell culture studies). The possible reactive metabolites and intermediates and their mechanisms of formation are presented. Special attention is directed to a very reactive metabolite, tetrachloro-1,4-benzoquinone, which has a high capacity to efficiently react with protein, thus possibly linking the oxidative biotransformation of hexachlorobenzene and the molecular mechanism of enzyme inactivation leading to hepatic porphyria.