Carbamazepine 10,11-epoxidation in human liver microsomes: influence of the CYP3A5*3 polymorphism.

Carbamazepine (CBZ) is a commonly prescribed antiepileptic drug, and is mainly metabolized to 10,11-CBZ epoxide in humans. Its biotransformation is catalyzed by cytochrome P450 (CYP) enzymes, with the predominant isoforms being CYP3A4 and CYP3A5. In the present study, the effects of the CYP3A5*3 (rs776746) polymorphism on CBZ 10,11-epoxidation in human liver microsomes genotyped as CYP3A5*3 were examined using a kinetic analysis. The kinetics for CBZ 10,11-epoxidation fit the Hill model with n of approximately 1.9-2.1 in all liver microsomes of the wild-type (CYP3A5*1/*1) and heterozygous (CYP3A5*1/*3) and homozygous (CYP3A5*3/*3) variants. The S50, Vmax, and CLmax values of wild-type liver microsomes were 263-327 µM, 793-1590 pmol/min/mg protein, and 1.51-2.95 µL/min/mg protein, respectively. The Vmax and CLmax values of liver microsomes of the heterozygous variant were approximately 15-40% those of wild-type liver microsomes. On the other hand, the Vmax and CLmax values of liver microsomes of the homozygous variant were more similar to those of the wild-type than the heterozygous variant. These results suggest that the CYP3A5*3 polymorphism has a negligible effect on CBZ 10,11-epoxidation in an in vitro system using human liver microsomes.

Species and sex differences in propofol glucuronidation in liver microsomes of humans, monkeys, rats and mice.

Propofol (2,6-diisopropylphenol) is a short-acting anesthetic commonly used in clinical practice, and is rapidly metabolized into glucuronide by UDP-glucuronosyltransferase (UGT). In the present study, propofol glucuronidation was examined in the liver microsomes of male and female humans, monkeys, rats, and mice. The kinetics of propofol glucuronidation by liver microsomes fit the substrate inhibition model for humans and mice, the Hill model for monkeys, and the isoenzyme (biphasic) model for rats. The K(m), V(max), and CL(int) values of human liver microsomes were 50 µM, 5.6 nmol/min/mg protein, and 110 µL/min/mg protein, respectively, for males, and 46 µM, 6.0 nmol/min/mg protein, and 130 µL/min/mg protein, respectively, for females. The rank order of the CL(int) or CL(max) (in vitro clearance) values of liver microsomes was mice humans > monkeys > rats (high-affinity phase) rats (low-affinity phase) in both males and females. Although no significant sex differences were observed in the values of kinetic parameters in any animal species, the in vitro clearance values of liver microsomes were males < females in humans, males = females in rats (low-affinity phase), and males > females in monkeys, rats (high-affinity phase), and mice. These results demonstrated that the kinetic profile of propofol glucuronidation by liver microsomes markedly differed among humans, monkeys, rats, and mice, and suggest that species and sex differences exist in the roles of UGT isoform(s), including UGT1A9, involved in its metabolism.

In vitro glucuronidation of propofol in microsomal fractions from human liver, intestine and kidney: tissue distribution and physiological role of UGT1A9.

Propofol (2,6-diisopropylphenol) is intravenously administered for anesthetic induction and maintenance, and is rapidly metabolized into its glucuronide, mainly by UDP-glucuronosyltransferase 1A9 (UGT1A9). In this study, propofol glucuronidation by liver microsomes (HLM), intestinal microsomes (HIM) and kidney microsomes (HKM) of humans were examined. The expression of UGT1A9 protein in HLM, HIM and HKM was analyzed by immunoblotting. The staining band intensities for UGT1A9 of HIM and HKM were 12% and 119% those of HLM, respectively. The kinetics of propofol glucuronidation by HLM and HKM exhibited substrate inhibition, whereas the kinetics by HIM followed the Michaelis-Menten model. The K(m), V(max) and CL(int) values of HLM were 41.8 µM, 5.21 nmol/min/mg protein and 126 µl/min/mg protein, respectively. The K(m) value of HIM was significantly higher (6.7-fold) than that of HLM, and the V(max) and CL(int) values were significantly lower (56% and 8.3%, respectively) than those of HLM. The K(m) value of HKM was comparable to that of HLM, and the V(max) and CL(int) values were significantly higher (2.1- and 3.7-fold, respectively) than those of HLM, respectively. These findings suggest that UGT1A9 expressed in the kidney as well as in the liver plays an important role in propofol glucuronidation. The information gained in this study should contribute to an appropriate use of drugs metabolized by UGT1A9.

Evaluation of animal models for intestinal first-pass metabolism of drug candidates to be metabolized by CYP3A enzymes via in vivo and in vitro oxidation of midazolam and triazolam.

1. To search an appropriate evaluation methodology for the intestinal first-pass metabolism of new drug candidates, grapefruit juice (GFJ)- and vehicle (tap water)-pretreated mice or rats were orally administered midazolam (MDZ) or triazolam (TRZ), and blood levels of the parent compounds and their metabolites were measured by liquid chromatography/MS/MS. A significant effect of GFJ to elevate the blood levels was observed only for TRZ in mice. 2. In vitro experiments using mouse, rat and human intestinal and hepatic microsomal fractions demonstrated that GFJ suppressed the intestinal microsomal oxidation of MDZ and especially TRZ. Substrate inhibition by MDZ caused reduction in 1'-hydroxylation but not 4-hydroxylation in both intestinal and hepatic microsomal fractions. The kinetic profiles of MDZ oxidation and the substrate inhibition in mouse intestinal and hepatic microsomal fractions were very similar to those in human microsomes but were different from those in rat microsomes. Furthermore, MDZ caused mechanism-based inactivation of cytochrome P450 3A-dependent TRZ 1'-hydroxylation in mouse, rat and human intestinal microsomes with similar potencies. 3. These results are useful information in the analysis of data obtained in mouse and rat for the evaluation of first-pass effects of drug candidates to be metabolized by CYP3A enzymes.

Separate evaluation of intestinal and hepatic metabolism of three benzodiazepines in rats with cannulated portal and jugular veins: comparison with the profile in non-cannulated mice.

Pharmacokinetic analyses of three kinds of benzodiazepines--midazolam (MDZ), triazolam (TRZ) and alprezolam (APZ)--were performed in rats with cannulated portal and jugular veins. Each drug was administered to the double-cannulated rats, and pharmacokinetic data for the parent drugs and their 1'- and 4-hydroxylated metabolites were compared with those obtained in non-cannulated mice. In bioavailability, the drugs ranked APZ >> TRZ = MDZ in rats, and APZ > TRZ >> MDZ in mice, with the values for MDZ remarkably different between rats and mice (19% in rats versus 2.3% in mice). In contrast, hepatic availability (Fh) was similar (APZ > TRZ > MDZ) in both species. Highly significant relationships were found between the ratio of the area under the plasma concentration-time curve (AUC) for the parent drugs in portal blood (AUC(por)) to that in systemic blood (AUC(sys)) and Fh in rats and mice. The double-cannulated rat is useful for estimating the hepatic availability of drug candidates by determining the AUC values for the parent drugs in portal and systemic blood samples.

Functional characterization of CYP3A4.16: catalytic activities toward midazolam and carbamazepine.

1. To assess the substrate-dependent effects of the low-activity allele of human CYP3A4, CYP3A4*16 (Thr185Ser), a recombinant wild-type (CYP3A4.1) or variant (CYP3A4.16) protein was co-expressed with human NADPH-P450 reductase in Sf21 insect cells using a baculovirus-insect cell system. 2. The holo-CYP3A4 protein level of CYP3A4.16 in insect microsomes was slightly higher than that of CYP3A4.1, while no difference in total (apo- and holo-) CYP3A4 protein levels was observed between them. 3. When midazolam was used as a substrate, K(m) and V(max) for 1'-hydroxylation in CYP3A4.16 were significantly higher and lower, respectively, than those in the wild-type, resulting in a 50% decrease in intrinsic clearance (V(max)/K(m)) of the variant. In contrast, intrinsic clearance for 4-hydroxylation of the variant was decreased by 30% due to a significant increase in K(m) without a difference in V(max). 4. Both the wild-type and variant exhibited sigmoidal kinetic profiles for carbamazepine 10,11-epoxide formation. When the modified two-site equation was applied for the analysis of kinetic parameters, K(m2) and V(max2) of CYP3A4.16 were approximately two times higher and lower than those of the wild-type, resulting in a 74% decrease in intrinsic clearance. 5. These results demonstrated that CYP3A4.16 shows the substrate-dependent altered kinetics compared with CYP3A4.1.

Induction of microsomal stearoyl-CoA desaturation by the administration of various peroxisome proliferators.

Male rats were fed a diet containing 0.5% p-chlorophenoxyisobutyric acid (clofibric acid), 1% acetylsalicylic acid, 2% di-(2-ethylhexyl)phthalate, 2% dibutylphthalate, 2% di-(2-ethylhexyl)adipate or 0.5% 4,4'-(isopropylidenedithio)bis(2,6-di-tert-butylphenol)) (probucol) for 7 days. Activity of microsomal stearoyl-CoA desaturation in rat liver was increased, ranging from 2.1 to 3.6 times, in association with an increase in the activity of peroxisomal beta-oxidation and catalase following the administration of clofibric acid, di-(2-ethylhexyl)phthalate, dibutylphthalate or di-(2-ethylhexyl)adipate. The increase in the activity of stearoyl-CoA desaturation by peroxisome proliferators appears to be due to an increase in activity only of the terminal desaturase, but not to changes in either NADH-cytochrome b5 reductase activity or cytochrome b5 content. Unlike peroxisome proliferators, probucol increased little the activity of either microsomal desaturation or peroxisomal beta-oxidation. The percentage of octadecenoic acid in total fatty acid of hepatic microsomes, homogenates and serum was increased markedly by the administration of peroxisome proliferators.

In vitro biotransformation of atrazine by rat liver microsomal cytochrome P450 enzymes.

We studied atrazine (ATZ) metabolism in male and female rat liver microsomes in vitro, and the major metabolite was deisopropylatrazine (DeiPr-ATZ) with deethylatrazine (DeEt-ATZ) and 1-hydroxyisopropylatrazine (iPrOH-ATZ) as minor metabolites in both sexes. The enzyme kinetics of ATZ biotransformation were examined by means of Eadie-Hofstee analyses. Although no remarkable sex difference of Michaelis Menten values for each pathway was observed, Cl(int)S (Vmax/Km) for DeiPr-ATZ, DeEt-ATZ and iPrOH-ATZ were slightly higher in female than in male rats. The formation of DeiPr-ATZ, DeEt-ATZ and iPrOH-ATZ from ATZ was substantially inhibited by SKF-525A, metyrapone, diallyl sulfide, 7-ethoxycoumarin, benzphetamine, nicotine, testosterone and lauric acid in both sexes. Cimetidine effectively inhibited the formation of all metabolites in male rats. On the other hand, the inhibition rates of the formation of DeiPr-ATZ and iPrOH-ATZ by cimetidine in female rats were lower than those in male rats, and DeEt-ATZ was hardly affected by the chemicals. In contrast with the results for cimetidine, the inhibition of ATZ biotransformation by bufuralol was more effective in female than in male rats. Anti-rat CYP2B1 and CYP2E1 antibodies effectively inhibited DeiPr-ATZ, DeEt-ATZ and iPrOH-ATZ formations in both sexes. Anti-rat CYP2C11 antibody also inhibited the three metabolites in both sexes, with the inhibition rates higher in male than in female rats, similar to cimetidine. In the case of anti-rat CYP2D1 antibody, the inhibitory effect on ATZ biotransformation in male rats was less than that in female rats. On the other hand, anti-rat CYP1A2, CYP3A2 and CYP4A1 antibodies did not affect the ATZ biotransformation in either sex. There was no significant correlation between the formation rate of ATZ metabolites and P450 isoform levels in either sex. These results may mean that CYP2B2, CYP2C11, CYP2D1 (only iPrOH-ATZ formation) and CYP2E1 in male rats, and CYP2B2, CYP2D1 and CYP2E1 in female rats are involved ATZ metabolism in liver, and that the substrate specificity of P450 isoforms for ATZ is broad.

Comparative cytotoxicity of the aqueous chlorination products of thiobencarb, a thiocarbamate herbicide, in cultured rat hepatocytes.

Thiobencarb (S-4-chlorobenzyl N, N-diethylthiocarbamate) has been widely used in the rice fields of Japan. This herbicide is reported to decompose in aqueous chlorination to the compounds 4-chlorotoluene, 4-chlorobenzyl chloride, 4-chlorobenzyl alcohol, 4-chlorobenzaldehyde and 4-chlorobenzoic acid. We compared their cytotoxicity and the inducibility of cytochrome P-450 (P450) in cultured rat hepatocytes. Of the six compounds including thiobencarb, 4-chlorobenzyl chloride was the most hepatotoxic (EC(50): 0.17 mm), followed by thiobencarb (0.69 mm) and 4-chlorotoluene (1.2 mm). 4-Chlorobenzyl alcohol (4.6 mm) and 4-chlorobenzaldehyde (4.6 mm) were less toxic than thiobencarb, and 4-chlorobenzoic acid was the least toxic ( > 6.0 mm). From the results of the TBARS (2-thiobarbituric acid reactive substance) assay, lipid peroxidation was shown to be involved in the hepatotoxicity of 4-chlorobenzyl chloride, and less probably in that of thiobencarb and 4-chlorobenzaldehyde. 4-Chlorobenzoic acid and 4-chlorobenzaldehyde induced hepatic ethoxyresorufin O-deethylase and pentoxyresorufin O-depentylase activities, respectively. The induction of enzyme activities was accompanied by the increase in the corresponding P-450 apoprotein. Furthermore, 4-chlorotoluene, 4-chlorobenzaldehyde and 4-chlorobenzoic acid also induced CYP2B1, which was not reflected in the enzyme activity. These results provide primary information on the toxicity of the thiobencarb degradation products in cultured rat hepatocytes.

Cytotoxic and porphyrinogenic effects of diphenyl ethers in cultured rat hepatocytes: chlornitrofen (CNP), CNP-amino, chlomethoxyfen and bifenox.

We studied the cytotoxic and porphyrinogenic effects of four diphenyl ethers (DPEs), chlornitrofen (CNP), CNP-amino, chlomethoxyfen and bifenox, in rat hepatocytes cultured on Matrigel. Cytotoxicity was determined as a decrease in viability measured by the release of lactate dehydrogenase. Of the DPEs examined. CNP-amino was the most cytotoxic, with an LC50 value of 0.36 mM (95% confidence interval, 0.33-0.40 mM). CNP also reduced the viability in a concentration-dependent manner at the concentrations of 0.50 mM or above. In contrast, no concentration-dependent decrease in viability was observed in the chlomethoxyfen- and bifenox-treated hepatocytes at the concentrations up to 1.0 mM. To identify the enzyme involved in the metabolic activation of CNP-amino, inhibition studies were carried out using SKF 525-A (0.050 mM) and methimazole (1.0 mM). SKF 525-A, a cytochrome P450 inhibitor. quickened the onset of cell killing by CNP-amino, while methimazole, an inhibitor of flavin-containing monooxygenase (FMO), partially suppressed the cytotoxicity of CNP-amino. These results suggest that FMO plays an important role in the cytotoxicity induced by CNP-amino, while cytochrome P450 participates in the detoxification, possibly via the ring-hydroxylation. The maximum porphyrin accumulation was observed at 0.13 mM for chlomethoxyfen (18-fold) and at 0.25 mM for CNP and bifenox (17- and 21-fold, respectively). In contrast to these DPEs, the porphyrinogenic effect of CNP-amino was weak, with the maximum accumulation at 0.13 mM (at least fivefold). The predominant species was protoporphyrin IX in all of the DPE-treated cultures. These results suggest that all of the DPEs examined, possibly including CNP-amino, inhibit protoporphyrinogen oxidase, resulting in the accumulation of protoporphyrin IX.

Determination of UDP-glucuronosyltransferase UGT1A6 activity in human and rat liver microsomes by HPLC with UV detection.

A simple and sensitive method for the determination of UDP-glucuronosyltransferase UGT1A6 activity using 4-methylumbelliferone (4-MU) and 4-nitrophenol (4-NP) as substrates in human and rat liver microsomes by high-performance liquid chromatography (HPLC) with uv detection is reported. The method was validated for the determination of 4-methylumbelliferyl beta-D-glucuronide (4-MUG) and 4-nitrophenyl beta-D-glucuronide (4-NPG) with respect to specificity, linearity, detection limit, recovery, stability, precision and accuracy. There was no interference from matrix and non-enzymatic reactions. Calibration curves for 4-MUG and 4-NPG are linear from 0.5 to 500 microM. Average recoveries ranged from 98 to 100% in spiked liver microsomes samples. 4-MUG and 4-NPG were stable at 4 degrees C for at least 72 h in spiked liver microsomes samples. The method was found to be more sensitive than previous methods using a spectrophotometer, a spectrofluorometer and HPLC. The detection limit for 4-MUG and 4-NPG (signal-to-noise ratio of 3) was 14 and 23 nM, respectively. The intra- and inter-day precision (relative S.D. (RSD)) and accuracy (relative mean error (RME)) was <5 and 9%, respectively. The intra- and inter-day reproducibility (RSD) of UGT1A6 enzyme assay in liver microsomes was <6%. With this improved sensitivity, the kinetics of UGT activities toward 4-MU and 4-NP in human and rat liver microsomes could be determined more precisely. In addition, the method could determine the non-inducible, and 3-methylcholanthrene- and phenobarbital-inducible activities of UGT1A6 in rat liver microsomes under the same assay conditions. Therefore, this method is applicable to in vivo and in vitro studies on the interaction of xenobiotic chemicals with UGT1A6 isoform in mammals using small amounts of biological samples.

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.

Interaction of bisphenol A with rat hepatic cytochrome P450 enzymes.

The effect of bisphenol A (BPA) on the kinetics of cytochrome P450 (P450)-dependent monooxygenases in rat liver microsomes was studied. Testosterone 16beta-hydroxylase (TS16BH) and testosterone 2alpha-hydroxylase (TS2AH) activities were extensively inhibited by BPA at 100 microM (69% and 74%, respectively). The inhibition type was mixed for both P450-dependent monooxyganases. The Ki of TS16BH and TS2AH from Lineweaver-Burk plots were 25.9 and 24.9 microM, respectively. The activities of acetanilide 4-hydroxylase (AA4H), 7-ethoxycoumarin O-deethylase (ECOD), bufuralol 1'-hydroxylase (BF1'H), chlorzoxazone 6-hydroxylase (CZ6H) and testosterone 6beta-hydroxylase (TS6BH) were also effectively inhibited by BPA at 100 microM (43-52%). The inhibition type of these P450-dependent monooxygenases was mixed or uncompetitive, and the K(i)s (50.5-88.5 microM) were higher than those of TS16BH and TS2AH. By contrast, the values of IC50 and Ki of testosterone 7alpha-hydroxylase (TS7AH) and lauric acid omega-hydroxylase (LAOH) for BPA were >1000 microM. These results suggest that BPA interacts with rat hepatic CYP1A2, CYP2A2, CYP2B2, CYP2C11, CYP2D1, CYP2E1 and CYP3A2 in vitro.

The effect of 1,2,3,4-tetrachlorodibenzo-p-dioxin on drug-metabolizing enzymes in the rat liver.

The effects of 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TCDD) on drug-metabolizing enzymes were studied in male and female rats. 1,2,3,4-TCDD (25, 50, 100 and 200 mumol/kg) was administered by i.p. injection once. Among the cytochrome P-450 (P450)-mediated monooxygenase activities tested, 7-ethoxyresorufin O-deethylase (EROD) activities in both male and female rats, which are associated with CYP1A1, were remarkably induced by all doses of 1,2,3,4-TCDD. The relative induction to each control activity were from 3.0- to 24.5-fold and from 2.2- to 16.5-fold, respectively. Also, 1,2,3,4-TCDD increased other CYP1A-mediated monooxygenase activities such as 7-ethoxycoumarin O-deethylase (ECOD) and 7-methoxyresorufin O-demethylase (MROD) in male and female rats dose-dependently (1.4- to 4.3-fold). Western immunoblotting showed that the levels of CYP1A1 and CYP1A2 proteins in liver microsomes were increased by 1,2,3,4-TCDD. Although the activities of other P450-mediated monooxygenases, namely 7-pentoxyresorufin O-depentylase (PROD), 7-benzyloxyresorufin O-debenzylase (BROD), aminopyrine N-demethylase (APND) and nitrosodimethylamine N-demethylase (NDAND) in both male and female rats were induced at high doses (> or = 50 mumol/kg) of 1,2,3,4-TCDD, the relative level was low compared with those of the CYP1A-mediated monooxygenase such as EROD, ECOD or MROD. In addition to P450-mediated monooxygenase, there was significant induction in the activities of the Phase II drug-metabolizing enzymes, UDP-glucuronyltransferase (UGT) activities towards 4-nitrophenol (4-NP) and 7-hydroxycoumarin (7-HC) and glutathione S-transferase (GST) towards 1-chloro-2,4-dinitrobenzene (CDNB), 1,2-dichloro-4-nitrobenzene (DCNB) and DT-diaphorase. These results indicate that 1,2,3,4-TCDD induces both Phase I (CYP1A-mediated monooxygenase) and Phase II drug-metabolizing enzymes (UGT, GST, DT-diaphorase) in the male and female rat liver, and that the alterations of drug-metabolizing enzyme are characteristic of PCDD toxicity.

Effect of 1,2,4-trichlorodibenzo-p-dioxin on drug-metabolizing enzymes in the rat liver.

The effects of 1,2,4-trichlorodibenzo-p-dioxin (1,2,4-TrCDD) on drug-metabolizing-enzymes have been studied in male Wistar rats. 1,2,4-TrCDD (0.1 mmol/kg per day) was administered by i.p. injection for 3 days. Among the cytochrome P-450 (P450)-mediated monooxygenase activities tested, 7-ethoxyresorufin O-deethylase, which is associated with CYP1A1, was remarkably induced by 1,2,4-TrCDD (0.1 mmol/kg). The relative induction to control activity was 32.9-fold. Also, 1,2,4-TrCDD increased other CYP1A-mediated monooxygenase activities such as 7-ethoxycoumarin O-deethylase, 4-nitroanisole O-demethylase, 7-methoxyresorufin O-demethylase and caffeine N-demethylase from 5.7- to 1.9-fold. Western immunoblotting showed that the levels of CYP1A1 and CYP1A2 proteins in liver microsomes were increased by 1,2,4-TrCDD. On the other hand, 7-pentoxyresorufin O-depentylase activity was induced 2.6-fold whereas aniline 4-hydroxylase, nitrosodimethylamine N-demethylase and erythromycin N-demethylase activities were increased slightly (1.3-, 1.6- and 1.3-fold, respectively) by 1,2,4-TrCDD. However, aminopyrine N-demethylase was not significantly induced by 1,2,4-TrCDD. Of the Phase II drug-metabolizing enzymes, DT-diaphorase and glutathione S-transferase (GST) activities towards 1-chloro-2,4-dinitrobenzene and 1,2-dichloro-4-nitrobenzene, and those of UDP-glucuronyltransferase (UGT) towards 4-nitrophenol and 7-hydroxycoumarin were increased from 2.7 to 1.4-fold by 1,2,4-TrCDD. These results indicate that 1,2,4-TrCDD induces both Phase I and Phase II drug-metabolizing enzymes in the rat liver.

Induction of hepatic drug-metabolizing enzymes by chlornitrofen (CNP) and CNP-amino in rats and mice.

The induction of hepatic drug-metabolizing enzymes by chlornitrofen (CNP) and CNP-amino was studied in the liver of male rats and mice. CNP-amino increased the activities of 7-pentoxyresorufin O-depentylase (PROD) and 7-benzyloxyresorufin O-debenzylase (BROD) as CYP2B1-dependent monooxygenase 3.6- and 4.1-fold in rats. On the contrary, these enzyme activities in mice were induced by CNP rather than by CNP-amino. Furthermore, immunoblotting showed that the protein levels of CYP2B subfamily cytochrome P450 (P450) in liver microsomes of rats and mice were increased by CNP or CNP-amino. Phase II drug-metabolizing enzymes, UDP-glucuronyltransferase (UGT) and glutathione S-transferase (GST) levels in mice were also significantly increased from 1.4 to 2.5-fold by CNP or CNP-amino. However, neither CNP nor CNP-amino affected UGT and GST in rats. These results suggest that CNP and or CNP-amino induce the P450 isoforms of CYP2B subfamily in the rat and mouse liver, and that the inducibility of drug-metabolizing enzyme by the compounds is different between rats and mice.

Interaction of 2,4,4'-trichloro-2'-hydroxydiphenyl ether with microsomal cytochrome P450-dependent monooxygenases in rat liver.

We studied the effects of 2,4,4'-trichloro-2'-hydroxydiphenyl ether (Irgasan DP300) on the kinetics of the cytochrome P450 (P450)-dependent monooxygenases in rat liver microsomes. The activities of 7-ethoxyresorufin O-deethylase (EROD) and 7-pentoxyresorufin O-depentylase (PROD) in rat liver microsomes exposed to 3-methylcholanthrene (MC) and phenobarbital (PB) respectively, were substantially inhibited by Irgasan DP300. The inhibition profile of EROD was competitive, whereas that of PROD was noncompetitive; the Ki values from Hanes plots were 0.24 and 1.48 microM for EROD and PROD, respectively. Phenacetin O-deethylase (PCOD) and 4-nitrophenol hydroxylase (4NPH) activities in rats exposed to PB were also inhibited by Irgasan DP300, at Ki values lower than those for other microsomes. Irgasan DP300 slightly inhibited testosterone 6 beta-hydroxylase (TS6BH) activities in some microsomes. No effect of Irgasan DP300 on lauric acid omega-hydroxylase (LAOH) activity was evident in any microsomal preparations. These results indicated that Irgasan DP300 inhibits MC- and PB-inducible P450-dependent monoxygenase in vitro competitively or noncompetitively, and that the P450 enzymes of the CYP1A or CYP2B subfamily may contribute to Irgasan DP300 toxicity.

[Toxicological assessment of 2,5,2',5'-tetrachlorobiphenyl and its major metabolite, 3-hydroxy-2,5,2',5'-tetrachlorobiphenyl in rats].

We observed previously that polychlorinated biphenyl (PCB) could be classified to two groups, 3-methylcholanthrene (MC)-type and phenobarbital (PB)-type, in term of inducibility of the hepatic enzymes. MC-type PCBs such as 3,4,3',4'-tetrachlorobiphenyl (TCB), 3,4,5,3',4'-pentachlorobiphenyl (PenCB) and 3,4,5,3',4',5'-hexachlorobiphenyl (HexCB) exhibited high acute toxicity in parallel with their induction ability of microsomal benzo[a]pyrene 3-hydroxylase and cytosolic DT-diaphorase. On the contrary, PB-type PCBs such as 2,5,2',5'-TCB and 2,4,5,2',4',5'-HexCB which induce microsomal benzphetamine N-demethylase and NADPH-cytochrome P-450 reductase activities showed virtually no or very low toxicity. In the present study, we examined effects of 2,5,2',5'-TCB and its major metabolite 3-hydroxy-2,5,2',5'-TCB on body weight gain, organ weights and activities of hepatic enzymes in rats and assessed acute toxicity of these compounds. As the results, in both 2,5,2',5'-TCB and 3-hydroxy-2,5,2',5'-TCB groups, the body weights were increased during the experiment, but the rate of growth was significantly suppressed after 3 days. Significant hypertrophy of the liver and decrease of total liver lipid content were observed in 2,5,2',5'-TCB group, but the atrophy of spleen and thymus was not affected in both groups. On the other hand, in 2,5,2',5'-TCB group, benzo[a]pyrene 3-hydroxylase and benzphetamine N-demethylase activities were increased to 2. 4-fold and 1.5-fold, respectively, but were not increased in 3-hydroxy-2,5,2',5'-TCB group. After injection of 2,5,2',5'-TCB, 45% of the dose was excreted as 3-hydroxy-2,5,2',5'-TCB in feces for 5 days.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of polycyclic aromatic hydrocarbons in food samples by automated on-line in-tube solid-phase microextraction coupled with high-performance liquid chromatography-fluorescence detection.

A simple and sensitive automated method, consisting of in-tube solid-phase microextraction (SPME) coupled with high-performance liquid chromatography-fluorescence detection (HPLC-FLD), was developed for the determination of 15 polycyclic aromatic hydrocarbons (PAHs) in food samples. PAHs were separated within 15 min by HPLC using a Zorbax Eclipse PAH column with a water/acetonitrile gradient elution program as the mobile phase. The optimum in-tube SPME conditions were 20 draw/eject cycles of 40 microL of sample using a CP-Sil 19CB capillary column as an extraction device. Low- and high-molecular weight PAHs were extracted effectively onto the capillary coating from 5% and 30% methanol solutions, respectively. The extracted PAHs were readily desorbed from the capillary by passage of the mobile phase, and no carryover was observed. Using the in-tube SPME HPLC-FLD method, good linearity of the calibration curve (r>0.9972) was obtained in the concentration range of 0.05-2.0 ng/mL, and the detection limits (S/N=3) of PAHs were 0.32-4.63 pg/mL. The in-tube SPME method showed 18-47 fold higher sensitivity than the direct injection method. The intra-day and inter-day precision (relative standard deviations) for a 1 ng/mL PAH mixture were below 5.1% and 7.6% (n=5), respectively. This method was applied successfully to the analysis of tea products and dried food samples without interference peaks, and the recoveries of PAHs spiked into the tea samples were >70%. Low-molecular weight PAHs such as naphthalene and pyrene were detected in many foods, and carcinogenic benzo[a]pyrene, at relatively high concentrations, was also detected in some black tea samples. This method was also utilized to assess the release of PAHs from tea leaves into the liquor.

Determination of aflatoxins in food samples by automated on-line in-tube solid-phase microextraction coupled with liquid chromatography-mass spectrometry.

A simple and sensitive automated method for determination of aflatoxins (B1, B2, G1, and G2) in nuts, cereals, dried fruits, and spices was developed consisting of in-tube solid-phase microextraction (SPME) coupled with liquid chromatography-mass spectrometry (LC-MS). Aflatoxins were separated within 8 min by high-performance liquid chromatography using a Zorbax Eclipse XDB-C8 column with methanol/acetonitrile (60/40, v/v): 5mM ammonium formate (45:55) as the mobile phase. Electrospray ionization conditions in the positive ion mode were optimized for MS detection of aflatoxins. The pseudo-molecular ions [M+H](+) were used to detect aflatoxins in selected ion monitoring (SIM) mode. The optimum in-tube SPME conditions were 25draw/eject cycles of 40 microL of sample using a Supel-Q PLOT capillary column as an extraction device. The extracted aflatoxins were readily desorbed from the capillary by passage of the mobile phase, and no carryover was observed. Using the in-tube SPME LC-MS with SIM method, good linearity of the calibration curve (r>0.9994) was obtained in the concentration range of 0.05-2.0 ng/mL using aflatoxin M1 as an internal standard, and the detection limits (S/N=3) of aflatoxins were 2.1-2.8 pg/mL. The in-tube SPME method showed >23-fold higher sensitivity than the direct injection method (10 microL injection volume). The within-day and between-day precision (relative standard deviations) at the concentration of 1 ng/mL aflatoxin mixture were below 3.3% and 7.7% (n=5), respectively. This method was applied successfully to analysis of food samples without interference peaks. The recoveries of aflatoxins spiked into nuts and cereals were >80%, and the relative standard deviations were <11.2%. Aflatoxins were detected at <10 ng/g in several commercial food samples.