Enzymatic activation of chemicals to toxic metabolites

Investigation of species differences in isobutene (2-methylpropene) metabolism between mice and rats

Metabolism of isobutene (2-methylpropene) in rats (Sprague Dawley) and mice (B6C3F1) follows kinetics according to Michaelis-Menten. The maximal metabolic elimination rates are 340 mumol/kg/h for rats and 560 mumol/kg/h for mice. The atmospheric concentration at which Vmax/2 is reached is 1200 ppm for rats and 1800 ppm for mice. At steady state, below atmospheric concentrations of about 500 ppm the rate of metabolism of isobutene is direct proportional to its concentration. 1,1-Dimethyloxirane is formed as a primary reactive intermediate during metabolism of isobutene in rats and can be detected in the exhaled air of the animals. Under conditions of saturation of isobutene metabolism the concentration of 1,1-dimethyloxirane in the atmosphere of a closed exposure system is only about 1/15 of that observed for ethene oxide and about 1/100 of that observed for 1,2-epoxy-3-butene as intermediates in the metabolism of ethene or 1,3-butadiene.

Bioactivation of xenobiotics by formation of toxic glutathione conjugates

Evidence has been accumulating that several classes of compounds are converted by glutathione conjugate formation to toxic metabolites. The aim of this review is to summarize the current knowledge on the biosynthesis and toxicity of glutathione S-conjugates derived from halogenated alkanes, halogenated alkenes, and hydroquinones and quinones. Different types of toxic glutathione conjugates have been identified and will be discussed in detail: (i) conjugates which are transformed to electrophilic sulfur mustards, (ii) conjugates which are converted to toxic metabolites in an enzyme-catalyzed multistep mechanism, (iii) conjugates which serve as a transport form for toxic quinones and (iv) reversible glutathione conjugate formation and release of the toxic agent in cell types with lower glutathione concentrations. The kidney is the main, with some compounds the exclusive, target organ for compounds metabolized by pathways (i) to (iii). Selective toxicity to the kidney is easily explained due to the capability of the kidney to accumulate intermediates formed by processing of S-conjugates and to bioactivate these intermediates to toxic metabolites. The influences of other factors participating in the renal susceptibility are discussed.

Interactions between N-acetyl-p-benzoquinone imine and fluorescent calcium probes: implications for mechanistic toxicology

Intracellular free calcium ([Ca2+]i) homeostasis has been implicated as an early target in both cellular necrosis and apoptosis. In this study, we have used peripheral blood mononuclear cells (PBMC) as target cells to investigate the effects of several reactive metabolites associated with drug toxicity on [Ca2+]i in order to delineate further early events in cytotoxicity. Compounds implicated in both drug-induced necrosis (N-acetyl-p-benzoquinone imine; NAPQI) and drug hypersensitivity (sulfamethoxazole hydroxylamine; SMX-HA) were examined and their effects on [Ca2+]i compared with those of the T cell mitogen phytohemagglutinin (PHA; 1.5 micrograms/ml) and the calcium ionophore ionomycin (2.5 microM). PHA and ionomycin produced characteristic elevations in [Ca2+]i as monitored by an increase in the fluorescence of fluo-3-loaded cells. SMX-HA did not significantly affect [Ca2+]i at concentrations previously shown to be cytotoxic to PBMC (100 and 500 microM), suggesting that Ca2+ homeostasis is not an early target for SMX-HA toxicity. Addition of NAPQI (250 microM) to fluo-3-loaded cells produced a marked decrease in fluorescence which was not reversed by ionomycin. Conversely, addition of NAPQI to cells loaded with indo-1 resulted in a rapid increase in fluorescence. This effect, however, was found to be attributable to NAPQI addition per se rather than to an increase in [Ca2+]i. HPLC and fluorescence analysis of samples generated from the decomposition of NAPQI revealed the presence of several products which fluoresced intensely at the excitation/emission wavelength pairs of a number of fluorescent probes commonly used to monitor [Ca2+]i.(ABSTRACT TRUNCATED AT 250 WORDS)

Covalent binding of o,p'-DDD in rabbit lung and isolated rabbit lung cells

The irreversible binding of o,p'-DDD was examined in isolated lung cells, in lung microsomes and in vivo in male New Zealand White rabbits. Non-ciliated bronchiolar (Clara) cells had the highest capacity to bind o,p'-DDD, followed by alveolar type II cells. A fraction of mixed unidentified lung cells was also able to bind o,p'-DDD while no binding was observed in alveolar macrophages. The activation of o,p'-DDD was shown to be mediated by cytochrome P-450 in both lung microsomes and isolated lung cells. In vivo, the binding was preferentially localized in the lung alveolar and bronchiolar regions. The binding of o,p'-DDD observed in vivo may thus be caused by the capacity of several cell types to activate o,p'-DDD.

Interspecies comparisons in toxicology: the utility and futility of plasma concentrations of the test substance

A classical dilemma in toxicology is how the dose administered relates to the dose delivered to the target site. Plasma concentrations of the test substance may be misleading since the concentration of any given substance in the plasma may not be representative of its concentration in tissues. Furthermore, a given tissue concentration of a xenobiotic can evoke responses which are highly species-dependent. While evaluating toxicity data within one species, plasma concentrations reflect the effects of route of administration, bioavailability, dose level, multiple dosing, age, gender, etc. However, when toxicity data is compared across species, the relevance of plasma concentrations depends on the nature of the toxicity. Reversible, pharmacodynamic effects often correlate with plasma concentrations, although there may be marked interspecies differences in dose-response relationships. Irreversible effects, if pharmacodynamic in origin, often correlate better with the intensity/duration of the pharmacodynamic response, rather than with plasma concentration. On the other hand, irreversible effects, if chemically mediated, may not correlate at all with plasma concentration, the lesions being caused by reactive metabolites of fleeting existence, which rarely survive long enough to leave their site of synthesis. They cannot be measured in the plasma nor predicted from plasma concentrations of the parent xenobiotic. The limitations of plasma concentrations in interpreting the toxicology of substances which are tissue-sequestered, which are subject to pharmacogenetic factors, or which show plasma concentrations that are not proportional to dose are also discussed. Mention is made of possible alternatives to plasma concentrations in assessing exposure in toxicology studies.

Bioactivation of dapsone to a cytotoxic metabolite: in vitro use of a novel two compartment system which contains human tissues

1. A two compartment system, comprising two adjacent teflon chambers separated by a semi-permeable membrane, has been devised with which to investigate the generation of drug metabolites that are toxic to human cells in vitro. 2. Compartment A contained a drug-metabolising system (human liver microsomes +/- NADPH) and compartment B contained target cells (human mononuclear leucocytes). The semi-permeable membrane retained protein (m.w. greater than 10,000) but allowed equilibration (within 1 h) of drug and drug metabolites, during which time cells remained viable. 3. Incubation of dapsone (100 microM) with human microsomal protein (2 mg ml-1) and NADPH (1 mM) in compartment A caused cell death (8.7 +/- 1.8%) in compartment B, which was reduced significantly (P less than 0.05) by the addition of glutathione (500 microM). Dapsone in the absence of NADPH was not cytotoxic. 4. Chemical analysis showed the presence of dapsone hydroxylamine as the only stable metabolite in both compartment A (5.2 +/- 0.4% incubated drug) and compartment B (3.5 +/- 0.5%). 5. Irreversible binding of dapsone to cells was significantly (P less than 0.05) reduced by omission of NADPH (85 +/- 13 pmol/10(6) cells) or addition of glutathione (103 +/- 9) compared with control values (153 +/- 51).

Early changes in hepatic redox homeostasis following treatment with a single dose of valproic acid

Changes in reduced glutathione (GSH) and pyridine nucleotide phosphate levels as well as in the activities of the glutathione peroxidase-reductase system and glucose-6-phosphate dehydrogenase have been studied in rats after a single i.p. administration of various doses of valproic acid (VPA). GSH level decreased in a dose-dependent relation. At the end of 180 min GSH levels either returned to control limits (lower doses) or showed a tendency to normalize (higher doses). GSH loss was paralleled by the reduction in glutathione reductase activity. A significant NADPH reduction was also seen after animal exposure to high VPA doses. At the end of 180 min a maximal NADPH decrease was reached. The activities of both glutathione peroxidase and glucose-6-phosphate dehydrogenase were suppressed irrespective of whether animals were given low or high VPA doses.

Altered benzo[a]pyrene metabolism in C3H/10T1/2 cells transformed by aflatoxin B1 or 3-methylcholanthrene

C3H/10T1/2 clone 8 (10T1/2) cells possess Phase I and Phase II xenobiotic metabolizing enzymes associated with the metabolism of polycyclic aromatic hydrocarbons to activated or detoxified species. We compared the metabolism of benzo[a]pyrene (BaP) by these cells to an aflatoxin B1 (AFB1)-transformed line (7SA) and a 3-methylcholanthrene (3-MC)-transformed line (MCA) isolated from carcinogen-treated 10T1/2 cells. Relative to 10T1/2 cells, basal levels of cytochrome P450-mediated aryl hydrocarbon hydroxylase (AHH) were significantly depressed in 7SA cells by about 30%. The inducibility of AHH by BaP treatment was depressed by 30-70% in MCA and 7SA cells over a 36-hr time course. 10T1/2 and MCA cells accumulated similar intracellular amounts of 3-OH-BaP by 12 and 24 hr, respectively; in contrast the accumulation of 3-OH-BaP in 7SA cells was 70% lower. During 36 hr of BaP treatment, total BaP-DNA adduct levels formed in 7SA and MCA cells, determined by 32P-postlabeling analysis, were 90 and 83% lower, respectively, than those found in 10T1/2 cells. These differences in response to BaP treatment were not related to cellular differences in the uptake or efflux of BaP. Relative to 10T1/2 or MCA cells, 7SA cells were found to have at least a twofold increase in UDP-glucuronyltransferase activity, which correlated with the lower intracellular accumulation of 3-OH-BaP and enhanced formation of extracellular polar metabolites. MCA cells had an almost twofold increase in glutathione S-transferase activity relative to parental 10T1/2 cells but produced lower levels of extracellular polar metabolites. These results demonstrate an association between chemical transformation of 10T1/2 cells and altered xenobiotic metabolism. This system may provide an in vitro model for examining the molecular events responsible for the biochemically altered phenotype of the malignantly transformed cell.

Metronidazole and antipyrine as probes for the study of foreign compound metabolism

The aim of the present work was to develop a tool for the study of the enzyme activities relevant for the biotransformation of foreign compounds, their elimination and/or activation to toxic substances. The activity of an enzyme may be assessed by the rate of metabolism of a preferably specific probe or model compound. The cytochrome P450'ies, the key enzymes for the elimination and/or activation of most foreign compounds, exist in multiple forms with variable substrate specificity and regulation. Some cytochrome P450'ies are under genetic control, whereas the activity of others is mainly regulated by the influence from factors in the environment. Only some of the cytochrome P450'ies are relevant for the formation of harmful metabolites. Thus, the activity of as many cytochrome P450 forms as possible should be assessable, preferably simultaneously. The present work evaluated metronidazole in a cocktail with antipyrine as a tool for the study of the regulation of foreign compound metabolism in the liver. The cytochrome P450 catalyzed metabolism of metronidazole and antipyrine was studied in humans and in isolated rat hepatocytes. In humans the influence of dose, route of administration, enzyme induction and inhibition and liver disease was investigated. Rats of either sex were studied with and without pretreatment with specific enzyme inducers and incubations included specific enzyme inhibitors. Evidence was provided that the oxidative formation of the five major metabolites, two from metronidazole and three from antipyrine, depends on different cytochrome P450'ies. In humans it was demonstrated that the clearance of metronidazole and antipyrine could be determined from the same saliva sample collected 16-24 hours after their oral administration and so could the clearance for formation of each metabolite if urine was collected for 48 hours. Thus, with the cocktail of metronidazole and antipyrine and simple non-invasive sampling the activity of five different cytochrome P450'ies can be assessed in vivo. In addition, metronidazole may also be used for assessment of the glucuronidation capacity although this is a minor pathway in man. Because the variation within subjects is much less than between them, the cocktail test is particularly suited for paired designs with measurements before and after an environmental change and the subjects serving as their own control. The metronidazole/antipyrine cocktail may have many applications in the study of the regulation of foreign compound metabolism in man and in animals, in vivo and in vitro.

Renal cysteine conjugate beta-lyase-mediated toxicity studied with primary cultures of human proximal tubular cells

The beta-lyase pathway has been shown to mediate the nephrotoxicity of S-cysteine conjugates of a variety of haloalkenes in a number of animal models in vitro and in vivo. However, there is no information available concerning this mechanism of bioactivation in human tissues. In this investigation a well-characterized model of human proximal tubule epithelial cells, the presumed target cell, was used to investigate the toxicity of a series of glutathione and cysteine conjugates of nephrotoxic haloalkenes. Both S-(1,2-dichlorovinyl)-glutathione (DCVG) and S-(1,2-dichlorovinyl)-L-cysteine (DCVC) caused dose-dependent toxicity over a range of 25 to 500 microM. DCVC was consistently found to be more toxic than DCVG, but the inclusion of gamma-glutamyltransferase (0.5 U/ml) increased the toxicity of DCVG to that observed with an equimolar concentration of DCVC, indicating that metabolism to the cysteine conjugate is an important rate-limiting step in this in vitro model. S-(1,2,3,4,4-Pentachlorobutadienyl)-L-cysteine, S-(2-chloro-1,1,2-trifluoroethyl)-L-cysteine, and S-(1,1,2,2-tetrafluoroethyl)-L-cysteine were also found to be toxic to human proximal tubular cells. Incubation with [35S]DCVC resulted in covalent binding of 35S-label, which increased linearly to a final level of 1.05 nmol/mg protein at 6 hr. Aminooxyacetic acid (250 microM), an inhibitor of pyridoxal phosphate-dependent enzymes such as beta-lyase, protected the cells from the toxicity of all of the cysteine conjugates and inhibited the covalent binding of 35S-label from [35S]DCVC to cellular macromolecules. The results of the present study provide the first evidence that human proximal tubular cells are sensitive to the toxicity of glutathione and/or cysteine conjugates of a variety of chloro- and fluoroalkenes which are activated via the beta-lyase pathway. The implications for human health are discussed.

Presence of functional cytochrome P-450 on isolated rat hepatocyte plasma membrane

Antibodies against cytochrome P-450 are found in some children with autoimmune hepatitis (antiliver/kidney microsome 1) and in patients with ticrynafen hepatitis (antiliver/kidney microsome 2). For an immune reaction against cytochrome P-450 to possibly destroy the hepatocytes, one must assume that cytochrome P-450 is present on the plasma membrane surface of hepatocytes. In a first series of experiments, plasma membranes were prepared with a technique based on the electrostatic attachment of isolated hepatocytes to polyethyleneimine-coated beads. After vortexing, beads were coated with a very pure plasma membrane fraction. Microsomal contamination, judged from the specific activities of glucose-6-phosphatase or NADH-cytochrome c reductase, was less than 1%. Nevertheless, the specific content (per milligram of protein) of CO-binding cytochrome P-450 was 20% of that in microsomes; the specific benzo(a)pyrene hydroxylase activity was 25%, and ethoxycoumarin deethylase 11%. Immunoblots showed the presence of cytochromes P-450 UT-A, UT-H, PB-B, ISF-G and PCN-E, the last three isoenzymes being inducible by, respectively, phenobarbital, 3-methylcholanthrene and dexamethasone. In a second series of experiments, nonpermeabilized isolated hepatocytes from untreated rats were incubated with anticytochrome P-450 antibodies. Immunofluorescence and immunoperoxidase staining confirmed the presence of cytochromes P-450 UT-A, PB-B and ISF-G on the membrane. In a last series of experiments, human antiliver-kidney microsomal 1 antibodies were found to react specifically with rat liver plasma membrane cytochrome P-450 UT-H (IID subfamily). We conclude that several cytochrome P-450 isoenzymes are present, active and inducible on the plasma membrane surface of hepatocytes. It is therefore conceivable that immunization against plasma membrane cytochrome P-450 might lead to the immunological destruction of hepatocytes in some patients.

Enzymatic oxidation of xenobiotic chemicals

Studies with biomimetic models can yield considerable insight into mechanisms of enzymatic catalysis. The discussion above indicates how such information has been important in the cases of flavoproteins, hemoproteins, and, to a lesser extent, the copper protein dopamine beta-hydroxylase. Some of the moieties that we generally accept as intermediates (i.e., high-valent iron oxygen complex in cytochrome P-450 reactions) would be extremely hard to characterize were it not for biomimetic models and more stable analogs such as peroxidase Compound I complexes. Although biomimetic models can be useful, we do need to keep them in perspective. It is possible to alter ligands and aspects of the environment in a way that may not reflect the active site of the protein. Eventually, the model work needs to be carried back to the proteins. We have seen that diagnostic substrates can be of considerable use in understanding enzymes and examples of elucidation of mechanisms through the use of rearrangements, mechanism-based inactivation, isotope labeling, kinetic isotope effects, and free energy relationships have been given. The point should be made that a myriad of approaches need to be applied to the study of each enzyme, for there is potential for misleading information if total reliance is placed on a single approach. The point also needs to be made that in the future we need information concerning the structures of the active sites of enzymes in order to fully understand them. Of the enzymes considered here, only a bacterial form of cytochrome P-450 (P-450cam) has been crystallized. The challenge to determine the three-dimensional structures of these enzymes, particularly the intrinsic membrane proteins, is formidable, yet our further understanding of the mechanisms of enzyme catalysis will remain elusive as long as we have to speak of putative specific residues, domains, and distances in anecdotal terms. The point should be made that there is actually some commonality among many of the catalytic mechanisms of oxidation, even among proteins with different structures and prosthetic groups. Thus, we see that cytochrome P-450 has some elements of a peroxidase and vice versa; indeed, the chemistry at the prosthetic group is probably very similar and the overall chemistry seems to be induced by the protein structure. The copper protein dopamine beta-hydroxylase appears to proceed with chemistry similar to that of the hemoprotein cytochrome P-450 and, although not so thoroughly studied, the non-heme iron protein P. oleovarans omega-hydroxylase.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of thermal acclimation on the activity of arylhydrocarbon hydroxylase in rainbow trout (Oncorhynchus mykiss)

1. The possibility that temperature acclimation (to 10 or 18 degrees C for 28 days) would alter the cytochromes P-450 of rainbow trout was addressed. 2. The specific content of LM4b (P-450 IA1), the trout isozyme responsible for activation of polynuclear aromatic hydrocarbons, was lower in 18 degrees C fish than it was in 10 degrees C fish. 3. Kinetic analysis of aryl hydrocarbon hydroxylase indicated that, while thermal acclimation caused no change in Vmax, it lowered the apparent Km of this enzyme for benzo[a]pyrene when assayed at acutely shifted temperatures. 4. Thermal acclimation of fish may have significance when feral populations are subjected to acute temperature shifts.

Localization, distribution, and induction of xenobiotic-metabolizing enzymes and aryl hydrocarbon hydroxylase activity within lung

The metabolism of xenobiotics within lung often leads to toxicity, although certain pulmonary cells are more readily damaged than others. This differential susceptibility can result from cell-specific differences in xenobiotic activation and detoxication. The localization and distribution of xenobiotic-metabolizing enzymes (cytochromes P-450, NADPH-cytochrome P-450 reductase, epoxide hydrolase, glutathione S-transferases, UDP-glucuronosyltransferases, and a sulfotransferase) and of aryl hydrocarbon (benzo[a]pyrene) hydroxylase activity determined immunohistochemically and histochemically, respectively, within lung are discussed. Findings reveal that xenobiotics can be metabolized in situ, albeit to different extents, by bronchial epithelial cells, Clara and ciliated bronchiolar epithelial cells, and type II pneumocytes and other alveolar wall cells and that enzymes and activities are not necessarily induced uniformly among these cells.

Structural requirements for bioactivation of anticonvulsants to cytotoxic metabolites in vitro

The formation of cytotoxic metabolites from the anticonvulsants phenytoin and carbamazepine was investigated in vitro using a hepatic microsomal enzyme system and human mononuclear leucocytes as target cells. Both drugs were metabolised to cytotoxic products. In order to assess the structural requirements for this bioactivation, a series of structurally related compounds was investigated. It was found that molecules which contain either an amide function or an aryl ring may undergo activation in vitro, but only the metabolism-dependent toxicity of the latter is potentiated by pre-treatment of the target cells with an epoxide hydrolase inhibitor. Taken collectively, these data are consistent with the concept that reactive epoxide metabolites of both phenytoin and carbamazepine may produce toxicity in individuals with an inherited deficiency in epoxide hydrolase.

Cytochrome P450 isozyme distribution in normal and tumor-bearing hepatic tissue from rainbow trout (Salmo gairdneri)

An immunohistochemical technique was used to localize the major constitutive cytochrome P450 isozyme, P450 LM2, and the major beta-naphthoflavone-inducible isozyme, P450 LM4b, in the livers of untreated and aflatoxin B1 (AFB1)-initiated, tumor-bearing rainbow trout. In hepatic tissue sections from untreated trout, no regular anatomical pattern within the hepatic parenchymal cells could be discerned for either isozyme. Immunostaining was observed for P450 LM2 along the sinusoidal border of some of the parenchymal cells, there was moderate staining within the cytoplasm of most cells, and there were focal areas of increased staining. There was intense, uniform immunostaining for P450 LM2 within the cytoplasm of the bile duct cells, in the endothelial lining of arterioles, and along the epithelial surface of the gall bladder. Staining for P450 LM4b in livers from untreated trout was barely detectable. In liver tissue sections from AFB1-treated tumor-bearing fish, P450 LM2 appeared to be reduced and P450 LM4b was absent in the hepatocellular carcinoma nodules. An apparent increase in immunostaining for P450 LM4b was observed in nonneoplastic cells juxtaposed next to neoplastic cells as well as in areas distant to the tumors. These results may indicate that the pattern of P450 isozymes is altered in nonneoplastic cells of tumor-bearing trout livers.

A stereochemical investigation of the cytotoxicity of mianserin metabolites in vitro

1. The metabolism of the enantiomers of mianserin to stable, chemically reactive and cytotoxic metabolites by human liver microsomes has been investigated in vitro. 2. Both enantiomers were metabolised to three major oxidation products: 8-hydroxymianserin, desmethylmianserin and mianserin 2-oxide. Hydroxylation occurred more readily with the S-enantiomer, whereas desmethylmianserin was always the major metabolite of the R-enantiomer. 3. The generation of chemically reactive metabolites exhibited a marginal degree of stereoselectivity, as assessed by irreversible binding of drug to microsomal protein (S greater than or equal to R; P less than or equal to 0.05). 4. The formation of metabolites which were cytotoxic towards human mononuclear leucocytes was greater (P less than or equal to 0.001] for R(-)-mianserin than for S(+)-mianserin and showed a significant correlation with N-demethylation (r = 0.84, P less than or equal to 0.01).

Activation of 8-methoxypsoralen by cytochrome P-450. Enzyme kinetics of covalent binding and influence of inhibitors and inducers of drug metabolism

The kinetics of covalent binding of reactive metabolites of 8-methoxypsoralen (8-MOP) to protein were measured in incubations of liver microsomes of rats pretreated for 3 days with i.p. injections of 80 mg/kg/day of beta-naphthoflavone (BNF), phenobarbital (PB), 8-MOP, or vehicle. Covalent binding of radioactivity derived from [14C]8-MOP (labeled at the metabolically stable 4-position in the coumarin ring) required NADPH, obeyed classical Michaelis-Menten kinetics, and was inducible by both PB and BNF. Plots of V versus V/[S] were linear in liver microsomes of rats pretreated with vehicle, PB, or 8-MOP; respective values for Km were 26, 24 and 13 microM and for Vmax were 0.61, 1.70 and 0.50 nmol bound/min/mg protein. In microsomes of rats pretreated with BNF, high- and low-affinity components of covalent binding were observed with respective values for Km of 4.7 and 117 microM and for Vmax of 0.77 and 1.71 nmol bound/min/mg protein. Addition of glutathione and cysteine to the incubations decreased covalent binding by 33 and 67%, respectively, presumably by trapping reactive electrophilic metabolites. Inhibition of epoxide hydrolase with 1,1,1-trichloropropene-2,3-oxide did not affect covalent binding of reactive metabolites of 8-MOP. SKF-525A was a potent inhibitor of both the metabolism of 8-MOP and covalent binding in microsomes from rats pretreated with PB, but had only a slight effect in microsomes from rats pretreated with BNF. In contrast, alpha-naphthoflavone almost completely inhibited metabolism of 8-MOP and covalent binding in BNF-induced microsomes but had no effect in PB-induced microsomes. Apparent covalent binding was reduced by 39% in incubations with 8-MOP labeled with tritium in the metabolically labile methoxy group. Collectively, these results indicate that 8-MOP is biotransformed by two or more isozymes of cytochrome P-450 to reactive electrophiles capable of binding to tissue macromolecules.

Presence of covalently bound metabolites on rat hepatocyte plasma membrane proteins after administration of isaxonine, a drug leading to immunoallergic hepatitis in man

Isaxonine and several other drugs transformed by cytochrome P-450 into reactive metabolites apparently lead to immunoallergic hepatitis in man. Protein epitopes modified by the covalent binding of the metabolites have been proposed as possible targets for the immune response. The purpose of this work was to determine whether covalently bound metabolites are indeed present on hepatocyte plasma membrane proteins. In a first series of experiments, rats were killed 15 or 60 min after administration of [2-14C]isaxonine (0.2 mmol.kg-1 i.p.), and various fractions were prepared from isolated hepatocytes; microsomal contamination of the plasma membrane fraction was 1.2% or less. At 60 min, the amount of isaxonine metabolite covalently bound per mg of protein was similar in plasma membranes (0.42 nmole metabolite.mg protein-1) and in microsomes (0.38); both values were decreased by about 70% in rats pretreated with piperonyl butoxide, an inhibitor of cytochrome P-450. At 15 min, however, covalent binding to plasma membrane proteins (0.06 nmole metabolite.mg protein-1) was only half of that to microsomal proteins (0.12). In a second series of experiments, [2-14C] isaxonine (0.1 mM) was incubated with NADPH, hepatic microsomes and plasma membranes. The reactive isaxonine metabolite became bound extensively to microsomal proteins, but not to plasma membrane proteins. These results show that administration of isaxonine leads to the presence of isaxonine adducts on the proteins of the hepatocyte plasma membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of enzyme induction on metabolite formation of bis(2-methoxyethyl) ether in the rat

The effect of enzyme induction on the metabolism of the reproductive toxicant bis (2-methoxyethyl) ether (diglyme) was studied in male Sprague-Dawley rats. Rats were given either daily doses of diglyme at 5.1 mmol/kg body wt. by gavage or 0.1% (w/v) phenobarbital (PB) in the drinking water for 22 consecutive days. In one study, a significant reduction in the hexobarbital sleeping time was determined for rats pretreated with diglyme or PB in comparison with that determined for naive rats. In a second study, naive and pretreated rats given single oral doses of 14C-diglyme at 5.1 mmol/kg body wt. showed similar urinary 14C excretion patterns. Urinary metabolites were separated and quantified by hplc to evaluate the influence of pretreatment with either diglyme or PB on the 14C-diglyme urinary metabolite profile. The amount of (2-methoxyethoxy) acetic acid, the principal metabolite, was similar for rats given no pretreatment and for rats pretreated with either diglyme or PB. However, both pretreatments resulted in significant increases in the formation of methoxyacetic acid, a recognized reproductive toxicant.

Identification of ethanol-inducible P450 isozyme 3a (P450IIE1) as a benzene and phenol hydroxylase

In this report, the identity of the cytochrome P450 isozyme(s) catalyzing the hydroxylation of benzene and the major hydroxylated metabolite of benzene, phenol, was investigated using rabbit hepatic microsomes and six purified isozymes of hepatic P450. Microsomes from acetone-treated rabbits showed about a 5-fold induction of benzene hydroxylation to phenol and hydroquinone. This increase correlated with the increase in form 3a determined immunochemically (about 7-fold). Antibody to isozyme 3a inhibited greater than 90% of the benzene and phenol hydroxylase activity of hepatic microsomes from acetone-treated rabbits. At high benzene concentrations (2 mM) in the presence of cytochrome b5, form 3a was 1.3 times more active than form 2 and 7- to 10-fold more active than forms 3b, 3c, 4, and 6. At lower benzene concentrations (about 0.3 mM) form 3a was 5-fold more active than form 2. Furthermore, form 3a was the only isozyme to produce significant quantities of hydroquinone as did microsomes from acetone-treated rabbits. When phenol was used as the substrate, hydroquinone was the only product detected, and acetone treatment induced its formation 4- to 5-fold. Purified form 3a was 20- to 30-fold more active than the next most active isozyme, form 6, depending on the presence or absence of cytochrome b5. These results suggest that isozyme 3a (P450IIE1) is a low-Km benzene hydroxylase and the principal phenol hydroxylase in rabbit hepatic microsomes. As a result, the induction of isozyme 3a could potentiate the toxicity of benzene by catalyzing an increase in the formation of both phenol and hydroquinone.

Genetically determined oxidation polymorphism and drug hepatotoxicity. Study of 51 patients

The influence of genetically determined oxidation polymorphism on drug hepatotoxicity has been poorly investigated and results are controversial. We studied drug oxidation capacity in 51 patients with hepatitis caused mainly by drugs undergoing oxidative metabolism, using dextromethorphan, a test compound recently proposed as a substitute for debrisoquine. Phenotyping was performed using the metabolic ratio (MR) calculated as MR = 0-10 h urinary output of dextromethorphan/0-10 h urinary output of dextrorphan (the main oxidative metabolite), after oral administration of 40 mg dextromethorphan hydrobromide. Dextromethorphan oxidation capacity was similar in patients and in 103 control subjects as judged by: (a) the prevalence of each phenotype (5.9% versus 3.9% for the poor metabolizer phenotype and 94.1% versus 96.1% for the extensive metabolizer phenotype; (b) the frequency distribution histograms of log metabolic ratio; (c) the mean values of dextromethorphan and dextrorphan urinary outputs and of log metabolic ratio for each phenotype. These results show that hepatotoxicity of several drugs, including amineptine, amodiaquine and Plethoryl, is related neither to an impairment in dextromethorphan oxidation capacity nor to an unusually high capacity to oxidize this drug.

Detection of reactive metabolites in vitro

The ability of 27 compounds to mediate depletion of glutathione (GSH) in a fortified liver microsome incubation via production of reactive metabolites generated by microsomal mono-oxygenase (MMO) metabolism has been studied. The majority of compounds tested of this type were positive in this assay, with the exception of iproniazid and naphthalene. Thioacetamide, the reactive metabolite of which binds via lysine residues, was negative. Acrolein and hexachlorobutadiene produced depletion in the absence of prior activation, as did menadione but for which enzyme induction increased the magnitude of depletion. Allyl alcohol produced MMO-mediated depletion of GSH. Some depletion of GSH occurred in the presence of common substrates of the MMO system. It is suggested that this depletion assay may be a valid test for detection of reactive metabolites generated by MMO metabolism.

Glutathione depletion and cytotoxicity by naphthalene 1,2-oxide in isolated hepatocytes

The ability of naphthalene 1,2-oxide to diffuse across intact cellular membranes, the subsequent biotransformation of this epoxide and its potential to produce losses in cellular viability have been examined in incubations of isolated hepatocytes. Addition of 1R,2S- or 1S,2R-naphthalene oxide enantiomers (15, 30 and 60 microM) to isolated hepatocytes resulted in a rapid depletion of intracellular glutathione. Depletion of glutathione was concentration dependent and maximal at 5-15 min. Addition of either of the enantiomeric oxides at 60 microM resulted in the loss of more than 20 nmol glutathione/10(6) cells (1 ml cells); thus more than a third of the added epoxide was available for conjugation with intracellular glutathione. The time course and concentration dependence of glutathione depletion corresponded to the rapid, concentration-dependent formation of naphthalene oxide glutathione conjugates. The levels of glutathione adduct were highest 1 min after addition of naphthalene oxide and declined to 25% of this level after 30 min. Loss of glutathione conjugates from incubations correlated with the formation of N-acetylcysteine adducts. In contrast, the levels of glutathione adducts added exogenously to hepatocytes were relatively stable over a 120-min incubation suggesting that although further metabolism of naphthalene oxide glutathione adducts formed intracellularly is possible, extracellular glutathione adducts cannot penetrate the hepatocellular membrane. Small amounts of radiolabel from [3H]naphthalene 1,2-oxide were bound covalently to macromolecules in hepatocytes; the rate of this binding slowed rapidly after the first minute of incubation. Severe blebbing of the surface of the hepatocytes was noted in cells incubated for 30 min with 480 microM naphthalene oxide. Many of the cells were vacuolated at 60 min and progressed to frank necrosis with pyknotic nuclei and inability to exclude trypan blue. Cells incubated with 1-naphthol responded in a qualitatively similar fashion to those cells incubated with epoxide; however, hepatocytes incubated with 1-naphthol progressed to frank cellular necrosis at a slower rate. In hepatocytes partially depleted of glutathione by pretreatment with buthionine sulfoximine, addition of 1S,2R-naphthalene oxide at a rate of 1 nmol/min/10(6) cells resulted in significant losses in cell viability. In contrast, no losses in cell viability were observed with the enantiomer, 1R,2S-naphthalene oxide. Both epoxides produced similar losses in cellular glutathione levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolism, toxicity, and carcinogenicity of trichloroethylene

Lifetime cancer or unit risk estimates for TRI have been calculated by the EPA on the basis of metabolized dose-tumor incidence relationships. Previously, it was common practice to directly extrapolate exposure dose-tumor incidence data from laboratory animal studies to predict cancer risks in humans. Such direct species-to-species extrapolations, however, do not take into account potentially important species differences in systemic uptake, tissue distribution, metabolism, deposition at the site(s) of action, and elimination. The consideration and use of pharmacokinetic and metabolic data can significantly reduce, though not eliminate, uncertainties inherent in species-to-species, route-to-route, and high- to low-dose extrapolations. The total amount of TRI metabolized was considered in the most recent EPA Health Assessment Document for Trichloroethylene to be the effective dose (EFD) producing tumors. Exposure dose-metabolism relationships were determined from direct measurement data in inhalation and oral dosing studies in mice and rats. The magnitude of TRI metabolism in these two species closely approximated body surface area. Thus, it was assumed that the amount of TRI metabolized per square meter of surface area was equivalent among species when calculating human equivalent doses from the animal data. Direct measurement data from an inhalation study in humans were used to calculate the amount of TRI metabolized and the unit risk estimate when a person inhales 1 microgram TRI per cubic meter continuously for 24 h. The EPA Cancer Assessment Group (CAG) elected to use this risk estimate for TRI in air, since it was calculated on the basis of a human metabolized dose rather than unit risk estimates based on animal studies. The current survey of literature and ongoing research uncovered no new animal or human studies in which TRI metabolites were directly measured, which would be any more suitable for use in estimating the total metabolized dose of TRI. On the basis of information now available, it is appropriate to continue to use the total amount of TRI metabolized as the EFD producing tumors in the liver. Use of the total amount metabolized represents an important "step in the right direction" in reducing uncertainties in interspecies extrapolations of data on a chemical such as TRI. TRI is believed to be metabolically activated to a reactive intermediate(s), although the identity of the intermediate(s) is unclear. There is evidence that formation of reactive intermediate(s) and TRI hepatotoxicity are directly proportional to the overall extent of TRI metabolism.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of phenobarbital and 3-methylcholanthrene on the hepatic cytochrome P-450 metabolism of various alkoxyresorufin ethers in the cotton rat (Sigmodon hispidus)

1. Microsomes isolated from phenobarbital and 3-methylcholanthrene induced cotton rats (Sigmodon hispidus) were tested for o-dealkylase activity with methoxy-, ethoxy-, pentoxy- and benzoxyresorufin ethers. 2. The activity of 3-methylcholanthrene induced microsomes was greater than controls. 3. Activity of phenobarbital induced microsomes was not different from controls. 4. There was a distinct difference between male and female animals. 5. The results obtained from cotton rats are markedly different from results obtained from Sprague-Dawley (S-D) rats.

An in vitro study of the microsomal metabolism and cellular toxicity of phenytoin, sorbinil and mianserin

1. The cytotoxicity of metabolites generated from phenytoin, sorbinil and mianserin by human and mouse liver microsomes was assessed by co-incubation with human mononuclear leucocytes as target cells. Cytotoxicity was determined by trypan blue dye exclusion. 2. Phenytoin and sorbinil were metabolised by NADPH-dependent murine microsomal enzymes to cytotoxic metabolites. Cytotoxicity produced by both drugs was significantly enhanced by the epoxide hydrolase inhibitor trichloropropane oxide (TCPO). No significant cytotoxicity was observed in the presence of human liver microsomes. 3. Mianserin was metabolised by both human and mouse liver microsomes to a cytotoxin. Cytotoxicity was greater in the presence of human liver microsomes (13.7 +/- 2.2%; mean +/- s.d. for four livers, compared with 6.0 +/- 2.4%, mean +/- s.d., n = 4, with mouse liver microsomes), and was unaffected by pretreatment with TCPO. 4. Stable metabolites were quantified by radiometric high performance liquid chromatography. Phenytoin and sorbinil were metabolised to 5-(p-hydroxyphenyl)-5-phenyl-hydantoin (0.3-0.5% of incubated radioactivity) and 2-hydroxysorbinil (0.4-2.7% of incubated radioactivity), respectively, by both human and mouse liver microsomes. 5. Mianserin was metabolised to 8-hydroxymianserin and desmethylmianserin by both human and mouse liver microsomes. Desmethylmianserin was the major product in incubations with human liver microsomes (32.3 +/- 12%, mean +/- s.d. for four livers), whereas 8-hydroxymianserin was the predominant metabolite generated by mouse liver microsomes (25.9 +/- 1.5%, mean +/- s.d., n = 4). 6. Generation of electrophilic metabolites was assessed by determination of the amount of radiolabelled material which became irreversibly bound to protein. Only mouse liver microsomes activated phenytoin to a chemically reactive metabolite, whereas both mouse and human liver microsomes generated reactive metabolites from sorbinil and mianserin. 7. These studies show that drug cytotoxicity can be mediated by low concentrations (circa microM) of metabolites generated by NADPH-dependent hepatic microsomal enzymes; however demonstration of cytotoxicity in vitro has not been established as a means of predicting in vivo toxicity.

The Detection of Reactive Metabolites Generated by the Microsomal Mixed Function Oxidase System

It is recognised that the cytotoxicity of a number of xenobiotics is mediated through the production of reactive metabolites (1). Given this, and the current emphasis on the development of in vitro tests for assessment of xenobiotic cytotoxicity (2), it is of considerable interest to develop systems for the detection in vitro of cytotoxic reactive metabolites. Indeed, it would seem appropriate to employ a limited battery of tests for detection of xenobiotic-mediated cytotoxicity, some of which would be dedicated to assessment of the role of metabolism in such toxicity. The aim of this review is to identify and discuss the approaches currently being employed to this end, and to suggest some possible future trends.

Toxicity of methylating agents in isolated hepatocytes

To investigate the pathogenesis of hepatotoxicity by methylating agents, we exposed isolated hepatocytes to N-nitrosodimethylamine (NDMA), N-methyl-N'-nitro-N nitrosoguanidine (MNNG), N-methyl-N-nitrosourea (MNU), or methyl methanesulfonate (MMS). Although NDMA is a potent in vivo hepatotoxicant in rats, no evidence of hepatocyte injury, measured by the leakage of lactate dehydrogenase (LDH) activity into the medium, was observed following exposure to a 1-100 mM concentration of either NDMA or MNU. In contrast, exposure of hepatocytes to MMS or MNNG resulted in greater than or equal to 90% LDH release. These differences in toxicity were not related to the extent of covalent binding to hepatocytes. Following MMS or MNNG, but not MNU or NDMA exposure, a significant rise in the generation of thiobarbiturate (TBA)-reactive species was observed. When hepatocytes were exposed to the antioxidant promethazine prior to the addition of MMS or MNNG, the formation of TBA-reactive species was inhibited completely. Although promethazine blocked MNNG-mediated cell injury, the antioxidant had no effect on MMS intoxication. These data suggest that methylating agents can cause hepatotoxicity by more than a single mechanism. For MNNG, lipid peroxidation may be involved in the pathogenesis of acute hepatotoxicity.

Selective inactivation by 21-chlorinated steroids of rabbit liver and adrenal microsomal cytochromes P-450 involved in progesterone hydroxylation

The inactivation by 21-chlorinated steroids of rabbit liver cytochromes P-450 involved in the hydroxylation of progesterone has been investigated in intact microsomes encompassing two phenotypes of 21-hydroxylase activity, two phenotypes of 16 alpha-hydroxylase activity, and three phenotypes of 6 beta-hydroxylase activity. In liver microsomes from outbred New Zealand White male rabbits exhibiting a high content of cytochrome P-450 1, 21,21-dichloropregnenolone caused a time- and NADPH-dependent loss of 21-hydroxylase activity. This loss of activity exhibited a number of characteristics of mechanism-based inactivation, including irreversibility, saturation with increasing inhibitor concentrations, and protection by substrate, and was also documented with purified P-450 1 in a reconstituted system. 21,21-Dichloropregnenolone caused no time-dependent loss of 6 beta-hydroxylase activity in microsomes from the New Zealand White rabbits or from control or rifampicin-treated rabbits of the inbred B/J strain. In contrast, in the microsomes from the B/J rabbits, some inactivation of the 16 alpha-hydroxylase was observed (k = 0.04 min-1), regardless of the rifampicin treatment. The other two compounds tested, 21-chloropregnenolone and 21,21-dichloroprogesterone, were less effective than the dichloropregnenolone as inactivators of cytochrome P-450 1. On the other hand, 21,21-dichloroprogesterone, but not 21,21-dichloropregneolone, caused a rapid time-dependent loss of 21-hydroxylase activity in rabbit adrenal microsomes. The results indicate that the introduction of a dichloromethyl group into a substrate bearing a methyl group normally hydroxylated by only one or a few forms of cytochrome P-450 may be a rational means of designing selective inhibitors of the enzyme.

Allylamine-induced vascular toxicity in vitro: prevention by semicarbazide-sensitive amine oxidase inhibitors

The present studies were designed to evaluate the role that metabolic activation plays in allylamine (AAM)-induced vascular toxicity. The effects of AAM were evaluated in primary cultures of rat vascular endothelial (VEC) and smooth muscle cells (SMC). Semicarbazide (SC) and diethyldithiocarbamate (DDC) were used as inhibitors of semicarbazide-sensitive amine oxidase (SSAO). Clorgyline and pargyline were used as inhibitors of monoamine oxidase (MAO) A and B, respectively. The effect of catalase, a hydrogen peroxide scavenger, on AAM-induced cytotoxicity was also evaluated. Lactate dehydrogenase (LDH) release and morphological alterations were chosen as indicators of cytotoxicity. Confluent cultures of VEC and SMC were exposed to various concentrations of AAM (2-200 microM) in the absence and presence of serum for 4, 12, or 24 hr. High concentrations of AAM (200 microM) alone produced a time-dependent increase in LDH release and morphologic alterations in cultures of both cell types. Lower concentrations of AAM did not compromise the structural integrity of the cells. Semicarbazide (200 microM) or DDC (2 mM), but not clorgyline (10 microM) or pargyline (10 microM), prevented the toxicity of AAM (200 microM). Allylamine-induced cytotoxicity was partially prevented by catalase (2500 U/ml). The presence of fetal bovine serum in the medium was not essential for the manifestation of cytotoxicity. Single cell suspensions of VEC or SMC formed acrolein (ACR) when incubated in the presence of AAM. The formation of ACR mediated by SMC was inhibited by SC (20 microM), but not clorgyline (10 microM). These results support the concept that AAM is oxidatively deaminated by an SSAO present in vascular cells to generate toxic metabolic by-products capable of causing extensive cellular injury.

Kinetic study in the transient phase of the suicide inactivation of frog epidermis tyrosinase

This paper deals with the kinetic study of a multisubstrate mechanism with enzyme inactivation induced by a suicide substrate. A transient phase approach has been developed that enables the deduction of explicit equations of product concentration vs. time. From these equations kinetic constants which characterize the suicide substrate can be obtained. This study with tyrosinase enzyme, which acts on L-dopa and catechol allowed us to determine the corresponding kinetic parameters, indicating that catechol is about 8-times more powerful as a suicide substrate than is L-dopa.

A direct, highly sensitive assay for cytochrome P-450 catalyzed O-deethylation using a novel coumarin analog

The microsomal O-deethylation of a novel coumarin analog, 7-ethoxy-4-trifluoromethylcoumarin (EFC), to a fluorescent product was characterized. Results indicate that this analog provides a rapid, convenient and highly sensitive means to assay cytochrome P-450-mediated metabolism. Like microsomal 7-ethoxycoumarin (7-EC) O-deethylation, EFC O-deethylation responded to both phenobarbital was greater than that seen with 7-EC (5- to 6-fold over control after 50 mg/kg/day for 4 days in Sprague-Dawley rats compared to approximately 2-fold for 7-EC). Since the reaction was monitored by direct fluorometry of the product, any departures from linearity under a particular set of reaction conditions (e.g. with highly induced samples) were immediately apparent. In the absence of an NADPH-regenerating system, background drift was very low (less than 0.01 fluorescent units), so the sensitivity of the assay was limited primarily by that of the fluorometer employed. This makes the assay particularly useful in situations where test material is limited, e.g. when measuring activity in cultured hepatocytes. Its simplicity, reproducibility, and response to a variety of inducing agents also make it suitable for a rapid screening assay for cytochrome P-450 induction.

Acute and chronic drug-induced hepatitis

Adverse drug reactions may mimic almost any kind of liver disease. Acute hepatitis is often due to the formation of reactive metabolites in the liver. Despite several protective mechanisms (epoxide hydrolases, conjugation with glutathione), this formation may lead to predictable toxic hepatitis after hugh overdoses (e.g. paracetamol), or to idiosyncratic toxic hepatitis after therapeutic doses (e.g. isoniazid). Both genetic factors (e.g. constitutive levels of cytochrome P-450 isoenzymes, or defects in protective mechanisms) and acquired factors (e.g. malnutrition, or chronic intake of alcohol or other microsomal enzyme inducers) may explain the unique susceptibility of some patients. Formation of chemically reactive metabolites may also lead to allergic hepatitis, probably through immunization against plasma membrane protein epitopes modified by the covalent binding of the reactive metabolites. This may be the mechanism for acute hepatitis produced by many drugs (e.g. amineptine, erythromycin derivatives, halothane, imipramine, isaxonine, alpha-methyldopa, tienilic acid, etc.). Genetic defects in several protective mechanisms (e.g. epoxide hydrolase, acetylation) may explain the unique susceptibility of some patients, possibly by increasing exposure to allergenic, metabolite-altered plasma membrane protein epitopes. Like toxic idiosyncratic hepatitis, allergic hepatitis occurs in a few patients only. Unlike toxic hepatitis, allergic hepatitis is frequently associated with fever, rash or other hypersensitivity manifestations; it may be hepatocellular, mixed or cholestatic; it promptly recurs after inadvertent drug rechallenge. Lysosomal phospholipidosis occurs frequently with three antianginal drugs (diethylaminoethoxyhexestrol, amiodarone and perhexiline). These cationic, amphiphilic drugs may form phospholipid-drug complexes within lysosomes. Such complexes resist phospholipases and accumulate within enlarged lysosomes, forming myeloid figures. This phospholipidosis has little clinical importance. In a few patients, however, it is associated with alcoholic-like liver lesions leading to overt liver disease and, at times, cirrhosis. Subjects with a deficiency in a particular isoenzyme of cytochrome P-450 poorly metabolize perhexiline and are at higher risk of developing liver lesions. Prolonged, drug-induced liver-cell necrosis may also lead to subacute hepatitis, chronic hepatitis or even cirrhosis. This usually occurs when the drug administration is continued, either because the liver disease remains undetected or because its drug aetiology is overlooked. Several autoantibodies may be present.(ABSTRACT TRUNCATED AT 400 WORDS)