Enzymatic activation of chemicals to toxic metabolites

Genotoxic effects of paracetamol in V79 Chinese hamster cells

Paracetamol was studied for possible genotoxic effects in V79 Chinese hamster cells. Paracetamol (0.5 mM for 30 min) reduced the rate of DNA synthesis in exponentially growing V79 cells to about 50% of control. A further decrease in the DNA synthesis was seen during the first 30 min after termination of paracetamol exposure. Paracetamol (3 and 10 mM for 2 h) caused a small increase in DNA single-strand breaks, as measured by the alkaline elution technique. After 16 h elution, the amount of DNA retained on the filters was 79 and 70% of controls in cells treated with 3 and 10 mM paracetamol respectively. No indication of DNA damage was seen in measuring the effect of paracetamol (0.25-10 mM for 2 h) on unscheduled DNA synthesis in growth-arrested cultures of V79 cells. At the highest concentrations (3 and 10 mM paracetamol), decreased unscheduled DNA synthesis was observed. Also UV-induced DNA-repair synthesis was inhibited by 3 and 10 mM paracetamol. DNA-repair synthesis was, however, inhibited at a much higher concentration than that inhibiting replicative DNA synthesis. The number of sister-chromatid exchanges (SCE) increased in a dose-dependent manner on 2 h exposure to paracetamol from 1 mM to 10 mM. At the highest dose tested (10 mM), the number of SCE increased to 3 times the control value. Co-culturing the V79 cells with freshly isolated mouse hepatocytes had no further effect on the paracetamol induced sister-chromatid exchanges. The present study indicates that paracetamol may cause DNA damage in V79 cells without any external metabolic activation system added.

Genetics of naphthalene catabolism in pseudomonads

In pseudomonads, naphthalene is catabolized in a series of reactions to salicylic acid, which is further degraded via the catechol meta-cleavage, ortho-cleavage, or gentisic acid pathway to Krebs cycle intermediates. The naphthalene catabolic genes have been located on self-transmissible plasmids, in most cases, and implicated to have chromosomal locations in other cases. The best-studied naphthalene catabolic plasmid is NAH7. It carries two operons, one of which enables the host to utilize naphthalene and the other to utilize salicylate as a carbon and energy source. The product of another NAH7 gene, nahR, is required to turn on both operons in the presence of the inducer, salicylate. Several different naphthalene and salicylate catabolic plasmids have been shown to share sequence homology with NAH7. These plasmids can undergo structural alterations involving insertions and deletions during conjugations and changes in nutritional conditions. Available evidence suggests that salicylate catabolic plasmids can form from the naphthalene catabolic plasmids by structural alterations of the plasmid DNA. The gene organization and regulation, as well as the genetic instability of the naphthalene catabolic plasmids, are reminiscent of the TOL plasmids and suggest that the naphthalene catabolic plasmids and other catabolic plasmids may have evolved in a short period of time by acquiring and modifying preevolved gene clusters from host chromosomes or other plasmids.

2,2,2-Trifluoroethanol toxicity in aged rats

Various anesthetics are metabolized by different forms of cytochrome P-450 yielding the same toxic metabolite, 2,2,2-trifluoroethanol (TFE). The toxicity of TFE is a consequence of its metabolism catalyzed by cytochrome P-450. Since a marked age difference exists in the composition and inducibility of the hepatic mixed function oxidase system, we have elucidated the toxicity of TFE in 36-month-old aged Wistar rats. The aged rats were injected with sublethal doses of TFE (0.10 g/kg once per week for 5 weeks), after which they were sacrificed for pathologic examination. The major TFE-related lesions observed were severe hepatocyte degenerative changes such as basophilic, eosinophilic, vacuolated hepatocytes, bile duct hyperplasia, accumulation of lipofuscin pigments, and preneoplastic nodules. Other changes seen were the hyalinization of gastric submucosal wall, generalized testicular atrophy due to the loss of seminiferous tubules, coagulation necrosis of intestinal mucosal wall, hyperpigmentation, and more advanced and severe chronic progressive glomerulonephropathy in the TFE-treated rats. More severe lipofuscin and vacuolation of white matter of thalamic area, pons, midbrain, and cerebellum reflective of enhanced aging were also seen. Ultrastructural studies of liver from TFE-treated rats revealed rather diffuse loss of glycogen, fragmentation of endoplasmic reticulum, mineralization of mitochondria, and loss of other organelles within the hepatocytes versus saline-treated aged rats. Hepatic cytochrome P-450 concentrations, measured as a possible index of endoplasmic reticulum damage, were not affected by TFE administration.

Cytochromes P-450

1. The cytochromes P-450 are of interest in fields as diverse as biochemistry, pharmacology, mechanistic and synthetic chemistry, carcinogenesis, toxicology, endocrinology, and nutrition. 2. Cytochrome P-450 enzymes are coded by a multi-gene family. 3. The many forms have some elements of similarity in their primary sequences and catalyze chemically equivalent reactions, either mixed-function oxidations or reductions. 4. The catalytic specificity of the individual forms of the enzyme is related to three-dimensional interactions between individual substrates and their binding sites, and the reactions are related in their chemistry. 5. Regulation of enzyme levels appears to involve transcriptional and post-transcriptional aspects.

Rat liver cytochrome P-450 isozymes as catalysts of aldrin epoxidation in reconstituted monooxygenase systems and microsomes

To explore which rat liver cytochrome P-450 species are involved in aldrin epoxidation, we have studied the catalytic activities of a series of cytochrome P-450 isozymes purified from untreated and inducer-treated Sprague-Dawley rats. Of ten cytochrome P-450 forms analyzed, seven isozymes, listed in order of decreasing activity, catalyzed aldrin epoxidation: P-450UT-A, P-450PB-C, P-450UT-H, P-450PB-B, P-450PCN-E, P-450UT-F, and P-450PB-D. P-450UT-I, P-450BNF-B, and P-450ISF-G were not very active at all. A novel aldrin metabolite, endo-dieldrin, was formed by cytochrome P-450UT-F in a 6-fold excess over dieldrin, which is the exo-isomer. The activity of aldrin epoxidase furthermore was assayed in liver microsomes from Sprague-Dawley rats of diverse physiological status and after pretreatment with various inducers resulting in a peculiar pattern of cytochrome P-450 isozymes. Untreated animals, at an age of 3 weeks, showed similar enzyme activities in both genders. During maturation, the activity of males increased by 3-fold, while the activity in females did not significantly change during this period. Pretreatment with pregnenolone-16-alpha-carbonitrile or dexamethasone strongly increased the activity in females. Pretreatment with dexamethasone did not increase the activity of males. A 50% depression of epoxidase activity was noted for males pretreated with 5,6-benzoflavone. Phenobarbital pretreatment increased the activity of females by 12-fold and of males by 2-fold. Males responded to pretreatment with polychlorinated biphenyls in a strain dependent fashion: enzyme activity was increased 2-fold in Sprague-Dawley rats but was not altered in Wistar rats. "Theoretical" values of microsomal epoxidase activity were calculated for weanling and adult Sprague-Dawley rats from turnover numbers and published data on the relative abundance of aldrin epoxidizing P-450 isozymes (Waxmann et al., Biochemistry 24, 4409, 1985). These values agreed with the activities determined. A similar statement can be made for male rats of both strains pretreated with inducers, when the ratio of enzyme activity of pretreated to control animals was used as a basis of comparison. The activity ratio of females pretreated with pregnenolone-16-alpha-carbonitrile, dexamethasone and phenobarbital, however, was much higher than the ratio calculated. Our results reveal that aldrin epoxidation is a reaction indicative of male specific and of phenobarbital-inducible cytochrome P-450 isozymes in rat liver.(ABSTRACT TRUNCATED AT 400 WORDS)

Reaction of the model thiol 2-mercaptoethanol and glutathione with methylvinylmaleimide, a Michael acceptor with extended conjugation

Many alpha,beta-unsaturated compounds are sufficiently reactive to condense with nucleophiles under physiological conditions and are potentially deleterious to cellular processes. These compounds react with thiols by nucleophilic attack to give 1,4 addition products. We have examined the products formed from the reaction of the model thiols HSCH2CH2OH and glutathione with methylvinylmaleimide, a Michael acceptor with extended conjugation. Glutathione produced exclusively a 1,6 addition product with methylvinylmaleimide. HSCH2CH2OH also formed a 1,6 nucleophilic addition product, as well as a disubstituted product resulting from apparent further 1,4 addition to the 1,6 addition product. Two other novel products which resulted from addition to the maleimide ring and addition at the vinyl carbon proximal to the maleimide ring were observed.

2,2,2-Trifluoroethanol-induced enteropathy in rats: chemically or bacterially mediated effects

The lethal effects of the fluorinated ether anesthetic, fluroxene, in rats are a consequence of its metabolism, which is catalyzed by cytochrome P-450 to the toxic metabolite 2,2,2-trifluoroethanol (TFE). The anesthetic or TFE (0.21 g/kg) caused decreased white blood cell counts, necrosis of bone marrow and lymphocytes, and decreased small intestine dry weight and was associated with septicemia. To elucidate the mechanism of TFE toxicity in rats we undertook histopathologic, ultrastructural and bacteriologic studies. TFE produced severe edema of intestinal lamina propria and submucosae, dilatation of crypts, loss of surface epithelium, vacuolation and necrosis of intestinal epithelial cells, and infiltration of polymorphonuclear leukocytes into the edematous lamina propria. Intestinal epithelial villi lost their cellular tissue integrity. Coccobacillary organisms were numerous in the ulcerated intestine. Hemolytic E. coli were isolated from intestinal tissue at a two-log increase in concentration relative to controls. Hemograms from TFE-treated rats exhibited marked leukopenia and morphologic differences. The platelets lost their discoid shape, extended pseudopods, and centralizing granules. Hemoglobin precipitation as Heinz bodies and crystalloid structures was observed in TFE-treated rats. Together the data suggest that TFE-induced enteropathy was most probably due to E. coli precipitated from TFE-mediated alterations in the population of small intestinal microbes. The antibiotics erythromycin, active against gram-positive bacteria, and streptomycin, active against gram-negative bacteria, and the antiendotoxin, polymyxin B, were administered to rats prior to TFE in an effort to differentiate between these mechanisms by altering the intestinal bacteria populations. The results indicate that the TFE-induced small intestinal lesions are initiated by the direct focal necrotic effect of TFE or its metabolites on the small intestinal epithelium. The focal coagulation necrosis produced by TFE predisposes the animals to lethal enteritis and systemic bacteremia.

2,2,2-Trifluoroethanol toxicity in hamsters (Mesocricetus auratus)

As part of an effort to further elucidate the mechanism whereby fluorinated ether anesthetics express their various toxic effects, Golden Syrian hamsters were utilized to determine acute and subchronic toxicities of the anesthetic metabolite 2,2,2-trifluoroethanol (TFE). The major lesion observed with the TFE (0.25 g/kg ip) was coagulation necrosis characterized by nuclear pyknosis in epithelial cells of intestinal villi, the deeper portion of gastric mucosae, lymphoid organs such as thymus, spleen and mesenteric lymph nodules and brain. Hepatocyte hypertrophy with diffuse vacuolar degeneration was also found. Lungs had a characteristic focal terminal bronchiolar epithelial hyperplasia with discrete mucous metaplasia in the terminal bronchioles. In subchronic studies, hamsters were injected with sublethal doses of TFE (0.20 g/kg once per week for 5 weeks). The major lesions observed were severe hepatocyte hyperplasia and hypertrophy with fatty degeneration, necrosis of hair germinal matrix, urinary bladder transitional epithelial cellular hypertrophy, and hyperplasia with vacuolar degeneration and submucosal edema. Ultrastructural studies of acutely treated animals indicate that fat vacuoles formed within the hepatic endoplasmic reticulum characterized by lamellar bodies forming multiple myelin figures within the fat vacuoles. Hepatic cytochrome P-450 concentrations measured as a possible index of endoplasmic reticulum damage, failed to reveal any significant differences between the treated and non-treated hamsters.

The detection of cytotoxicity produced by short-lived reactive intermediates: a study with bromobenzene

1. A V79 cell incubation incorporating rat liver 9000 g supernatant (S9) fractions, used previously to detect the toxicity due to long-lived, stable metabolites of cyclophosphamide, has been used to study the toxicity of short-lived, reactive metabolites generated from bromobenzene. 2. Cytotoxicity was observed in the presence of S9 fractions from rats treated with phenobarbitone but not in the presence of S9 fractions from untreated or beta-naphthoflavone-treated animals. This toxicity was enhanced by depletion of the glutathione in the S9 fraction by prior treatment of the animals with diethyl maleate and was reduced by SKF 525 A, in agreement with results in vivo on the mechanism of bromobenzene-induced hepatotoxicity. 3. This study demonstrates that cytotoxicity due to the generation of short-lived, reactive metabolites can be detected in this system in vitro provided that procedures are used to modify the activating and detoxifying enzyme systems within the S9 fraction.

Polymorphism of human cytochrome P-450

The cytochrome P-450 forms involved in debrisoquine 4-hydroxylation (P-450DB), phenacetin O-deethylation (P-450PA), S-mephenytoin 4-hydroxylation (P-450MP), and nifedipine 1,4-oxidation (P-450NF) have been purified to electrophoretic homogeneity from human liver microsomes. All of these reactions show in vivo polymorphism in humans. Evidence for the roles of the purified proteins in these processes comes from in vitro reconstitution and immunoinhibition studies. The rat orthologs of these enzymes are as follows--P-450DB: P-450UT-H; P-450PA: P-450ISF-G; P-450MP: P-450UT-I; P-450NF: P-450PCN-E. Only in the case of P-450UT-H is the primary rat ortholog the same cytochrome P-450 which catalyses the catalytic reaction under consideration. Reconstitution and immunochemical studies establish that the following reactions are catalysed by the individual P-450s--P-450DB: debrisoquine 4-hydroxylation, sparteine delta 5-oxidation, bufuralol 1'-hydroxylation, encainide O-demethylation, and propanolol 4-hydroxylation; P-450PA: phenacetin O-deethylation; P-450MP: S-mephenytoin 4-hydroxylation and tolbutamide methyl hydroxylation; P-450NF: oxidation of nifedipine and 16 other substituted dihydropyridines, estradiol 2- and 4-hydroxylation, aldrin epoxidation, benzphetamine N-demethylation and 6 beta-hydroxylation of testosterone, androstenedione and cortisol. A cDNA clone has been isolated that corresponds to rat P-450UT-H, as shown by a number of criteria. Studies with this probe establish that the sex and strain variation in debrisoquine 4-hydroxylase and related activities is related to differences in the levels of a 2.0 kb length mRNA present.(ABSTRACT TRUNCATED AT 250 WORDS)

Aflatoxin B1 activation to a plasmid mutagen by a chemical model of cytochrome P-450

Aflatoxin B1 (AFB1) was oxidised by a chemical model of cytochrome P-450 and the products obtained analysed by reversed-phase hplc. The oxidation system employs a water-insoluble iron(III)porphyrin catalyst (tetraphenylporphinatoiron(III) chloride; FeTPPCl) and an oxygen donor, iodosylbenzene (PhIO). The two AFB1 products obtained from this reaction were derived by initial oxidation across the 8-9 double bond of AFB1 and subsequent breakdown of AFB1-8,9-epoxide, the compound postulated to be the ultimate carcinogenic and mutagenic derivative of AFB1. Oxidation of AFB1 by the porphyrin-catalysed system in the presence of calf-thymus DNA, resulted in the formation of AFB1-DNA adducts identical to those formed in vivo and in vitro after liver mono-oxygenase activation. The chemical model system was therefore used on a microscale to react AFB1 with plasmid pHR1800, a plasmid containing the genes for beta-lactamase (amp-r) and galactokinase (galK). AFB1-modified plasmid DNA was then transformed into E. coli AB1886 (uvrA-) and the effects of the AFB1-DNA adducts on plasmid survival and the ability of pHR1800-transformed bacteria to metabolise galactose, were then studied. The results showed an AFB1 dose-dependent decrease in plasmid survival and increase in the mutation frequency of the galK gene only when the complete oxidation system was used to generate the reactive AFB1 metabolite.

Ethanol oxidation and toxicity: role of alcohol P-450 oxygenase

The isolation and characterization of ethanol-inducible rabbit liver microsomal cytochrome P-450, termed P-450 3a or P-450ALC, has provided definitive evidence for the role of this enzyme in alcohol oxidation. From findings on the distribution, substrate specificity, and mechanism of action of P-450ALC we have suggested "alcohol P-450 oxygenase" as a more biochemically accurate name than "microsomal ethanol-oxidizing system." The present review is concerned with studies in this and other laboratories on activities and inducers associated with this versatile enzyme. Numerous xenobiotics, including alcohols and ketones, nitrosamines, aromatic compounds, and halogenated alkanes, alkenes, and ethers, are known to undergo increased microsomal metabolism after chronic exposure of various species to ethanol. Diverse compounds and treatments may induce P-450ALC, including the administration of ten or more chemically different compounds, fasting, or the diabetic state. Whether a common mechanism of induction is involved is unknown at this time. As direct evidence that P-450ALC catalyzes numerous metabolic reactions, the purified rabbit enzyme has been used in a reconstituted system to demonstrate various metabolic transformations, including the oxidation of various alcohols, acetone, acetol, p-nitrophenol, and aniline, the dealkylation of substituted nitrosamines, the reductive dechlorination of carbon tetrachloride, carbon tetrachloride-induced lipid peroxidation, and acetaminophen activation to form the glutathione conjugate.

Formation of reactive 1-nitropyrene metabolites by lung microsomes and isolated lung cells

The metabolism and activation of 1-nitropyrene (1-NP) to reactive intermediates by lung microsomes and isolated lung cells was studied. Mutagenicity of 1-NP metabolites was assayed in Salmonella typhimurium TA98NR, a strain lacking a major component of nitroreductase activity. In the presence of NADPH, microsomes from rabbit, rat and hamster lung metabolized 1-NP to mutagenic products to a similar degree. Pretreatment with a mixture of polychlorinated biphenyls (PCB) decreased the formation of mutagenic metabolites by rabbit lung microsomes, but did not affect the production of mutagens by rat or hamster lung microsomes. 3H-1-NP was metabolized to covalently bound protein products at a rate of 82 and 10 pmol/mg by rabbit and hamster lung microsomes, respectively, whereas no binding was detected in rat lung microsomes. PCB-pretreatment increased covalent protein binding of 3H-1-NP in lung microsomes from hamster and rat, but decreased the binding in rabbit lung microsomes. High performance liquid chromatography analysis indicated that 3H-1-NP was readily converted to ring-hydroxylated products by rabbit and hamster lung microsomes; the rate was much lower with rat lung microsomes. 3H-1-NP was activated to metabolites that covalently bound to protein in isolated rabbit lung cells, with the following rates being observed: Clara cells greater than lung digest greater than type II cells. In contrast, covalent protein binding in cells isolated from rat lung was very low. 1-NP was not activated to products mutagenic for S. typhimurium TA 98NR when co-incubated with cells isolated either from rabbit or rat lung.

Control of 5-aminolevulinate synthase in animals

The proposed mechanism by which hepatic ALV-synthase mitochondrial levels are regulated is outlined in Fig. 2. ALV-synthase catalyzes the first and rate-limiting step in the heme pathway and is normally present in low amounts. A cytosolic, regulatory free heme pool tightly controls the amount of ALV-synthase in two ways. In the primary mechanism of regulation, heme is proposed to inhibit the synthesis of ALV-synthase mRNA. Most likely this would be mediated through the action of specific heme-binding protein(s) which recognize regulatory control regions of the ALV-synthase gene. Gene activity therefore is significantly repressed most of the time. When there is an increased demand for heme by newly synthesized cellular hemoproteins, the free heme pool is reduced, leading to a derepression of ALV-synthase mRNA synthesis. Once the need for increased heme synthesis is satisfied, inhibitory heme levels build up again. When drugs such as phenobarbital are administered to animals, there is a rapid induction in the liver of both cytochrome P-450 and ALV-synthase. It is proposed that the heme pool governing ALV-synthase levels is lowered by the increased heme demand due to cytochrome P-450 apoprotein formation. The primary event in the drug induction of ALV-synthase is therefore the increased synthesis of cytochrome P-450 apoprotein. However, the mechanism by which this occurs is unknown, although drugs do increase the synthesis of mRNA for cytochrome P-450 (Fig. 2). (There is evidence that for the aromatic hydrocarbons a specific cytosolic receptor exists.) In the acute hepatic porphyria diseases, uncontrolled synthesis of hepatic ALV-synthase occurs. The various forms are characterized by reduced levels of one of the heme pathway enzymes other than ALV-synthase. Attacks of the disease are commonly precipitated by drugs which induce cytochrome P-450, and the uncontrolled accumulation of ALV-synthase which accompanies these attacks results from the combined action of the block in the heme pathway and the increased cytochrome P-450 levels. A major challenge which now exists is to understand at the molecular level how the genes for ALV-synthase and cytochrome P-450 are regulated in the liver and other tissues.(ABSTRACT TRUNCATED AT 400 WORDS)