Mutagenicity and α-hydroxylation of N-nitrosopyrrolidine and N-nitrosopiperidine: A possible corelation

N-Nitrosopyrrolidine (NPyrr) and N-nitrosopiperidine (NPip)are carcinogenic and mutagenic cyclic nitrosamines. Their biotransformation by rat liver post-mitochondrial fraction into 1,4-butanediol and 1,5-pentanediol, respectively, is evaluated by determining these ultramate metabolites with a sensitive and suitable method. Their mutagenic activity towards the Salmonella typhimurtum strain TA 1530 was simultaneously observed. A relationship exists between their metabolism and their mutagenicity. α-Hydroxylation is probably the critical metabolic mechanism of cyclic nitrosamines.     

Genotoxic and mutagenic activation of aflatoxin B1 by constitutive forms of cytochrome P-450 in rat liver microsomes

Cytochrome P-450-mediated activation of aflatoxin B1 (AFB1) to genotoxic and mutagenic products which subsequently cause induction of an umu gene expression in Salmonella typhimurium TA1535/pSK1002 has been studied in a rat liver microsomal or reconstituted monooxygenase system. Liver microsomes from male Sprague-Dawley rats had a 1.5-fold higher activity to catalyze AFB1 than did those from female rats. In addition, the activation was not increased in liver microsomes from rats pretreated with phenobarbital, 3-methylcholanthrene, a polychlorinated biphenyl mixture, or dexamethasone, suggesting that the constitutive forms of cytochrome P-450 have important roles for the activation of AFB1 in rat liver microsomes. Using 15 forms of cytochrome P-450 purified from liver microsomes of untreated and phenobarbital- and 3-methylcholanthrene-treated rats, three isozymes from untreated male rats and one isozyme from untreated female rats were found to have high reactivities in metabolizing AFB1 to genotoxic products. Cytochrome P-450 forms isolated from inducer-treated rats were relatively less active. The close correlation between induction of umu gene expression and mutagenicity with Ames/S. typhimurium TA98 system by activated metabolites of AFB1 in the reconstituted monooxygenase system suggested that the constitutive forms of cytochrome P-450 had major roles for genotoxic and mutagenic activation of AFB1 in rat liver microsomes.     

Human hepatic cytochrome P-450 composition as probed by in vitro microsomal metabolism of warfarin

Human liver microsomal fractions from 27 renal donors (tissue obtained post mortem) and from six cancer patients (tissue obtained during surgery) were used to investigate human hepatic cytochrome P-450 isozyme compositions. In vitro microsomal metabolism of the R and S enantiomers of warfarin to dehydrowarfarin and 4'-, 6-, 7-, 8-, and 10-hydroxywarfarin is catalyzed by cytochrome P-450 isozymes and was used as the basis for evaluating similarities and differences between human cytochrome P-450 isozyme compositions. The mean hepatic cytochrome P-450 concentration from postmortem samples was not significantly different from that of surgical patients (0.51 +/- 0.16 vs. 0.35 +/- 0.14 nmol/mg protein), but the NADPH-cytochrome P-450 reductase activity of the former was significantly higher than that of the latter (141 +/- 56 vs. 29 +/- 6 nmol cytochrome c reduced/min/mg protein). In general, the microsomal preparations were overall stereoselective for R warfarin metabolism. The stereoselectivities for formation of the individual metabolites of the R enantiomer were 6-, 8-, and 10-hydroxywarfarin and the S enantiomer were 4'- and 7-hydroxywarfarin. Of the 33 microsomal preparations, 21 exhibited qualitatively similar warfarin metabolite profiles with 6R- and 7S-hydroxywarfarin having the highest formation rates. Some of the preparations exhibited markedly different metabolite profiles, the most notable having 10R-hydroxywarfarin as the major metabolite. Based on the known warfarin metabolite profiles of five purified cytochrome P-450 isozymes, the isozyme composition of the microsomes can be estimated. The majority of the microsomal preparations apparently had similar isozyme compositions but some preparations were markedly different.     

Effects of erythromycin on hepatic drug-metabolizing enzymes in humans

In rats, erythromycin has been shown to induce microsomal enzymes and to promote its own transformation into a metabolite which forms an inactive complex with reduced cytochrome P-450. To determine whether similar effects also occur in humans, we studied hepatic microsomal enzymes from six untreated patients and six patients treated with erythromycin propionate, 2 g per os daily for 7 days In the treated patients, NADPH-cytochrome c reductase activity was increased; the total cytochrome P-450 concn was also increased but part of the total cytochrome P-450 was complexed by an erythromycin metabolite. The concn of uncomplexed (active) cytochrome P-450 was not significantly modified and the activity of hexobarbital hydroxylase remained unchanged. We also measured the clearance of antipyrine in six other patients; this clearance was not significantly decreased when measured again on the seventh day of the erythromycin propionate treatment. We conclude that the administration of erythromycin propionate induces microsomal enzymes and results in the formation of an inactive cytochrome P-450-metabolite complex in humans. However, the concn of uncomplexed (active) cytochrome P-450 and tests for in vitro and in vivo drug metabolism were not significantly modified.     

Mutagenic activation of tris(2,3-dibromopropyl)phosphate: the role of microsomal oxidative metabolism.

The flame retardant tris(2,3-dibromopropyl)phosphate (Tris-BP) is converted to products which are mutagenic for Salmonella typhimurium TA 100 in the presence of rat liver microsomes, NADPH and oxygen. Other bromopropyl-compounds were also mutagenic; 2,3-dibromopropene and 2,3-dibromopropionic acid were directly mutagenic, whereas 2,3-dibromopropanol and tris(2-bromopropyl)phosphate were weakly mutagenic after addition of liver microsomes and cofactors. Typical in vivo and in vitro inhibitors of cytochrome P-450 inhibited Tris-BP mutagenicity. The effects of inducers of cytochrome P-450 on Tris-BP mutagenicity was dependent on the concentration of mutagen and microsomal protein in the assay, indicating complexity in the kinetics involved when dealing with possible multiple pathways that lead to mutagenicity. Addition of glutathione strongly inhibited Tris-BP mutagenicity. It is suggested that Tris-BP is oxidized to a reactive electrophile, possibly the 2-keto derivative, which could react with nucleophilic groups in DNA and thus lead to mutagenic events.     

Antimutagenicity of ellagic acid towards the food mutagen IQ: investigation into possible mechanisms of action.

The ability of the plant phenol ellagic acid to inhibit the mutagenicity of the food mutagen IQ was evaluated using Salmonella typhimurium strain TA98 in the Ames mutagenicity test. Ellagic acid caused a concentration-dependent decrease in the S-9- and microsome-mediated mutagenicity of IQ. The plant phenol did not interact directly with the IQ-derived mutagenic species and did not modify the cytosol-mediated activation of the promutagen. At the concentrations used in the mutagenicity studies, ellagic acid failed to inhibit microsomal mixed-function oxidase activity, including that mediated by the P450I family responsible for the bioactivation of IQ, despite being an essentially planar molecule as indicated by computer-graphic analysis. The inhibitory effect of ellagic acid was independent of its ability to chelate Mg2+. However, pre-incubation of ellagic acid with the bacteria, followed by removal of the plant phenol, did not completely prevent the inhibitory effect of the phenol on the mutagenicity of IQ. Intraperitoneal administration of ellagic acid to rats caused a decrease in total cytochrome P-450 levels and related activities as well as in cytosolic glutathione S-transferase activity. Finally, the possibility that the reported anticarcinogenic action of ellagic acid reflects nothing more than non-selective destruction of hepatic cytochromes P-450, and thus reduced chemical activation, is considered.     

Differential mutagenicities of triamino benzenes against Salmonella typhimurium TA98 in the presence of S9 fractions from polychlorinated biphenyls-, phenobarbital- or 3-methylcholanthrene-pretreated rats, hamsters and mice.

Mutagenicity of 6 aminobenzene derivatives against Salmonella typhimurium TA98 was studied in the presence of various S9 fractions. S9, which has been prepared form the livers of rats, hamsters and mice after pretreatment with different types of inducers; polychlorinated biphenyls, phenobarbital and 3-methylcholanthrene, was used as the methabolic activating enzyme in this mutation assay. The S9 fractions from 3-methylcholanthrene-treated rats and mice are most useful for mutation induction by the all aminobenzenes used. The mutagenic activity of the compounds was clearly correlated to 3-methylcholanthrene-induced cytochrome P-450. However, any significant correlation between aniline hydroxylase activity and the mutagenesis was not observed.     

Oxidative activation of the thiophene ring by hepatic enzymes. Hydroxylation and formation of electrophilic metabolites during metabolism of tienilic acid and its isomer by rat liver microsomes

Tienilic acid (TA) is metabolized by liver microsomes from phenobarbital-treated rats in the presence of NADPH with the major formation of 5-hydroxytienilic acid (5-OHTA) which is derived from the regioselective hydroxylation of the thiophene ring of TA. During this in vitro metabolism of TA, reactive electrophilic intermediates which bind irreversibly to microsomal proteins are formed. 5-Hydroxylation of TA and activation of TA to reactive metabolites which covalently bind to proteins both required intact microsomes, NADPH and O2 and are inhibited by metyrapone and SKF 525A, indicating that they are dependent on monooxygenases using cytochromes P-450. Microsomal oxidation of an isomer of tienilic acid (TAI) bearing the aroyl substituent on position 3 (instead of 2) of the thiophene ring also leads to reactive intermediates able to bind covalently to microsomal proteins. Covalent binding of TAI, as that of TA, depends on cytochrome P-450-dependent monooxygenases and is almost completely inhibited in the presence of sulfur containing nucleophiles such as glutathione, cysteine or cyteamine. These results show that 5-OHTA, which has been reported as the major metabolite of TA in vivo in humans, is formed by liver microsomes by a cytochrome P-450-dependent reaction. They also show that two thiophene derivatives, TA and TAI, bind to microsomal proteins after in vitro metabolic activation, TAI giving a much higher level of covalent binding than TA (about 5-fold higher) and a much higher covalent binding: stable metabolites ratio (4 instead of 0.5).     

A reactive metabolite of furan, cis-2-butene-1,4-dial, is mutagenic in the Ames assay

Furan is classified as a nongenotoxic hepatocarcinogen. It is thought to be activated to a toxic metabolite, cis-2-butene-1,4-dial, which is acutely toxic to liver cells. The resulting cytotoxicity is followed by compensatory cell proliferation, increasing the likelihood of tumor production. We examined the genotoxic activity of cis-2-butene-1,4-dial in several strains of Salmonella typhimurium commonly used in the Ames assay. This reactive compound tested positive in TA104, a strain that is sensitive to aldehydes. Mutagenic activity was concentration-dependent (1000 +/- 180 revertants/micromol). Incubation of cis-2-butene-1,4-dial with glutathione prior to addition of bacteria inhibited both the acute toxic and genotoxic activity of this compound. No evidence of mutagenic activity was seen at nontoxic concentrations in TA97, TA98, TA100, and TA102. Our findings are consistent with the hypothesis that cis-2-butene-1,4-dial reacts with DNA to form mutagenic adducts. Our data suggest that cis-2-butene-1,4-dial may be an important genotoxic as well as toxic intermediate in furan-induced tumorigenesis.     

Effect of the suicide substrate 3,5-diethoxycarbonyl-2,6-dimethyl-4-ethyl-1,4-dihydropyridine on the metabolism of xenobiotics and on cytochrome P-450 apoproteins

Treatment of rats with the cytochrome P-450 suicide substrate, 3,5-diethoxycarbonyl-2,6-dimethyl-4-ethyl-1,4-dihydropyridine (DDEP), produced a 95% inhibition of the in vivo demethylation of either aminopyrine or morphine within 2 hr. One-carbon metabolism of formaldehyde or formate to carbon dioxide was not altered. DDEP also produced a time-dependent decrease in total hepatic microsomal cytochrome P-450 but had no effect on either NADPH-cytochrome c reductase or p-nitrophenol glucuronyl-transferase activities up to 24 hr after administration. A rapid decrease in rat liver microsomal aniline hydroxylation and ethoxyresorufin deethylation was observed in vitro following DDEP administration. Although in vitro testosterone metabolism to 16 alpha-, 16 beta-, and 2 alpha-hydroxy metabolites was depressed profoundly by DDEP in microsomes from untreated and 3-methylcholanthrene-treated animals, 7 alpha-hydroxylation of testosterone was much less affected. Immunochemical quantification of various microsomal cytochrome P-450 protein moieties showed that cytochromes P-450 beta NF-B, P-450UT-A, P-450PCN-E, and P-450PB-C were decreased in hepatic microsomes from DDEP-treated rats. However, the protein moiety of cytochrome P-450UT-H was not diminished and the immunoreactive protein for cytochromes P-450UT-F, P-450PB-B, and P-450ISF-G was only slightly decreased. These results show that DDEP treatment leads to marked decreases in holoprotein and apoproteins of many but not all hepatic microsomal cytochrome P-450 isozymes.     

The role of cytochromes P-450 and flavin-containing monooxygenase in the metabolism of (S)-nicotine by rabbit lung

Rabbit lung microsomes metabolize (S)-nicotine primarily to (S)-nicotine delta 1',5'-iminium ion, which is the precursor of (S)-cotinine, the major urinary metabolite of (S)-nicotine in mammals. (S)-Nicotine-N'-oxide and normicotine are also produced as minor metabolites. alpha-Methylbenzylaminobenzotriazole, a mechanism-based suicide inhibitor of rabbit lung cytochromes P-450 2 and 6, inhibited (S)-nicotine oxidation in parallel with inhibition of benzphetamine N-demethylation and ethoxyresorufin O-deethylation. Pretreatment of rabbits with TCDD or Aroclor 1260 had no effect and markedly inhibited (S)-nicotine oxidation, respectively, strongly suggesting that alpha-methylbenzylaminobenzotriazole inhibition was due to inactivation of rabbit lung P-450 2. Reconstitution with cytochromes P-450 2 and 5 demonstrated that only P-450 2 was active toward (S)-nicotine, yielding predominantly the iminium ion, with smaller amounts of nornicotine, (S)-nicotine N'-oxide, and an unknown metabolite also detected. The purified rabbit lung P-450 2-catalyzed oxidation of (S)-nicotine to (S)-nicotine delta 1',5'-iminium ion exhibited a Km of 70 microM and a Vmax of 1.5 min. Covalent binding of (S)-5-3H-nicotine to rabbit lung macromolecules was dependent upon rabbit lung P-450 2-catalyzed formation of the iminium ion. Antibodies raised against P-450 2 inhibited the rabbit lung microsomal metabolism of (S)-nicotine to (S)-nicotine delta 1',5'-iminium ion by almost 95%. Titration of reconstituted P-450 2 with cytochrome b5 produced a concentration-dependent inhibition of nicotine oxidase activity. Increasing the ratio of NADH to NADPH in incubations containing lung microsomes and (S)-nicotine decreased the yield of the iminium ion, confirming the inhibitory effect of cytochrome b5 on the P-450 2-catalyzed alpha-carbon oxidation reaction. NADH alone did not support the lung microsomal metabolism of (S)-nicotine. N'-oxidation of (S]-nicotine is catalyzed by purified pig liver flavin-containing monooxygenase. A number of experiments involving the use of P-450 inhibitors, titration with NADPH-cytochrome P-450 reductase antibodies, and determination of the pH-enzyme activity profile suggested that rabbit lung flavin-containing monooxygenase contributes to a small amount of the N'-oxide produced by rabbit lung microsomes. Further examination with purified flavin-containing monooxygenase isolated from rabbit lung microsomes demonstrated that (S)-nicotine is a poor substrate for this enzyme. The low yield of N'-oxide, relative to other metabolites, in rabbit lung is uncharacteristic for most mammalian tissues and presumably reflects the unusual substrate specificity of rabbit lung flavin-containing monooxygenase.     

Alterations in hepatic heme and cytochrome P-450 metabolism in murphy-sturm lymphosarcoma-bearing rats implications for drug metabolism

Previous studies have shown that tumor-bearing rats have significantly decreased hepatic microsomal cytochrome P-450 content and NADPH-cytochrome c reductase activity with, consequently, significantly decreased capacity for microsomal oxidative drug metabolism. Subsequent investigations have revealed that the rates of hepatic cytochrome P-450 apo-protein synthesis and degradation are decreased significantly and hepatic microsomal heme oxygenase activity is increased significantly in rats bearing an extra-hepatic tumor. Further studies have been done to attempt to clarify the pathogenesis and significance of these observations. Hepatic delta-aminolevulinic acid (ALA) synthetase activity in male Wistar rats declined to a nadir of 162 ± 34 (S.E.) pmoles ALA per mg protein per 30 min 6 days following i.m. transplantation of Murphy-Sturm lymphosarcoma (vs control = 218 ± 36 pmoles per mg per 30 min). Turnover of ³H-labeled heme in microsomal CO-binding particles (i.e. cytochrome P-450 heme) was increased significantly 8 days following i.m. transplantation of Murphy-Sturm lymphosarcoma with a T_{$\text{1}\text{2}$} of 5.5 hr for the fast phase of hepatic cytochrome P-450 heme disappearance in tumor-bearing rats as compared with a T_{$\text{1}\text{2}$} of 7 hr in control rats. Hepatic cytochrome P-450 apo-protein concentration was slightly, but not significantly, increased in Murphy-Sturm lymphosarcoma-bearing rats as compared with control rats up to 10 days following tumor transplantation. These results suggest that, in Murphy-Sturm lymphosarcoma-bearing rats, decreased microsomal cytochrome P-450 concentration is the result of both decreased cytochrome P-450 apo-protein synthesis and increased cytochrome P-450 heme turnover. Apo-cytochrome P-450 concentration was not appreciably altered because increased cytochrome P-450 heme turnover and decreased cytochrome P-450 apo-protein degradation were balanced by decreased cytochrome P-450 apo-protein synthesis. Because of their effects on cytochrome P-450 concentration and action, these alterations in heme and hemoprotein metabolism may be of importance in regulating oxidative drug metabolism in the tumor-bearing state.     

Tetrachloroethylene metabolism by the hepatic microsomal cytochrome P-450 system

The interaction of tetrachloroethylene with hepatic microsomal cytochromes P-450 has been investigated using male Long-Evans rats. The spectral binding of tetrachloroethylene to cytochromes P-450 in hepatic microsomes from uninduced rats was characterized by a K_s of 0.4 mM. The K_s was not affected by phenobarbital induction, but was increased following pregnenolone-16α-carbonitrile induction. The K_M of 1.1 mM, calculated for the conversion of tetrachloroethylene to total chlorinated metabolites by the hepatic microsomal cytochrome P-450 system, was decreased by phenobarbital induction and increased by pregnenolone-16α-carbonitrile induction. The maximum extents of binding (ΔA_max) and metabolism (V_max) of tetrachloroethylene were increased by both phenobarbital and pregnenolone-16α-carbonitrile induction. Induction with β-naphthoflavone was without effect on any of the above parameters. The effects of the inducing agents on tetrachloroethylene-stimulated CO-inhibitable hepatic microsomal NADPH oxidation followed the same trend as their effects on V_max for the metabolism of tetrachloroethylene, although in all cases the extent of NADPH oxidation was 5- to 25-fold greater than the extent of metabolite production. The inhibitors of cytochromes P-450, viz. metyrapone, SKF 525-A, and CO, inhibited the hepatic microsomal binding and metabolism of tetrachloroethylene. Free trichloroacetic acid was found to be the major metabolite of tetrachloroethylene from the hepatic microsomal cytochrome P-450 system. Neither 2.2,2-trichloroethanol nor chloral hydrate was produced in measurable amounts from tetrachloroethylene. A minor but significant metabolite of tetrachloroethylene by cytochrome P-450 was the trichloroacetyl moiety covalently bound to components of the hepatic microsomes. Incubation of tetrachloroethylene. an NADPH-generating system. EDTA and hepatic microsomes was without effect on the levels of microsomal cytochromes P-450, cytochrome b₅, beme, and NADPH-cytochrome c reductase. It is concluded that hepatic microsomal cytochromes P-450 bind and metabolize tetrachloroethylene. The major product of this interaction is trichloroacetic acid, which is also the major urinary metabolite of tetrachloroethylene in vivo. The forms of cytochrome P-450 that bind and metabolize tetrachloroethylene include those induced by pregnenolone-16α-carbonitrile and by phenobarbital. Cytochrome P-448. which was induced in rat liver by β-naphthoflavone, does not appear to spectrally bind or metabolize tetrachloroethylene. The metabolism and toxicity of tetrachloroethylene are considered in relation to other chlorinated ethylenes.     

Loss of rat liver microsomal cytochrome P-450 during methimazole metabolism. Role of flavin-containing monooxygenase

The role of flavin-containing monooxygenase (FMO) in the decrease in cytochrome P-450 content during the microsomal metabolism of methimazole (N-methyl-2-mercaptoimidazole) was investigated by heat inactivation of FMO. Incubation of liver microsomes from untreated Fischer 344 rats with NADPH and methimazole resulted in a 25% loss of cytochrome P-450 detectable as its ferrous-carbon monoxide complex. The same extent of cytochrome P-450 loss was observed with 1 and 20 mM methimazole, suggesting saturation of the process. There was no significant loss of cytochrome P-450 when microsomal FMO was heat-inactivated prior to incubation with NADPH and methimazole. Heat pretreatment of the microsomes did not affect cytochrome P-450 concentrations and cytochrome P-420 was not observed. These results indicate that FMO-catalyzed metabolism of methimazole is necessary for the loss of cytochrome P-450 in microsomes from untreated rats. Sulfite and N-methylimidazole, the ultimate products of methimazole metabolism, did not cause a significant loss of cytochrome P-450. There was no loss of cytochrome P-450 when glutathione was included in the incubation with methimazole, suggesting that cytochrome P-450 loss was due to an interaction with oxygenated metabolites of methimazole formed by FMO. Losses of cytochrome P-450 were also observed after incubation of microsomes from phenobarbital- (31%) of beta-naphthoflavone-pretreated rats (44%) with NADPH and methimazole. In contrast to microsomes from untreated rats, heat inactivation of FMO did not prevent the loss of cytochrome P-450 in microsomes from the pretreated rats. These results indicate that both phenobarbital and beta-naphthoflavone induce isozymes of cytochrome P-450 capable of directly activating methimazole.     

Identification of the hepatic cytochrome P-450 isozymes induced and decreased by picloram

Microsomes from male rats treated with picloram (100 mg/kg/day) for 7 days showed a 48% decrease in 16 alpha-hydroxylase activity when incubated with (4-14C) androstenedione. These data are consistent with the assertion that picloram decreases the titer of hepatic male specific cytochrome P-450h. Several lines of evidence suggested that picloram is an inducer of hepatic cytochrome P-450 in male rats. First, SDS polyacrylamide gel electrophoresis revealed an intensified hepatic microsomal polypeptide (MW 54,000) following picloram pretreatment. This polypeptide co-migrated with protein bands which were correspondingly intensified after pretreatment with known inducers of cytochrome P-450d (3-methylcholanthrene and isosafrole). Second, no increase in the binding of metyrapone to picloram treated microsomes was noted compared with controls, suggesting no increase in phenobarbital-inducible forms of cytochrome P-450. Third, hepatic microsomes from picloram treated rats activated 2-amino-3-methylimidazo [4,5-f] quinoline (a cytochrome P-450d mediated catalysis) causing a 5-fold increase in the number of induced Salmonella typhimurium TA98 revertant colonies formed compared with control microsomes. Fourth, the binding of n-octylamine to hepatic microsomes from picloram-treated rats showed, like microsomes from 3-methylcholanthrene-treated rats, an increase in the proportion of high-spin cytochrome P-450 present. Cytochrome P-450d is known to be a high spin haemoprotein.     

The use of new methods to measure: the effect of diet and inducers of microsomal enzyme synthesis on cytochrome P-450 in liver homogenates, and on metabolism of dimethyl nitrosamine

Modified methods for measurement of cytochrome P-450 content of liver homogenates, and of formaldehyde produced in demethylation reactions are described. These methods have been used to measure cytochrome P-450 content, and metabolism of dimethyl nitrosamine in rat liver. Over 80 per cent of cytochrome P-450 present in liver homogenate of phenobarbitone treated rats could be recovered in the microsomal fraction. Feeding a low protein-low fat diet reduced the P-450 content of homogenate. and also reduced the recovery of cytochrome P-450 in the microsomal fraction to 50 per cent or less. The rate of metabolism of dimethyl nitrosamine in vivo and in vitro was increased by fasting and by phenobarbital treatment, and decreased by feeding low protein diet. Benzo(α)pyrene treatment caused a slight increase in the rate of DMN metabolism in vivo and in microsomes. The toxicity of dimethyl nitrosamine is not altered in parallel with changes in the rate of metabolism. It is suggested that the amount of toxic metabolite is more important than the rate at which it is formed.     

Detection and mechanism of formation of the potent direct-acting mutagen 2-bromoacrolein from 1,2-dibromo-3-chloropropane

The nematocide 1,2-dibromo-3-chloropropane (DBCP) was converted to products which are mutagenic for Salmonella typhimurium TA 100 in the presence of rat liver microsomes, NADPH, and oxygen. Typical in vivo and in vitro inhibitors of cytochrome P-450 decreased DBCP mutagenicity in the presence of microsomes. Addition of glutathione to cytosolic preparations failed to bioactivate DBCP to mutagenic metabolites. Mutagenicity studies with selectively deuterated analogs showed that substitution of deuterium for hydrogen at C-1 or C-3 of DBCP modestly decreased mutagenicity, but that deuteration at both C-1 and C-3 markedly decreased mutagenicity. The formation rates of the potent direct-acting mutagen, 2-bromoacrolein (2-BA), in incubations of DBCP and its deuterated analogs with rat liver microsomes, correlated with the isotope effects on mutagenicity. Characterization of 2-BA was accomplished by gas chromatography-mass spectrometry using positive-ion chemical ionization. Mass spectral analysis of 2-BA formed from specifically deuterated analogs of DBCP indicated that initial oxidative dehalogenation at C-1 followed by a spontaneous beta-elimination reaction was the preferred pathway in the formation of 2-BA from DBCP. These results demonstrate that mutagenic metabolites of DBCP are formed by cytochrome P-450-mediated oxidative metabolism, and that 2-BA is a major mutagen formed.     

Inhalation of isopropanol: induction of activating and deactivating enzymes in rat kidney and liver. Increased microsomal metabolism of n-hexane

Rats were exposed to isopropanol by inhalation of 200,2000 and 8000 ppm for 2 weeks with a daily exposure of 6 h. A pilot group exposed to 8000 ppm was given a recovery period of 4 weeks. Kidney and liver microsomal metabolism of n-hexane was investigated in vitro concomitant with activities of cytochrome P-450 and GSH enzymes and blood concentration of isopropanol and its metabolite acetone. A dose dependent increase was observed in the formation of all metabolites of n-hexane in both organs. Of special interest was the 9%, 80% and 198% increase of the preneurotoxic metabolite 2-hexanol in kidney microsomes. Cytochrome P-450 was increased 14%, 40% and 43% in kidney after 200, 2000 and 8000 ppm, respectively, and 10% and 19% in liver at 2000 and 8000 ppm. The activity of glutathione S-transferase was unaffected in kidney but elevated in liver, while GSH levels were elevated in both organs. The elevated level of kidney cytochrome P-450 did not return to normal during the 4-week-recovery period in contrast to liver cytochrome P-450. It is thus indicated that cytochrome P-450 and associated microsomal enzymes are more easily inducible and the changes more persistent in kidney than in liver. Our observations suggest that cytochrome P-450-mediated metabolic activation of n-hexane in the kidneys may have toxicological relevance in addition to liver metabolism, and that coexposure to isopropanol and n-hexane may represent an enhancement of the health hazard from n-hexane, possibly due to the isopropanol metabolite acetone.     

Constitutive forms of rabbit-liver microsomal cytochrome P-450: enzymatic diversity, polymorphism and allosteric regulation

Large, independent variations occur among New Zealand White rabbits in the 21- and and 6 beta-hydroxylation of progesterone as catalysed by liver microsomes. These reactions are catalysed respectively by two electrophoretically distinct types of rabbit-liver microsomal cytochrome P-450, 1 and 3b, as judged by their catalytic efficiency and the capacity of specific monoclonal antibodies to extensively inhibit the respective microsomal hydroxylases. The relatively large variations in progesterone 6 beta-hydroxylase activity do not appear to be associated with differences in microsomal content of cytochrome P-450 3b, whereas differences in the microsomal concentration of cytochrome P-450 1 may underlie variations in 21-hydroxylase activity. Preparations of cytochrome P-450 3b contain at least two catalytically distinct subforms, one of which catalyses both 6 beta- and 16 alpha-hydroxylation of progesterone with a low Km while the other subform catalyses predominantly 16 alpha-hydroxylation with a significantly greater Km. The two catalytic subforms of cytochrome P-450 3b can be independently modulated in vitro by positive and negative effectors that can arise in vivo from the metabolism of progesterone. The 6 beta-hydroxylase subform of cytochrome P-450 3b is not expressed in a genetically defined strain of rabbits, IIIVO/J, indicating a heritable basis for the differential expression of the two subforms of cytochrome P-450 3b. These results indicate that the extent of cytochrome P-450 multiplicity may be greater than is evident from the isolation of electrophoretically distinct forms of cytochrome P-450, and that small differences in structure may underlie large differences in catalytic properties. It is not known whether the differences among outbred New Zealand White rabbits in the expression of either cytochrome P-450 1 or the subforms of cytochrome P-450 3b reflect regulatory phenomena or genetic polymorphism.     

Adrenal mitochondrial metabolism of spironolactone. Absence of metabolic activation

Previous investigations have established that spironolactone (SL) administration to guinea pigs decreases adrenal mitochondrial and microsomal cytochrome P-450 content, and that the latter requires microsomal activation of the drug. Studies were carried out to determine if adrenal mitochondrial metabolism (activation) of SL was similarly involved in the effects of the drug on mitochondrial cytochrome P-450 destruction. Incubation of guinea pig adrenal mitochondria with SL in the absence of NADPH resulted in the formation of 7 alpha-thio-SL as the only metabolite. In the presence of an NADPH-generating system, an unknown polar metabolite was also produced. The mass spectrum of the unknown compound suggested that it was a hydroxylated derivative of SL. Incubation of mitochondrial preparations with 7 alpha-thio-SL also resulted in the formation of a polar metabolite, but the latter had a different HPLC retention time than that of the SL metabolite. Formation of the polar SL metabolite was prevented by metyrapone, an 11 beta-hydroxylase inhibitor, and was greatest in mitochondria from the adrenal zone having the highest 11 beta-hydroxylase activity. Steroid substrates for 11 beta-hydroxylation inhibited the production of the SL metabolite. Mitochondrial incubations with SL or with 7 alpha-thio-SL in the presence or absence of an NADPH-generating system did not affect cytochrome P-450 concentrations. The results indicate that, unlike the microsomal effects of SL, local activation of SL is not responsible for the destruction of adrenal mitochondrial cytochromes P-450. The major adrenal mitochondrial metabolites of SL appear to be 11 beta-hydroxy-SL and 7 alpha-thio-SL.