Effects of mammalian CYP3A inducers on CYP3A-related enzyme activities in grass carp (Ctenopharyngodon idellus): Possible implications for the establishment of a fish CYP3A induction model

Unexpected drug-drug interactions in fish are generally associated with the induction of CYP3A activity and may lead to the formation of drug residues and thus threaten the safety of fishery products. However, little information is available about CYP3A induction in fish. In the present study, we determined the in vivo and in vitro effects of typical mammalian CYP3A inducers (rifampicin, phenobarbital and dexamethasone) on CYP3A-related enzyme activities in a freshwater teleost, the grass carp (Ctenopharyngodon idellus). Our results showed that the response to rifampicin was similar for grass carp liver cell line (GCL), liver microsomes and the primary hepatocytes of grass carp, as indicated by the activity of aminopyrine N-demethylase (APND). When erythromycin N-demethylase (ERND) and 6beta-testosterone hydroxylase (6beta-TOH) were taken into consideration, the GCL displayed a greater capacity for conducting CYP3A metabolism and induction than the C. idellus kidney cell line (CIK). Using erythromycin and testosterone as substrates, we demonstrated that CYP3A catalysis exhibited non-Michaelis-Menten kinetics in GCL cells, and that V(max)/K(m) values were significantly increased due to rifampicin-treatment. Overall, this study may have implications for the use of GCL as a CYP3A induction model to identify physiological changes in fish as well as the similarities or differences between fish and mammals.

Effect of phenobarbitone treatment against signal grass (Brachiaria decumbens) toxicity in sheep

The effect of phenobarbitone against signal grass (Brachiaria decumbens) toxicity was studied in 26 male crossbred sheep. Grazing on signal grass significantly decreased the concentration of cytochrome P-450 and the activity of drug metabolizing enzymes, viz. aminopyrine-N-demethylase, aniline-4-hydroxylase, UDP- glucuronyltransferase and glutathione-S-transferase in liver and kidneys of affected sheep.Oral administration of phenobarbitone (30 mg/kg body weight) for five consecutive days before grazing on B. decumbens pasture, and thereafter, for three consecutive days every two weeks, resulted in significant increases in hepatic and renal activities of drug-metabolizing enzymes. The induction of drug metabolizing activity in sheep grazing on signal grass group was found to be lower than in animals given phenobarbitone alone. Induction by phenobarbitone provided a degree of protection against the toxic effects of B. decumbens as indicated by the delay in the appearance of signs of toxicity. Furthermore, these were much milder compared to those in the sheep not treated with phenobarbitone. The present study suggests that phenobarbitone-type cytochrome P-450 isoenzyme-induction may increase resistance against signal grass (B. decumbens) toxicity in sheep.

Effects of clonidine in a primed rat model of acute hepatic porphyria

Acute hepatic porphyrias can be induced by several drugs and acute attacks of porphyrias are often associated with severe hypertension. Therefore it is important to know if an antihypertensive drug used has porphyrogenic potency or not. As previously demonstrated in normal rats the alpha-receptor blocker clonidine (CAS 4205-90-7) has no significant influence on the porphyrin metabolism. Pretreatment of rats with 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) or allyl-isopropyl-acetamide (AIA) induces hepatic delta-aminolaevulinic acid synthase (ALA-S) and increases the urinary excretion of porphyrin precursors (ALA and PBG) comparable to the latent phase of acute hepatic porphyrias in humans. Clonidine did not induce hepatic ALA-S or urinary excretion of ALA or PBG in normal as well as in DDC or AIA pretreated rats. Moreover the induction of P4501A1 (7-ethoxyresorufin-O-deethylase) by DDC was abolished by simultaneous application of clonidine. From these findings one can probably conclude that clonidine is a safe drug in human acute hepatic porphyria.

Effect of 2,4,4'-trichloro-2'-hydroxydiphenyl ether on cytochrome P450 enzymes in the rat liver

We examined the effect of 2,4,4'-trichloro-2'-hydroxydiphenyl ether (Irgasan DP300) on microsomal cytochrome P450 (P450) enzymes in rat liver. Rats were treated intraperitoneally with Irgasan DP300 daily for 4 days, at doses of 0.2, 0.4 and 0.8 mmol/kg. Among the P450-dependent monooxygenase activities, 7-benzyloxyresorufin O-debenzylase (BROD) and 7-pentoxyresorufin O-depentylase (PROD) in rats, which are associated with CYP2B1, were remarkably induced by all doses of Irgasan DP300. The relative induction to each control activity were from 5.6- to 22.3-fold and 4.9- to 20.2-fold, respectively. Furthermore, immunoblotting showed that CYP2B1/2 protein level in rat liver microsomes was increased from 10.8- to 34.4-fold by Irgasan DP300. In addition, 7-ethoxycoumarin O-deethylase (ECOD) and p-nitrophenol hydroxylase (PNPH) activities were significantly increased by Irgasan DP300 at all doses (from 1.4- to 4.9-fold). Although the activities of other P450-dependent monooxygenases, namely aminopyrine N-demethylase (APND), aniline p-hydroxylase (ANPH), erythromycin N-demethylase (EMND), lauric acid omega-hydroxylase (LAOH) and testosterone 6 beta-hydroxylase (TS6BH) were increased at high doses (> or = 0.4 mmol/kg) of Irgasan DP300, the relative level was lower than those of the CYP2B1-dependent monooxygenases such as BROD and PROD. However, 7-ethoxyresorufin O-deethylase (EROD), 7-methoxyresorufin O-demethylase (MROD), testosterone 2 alpha-hydroxylase (TS2AH) and testosterone 7 alpha-hydroxylase (TS7AH) activities were not affected by any doses of Irgsan DP300. Immunoblotting showed that CYP3A2/1 and CYP4A1 protein levels were significantly induced from 1.3- to 2.2-fold by Irgasan DP300 (> or = 0.4 mmol/kg), whereas those of CYP1A1/2, CYP2C11/6 and CYP2E1 were not affected by any doses of Irgasan DP300. These results suggest that Irgasan DP300 induces the P450 isoforms of CYP2B subfamily in the rat liver, and that the induced P450 isozymes closely relates to the toxicity of Irgasan DP300 or its chlorinated derivatives.

Molecular non-genetic biomarkers related to Fenarimol cocarcinogenesis: organ- and sex-specific CYP induction in rat

Selective biochemical markers of effect have been used to evaluate some non-genotoxic cocarcinogenic properties of Fenarimol. Several CYP-dependent reactions have been monitored in liver, kidney and lung microsomes of male and female Sprague-Dawely rats treated (i.p.) with 200 or 400 mg/kg body wt dose of this pesticide. Highly specific substrates were used as probes of various isoforms, such as CYP1A1, 1A2, 2B1, 2E1 and 3A. A complex pattern of CYP induction, including organ- and sex-related differences in the inductive response by Fenarimol, has been recorded in this investigation, the kidney (mainly male) being more responsive when compared to other tissues. A 6.6-fold increase in the 2B1-like activity, probed by dealkylation of pentoxyresorufin was observed in the liver at a higher dose. On the contrary, a marked induction of CYP1A1 mediated ethoxyresorufin O-deethylase activity, ranging from 20- to 35-fold in female and male, respectively, was observed in the kidney at a lower dose tested. In the lung, at a higher dose, the p-nitrophenol hydroxylase activity (2E1) was enhanced up to 3.5-fold in male animals, whereas the 3A-like activity, probed by the N-demethylation of aminopyrine, was induced up to 2.6-fold in females. A weak, although significant reduction of CYP2B1 isoforms in lung was also recorded. Taken together, these data corroborated by means of Western immunoblotting analysis (using rabbit polyclonal antibodies anti-CYP 2B1/2, 1A1, 2E1, and 3A1/2) indicate a possible cotoxic, comutagenic cocancerogenic and promoting potential of this fungicide.

Toxicity of PCB 156 in the rat liver: an ultrastructural and biochemical study

PCB 156 (2,3,3',4,4',5-hexachlorobiphenyl) congener was given to weanling Sprague-Dawley rats in diets prepared by mixing it in 4% corn oil. The animals were placed in eight groups, each comprising 10 males or females and received diets that contained 0.01, 0.1, 1, 10 ppm PCB; in addition, two control groups of rats of each gender were given diets mixed with corn oil. Thirteen weeks after commencement of dosing, animals were euthanized and liver specimens were harvested and prepared for transmission electron microscopy. Hepatocyte architectural alterations comprised augmentation of smooth reticulum profiles and mitochondria with unorthodox cristae in animals regardless of gender from 1 and 10 ppm groups. Hepatic microsomal aminopyrine-N-demethylase was elevated significantly in both genders at highest (10 ppm) congener concentration. Based on our previous work, PCB 156 is estimated to be more toxic than PCB 153 or 28 in terms of liver morphologic expressions.

Induction of hepatic cytochrome P450 by phenobarbitone in rhesus monkey (Macaca mulatta)

An intraperitoneal administration of PB at a daily dose of 50 mg Kg-1 body wt for 4 days increased the specific content of hepatic microsomal heme, cytochrome P450 and the activity of aminopyrine N-demethylase by 1.8, 2.8 and 3.5 fold respectively. These results were substantiated by the intensification of the 52.5 KDa polypeptide in the electrophoretogram of sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the hepatic microsomes obtained from PB-pretreated versus control macaques. PB did not affect the hepatic content of cytochrome b5 and the activity of NADPH cytochrome c reductase, whereas it decreased the activity of NADH cytochrome c reductase in the rhesus monkeys. To the best of our knowledge this is a first report on the induction of hepatic cytochrome P450 and related enzymes by PB in rhesus monkeys.

The effect of phenobarbital and dexamethasone coadministration on the activity of rat liver P450 system

Phenobarbital, the potent inducer of CYP2B and CYP3A, and dexamethasone, that induces CYP3A, are not able to elevate p-nitrophenol hydroxylase activity of CYP2E1. However, rats treated with phenobarbital and dexamethasone in combination showed threefold increase in p-nitrophenol hydroxylation and the activity correlates with an elevated amount of a 53.000 dalton protein. Biosynthesis of mRNA and P450 protein is required for the induction. 3-amino-1,2,4-triazole and anti CYP2E1 IgG inhibition studies show that CYP2E1 is not responsible for enhanced p-nitrophenol hydroxylation, but the residual activity indicates the participation of other isozyme(s). As a result of double induction, changes in the amount of CYP2E1 of microsomes were not detected by Western blot analysis compared to untreated rat liver microsomes.

Induction of hepatic microsomal cytochrome P450 by dexamethasone in rhesus monkey (Macaca mulatta)

Intraperitoneal administration of 150 mg dexamethasone (DEX) Kg-1 body wt for four days to rhesus monkeys resulted in statistically significant increases in the activities of hepatic tyrosine aminotransferase (3 fold), microsomal cytochrome P450 (2 fold) and erythromycin N-demethylase (4 fold), but no change in the activities of aminopyrine N-demethylase and NADPH cytochrome c reductase. Three peaks were obtained from control or DEX-treated monkey livers on fractionation of detergent solubilized microsomes by anion exchange chromatography on DE-52. Peak II obtained from DEX-treated monkey microsomes on DE-52 demonstrated the highest specific activity of cytochrome P450 (5.84 nmol mg-1 protein) as compared to other peaks from the same microsomes or any of the peaks obtained from the control microsomes. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of the microsomes from control and experimental animals and peak II obtained after anion exchange chromatography of DEX-treated microsomes demonstrated the intensification of two polypeptides of 52.5 and 50 kDa. The results indicate that DEX is an inducer of cytochrome P450 and dependent erythromycin N-demethylase in non-human primate, Macaca mulatta.

Influence of oral isotretinoin on hepatic and cutaneous P-450-dependent isozyme activities

Oral administration of isotretinoin (13-cis-retinoic acid) (6 mg/kg per day), 0.05% hexachlorobenzene (HCB) or both drugs simultaneously for 10 days to female Wistar rats caused a statistically significant induction of aminopyrine-N-demethylase (ADM), 7-ethoxyresorufin-O-deethylase (7-ERO-D) and erythromycin-N-demethylase (EMDM) in the liver microsomes. Oral administration of isotretinoin alone or together with HCB induced a marked induction of 7-ERO-D and EMDM in the skin. Administration of isotretinoin alone for 60 days resulted in the induction of EMDM in the liver microsomes, and in combination with HCB caused a statistically significant induction of all hepatic isozymes. HCB alone caused a marked induction of only 7-ERO-D in the skin. These results clearly show that oral isotretinoin is capable of inducing hepatic and cutaneous microsomal P-450-dependent catalytic activities. It remains to be elucidated whether the induction of these enzymes is of importance for the therapeutic action of isotretinoin.

Interaction of caffeine with acetaminophen. 1. Correlation of the effect of caffeine on acetaminophen hepatotoxicity and acetaminophen bioactivation following treatment of mice with various cytochrome P450 inducing agents

The combination of caffeine with acetaminophen (APAP) is used widely in the treatment of headache. The effects of caffeine on APAP-induced hepatotoxicity and APAP bioactivation by liver microsomes from uninduced mice and from mice pretreated with various agents that induce cytochrome P450 were studied. When 1 mM caffeine was included, the rate of glutathione-APAP conjugate (GS-APAP) formation was increased significantly by 33 and 39% in microsomes from phenobarbital (PB)- and dexamethasone (DEX)-treated mice, respectively, whereas this parameter was decreased 39 and 12% by caffeine in microsomes from beta-naphthoflavone (beta NF)- and acetone-treated mice, respectively. A 5 mM concentration of caffeine increased GS-APAP formation by 47, 107 and 117% in microsomes from control, PB-, and DEX-treated mice, respectively, and decreased it 39 and 25% in microsomes from beta NF- and acetone-treated mice, respectively. Caffeine was a competitive inhibitor of APAP bioactivation in microsomes from beta NF- and acetone-treated mice. While caffeine increased APAP bioactivation in microsomes from uninduced, PB-, and DEX-treated mice, the apparent Km values for APAP were increased by caffeine, indicating that this enhancement was not due to a direct effect of caffeine on APAP binding to cytochrome P450 but may be due to an effect of caffeine on the substrate-enzyme complex. The variable effect of caffeine on APAP hepatotoxicity correlated with the effect of caffeine on APAP bioactivation by liver microsomes, regardless of pretreatment. Lack of correlation of aminopyrine N-demethylase, but good correlation of erythromycin N-demethylase activity with the extent of caffeine enhancement of APAP bioactivation following PB or DEX treatment suggests that a murine P450 subfamily similar to the rat P450 3A subfamily may be the candidate in mediating the stimulatory effect of caffeine on APAP bioactivation and APAP-induced hepatotoxicity.

Elevation of serum triacylglycerol concentration in association with hepatic microsomal enzyme induction after treatment with phenylbutazone and diclofenac sodium in rats

1. The relationship between serum triacylglycerol concentration and hepatic microsomal enzyme activity was examined in rats. 2. Two groups of rats were injected with diclofenac sodium at doses of 2.5 and 5 mg day-1 kg-1. A third group was injected with phenylbutazone at a dose of 20 mg day-1 kg-1. The treatment was continued for 15 days and the rats were killed 24 h after the last dose. 3. In all drug-treated rats, the serum triacylglycerol concentration and the hepatic microsomal activities of aminopyrine N-demethylase and aniline hydroxylase were significantly increased as compared with the corresponding values in control rats. The correlations between the serum triacylglycerol concentrations and the activities of the two enzymes, as indices of the hepatic microsomal activity, were highly significant. 4. These results indicate that the possibility of hypertriglyceridaemia as an adverse effect of the induction of the hepatic microsomal enzymes after the administration of phenylbutazone and diclofenac sodium should be considered.

Further differentiation of 2'- and 4'-chloroflavones as inducers of hepatic microsomal monooxygenases in rats

2'-Chloroflavone (CF) and 4'-CF were studied as inducers of cytochrome P-450-mediated monooxygenases in rat hepatic microsomes by comparison with phenobarbital (PB) and 3-methylcholanthrene (MC). As a result of interaction with the substrate 7-ethoxycoumarin (7-EC), 2'-CF-induced microsomes showed a difference spectrum different from usual type I, whereas 4'-CF-induced microsomes showed a typical type I spectrum. 2'-CF-induced microsomes increased the Vmax of aminopyrine (AM) N-demethylase activity about 1.6-fold while not affecting the apparent Km value. In contrast, 4'-CF-induced microsomes decreased the Km of 7-EC O-deethylase activity to about one fourth while significantly increasing the Vmax value. All the activities of AM N-demethylase, 7-EC O-deethylase and 6,7-dimethoxycoumarin O-demethylase in 2'-CF-induced microsomes were inhibited strongly by metyrapone and SKF 525-A, and significantly by diphenhydramine. The three enzyme activities in 4'-CF-induced microsomes were inhibited markedly by alpha-naphthoflavone and significantly by 2-bromo-4'-nitroacetophenone. These results further support the previous proposal that 2'-CF is a PB-type inducer of cytochrome P-450 while 4'-CF is a MC-type inducer.

Effect of carrageenan-induced inflammation on the induction of hepatic microsomal enzymes by phenobarbital and benzo[a]pyrene in male rats

The inhibitory effect of carrageenan-induced inflammation was examined by utilizing rats treated with inducers of this drug metabolizing system. Animals were given sodium phenobarbital (PB, 80 mg/kg, i.p., 24 h prior to death), or benzo[a]pyrene (BP, 40 mg/kg, i.p., 24 h prior to death) as inducers. Some animals were also given carrageenan 24 h prior to death. Non-induced male rats exhibited significant decreases in hepatic 9000 x g supernatant (S-9) cytochrome P-450 and aminopyrine (AM) N-demethylase, benzphetamine (BenzP) N-demethylase and meperidine (MP) N-demethylase activity following carrageenan treatment. Carrageenan also depressed the induction of hepatic S-9 cytochrome P-450 content caused by PB treatment, and suppressed the induction of AM, BenzP, MP, arylhydrocarbon and 7-ethoxycoumarine metabolism by PB treatment. Cytochrome P-450 levels and related biotransformation activity which are elevated by BP treatment were not decreased by the injection of BP and carrageenan simultaneously to male rats. Non-induced, PB-treated and BP-treated female rats did not show inhibited carrageenan-induced reduction in hemoprotein content or inhibition of AM-N-demethylase, BenzP N-demethylase and aniline hydroxylase activities. These results demonstrate the selective nature of the inhibitory effects of carrageenan-induced inflammation upon drug metabolism in the rats.

Unimpaired induction of drug-metabolizing enzymes in hepatocytes isolated from rats with micronodular cirrhosis

To test further the competence of the cirrhotic liver to metabolize xenobiotics, hepatocytes were isolated from control and CCl4-induced cirrhotic male or female rats. Histologically micronodular cirrhosis was present in all CCl4-treated rats, while control rats had normal livers. Portal perfusion pressure and intrahepatic collagen content were also significantly increased by CCl4 administration. In male rats, no significant differences in levels of circulating transaminases nor in alkaline phosphatase was observed between cirrhotic and control rats, while CCl4-treated females had slightly higher than normal serum transaminase levels at the time of the studies. Hepatocytic cytochrome P-450 and basal xenobiotic biotransformation were unaffected by micronodular cirrhosis in both genders; calculation of the aminopyrine and 7-ethoxycoumarin intrinsic clearances (Cli) revealed, however, a slightly decreased transformation potential in hepatocytes obtained from cirrhotic females, a phenomenon not observed in cirrhotic male rats. It is speculated that the observed reduction in Cli may have been independent of cirrhosis per se, owing to the perduring cytotoxic effect of CCl4 as evidenced by the higher than normal level of transaminases in female rats. Finally, male rats were subjected to in vivo administration of phenobarbital or 3-methylcholanthrene; both compounds led to significant induction of the mixed-function oxidase system, which was similar in magnitude and in selectivity in control and cirrhotic rats as illustrated by calculation of the Michaelis-Menten kinetic parameters for aniline p-hydroxylation, aminopyrine-N-demethylation, 7-ethoxycoumarin-O-deethylation, and p-nitrophenol UDP-glucuronyl transferase. We conclude that in well-established but compensated and hepatolysis-free micronodular cirrhosis, hepatocytes are fully able to transform xenobiotics and to respond normally and selectively to inducers of drug metabolism.

Pharmacokinetics of 4-acetylaminophenylacetic acid. 2nd communication: tissue accumulation and enzyme induction in rats after repeated administration, and placental and milk transfer after single dosing

The absorption, distribution, metabolism and excretion of 14C-labeled 4-acetylaminophenylacetic acid (MS-932) were studied in male rats after administration of an oral dose of 10 mg/kg once a day for 21 days. Comparison with the single dosing showed no marked alterations in absorption, distribution, metabolism and excretion. There were no significant differences in the activities of hepatic aniline hydroxylase and aminopyrine N-demethylase between the MS-932 treated group (10 mg/kg for 8 days) and the 0.5% aqueous sodium carboxymethyl cellulose control group (p greater than 0.05). Placental transfer of radioactivity was studied after single oral administration of 10 mg/kg of 14C-MS-932 to pregnant rats on the 12-13th and 19-20th days of gestation. Radioactivity concentrations were highest in the maternal plasma and lowest in the amniotic fluid and fetus for both middle and late pregnancies. The concentrations in the amniotic fluid and fetus decreased more slowly than did the concentration in the maternal plasma. Excretion of radioactivity to milk was studied after single oral administration of 10 mg/kg of 14C-MS-932 to lactating rats on the 10th day after parturition. Radioactivity concentrations in the rat milk were maximal at 1 h after dosing and were lower than in the maternal plasma at all the sampling times.

Dose-related induction of rat hepatic drug-metabolizing enzymes by diuron and chlorotoluron, two substituted phenylurea herbicides

Dose-related induction of various hepatic drug-metabolizing enzymes has been investigated after short-term treatment of rats by diuron and chlorotoluron, a dichlorinated and a monochlorinated phenylurea herbicide, respectively. Results suggest that 'saturation' of the induction system of benzo(a)pyrene monooxygenase, 7-ethoxycoumarin O-deethylase and 7-ethoxyresorufin O-deethylase activities may occur in the same range of the molar doses of both compounds, and with the dichlorinated herbicide at much higher activities. Induction of epoxide hydrolase, UDP-glucuronyltransferase and glutathione S-transferases also shows saturation curves in the function of molar doses. However, the structural difference is not reflected in the enhancement of enzyme activities.

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 a 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 micrograms 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 micrograms/kg for C57BL/6J and 300 micrograms/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 LD50 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 micrograms/kg) and DBA/2J (30 and 600 micrograms/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 P1-450 and P3-450 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-450BNF-B (P1-450) and anti-P-450BNF/ISF-G (P3-450) 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 P1-450 and P3-450 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.

Induction of cytochrome P450 isozymes in human amnion FL cells

The human epithelial FL cell line contains low levels of constitutive ethoxyresorufin O-deethylase, ethoxycoumarin O-deethylase and aminopyrine N-demethylase activities, of which ethoxyresorufin O-deethylase could be induced by norepinephrine, beta-naphthoflavone and 3-methylcholanthrene, but ethoxy-couarin O-deethylase and aminopyrine N-demethylase activities could be induced by norepinephrine, beta-naphthoflavone and 3-methylcholanthrene as well as phenobarbital. Inducibility of ethoxyresorufin O-deethylase, ethoxycoumarin O-deethylase and aminopyrine N-demethylase activities was 3.1-6.7-, 1.8-3.1- and 1.4-2.0-fold respectively. Co-treatment with 3-methylcholanthrene and norepinephrine resulted in higher ethoxyresorufin O-deethylase and aminopyrine N-demethylase activities than treatment with 3-methylcholanthrene alone. The induced ethoxyresorufin O-deethylase and ethoxycoumarin O-deethylase activities remained at a high level for 24-36 h after removal of the inducer from the medium. The metabolizing ability of induced FL cells is stronger than that of non-induced ones in a given time following induction. The results further confirm that it is feasible to use the cytochrome P450 isozyme-induced FL cell as a biological indicator in short-term tests for screening promutagens/procarcinogens.

Long-term effects of phenobarbital on rat liver microsomal drug-metabolizing enzymes and heme-metabolizing enzyme

Long-term effects of phenobarbital on changes of rat hepatic enzymes involved in drug and heme metabolism were examined by employing two different treatment schedules which produce tolerance (tolerant group) and/or dependence (dependent group) against the drug. In both treatment groups, phenobarbital produced a marked and persistent induction of cytochrome P-450 and aminopyrine N-demethylase; the response was greater in the tolerant group than that in the dependent group at the early time periods. Thereafter, the magnitude of the induction of drug-metabolizing enzymes gradually decreased and finally was maintained at two-fold of the corresponding control levels. Likewise, the induction of cytochrome P-450b+e content, one of the major phenobarbital inducible species, was maintained at 40% of the total cytochrome P-450 content. Phenobarbital treatment resulted in increase of delta-aminolevulinate synthetase (ALAS) activity and a decrease in heme oxygenase (HO) activity. Changes of these enzymes involved in heme metabolism coincided with changes of the drug-metabolizing enzymes. Finally, the changes of enzyme content and activity after terminating phenobarbital treatment were determined and enzyme biological half-lives were calculated employing protein synthesis inhibitors. No significant differences in the changes of cytochrome P-450 and cytochrome P-450b+e content between the groups were observed after terminating phenobarbital treatment. However, changes of ALAS and HO activities after terminating the drug were significantly different between the groups; changes in the dependent group corresponded to the decrease in serum phenobarbital levels. On the other hand, biological half-lives of microsomal drug-metabolizing enzymes were longer in the tolerant group than in the dependent group.

Aminopyrine-N-demethylase activity of rat liver after administration of crude cannabis extract

The effect of cannabis extract, on the hepatic aminopyrine-N-demethylase activity was studied in rats. Daily administration of cannabis extract for 15 consecutive days increased the aminopyrine-N-demethylase activity which was significant on day 15 post-treatment at 2 and 10 mg/kg doses. At 20 mg/kg, a significant increase was observed from day 7 which continued up to day 15. These findings suggest that cannabis extract can induce hepatic aminopyrine-N-demethylase activity.

Inhibitory effect of zinc-protoporphyrin on the induction of heme oxygenase and the associated decrease in cytochrome P-450 content in rats

Pretreatment of rats with zinc-protoporphyrin, which has shown to be a potent competitive inhibitor of heme oxygenase, resulted in the inhibition of bromobenzene-mediated induction of heme oxygenase and decreases of the cytochrome P-450 content, aminopyrine demethylase and aniline hydroxylase activities. Such an inhibitory effect of zinc-protoporphyrin on the induction of heme oxygenase and concomitant decreases of drug-metabolizing enzymes occurred in a dose-dependent manner with complete inhibition of these effects at a dose of 40 mumol/kg. The effects of zinc-protoporphyrin were also observed in thioacetamide- and BCG-treated rats and ascitic tumor AH 66-bearing rats. Likewise, a decrease of cytochrome b5 content observed under these experimental conditions was also restored significantly by zinc-protoporphyrin. These results strongly suggest that the induction of heme oxygenase is a primarily important early event which consequently leads to the decrease in cytochrome P-450 content and associated enzyme activities.

Effect of four-day treatment with carbamazepine at different dose levels on microsomal enzyme induction, drug metabolism and drug toxicity

The effect of intraperitoneal injections of 0, 30, 60 and 100 mg/kg of carbamazepine (CBZ), twice a day for 4 days, was studied in 4 groups of 6 male Sprague-Dawley rats per group to evaluate its hepatic enzymatic induction, toxicity and metabolism. Rats were sacrificed on the fifth day and the urines of the last 24 hours were collected. While the activities of hepatic microsomal aminopyrine N-demethylase and epoxide hydratase tended to increase with the dose of CBZ, the cytochrome P-450 content and the activity of aniline hydroxylase however reached a maximum at 60 mg/kg. The percentage of the administered daily dose of CBZ excreted as unchanged CBZ in the urine increased considerably with the dose, while that of metabolites such as carbamazepine-10,11-epoxide (CBZ-E), trans-10,11-dihydrodihydroxycarbamazepine (TDC), and thioethers (T) did not markedly change. These data not only corroborate a maximum in enzyme induction but also suggest a saturation of the induced hepatic enzymes. Urinary T and TDC, representing more than 50% and less than 10%, respectively of the total amount recovered, tend to demonstrate that the glutathione conjugation with the intermediates of CBZ leading to the formation of higher mercapturates could be more important than the epoxide-diol pathway for the metabolism of CBZ under conditions of repeated dosing.

[Multiple forms of cytochrome P-450. Characteristics of isolated aminopyrine-N-demethylase (cytochrome P-450AP)]

A previously unidentified cytochrome P-450AP possessing the highest aminopyrine-N-demethylase activity has been isolated from liver microsomes of 4-isopropylaminoantipyrine-induced rats, using affinity chromatography in combination with ion-exchange chromatography with subsequent separation on hydroxyl apatite. Using radioisotope techniques, it was found that 4-isopropylaminoantipyrine induces cytochrome P-450AP synthesis de novo. The isolated cytochrome P-450AP has the following characteristics: Mr = 49,000 Da. CO-peak maximum at 450.5 mm, rate of aminopyrine demethylation in a reconstituted system-20 nmol HCHO/min/nmol of cytochrome P-450, benzphetamine-15. The hemoprotein synthesis is paralleled with the synthesis of a protein with Mr of 51,000 Da. Immunochemical analysis permitted to identify the latter protein as cytochrome P-450b. It was demonstrated that cytochrome P-450AP does not interact with the antibodies to the major phenobarbital-induced form, i.e., with cytochrome P-450b.

Ontogeny of the chicken cytochrome P-450 enzyme system. Expression and development of responsiveness to phenobarbital induction

The sensitivity of the developing embryo to toxins and drugs is highly dependent on the state of development of the cytochrome P-450 system. Previous work in this laboratory has demonstrated the genotoxicity of aflatoxin B1 (AFB1) to the chicken embryo at 3 days of incubation (DI) and induction of AFB1 genotoxicity by phenobarbital at 7 DI. In this study, the basal and 24-hr phenobarbital (PB) induced levels of aminopyrine-N-demethylase (AMPD) and cytochrome P-450 were assayed in hepatic microsomes from 7 DI to 36 days posthatching (PH) and in microsomes from whole embryos at 5 DI. A dose-response for induction by PB was observed in embryonic hepatic microsomes as early as 7 DI, whereas a low level of cytochrome P-450 was detected in control 7 DI microsomes using the reduced CO vs oxidized CO difference spectrum. Basal levels of AMPD and cytochrome P-450 in hepatic microsomes increased steadily throughout development as did the responsiveness of the embryonic liver to induction with PB. Hepatic microsomes from control and PB-induced chickens had the highest AMPD activities posthatching particularly from 1 to 3 days PH. Maximal induced levels, which were 2- to 3-fold over control throughout development, ranged from 1.22 at 7 DI to 12.72 nmol HCHO/mg protein/min at 2 days PH. The potency of PB as an inducer increased about 1000-fold between 7 DI and hatching. PB induction did not increase the specific activity of AMPD at any period of development. The specific activity of AMPD posthatching increased about 3-fold above embryonic levels, indicating the development of a cytochrome P-450 complex more active toward aminopyrine in the neonatal period.

Induction of rat hepatic cytochromes P-450 by environmental nitropolycyclic aromatic hydrocarbons

Nitrated polycyclic aromatic hydrocarbons (PAHs) are environmental contaminants that result from various incomplete combustion processes. We have examined the activity of hepatic microsomal enzymes in rats pretreated with a series of environmentally occurring nitrated PAHs including: 1- and 4-nitropyrene, 1,3-, 1,6- and 1,8-dinitropyrene, 6-nitrochrysene, 7-nitrobenz[a]anthracene, 3-nitrofluoranthene, and 1-, 3-, and 6-nitrobenzo[a]pyrene. None of the compounds increased the cytochrome P-450 content more than 2-fold. 1,8-Dinitropyrene, 6-nitrochrysene, and 1- and 3-nitrobenzo[a]pyrene significantly increased arylhydrocarbon hydroxylase activity 2- to 8-fold higher than solvent-treated controls. The induction of 7-ethoxycoumarin O-deethylase activity paralleled that found with arylhydrocarbon hydroxylase. The maximum induction of aminopyrine N-demethylase was only 1.5-fold, and none of the nitrated PAHs caused significant increases in epoxide hydrase or NADPH-cytochrome c reductase. 1-Nitropyrene reductase activity was induced by each of the compounds with the exception of 6-nitrobenzo[a]pyrene. The greatest increase was caused by 1-nitrobenzo[a]pyrene followed by 1,3-dinitropyrene, 3-nitrobenzo[a]pyrene and 6-nitrochrysene. These data suggest that nitrated PAHs may potentiate the effects of subsequent exposures to various chemical carcinogens.

Short-term oral administration of polybrominated biphenyls enhances the development of hepatic enzyme-altered foci in initiated rats

FireMaster BP-6 (FM), a commercial mixture of polybrominated biphenyls (PBB), has been shown to act as a tumor promoter in hepatocarcinogenesis assays in rats. Most hepatic tumor promoters must be administered for many weeks or months. Because FM is highly persistent in animal tissues, it was hypothesized that very short-term administration of FM would result in tumor promotion. Female Sprague-Dawley rats weighing 185-215 g were initiated by a two-thirds partial hepatectomy followed by 10 mg diethylnitrosamine/kg body weight (BW) 24 h later. Thirty days later, rats were gavaged with FM in corn oil, at total doses of 0, 13, or 130 mg FM/kg BW. Half the dose was given on d 30, and the remaining half was given 24 h later. At 120 d after gavage the rats were killed and necropsied. Five liver sections from each animal were histochemically stained for gamma-glutamyl transpeptidase-positive enzyme-altered foci (EAF). EAF were significantly increased over control values in initiated rats given 130 mg FM/kg. In animals given 13 mg FM/kg, EAF were increased to a lesser extent but not significantly above controls. Enhancement of these EAF in initiated rats reflects tumor-promoting activity. In this study, 24-h administration of FM in initiated rats was sufficient to enhance hepatic EAF measured 120 d later in an rats was sufficient to enhance hepatic EAF measured 120 d later in an initiation-promotion protocol, and a dose of 13 mg FM/kg was apparently close to a possible no-effect threshold level for enhancement of EAF.

Induction of rat hepatic monooxygenase activity by polychlorinated diphenyl ethers

Polychlorinated diphenyl ethers are recognized environmental contaminants. Twelve of these compounds were tested for their ability to induce liver cytochrome P-450 and monooxygenase activities in Sprague-Dawley rats. All the compounds increased P-450 levels or increased monooxygenase activities in a manner resembling 3-methylcholanthrene, phenobarbital or a combination of both (mixed). The responses obtained resembled those of the polychlorinated biphenyls, some of which are known to be toxic.

The effects of organochlorine pesticides as inducers of testosterone and benzo[a]pyrene hydroxylases

p,p'-DDE, phenobarbital, dieldrin heptachlor, chlordane and toxaphene induced rat liver microsomes exhibited increased formation of the 4,5-dihydrodiol, 3,6-quinone, 9- and 3-hydroxymetabolites of benzo[a]pyrene and the latter three compounds also induced an increase in the rate of formation of the 9,10-dihydrodiol metabolite. Lindane was inactive as an inducer of benzo[a]pyrene hydroxylase. With the exception of lindane, all the organochlorine pesticides and PB induced testosterone 16 alpha- and 16 beta-hydroxylases; in contrast lindane induced testosterone 6 alpha-, 7 alpha- and 6 beta-hydroxylases and PB also induced testosterone 15 beta-hydroxylase and androstenedione formation. Using a battery of monooxygenase enzyme assays it was evident that there were significant differences between PB and several organochlorine pesticides as inducers of rat hepatic cytochrome P-450-dependent monooxygenases.

Effect of pretreatment with hepatic microsomal enzyme inducers on the toxicity of diazinon in calves

The pretreatment of calves with a single dose of 10 mg kg-1 dieldrin or 21 daily doses of 10 mg kg-1 phenobarbitone increased the toxicity of diazinon as reflected by the development of more severe clinical signs and greater depression in whole blood cholinesterase activity in the pretreated calves. Induction by dieldrin or phenobarbitone of the hepatic microsomal enzyme amidopyrine-N-demethylase was also accompanied by a concurrent rise in the liver carboxylesterase activity.

Ascorbic acid requirement for the induction of microsomal drug-metabolizing enzymes in a rat mutant unable to synthesize ascorbic acid

We investigated the requirement of ascorbic acid for the induction by polychlorinated biphenyls (PCB) of hepatic drug-metabolizing enzymes in ODS-od/od rat (OD rat) which is a rat mutant unable to synthesize ascorbic acid. ODS- +/+ rats (+/+ rat), which can synthesize ascorbic acid, were used as controls. In OD rats, the dietary requirement of ascorbic acid to maintain normal growth and prevent any signs of scurvy is about 300 mg of ascorbic acid per kilogram diet. In this study, dietary levels of ascorbic acid tested were 0, 50, 300, 1000 and 3000 mg ascorbic acid per kilogram diet with or without 200 mg of PCB per kilogram diet. Feeding PCB did not affect growth in rats of either genotype. When statistical analysis was done within groups fed diets without PCB, ascorbic acid deficiency caused significant decreases in body weight gain, hepatic activities of drug-metabolizing enzymes and level of hepatic cytochrome P-450. When OD rats were fed a diet without PCB, the supplementation of about 300 mg ascorbic per kilogram diet was sufficient to maintain normal activities of hepatic aminopyrine N-demethylase, aniline hydroxylase, cytochrome c reductase and reduction of cytochrome P-450 and a normal level of hepatic cytochrome P-450. However, when OD rats were fed a diet supplemented with 200 mg PCB per kilogram of diet, significantly higher activities of hepatic aminopyrine N-demethylase and aniline hydroxylase and significantly higher level of hepatic cytochrome P-450 were observed in OD rats fed a diet supplemented with 1000 mg or 3000 mg ascorbic acid per kilogram of diet than in rats fed a diet supplemented with 300 mg of ascorbic acid. It is concluded that the dietary requirement of ascorbic acid is increased severalfold by the administration of xenobiotics, such as PCB, for the maximum induction of hepatic drug metabolism.

Induction of cytochrome P-450 and related drug-metabolizing activities in the livers of different rodent species by 2-acetylaminofluorene or by 3-methylcholanthrene

In general, large differences in the control levels of different cytochrome P-450-catalyzed activities (aminopyrine N-demethylase, benzo(a)pyrene monooxygenase, ethoxyresorufin O-deethylase, ethoxycoumarin O-deethylase and total 2-acetylaminofluorene metabolism and metabolite pattern) and in the inducibility of these activities in different rodent species (rat, hamster, guinea pig and mouse) and sexes were observed. For all the activities measured the lowest levels were observed in untreated rats. With a few minor exceptions, the only species tested in which cytochrome P-450-catalyzed activities were induced by treatment with 2-acetylaminofluorene was the rat. A larger number of the species tested were susceptible to induction by 3-methylcholanthrene. However, this xenobiotic proved also to induce most potently in the rat. There are relatively large differences between the male and female rat both in terms of control cytochrome P-450-catalyzed activities and in the inducibility of these activities by 2-acetylaminofluorene and 3-methylcholanthrene. In general, both of these xenobiotics proved to be more potent inducers in the female than in the male. Thus, it is quite clear that in quantitative terms the hepatic microsomal cytochrome P-450-catalyzed activities and their inducibility by 2-acetylaminofluorene or 3-methylcholanthrene in the male Sprague-Dawley rat are not representative for other rodent species or even for the female of the same species.

Induction of the hepatic cytochrome P-450 dependent monooxygenase system by cis- and trans-5,10-dihydrogen mirex

The hepatic induction of cytochrome P-450-dependent monooxygenase components by cis- and trans-5,10-dihydrogen mirex was studied in male and female laboratory rats. There were sex-dependent differences between the two isomeric derivatives of mirex. The cis-isomer significantly increased aniline hydroxylase activity in the female, but not in the male. In contrast, aminopyrine N-demethylase was significantly increased by the cis-isomer in both sexes. The trans-isomer increased the hydroxylase and N-demethylase activities in both sexes. The cis-isomer induced NADPH-cytochrome c reductase in the female, and the trans-isomer did not. Both isomers induced hepatic reductase activity in the male rat. No sex-dependent differences in the hepatic induction of cytochrome P-450 were observed for either isomer.

Tumor-promoting activity of benzodiazepine tranquilizers, diazepam and oxazepam, in mouse liver

The widely used benzodiazepine tranquilizers diazepam and oxazepam promoted development of hepatocellular hyperplastic foci and hepatocellular neoplasms (adenomas and carcinomas) when they were fed in diet to male B6C3F1 mice after initiation by N-nitrosodiethylamine. Diazepam was more effective than oxazepam and its effect was proportionate to dose. Both diazepam and oxazepam induced hepatomegaly, cytochrome P-450 and cytochrome P-450-dependent aminopyrine N-demethylase activity in hepatocytes, effects similar to those produced by a well-known rodent liver tumor promoter, phenobarbital. In view of the importance and widespread use of this class of compounds, more work is warranted to examine their effects on tumor development in different mammalian species.

[Characteristics of induced aminopyrine N-demethylase]

The study of the inducing effect of phenobarbital on the activity of N-demethylase of aminopyrine in the rat liver microsomes revealed the correlation of the rate of enzymic reaction with the changes in the maximum binding of aminopyrine by cytochrome P-450 rather than with the content of hemoprotein itself. The comparison of the activity of N-demethylation of aminopyrine in control and phenobarbital-induced rats showed the quantitative correspondence of the yield of products of the reaction--formaldehyde in the liver microsomes and 4-aminoantipyrine in the urine samples. Self-induction of N-demethylating activity at repeated aminopyrine administration to rats and the dose dependence of this phenomenon were found.

Evidence that methylsulfonyl metabolites of m-dichlorobenzene are causative substances of induction of hepatic microsomal drug-metabolizing enzymes by the parent compound in rats

This study was undertaken to clarify the relationship between the formation of 2,4- and 3,5-dichlorophenyl methyl sulfones, metabolites of m-dichlorobenzene (DCB), and their inducing effect on hepatic microsomal drug-metabolizing enzymes in rats. When m-DCB was injected ip into bile duct-cannulated rats, little or no methyl sulfones were detected in blood, liver, kidneys, adipose tissue, or bile. In the antibiotic-pretreated rats dosed with m-DCB, metabolite concentrations in the blood and the three tissues markedly decreased. These findings suggest that the formation of methylsulfonyl metabolites from m-DCB depends largely upon the metabolism of some precursor(s) excreted in the bile by intestinal microflora. The increasing effects of m-DCB administration on the activities of aminopyrine and aniline metabolizing enzymes and the contents of cytochromes P-450 and b5 in hepatic microsomes were scarcely observed in the bile duct-cannulated and antibiotic-pretreated rats, in which the drug-metabolizing enzymes were able to be induced by phenobarbital treatment. On the other hand, in rats administered 2,4- or 3,5-dichlorophenyl methyl sulfone hepatic distribution of each methyl sulfone was similar to that in intact rats, and the degree of increase of the above four parameters was nearly the same as that in the intact rats. These findings provide evidence that the induction of drug-metabolizing enzymes by m-DCB is not due to the action of m-DCB but is due to its methylsulfonyl metabolites.

Effects of a new fluorinated macrolide (P-0501A) and other erythromycins on drug metabolizing enzymes in rat liver

The effects of a new fluorinated macrolide (P-0501A) on drug metabolizing enzymes of rat liver were compared with three erythromycins--the base, the stearate and the estolate--after 7 days of dosing (1.36 mmol/kg po daily). The three erythromycins induced the synthesis of microsomal enzymes, but the products of their metabolism inactivated cytochrome P-450 in the order base less than or equal to stearate less than estolate. N-Demethylation of erythromycin and aminopyrine increased, while O-demethylation of 4-nitroanisole was reduced and hydroxylation of aniline was not changed after in vivo treatment. Pentobarbital sleeping time was prolonged and liver glutathione levels were lower in treated rats than in controls. In contrast to the three erythromycins, P-0501A did not induce the synthesis of microsomal enzymes, did not form an inactive complex with cytochrome P-450 and did not affect mono-oxygenase activities or pentobarbital narcosis.

[Pharmacokinetics and disposition of a new antitumor antibiotic (2''R)-4'-O-tetrahydropyranyladriamycin in rats. Distribution and excretion after multiple administration]

The accumulation of (2''R)-4'-O-tetrahydropyranyladriamycin (THP) was studied in rats received intravenous administration of 14C-THP at a dose of 0.5 mg/kg/day for 14 consecutive days by determining blood and tissue levels and the excretion of the radioactivity. The radioactivity levels in plasma and blood cells after the multiple administration were higher than those after single administration. The half-life of the radioactivity after the multiple administration was longer in the blood cells but not in the plasma than the half-life after a single administration. Tissue levels of the radioactivity after the multiple injection were 2 to 4 times as high as the levels after a single injection except for the brain and testes in which a large accumulation of the radioactivity was observed. However, little accumulation of unlabeled THP was found in most tissues when determined by HPLC. The accumulation of radioactivity in tissues, therefore, was due to metabolites of THP. The disposition of 14C-THP was also examined in rats which had previously received unlabeled THP (0.5 mg/kg/day) for 13 days. The pretreatment did not affect the disposition of 14C-THP seriously, although the pretreatment raised tissue levels slightly and a rebound of plasma level of 14C-THP, and lowered the fecal excretion ratio. No induction of hepatic drug metabolizing enzymes was observed in rats after repeated administrations of THP for consecutive 14 days.

[Monooxygenase system activity and rate of lipid peroxidation in rat liver microsomes in reinduction with polychlorinated biphenyls]

Single oral administration of sovol (mixture of polychlorinated diphenyls) caused a significant induction of the liver monooxygenase system (MOS) in rats. The inducing effect persisted for 5 months. Liver MOS responses were similar in repeated and primary sovol administrations. Differences in the morphological liver changes have been detected following single and repeated administrations of sovol.

Structure-activity relationship of bispyridyloxybenzene for induction of mouse hepatic aminopyrine N-demethylase activity. Chemical, biological, and X-ray crystallographic studies

1,4-bis-[2-(3,5-Dichloropyridyloxy)]-benzene (TCPOBOP) was previously shown to be an extremely potent phenobarbital-like inducer of hepatic microsomal monooxygenase activity in the mouse. To examine the structure-activity relationship, 31 congeners of TCPOBOP were synthesized and tested for their potency to induce hepatic aminopyrine N-demethylase activity in B6D2F1/J mice. For biological activity, the minimum requirement is a) a central 1,4-dioxygenated benzene ring, b) lateral pyridine rings linked to the central ring by ether bonds, but with other lateral heteroaromatic rings, e.g., quinoline or pyrimidine, also active, c) 5,5'-substituents of Cl, Br, or NO2 on the pyridine rings. For a series of 5,5'-substituted and 3,3'-dichloro,5,5'-substituted bispyridyloxybenzenes, no correlation was observed for Hansch pi and sigma p values. To account for this lack of correlation and conformational variability produced by the two ether bonds, we performed x-ray structure determinations on three compounds: a) TCPOBOP, b) the 5,5'-dichloro analogue, and c) the biologically inactive, 3,3'-dichloro analogue. In the two biologically active congeners the positioning of the pyridine rings is anti to the plane of the central benzene ring, and the dihedral angle between the central ring and the pyridines is approximately 60 degrees. In the inactive analogue the pyridine rings are syn and the dihedral angle is 84 degrees. The x-ray crystallographic data are consistent with the ether oxygen having an sp2-bonding conjugating with the heterodipolar bond of the pyridine C(2)--N(1), which strongly restricts rotation about the ether bonds. The potency of TCPOBOP and other bispyridyloxybenzene analogues to induce a phenobarbital-like pleiotropic response and the sharply defined structure-activity relationship among these congeners support the hypothesis that they act by binding to a specific recognition site.

Nuclear ADP-ribosyl transferase activity correlates with induction of P-450 monooxygenases by phenobarbital in rat liver microsomes

The effect of phenobarbital treatment on the nuclear ADP-ribosyl transferase activity has been studied in parallel with microsomal cytochrome P-450 concentration and related mono-oxygenase activities, in rat liver. A marked activation of the ADP-ribosyl transferase was observed 24 h after phenobarbital administration. The chronological study performed between 0-6 days after phenobarbital treatment showed a sharp increase in this nuclear enzyme activity, to approximately equal to 270% of the control value produced in 48 h. The administration of 5'-methylnicotinamide in vivo, an inhibitor of ADP-ribosyl transferase activity in vitro, produced a decrease both of the induction of liver microsomal cytochrome P-450 mono-oxygenases and nuclear ADP-ribosyl transferase activity. The role of nuclear ADP-ribosyl transferase in the adaptative response of the liver cell to phenobarbital is discussed.

[Function of the liver monooxygenase system after exposure to ionol and polychlorinated biphenyls]

After a single administration of polychlorinated diphenyls (sovol) at a dose of 50 mg/kg content of hemoproteins was increased simultaneously with an increase in the rate of N- and O-demethylation in rat liver microsomes. Repeated administrations of a synthetic antioxidant 2,6-ditret-butyl-4-methylphenol (ionol) at a dose of 500 mg/kg stimulated also the liver tissue monooxygenase system; in this case, an increase in content of cytochromes P-450 and b5 was accompanied by a distinct acceleration in oxidation of the first and second types of substrates in microsomes. A combined effect of sovol and ionol maintained the rate of microsomal enzymes induction, which exceeded markedly the values caused by individual effects of the compounds.

Cytochrome P-450 and monooxygenase activity in hepatic microsomes from N-phenylimidazole-treated rats

Repeated administration of N-phenylimidazole (PI) to rats (3 daily doses of 200 mumol/kg/day) enhanced hepatic microsomal cytochrome P-450 levels (approx. 130%) and aminopyrine N-demethylase (APDM) and aniline p-hydroxylase (APH) activities (approx. 140%); aryl hydrocarbon (benzo[a]pyrene) hydroxylase (AHH) and 7-ethoxycoumarin O-deethylase (ECOD) activities were not enhanced over control values under similar conditions. Spectral studies with PI-induced microsomes indicated that although type II PI-binding characteristics were similar to those observed in controls, the 427 nm/455 nm absorbance ratio of the type III dihydrosafrole metabolite-cytochrome P-450 complex was lower than that in control microsomes. The results suggest that the inducing characteristics of PI bear some resemblance to those of phenobarbital (PB).

Role of 3,5-dichlorophenyl methyl sulfone, a metabolite of m-dichlorobenzene, in the induction of hepatic microsomal drug-metabolizing enzymes by m-dichlorobenzene in rats

The increases in the hepatic microsomal aminopyrine N-demethylase activity and in the content of cytochrome P-450 produced by m-dichlorobenzene (m-DCB) occurred after increases in the hepatic concentration of 3,5-dichlorophenyl methyl sulfone, a minor metabolite. The extent of increases in aminopyrine N-demethylase activity and in the content of cytochrome P-450 at 48 hr after po administration of 200 mg/kg (1.36 mmol/kg) of m-DCB was almost equal to that 72 hr after the ip administration of 25 mumol/kg of the sulfone (Kimura et al., 1983). m-DCB in liver was not detectable at that time, and the concentration of sulfone was 63 to 70% of that 48 to 72 hr after the ip administration of 50 mumol/kg of sulfone. Administration of m-DCB (200 mg/kg) produced a significant reduction in hexobarbital sleeping time, but this reduction was less than that produced by administration of the sulfone (50 mumol/kg). The protein band patterns by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the microsomes from rats treated with the sulfone and m-DCB were similar to those of phenobarbital-treated rats but were different from those of 3-methylcholanthrene-treated rats. The sulfone showed type I interaction with the cytochrome P-450 (Ks, 0.17 mM). The sulfone was formed from the sulfide but reduction of the sulfone was not observed when it was incubated in a hepatic microsomal preparation. The pattern of induction by the sulfone and m-DCB was similar to that by phenobarbital and differed from that by 3-methylcholanthrene. From these results, 3,5-dichlorophenyl methyl sulfone is considered to be a major contributing factor of the inducing activity of m-DCB and to be a potent phenobarbital-like inducer.

Dealkylation of pentoxyresorufin: a rapid and sensitive assay for measuring induction of cytochrome(s) P-450 by phenobarbital and other xenobiotics in the rat

The O-dealkylation of pentoxyresorufin (7-pentoxyphenoxazone) by rat liver microsomes was examined. The reaction appeared highly specific for certain phenobarbital inducible forms of cytochrome P-450 and was increased 95- to 140-fold by animal pretreatment with phenobarbital (75 mg/kg/day, four ip injections) and approximately 50-fold by Aroclor 1254 (500 mg/kg, one ip injection) while animal pretreatment with 3-methylcholanthrene (50 mg/kg/day, three ip injections) resulted in less than a 2-fold increase over the rate detected in control microsomes. It was observed that this activity, in microsomes for Aroclor-pretreated rats, was dependent on O2 and was inhibited by metyrapone and SKF 525-A, indicative of cytochrome(s) P-450 mediation in the reaction. When antibodies directed against purified cytochrome(s) P-450s were employed to inhibit the pentoxyresorufin O-dealkylation reaction, antibodies to P-450PB-B greatly inhibited the reaction (greater than 90%), while antibodies to P-450PB-C or P-450PB/PCN-E had minimal effects. Assay of hepatic microsomes from rats which were pretreated with varying doses of phenobarbital (0.9-75 mg/kg/day, four ip injections) indicated that while aminopyrine-N-demethylase activity was induced only 2-fold at the maximum dose (75 mg/kg/day), pentoxyresorufin O-dealkylase activity was induced approximately 140-fold at this dose and approximately 4-fold by a dose of phenobarbital as low as 0.9 mg/kg.