Microsomal Monooxygenase System Research Articles

Polychlorinated biphenyl (PCB) congener effects on the longevity of the housefly.

The microsomal monooxygenase system in the housefly is well-characterized, active, inducible and inhibited by the same types of compounds as in vertebrate systems. Flies metabolize PCBs to polar products and retain congeners from a commercial mixture in proportions consistent with PCB biotransformation and elimination in vertebrates. Although TCDD has been shown to have a high affinity for, and be an effective agonist of the juvenile hormone receptor in insects, toxic manifestations in adult insects may differ from those in certain highly sensitive vertebrate species. This study was undertaken to determine if the housefly bioassay could effectively compare the net adverse biological activity of a group of PCB congeners independent of Ah receptor effects.

Effects of dimethylformamide on hepatic microsomal monooxygenase system and glutathione metabolism in rats

The effects of repeated exposure to N, N-dimethylformamide (DMF) on hepatic microsomal monooxygenase system and glutathione metabolism were investigated. DMF was administered to Wistar male rats by subcutaneous (s.c.) injection at 0.5 ml/kg body weight daily for 1 week. Macroscopically, mild liver swelling was observed and liver weights significantly increased after 1 week of exposure to DMF. Hematological changes were not detected. In exposed rats, glutamic oxaloacetic transaminase, glutamic pyruvic transaminase, cholinesterase and total cholesterol significantly increased. Hepatic microsomal cytochrome P-450 and protoheme decreased by 34% and 24%, respectively, while microsomal protein and cytochrome b 5 were not affected. NADH-ferricyanide reductase activity decreased by 24% while NADPH-cytochrome c reductase activity showed no change. Glutathione reductase (GR) activity showed a significant decrease after the first injection and remained depressed throughout the study, with no change in glutathione peroxidase (GPx) activity. Glutathione S-transferase (GST) activity showed a significant increase at 3 days after DMF treatment and gradually increased by 66% at 1 week. In a subsequent experiment with a single administration of DMF (4 ml/kg), reduced glutathione (GSH) in the liver was decreased by 28% at 8 h, but recovered to control levels by 24 h. These results indicate that DMF alters the hepatic microsomal monooxygenase system and glutathione metabolism. These findings may greatly contribute to the elucidation of the pathogenesis of DMF hepatotoxicity.

Characterization of cytochrome P450-dependent dimethylhydrazine metabolism in human colon microsomes

Polyclonal antibodies to components of the rat liver cytochrome P450 system were used to examine the composition and function of the microsomal cytochrome P450-dependent monooxygenase system of human colonic mucosal cells. Anticytochrome P450 reductase antibody gave a strong band of immunocross-reactivity in human colon microsomes at the same molecular weight level as purified cytochrome P450 reductase from rat liver, as well as hepatic microsomes isolated from untreated or phenobarbital-treated rats. These results demonstrate the presence of cytochrome P450 reductase in human colon cells. Similarly, cytochromes P450 IIB1 and IIA1 also appear to be present in Western blots of human colon microsomes. These antibodies, as well as antibodies to reductase and cytochrome b5, inhibit dimethylhydrazine metabolism in human colon microsomes to varying degrees. These data argue for a functional P450-dependent drug metabolism system in colon capable of activating/metabolizing the colon-specific model carcinogen, 1,2-dimethylhydrazine.

Effects of tributyltin chloride in vitro on the hepatic microsomal monooxygenase system in the fish Stenotomus chrysops

Tributyltin (TBT) has been shown to be highly toxic to a number of aquatic species, but the mode(s) of action on a biochemical level remain(s) to be elucidated. Here we investigate the interaction in vitro of TBT with hepatic microsomal cytochrome P-450 and associated enzyme activity in scup ( Stenotomus chrysops). Hepatic microsomes from β-naphthoflavone (βNF) induced scup were incubated in vitro with TBT and various components analyzed. TBT led to a time- and concentration-dependent decrease in total microsoinal P-450 content. This decrease was accompanied by the formation of cytochrome P-420. A complete loss of P-450 occurred with 1 mM TBT after 30 min incubation. Ethoxyresorufin- O-deethylase (EROD) activity was strongly inhibited by TBT in a concentration-dependent manner, with a complete inhibition at 0.3 mM TBT after 15 min incubation. Other components of the microsomal electron transport system were also investigated. Neither cytochrome b 5 content, nor NADPH-cytochrome c reductase activity were affected by TBT at concentrations up to 0.5 mM TBT. NADH-cytochrome c reductase activity, however, showed a concentration-dependent increase, with more than a doubling of activity at 0.5 mM TBT. This indicates that TBT has different effects on these reductases. In conclusion, TBT can strongly interact with hepatic microsomal P-450 in fish leading to destruction of native enzyme and inhibition of enzyme activity. Microsomal monooxigenase system; Cytochrome P-450; Tributyltin chloride: Interaction; Scup ( Stenotomus chrysops)

Effects of Short‐Term Ethanol and Nutrition on the Hepatic Microsomal Monooxygenase System in a Model Utilizing Total Enteral Nutrition in the Rat

The majority of studies examining the effects of ethanol on the hepatic microsomal monooxygenase system, (HMO), have utilized the liquid diet regimen of Lieber and DeCarli. While much has been learned with this useful model, there are some concerns associated with diminished nutrient intake. Decreased food intake in the presence of high levels of ethanol could give rise to at least three effects; primary ethanol effects, primary nutritional effects and/or effects resulting from interactions between nutritional deficiencies and ethanol (i.e., synergistic effects). A model similar to that developed by Tsukomoto and French is used in the current study, in which ethanol is infused directly into the stomach as part of a total enteral nutrition system (TEN). This assured that nutrients sufficient for normal growth were consumed. Two clinically relevant diets were selected for study. One diet is very similar to that used for intravenous feeding of human patients (diet A) and the other similar to that used for intragastric feeding of patients (diet B). The present study was conducted to determine the effects of different diets on HMO and to determine whether ethanol has demonstrable effects in the presence of dietary sources that promote normal growth rates. The effects of the two liquid diets alone or of TEN where 35% of the total calories in the diets were replaced by ethanol for 8 days were examined on HMO of adult male Sprague-Dawley rats. HMO substrates examined included testosterone, the alkoxyresorufins, carbon tetrachloride, and p-nitrophenol. Levels of cytochrome P-450 apoproteins were studied by Western blot analysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of Ugilec-141®, a PCB substitute based on tetrachlorobenzyltoluenes, on hepatic mono-oxygenase induction in estuarine fish

Ugilec-141® is a PCB substitute based on chlorinated benzyltoluenes which has been reported as an environmental contaminant in some German rivers. It is structurally similar to some phenylxylylethanes which induce hepatic microsomal mono-oxygenases in fish. When injected i.p. into dab ( Limanda limanda) or flounder ( Platichthys flesus) Ugilec-141 did not consistently induce hepatic microsomal ethoxyresorufin O-de-ethylase (EROD), ethoxycoumarin O-de-ethylase (ECOD) or pentoxyresorufin O-de-alkylase (PROD). When injected into winter flounder ( Pseudopleuronectes americanus) to yield tissue concentrations in the 2–40 μg/g range, Ugilec-141 slightly (but significantly) induced EROD (2·4-fold), ECOD (2·5-fold) and 3-cyano-7-ethoxycoumarin O-de-ethylase (4·4-fold). The classic inducer, β-naphthoflavone, consistently induced all these indices of mono-oxygenase activity in all species. Components of Ugilec-141 therefore appear to be only mild inducers of fish hepatic microsomal mono-oxygenase systems.

Effect of Cold Acclimation on the Broiler Chicks' Resistance to Acute Aflatoxicosis

The effect of acclimation to environmental temperatures of 10 to 12, 14 to 16, or 28 to 30 C on the resistance of broiler chicks to acute aflatoxicosis was studied in a series of four experiments. Chicks acclimated to 10 to 12C were more resistant to a single oral dose of aflatoxin (8mg per kg of BW) after 10 and 20 days than chicks acclimated to 28 to 30 C. However, chicks acclimated to 14 to 16 C required 20 days of acclimation before an increase in resistance was observed. Acclimation to 10 to 12 C for 14 days followed by acclimation to 28 to 30 C for 7 days resulted in no increase in the resistance of the chicks to an aflatoxin dose (8 mg per kg of BW) at 21 days, indicating that the resistance conveyed by cold acclimation was temporary. Sodium-pentobarbital sleeping time was significantly decreased and the cytochrome P-450 content was significantly increased after acclimating broiler chicks to 10 to 12 C. These observations suggest that stimulation of the microsomal mono-oxygenase system after cold acclimation may be responsible for the increased resistance of the broiler chick to acute aflatoxicosis.

Induction of cytochrome P-450 isozymes during long-term nifedipine administration

The wide substrate specificity of the microsomal monooxygenase system, catalyzing oxidation of many xenobiotics and endogenous compounds, is determined by the existence of multiple forms of cytochrome P-450 [13]. A modern approach to the characterization and identification of the individual cytochrome P-450 isozymes is to study specific reactions catalyzed by these hemoproteins. There are substrates whose metabolism is more specific for a particular cytochrome P-450 isozyme. For example, the O-de-ethylation of 7-ethoxyresorufin is catalyzed by cytochrome P-450 c [3] . The steroid hormones testosterone, androstenedione (AD), and progesterone are utilized as specific substrates by the microsomal monooxygenase of the liver, with well-marked regional and stereospecificity of metabolism [16]. The rate of formation of hydroxylated metabolites gives information on activity of different forms of cytochrome P-450. For example, the formation of 16a-OH-AD is catalyzed by cytochrome P-450h, which is characterized only of male rats. The formation of 16~-OH-AD is catalyzed by P-45%, that of 7a-OH-AD by cytochrome P-450a, and that of 6fl-OH-AD by cytochrome P-450p [15]. Thus the specificity of the reaction of metabolism of certain substrates can serve as an important criterion for the identification of many forms of cytochrome P-450. On the other hand, the immunochemical method using monospecific antibodies against individual cytochrome P-450 isozymes makes it quantitative and qualitative determination in microsomes possible. This combined approach can be used in pharmacologic investigations, especially those in which the drug influences the microsomal monooxygenase system. The calcium antagonist nifedipine, widely used in cardiologic practice, is metabolized by cytochrome P-450p in rat liver microsomes [6] or by P-450NF in human liver microsomes [14], which are induced by the synthetic steroid pregnenolone-16carbonitrile and steroids of the glucocorticoid class, such as dexamethasone and macrolide antibiotics [12]. Previously the writers described induction of a cytochrome P-450-dependent monooxygenase system after long-term administration of nifedipine to rats [7]. The aim of the present investigation was to discover what forms of cytochrome P-450 can be induced under these conditions.

Electrophoretic, spectral, catalytic and immunochemical properties of highly purified cytochrome P-450 from sheep lung

1. 1. Cytochrome P-450LgM2 was purified from sheep lung microsomes in the presence of detergents, Emulgen 913 and cholate. 2. 2. The purification procedure involved the chromatography of the detergent solubilized microsomes on DEAE-cellulose and hydroxylapatite. 3. 3. Cytochrome P-450LgM2 was further purified on second DEAE-cellulose and hydroxylapatite columns. 4. 4. The specific content of the highly purified P-450LgM2 was 16–18 nmol P-450/mg protein and purified 164-fold. 5. 5. The yield was 16% of the initial content in microsomes. 6. 6. The SDS-polyacrylamide slab gel electrophoresis (PAGE) of the purified lung cytochrome P-450LgM2 showed one protein band having the monomer molecular weight of 49,500. 7. 7. The absolute CO-difference spectrum of dithionate-reduced P-450LgM2 gave a peak at 451 nm. 8. 8. When sheep lung cytochrome P-450LgM2 and P-450LM2 purified from liver of phenobarbital (PB)-induced rabbit were subjected to Western Blotting and visualized immunochemically with anti P-450LM2, they showed identical mobilities. 9. 9. P-450LgM2 was found to be very active in N-demethylation of benzphetamine in a reconstituted system containing purified sheep lung reductase and synthetic lipid. 10. 10. Turnover numbers (min −1) for benzphetamine, aniline, ethylmorphine and p-nitrophenol were determined to be 273, 1.2, 15.5 and 1.05, respectively, in a reconstituted microsomal lung monooxygenase system. 11. 11. Spectral, electrophoretic, biocatalytic and immunochemical properties of sheep lung P-450LgM2 were found to be similar to those of P-450 isozyme 2, purified from PB-treated rabbit liver and of rabbit lung microsomes.

Biphasic response for hepatic microsomal enzyme induction by 2,3,7,8-tetrachlorodibenzo- p-dioxin in C57BL/6J and DBA/2J mice

The induction of the murine hepatic microsomal cytochrome P-450 monooxygenase system by 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) was studied over a wide range of doses, including those associated with acute toxicity. Studies were conducted in two inbred strains of mice which vary at the Ah receptor and at a number of other genetic loci. C57BL/6J mice possess at high-affinity Ah receptor and are responsive to enzyme inductive effects of TCDD, whereas DBA/2J mice do not possess a high-affinity receptor and are less responsive to TCDD. In a dose-response study, 7-ethoxyresorufin O-deethylase (EROD) activity appeared to be maximally induced in C57BL/6J and DBA/2J mice at 7 days following exposure to 3 and 30 μg of TCDD/kg respectively. Very similar results were reported previously for the induction of aryl hydrocarbon hydroxylase activity in these strains of mice. However, at higher doses of TCDD (at least 45 μg/kg for C57BL/6J and 300 μg/kg for DBA/2J), EROD activity was further increased (2-fold) from the apparent maximal (plateau) level, resulting in an unusual biphasic log dose-response relationship. EROD activity remained at these elevated rates in both strains for doses approaching and exceeding the respective ld 50 values for each strain. To further characterize this biphasic induction phenomenon, cytochrome P-450 content, benzo[ a]pyrene metabolism, and EROD and NADPH-cytochrome P-450 reductase activities were measured 1, 3 and 7 days after TCDD administration to C57BL/6J (3 and 150 μg/kg) and DBA/2J (30 and 600 μg/kg) mice. Maximal responses occurred in both strains at 3 days for all doses. In both strains, TCDD produced a dose-dependent increase in cytochrome P-450 content, EROD, and benzo[ a]pyrene metabolism. Furthermore, a 2-fold induction of reductase activity was observed in each strain following exposure to the respective high doses. Induction of cytochrome P 1-450 and P 3-450 was also measured by Western immunoblot, using antisera raised against the homologous rat isozymes. In both strains, TCDD produced a dose-related increase in two protein-staining bands recognized by anti-P-450 BNF-B (P 1-450) and anti-P-450 BNF/ISF-G (P 3-450) respectively. The extended induction of hepatic microsomal monooxygenase activities at the respective high doses of TCDD appears to be due, in part, to increases in NADPH-cytochrome P-450 reductase activity and cytochromes P 1-450 and P 3-450 content. Significant alterations in the expression of the cytochrome P-450 monooxygenase system following exposure to high doses of TCDD may be associated, in part, with the delayed acute toxicity reported at this level of exposure.

Microsomal cytochrome P450 monooxygenases in the house fly ( Musca domestica L.): Biochemical changes associated with pyrethroid resistance and phenobarbital induction

Differences in the microsomal cytochrome P450 monooxygenase system and its inducibility by sodium phenobarbital (PB) were investigated in insecticide-susceptible (S+) and pyrethroid-resistant (LPR) house fly strains. In comparison to the S+ strain, LPR had higher levels of total cytochrome P450s, cytochrome b 5, NADPH-cytochrome c (P450) reductase, and six monooxygenase activities: aryl hydrocarbon hydroxylation (AHH), aldrin epoxidation (AE), 7-ethoxycoumarin O-deethylation (ECOD), p-chloro- N-methylaniline N-demethylation (PCMA), ethoxyresorufin O-deethylation (EROD), and methoxyresorufin O-demethylation (MROD). The largest differences between strains were noted for MROD (64×) and EROD (56×) activities, and these appeared to be correlated with the monooxygenase-mediated pyrethroid resistance in the LPR strain. Comparisons between susceptible and resistant strains by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) revealed that the LPR strain showed a marked increase in the 54,400-Da protein band, suggesting this may be a cytochrome P450 isozyme involved in conferring part of the monooxygenase-mediated pyrethroid resistance. In vivo administration of 0.25% PB in sugar water to S+ flies induced higher levels of three components of the microsomal monooxygenase system, as well as increased levels of all six monooxygenase activities. In contrast, PB treatment had no significant effect on the level of cytochrome P450 or on EROD, AHH, or AE activity in LPR flies. Additionally, PB caused a decrease in the level of cytochrome b 5 and MROD and PCMA activities, while ECOD activity and the level of NADPH-cytochrome c (P450) reductase were significantly increased. SDS-PAGE indicated that induction of S+ flies with PB caused an increase in at least seven proteins with apparent molecular masses of 47,400; 49,400; 51,500; 52,200; 54,400; 56,000; and 57,500 Da. However, no observable difference was noted in PB-induced LPR flies. Differences in monooxygenase activities, in conjunction with the changes in turnover numbers of ECOD and AE activities in S+ flies and ECOD and MROD activities in LPR flies, suggest that PB treatment induced both quantitative and qualitative changes in the S+ and LPR cytochrome P450 monooxygenases.

Characterization of the induction of rat hepatic microsomal drug-metabolizing enzymes by 1-nitropyrene metabolites, 1-aminopyrene and N-acetylaminopyrene

The effect of 1-aminopyrene (1-AP) and N-acetylaminopyrene (1-NAAP) on rat hepatic microsomal monooxygenase system was investigated. Both drugs increased the total content of cytochrome P-450 (cyt. P-450). The substrate specificity and the electrophoretic pattern of 1-AP and 1-NAAP induced cytochrome(s) were compared with those of the major forms of cyt. P-450 induced by 3-methylcholanthrene (3-MC) and phenobarbital (PB). The results suggest that the form of cyt. P-450 induced by 1-AP and 1-NAAP resembles that one induced by 3-MC. Furthermore the abilities of liver microsomes from control or differently induced rats to ring hydroxylate and to activate 1-nitropyrene (1-NP) metabolites to species mutagenic for bacteria were compared. It was observed that: (1) 1-NAAP is a good substrate for microsome-mediated ring hydroxylation, whereas 1-AP is oxygenated only at a low extent; (2) 3-MC, 1-AP and 1-NAAP-stimulated microsomes are more active than control or PB-ones to ring hydroxylate 1-NAAP. As phenolic derivatives of 1-NAAP show high mutagenic activity, these results indicate that 1-AP and 1-NAAP induce toxification pathways of 1-NP in similar way, even if in less extent, as compared to 3-MC.

Effects of ethanol on microsomal drug metabolism in aging female rats. I. Induction

The purpose of this study was to determine how aging affects the induction by ethanol or acetone of the hepatic microsomal monooxygenase system of female Fischer 344 rats. Young-adult, middle-aged and old rats (4, 14 and 25 months) were fed an ethanol-containing or control liquid diet for 15 days. Cytochrome P-450, cytochrome c reductase, aniline hydroxylase, nitrophenol hydroxylase, nitroanisole O-demethylase and benzphetamine N-demethylase activities were measured in hepatic microsomes. All of the drug metabolism activities except benzphetamine N-demethylase were 20–35% lower in old than in young-adult rats fed the control diet. In addition, the increase in drug metabolism produced by feeding the regular ethanol diet (36% of calories as ethanol) was 50–60% lower in the old rats. However, there was no difference in the magnitude of ethanol induction when ethanol intakes were matched. The effects of chronic acetone consumption (1.2 g/day per kg body weight for 15 days) paralleled those of ethanol consumption, except that the extent of induction was greater with acetone. Acetone-induced levels of hepatic microcosomal cytochrome P-450, nitrophenol hydroxylase, nitroanisole O-demethylase and aniline hydroxylase were similar in all three age groups. The results of this study indicate that induction of hepatic microsomal drug metabolism by ethanol or acetone is unaffected by the aging process.

Effect of cimetidine on hepatic biochemical changes, liver toxicity and major urinary metabolite excretion of trichloroethylene in rats.

The effects of cimetidine (CIM) (an inhibitor of the hepatic microsomal monooxygenase system) on the metabolism and hepatotoxicity of trichloroethylene (TRI) were studied in male Sprague-Dawley rats. Rats were given three doses of 120 mg/kg i.p. (low-dose regimen) of CIM at 0, 6 and 11 h for 1 day, or ten doses of 200 mg/kg (high-dose regimen) at 8, 11, 14 and 17 h for 2 days and 8 and 11 h on 3rd day. Trichloroethylene (0.5 or 0.65 ml/kg) was administered i.p. 1 h after 2nd dose (low-dose regimen) or 9th dose (high-dose regimen) of CIM. In the low-dose regimen study, the activity of hepatic microsomal aminopyrine N-demethylase was decreased 1 and 5 h after the second dose and 7 h after the third dose of CIM, but became normal 20 h after the last dose. The cytochrome P-450 content and the activities of aniline hydroxylase and epoxide hydratase remained unchanged. Trichloroethylene at both dose levels produced liver toxicity, as verified by increase in activities of SDH and SGPT as well as by liver histology. Cimetidine alone had no such effect. An apparent reduction in TRI toxicity by CIM (at both dose regimens) could be observed histologically. The biochemical tests (SDH and SGPT) corroborated the histological changes only when TRI was given at a dose of 0.5 ml/kg combined with a high-dose regimen of CIM. Cimetidine at both dose regimens had a tendency to decrease the in vivo metabolism of TRI.(ABSTRACT TRUNCATED AT 250 WORDS)

The monooxygenases of birds, reptiles and amphibians.

1. Microsomal monooxygenase systems which contain cytochrome P-450 forms as their active centres are found in birds, reptiles and amphibians. Liver provides a rich source of monooxygenases but they are also present in other tissues. 2. In the hepatic microsomes of these species, levels of cytochrome P-450 and, in most cases monooxygenase activities, are lower than are found in hepatic microsomes of mammals. 3. Amongst birds, the lowest hepatic microsomal monooxygenase activities have been reported for specialized predators (fish-eaters and raptors). 4. When birds are dosed with inducers of the 3-methylcholanthrene (MC) type, the pattern of induction is similar to that in mammals. In contrast, phenobarbitone-type inducers are less effective in birds than in mammals--in some cases having no action at all. Prochloraz is a stronger inducer in birds than in the rat; it is an inducer of mixed type in birds. 5. Partial purification of avian cytochromes P-450 indicates substantial differences in properties from those of mammals. Further differences between birds and mammals in regard to the immunochemical properties of isoforms are evident from comparative studies using Western blotting. 6. On the evidence of Western blotting and response to inducers, cytochromes of family II (P450 II) seem less well represented in birds than in mammals. 7. The low monooxygenase activities of certain species of birds may make them relatively susceptible to lipophilic environmental chemicals that are detoxified by this system.

Separation and purification of component proteins of the cytochrome P-450-dependent microsomal monooxygenase system by high-performance liquid chromatography

The component proteins of the hepatic microsomal monooxygenase system, including various cytochrome P-450 isozymes, were separated and isolated from liver microsmes of untreated rabbits by Aminohexyl Sepharose and high-performance liquid chromatography (TSK preparative DEAE 5-PW, Bio-Rad HPHT). In addition to the known cytochrome P-450 isozymes, two new isozymes and one variant of the major isozyme were isolated. The monooxygenase activity was reconstituted by incorporating the purified proteins into liposomal membranes.

Mechanism of biological formation of cannabielsoin from cannabidiol in the guinea-pig, mouse, rat and rabbit.

Biological formation of cannabielsoin (CBE) from cannabidiol (CBD) was studied in the guinea pig, mouse, rat and rabbit in vitro. Emphasis was placed on the elucidation of this formation mechanism. The enzyme activity of CBE formation was localized in hepatic microsomes. The enzymatic reaction required nicotinamide adenine dinucleotide phosphate (NADPH) and molecular oxygen, and showed an optimal pH around 7.3. The microsomal CBE-forming activities decreased in the following order; guinea pig greater than mouse greater than or equal to rabbit greater than or equal to rat. The CBE formation in the guinea pig hepatic microsomes was suppressed with various inhibitors of cytochrome P-450 such as SKF 525-A, alpha-naphthoflavone and carbon monoxide, but not by disodium ethylenediamine tetraacetate. When incubated with the microsomes either in the presence or absence of NADPH, a synthetic epoxide of CBD, 1S, 2R-epoxy-CBD-2',6'-diacetate was easily and exclusively converted to CBE. On the other hand, 1R, 2S-epoxy-CBD was not changed to CBE at all, but to several oxidized metabolites. These results suggest that CBD is biotransformed to 1S,2R-epoxy-CBD with hepatic microsomal monooxygenase system including cytochrome P-450, and the epoxide is immediately converted to CBE.

Ozagrel hydrochloride monohydrate, a thromboxane synthase inhibitor, and its metabolites as inhibitors of hepatic microsomal drug metabolism.

The change in the hepatic oxidative drug-metabolizing capacity in humans treated with ozagrel hydrochloride monohydrate (OZA), an imidazole derivative and a new thromboxane A2 synthase inhibitor, was studied and the inhibitory potencies of the metabolites of OZA (M-1 and M-2) on the mouse hepatic microsomal monooxygenase system in vitro were compared with that of OZA. In vitro, M-1 and M-2, which are the beta-oxidized form and the reduced form of OZA, respectively, inhibited aminopyrine N-demethylation, aniline hydroxylation and testosterone hydroxylations in mouse hepatic microsomes and produced type II difference spectra in the same manner as OZA. The kinetic data indicated that the inhibitory potencies and the affinities of these compounds for cytochrome P-450 were decreased in the order of M-2 greater than OZA greater than M-1. The ratio of 6 beta-hydroxycortisol (6 beta-OHF) to cortisol (F) in urine, used as an indicator of oxidative drug-metabolizing capacity in humans, did not change significantly during oral treatment with 400 mg/d of OZA, while the ratio decreased to 80-85% of the original level during treatment with 800 mg/d of OZA. Although the participation of the metabolites of OZA in the reduction of drug-metabolizing capacity in vivo is not yet clear, the results suggest that hepatic oxidative drug-metabolizing enzyme activities in humans are inhibited by treatment with a relatively high dose of OZA.

Interactions of heterocyclic Maillard products with the hepatic microsomal monooxygenase system.

1. Interactions of methyl-substituted pyrazines, and other constituents of Maillard products generated during heat treatment of food, with hepatic microsomal mixed-function oxygenases were studied in vitro. 2. Spectral interactions of N-containing heteroaromatic compounds with the cytochrome P-450 system are type I or type II depending on the state of induction, and are relatively weak. Inhibition of 7-ethoxycoumarin O-deethylation by these compounds is ten times lower than that of metyrapone, agreeing with the weak spectral interaction. Inhibition is competitive for 2,3-dimethylquinoxaline, and complex for 2,5-dimethylpyrazine and 2,3,5,6-tetramethylpyrazine. 3. Spectral and inhibitory interactions indicate biotransformation. This was studied with 2,3,5,6-tetramethylpyrazine; the metabolite formed was identified as 2-hydroxymethyl-3,5,6-trimethylpyrazine. Metabolism to the N-oxide did not occur.

Effects of 2-arylbenzimidazoles on rat hepatic microsomal monooxygenase system

1. The effects of eight newly synthesized 2-aryl substituted benzimidazole derivatives on control and phenobarbital (PB) treated rat liver microsomal aniline 4-hydroxylase and ethylmorphine N-demethylase activities, and their binding to control and PB-treated rat liver microsomal oxidized cytochrome P-450 are presented. 2. All compounds inhibited ethylmorphine N-demethylase activity with I 50 values ranging from 8.50 × 10 −4 M to 27.83 × 10 −4 M in control and ranging from 2.80 × 10 −4 M to 15.79 × 10 −4 M in PB-treated rats. 3. Aniline 4-hydroxylase activity was inhibited by all of the compounds tested having I 50 values in the range of 7.04 × 10 −4 M-31.37 × 10 −4 M in PB-treated rats, but only five of the compounds showed inhibitory activity in control rats. 4. Only a few significant regression coefficients could be found between the parameters of the chemicals studied and their inhibitory patterns. 5. No correlation has been observed between the binding of the derivatives and their inhibitory pattern.