Identification and partial purification of hamster microsomal cytochrome P-450 isoenzymes

Mode of action and human relevance analysis for nuclear receptor-mediated liver toxicity: A case study with phenobarbital as a model constitutive androstane receptor (CAR) activator

The constitutive androstane receptor (CAR) and pregnane X receptor (PXR) are important nuclear receptors involved in the regulation of cellular responses from exposure to many xenobiotics and various physiological processes. Phenobarbital (PB) is a non-genotoxic indirect CAR activator, which induces cytochrome P450 (CYP) and other xenobiotic metabolizing enzymes and is known to produce liver foci/tumors in mice and rats. From literature data, a mode of action (MOA) for PB-induced rodent liver tumor formation was developed. A MOA for PXR activators was not established owing to a lack of suitable data. The key events in the PB-induced liver tumor MOA comprise activation of CAR followed by altered gene expression specific to CAR activation, increased cell proliferation, formation of altered hepatic foci and ultimately the development of liver tumors. Associative events in the MOA include altered epigenetic changes, induction of hepatic CYP2B enzymes, liver hypertrophy and decreased apoptosis; with inhibition of gap junctional intercellular communication being an associative event or modulating factor. The MOA was evaluated using the modified Bradford Hill criteria for causality and other possible MOAs were excluded. While PB produces liver tumors in rodents, important species differences were identified including a lack of cell proliferation in cultured human hepatocytes. The MOA for PB-induced rodent liver tumor formation was considered to be qualitatively not plausible for humans. This conclusion is supported by data from a number of epidemiological studies conducted in human populations chronically exposed to PB in which there is no clear evidence for increased liver tumor risk.

Cytochrome P450 induction in feral Cricetid rodents: a review of field and laboratory investigations

The constitutive and inducible hepatic cytochromes P450 of various feral Cricetid rodents (family Cricetidae, comprising various New World rats and mice, hamsters, gerbils and voles), have been examined in a relatively limited number of field and laboratory investigations. These studies, reviewed herein, have employed substrates and immunochemical reagents that are diagnostic for individual P450 subfamilies of Rattus norvegicus (the common laboratory species derived from the Norway rat, a member of the family Muridae). The results have demonstrated that the feral rodents display hepatic responses to prototypic CYP1A inducers (3-methylcholanthrene, beta-naphthoflavone) similar to those displayed by R. norvegicus and Mus musculus (the common laboratory species derived from the house mouse, another member of the family Muridae). At least one study has demonstrated the induction, by ethanol, of a protein immunochemically similar to CYP2E1 in a Cricetid rodent. In Cricetid rodents, phenobarbital-type inducers cause the induction of a hepatic protein immunologically similar to that primarily induced (CYP2B) in R. norvegicus and M. musculus. The proteins induced in the Cricetid rodents, however, exhibit striking differences in substrate specificity, compared to the proteins induced in R. norvegicus. These results indicate that the previously described differences between the P450 induction responses exhibited by the commonly utilized laboratory species R. norvegicus and M. musculus (family Muridae) and the Syrian hamster and gerbil (family Cricetidae) are observed as a generality for members of the Cricetid family of rodents.

Cytochrome P450 metabolism of estradiol in hamster liver and kidney

Estradiol induces kidney tumors in Syrian hamsters. The elevated conversion of estradiol to 4-hydroxylated metabolites in kidney compared to the predominant 2-hydroxylation in liver and other organs, where tumors are not induced by this treatment, has been proposed to be the basis of estrogen-induced carcinogenesis. In this study, we examined the hepatic and renal enzymes catalyzing the formation of catecholestrogens to understand the differences in estrogen metabolism in these organs. In liver, 2-hydroxylation of estradiol is the major metabolic pathway with 4-hydroxylation a minor by-product and with the formation of both catechols responding coordinately to the same inhibitors. Western blot analysis and inhibition studies suggest that the major form catalyzing hepatic estrogen 2-hydroxylation is a member of the CYP3A family, as previously observed with rat liver microsomes, and that 4-hydroxylation is a by-product of this metabolism. In the kidney, 4-hydroxylation of estradiol appears to be catalyzed by more than one enzyme according to the Eadie-Hofstee analysis. Both 2- and 4-hydroxylation in the kidney are affected differentially by inhibitors and are induced by beta-napthoflavone. Western blots of renal microsomes reveal that CYP1A2 is induced whereas CYP1A1 is detectable in kidney, but not induced by this treatment. Finally, a part of the 2-hydroxylation and a small part of the 4-hydroxylation by kidney microsomes may be catalyzed by a member of the CYP3A family, since these reactions are partially inhibited by CYP3A inhibitors such as progesterone and other progestins, although renal enzyme levels are much lower than those in the liver as revealed by Western blot. Our data suggest that estrogen 2-hydroxylation in the hamster kidney is catalyzed by members of the CYP1A and CYP3A families, which also contribute to 4-hydroxylation. The majority of 4-hydroxyestradiol formation in the hamster kidney may be catalyzed by a form(s) of the newly discovered CYP1B family that has yet to be characterized.

Induction and suppression of renal and hepatic cytochrome P450-dependent monooxygenases by acute and chronic streptozotocin diabetes in hamsters

The acute and chronic effects of streptozotocin diabetes on kidney and liver microsomal monooxygenases were studied using hamsters 2 days and 6 weeks following treatment with the diabetogen, respectively. Acute diabetes increased aniline hydroxylation and N-nitrosodimethylamine demethylation, decreased pentoxyresorufin O-dealkylation, without affecting benzo(a)pyrene hydroxylation and 7-ethoxycoumarin O-deethylation in kidney and liver microsomes. The effects of chronic diabetes on the microsomal monooxygenases were similar to the effects of acute diabetes, except that the chronic diabetic condition markedly decreased benzo(a)pyrene and 7-ethoxycoumarin oxidations in kidney microsomes. Total cytochrome P450 content and NADPH-cytochrome P450 reductase activity in kidney and liver microsomes of the diabetic hamsters were similar to the controls. Gel electrophoresis of microsomes from control and streptozoptocin treated hamster tissues revealed that diabetes enhanced the intensity of protein band(s) in the P450 molecular weight region. Immunoblotting of microsomal proteins showed that acute and chronic streptozotocin diabetes induced proteins immunorelated to P450s 2E1 and 1A in kidney and liver. In marked contrast, the acute and chronic diabetic conditions decreased the level of a P450 2B-immunorelated protein(s) in kidney and liver. The present study demonstrates that acute and chronic streptozotocin diabetes has the ability to induce P450 2E1 and 1A and suppress P450 2B in hamster kidney and liver and that the hamster monooxygenase responds to diabetes differently from the rat enzyme.

Induction of CYP2B1 mediated pentoxyresorufin O-dealkylase activity in different species, sex and tissue by prototype 2B1-inducers

The induction of CYP2B1 mediated pentoxyresorufin O-dealkylase (PROD) activity by various xenobiotics was explored in liver, kidney and lung from a variety of animal species of both sexes, in order to gain insights into the substrate specificity of induced CYPs. Marked species- and sex-related differences in the inducibility of PROD activity by tested chemicals were observed, the mouse being always more responsive when compared to hamster or rat. Induction by sodium phenobarbital (NaPB) led to a conspicuous increase in all situations, up to approximately 38-fold in female rat and mouse liver, with the exception of hamster kidney where PROD activity was only slightly affected. Unexpectedly, both sodium barbital (NaB) and phorone (PHR) moderately induce CYP2B1 isoforms in rat, the extent being highest in female kidney (PHR, 14-fold increase) and male lung (NaB, 4.5-fold). The degree of induction was maximal in the liver with some exceptions occurring in male mice where NaB induced up to 46- and 115-fold increases in lung and kidney and PHR up to 115-fold in kidney. Minimal, although significant induction of PROD activity following treatment with trans-1,2-dichloroethylene (1,2-DCE) occurred in all situations with the exception of hamster kidney and lung. Therefore, caution should be exercised when using PROD activity as specific enzymatic assay to probe CYP2B1-like induction.

Characterization of cytochrome P450-monooxygenases of Chinese hamsters with respect to aflatoxin B1 activation

Monooxygenases of Chinese hamster livers were characterized with respect to aflatoxin B1 activation and cytochrome P450 isozymes. Polychlorinated biphenyl (PCB)-treatment induced higher activity for aflatoxin B1 activation in Chinese hamsters than in rats and this activity was more elevated by the treatment with phenobarbital than with 3-methylcholanthrene, which is contrary to the observation in Syrian hamsters and rather similar to that in rats. Western blot analysis demonstrated that the treatment of Chinese hamsters with phenobarbital markedly elevated the level of cytochrome P450 isozyme belonging to the CYP2A family and only slightly that immuno-related to CYP2B1. In parallel with this, the activities of testosterone 15 alpha-hydroxylase and 7-ethoxycoumarin O-deethylase were significantly induced by phenobarbital treatment. These suggest that phenobarbital mainly induces an isozyme related to CYP2A2 which is responsible for aflatoxin B1 activation in Chinese hamsters.

Differential modulation of hepatic cytochrome P-450 enzymes in rat and Syrian hamster by 4'-trifluoromethyl-2,3,4,5-tetrachlorobiphenyl

The effects of a single injection (40 mg/kg) of 4'-trifluoromethyl-2,3,4,5-tetrachlorobiphenyl (CF3) on hepatic cytochrome P-450 monooxygenases were assessed in rat and syrian hamster. The CF3 treatment significantly increased the total amount of cytochrome P-450 in both species. In rats, CF3 treatment caused marked increases in ethoxyresorufin O-deethylase (EROD), arylhydrocarbon hydroxylase (AHH), and testosterone 7 alpha-hydroxylase activities but significantly reduced the activities of benzphetamine N-demethylase (BzND), erythromycin N-demethylase (ErND), testosterone 6 beta, 16 alpha, and 16 beta-hydroxylase, and formation of androstenedione. Administration of CF3 to hamsters strongly induced the activities of EROD, AHH, BzND, testosterone 15 alpha, and 16 alpha-hydroxylases, and androstenedione production, whereas ErND, testosterone 6 beta, and 7 alpha-hydroxylases were decreased. Administration of CF3 to rats induced the CYP1A family proteins and CYP2A1, while CF3 reduced the level of CYP2B1, and, to a lesser extent, of CYP6 beta 2. In hamsters, CF3 treatment significantly induced the CYP1A2, CYP2A1, CYP2A8, and CYP2B1 isozymes, whereas the CYP6 beta 2 level was decreased. The ability of hepatic microsomes to activate aflatoxin B1 and benzo(a)pyrene was elevated by CF3 treatment in hamsters, while activation of aflatoxin B1 was decreased in microsomes from CF3-treated rats. These results showed differences in the CF3-induced pattern of rat and hamster cytochrome P-450 monooxygenases.

Purification and characterization of an acetone-inducible cytochrome P-450 from hamster liver microsomes

A form of cytochrome P-450 has been purified to electrophoretic homogeneity from the hepatic microsomes of Syrian golden hamsters treated with acetone. This P-450 form, designated ha P-450j, had an M(r) of approximately 55,000, bound dimethyl sulphoxide and exhibited a CO-reduced absorbance maximum at 451 nm. The absolute spectra of its oxidized form indicated that ha P-450j was predominantly in the low-spin state. In a reconstituted system, ha P-450j showed relatively low catalytic activities towards 7-ethoxycoumarin, 7-ethoxyresorufin, aminopyrine, ethylmorphine and benzphetamine, whereas it catalysed the oxidation of aniline, acetone and thiobenzamide with a high catalytic-centre activity. In addition, ha P-450j catalysed at a high rate the high-affinity component of dimethylnitrosamine N-demethylase; in contrast, only the low-affinity component of diethylnitrosamine N-de-ethylase was efficiently catalysed. The addition of cytochrome b5 to the reconstitution system decreased the Km value for dimethylnitrosamine N-demethylase by a factor of 5 and increased the Vmax. value, and slightly enhanced the other activities. Thiobenzamide and diethyldithiocarbamate were found to be the most effective inhibitors of the ha-P-450j-dependent aniline hydroxylation. Polyclonal antibodies against rat P-450j recognized ha P-450j in immunoblots of control and treated hamster liver microsomes. Treatment of hamsters with acetone increased the apparent abundance of ha P-450j in microsomes, whereas phenobarbital and beta-naphthoflavone did not induce it. Analysis of N-terminal amino acid sequences demonstrated that ha P-450j has a high degree of sequence identity with rat P-450j. All the evidence presented in this study indicates that ha P-450j could represent the hamster orthologue of the previously described CYP2E1(s) of other species.

The effect of IFN-alpha-Con1 on hepatic cytochrome P-450 and protein synthesis and degradation in hepatic microsomes

Interferon and its inducers are well known to depress drug biotransformation in the liver by decreasing the levels of cytochrome P-450 in that organ. We now report that IFN-alpha-Con1, which was constructed from the most frequently observed amino acid sequences in human alpha-interferon subtypes, causes a loss in cytochrome P-450 which could be prevented by pretreating animals with either puromycin or actinomycin D. This suggests that the loss in drug biotransformation is mediated via the production of an intermediate protein. When the turnover of microsomal protein was examined this interferon appeared to depress the synthesis of proteins with molecular weights 46-60 kd and had little effect on the synthesis of other proteins. The in vitro translation of proteins of molecular weights 45-60 kd was also depressed in an in vitro translation system using mRNA isolated from the livers of interferon treated hamsters. Interferon had no effect on the degradation of microsomal proteins of all molecular weights. It is concluded that interferon probably depresses the levels of cytochrome P-450 in the liver by decreasing the synthesis of the apoprotein and that interferon has little effect on the degradation of the hemoprotein.

Cloning and characterization of two major 3-methylcholanthrene inducible hamster liver cytochrome P450s

We have studied the immunochemical properties of two major 3-methylcholanthrene inducible hamster liver cytochrome P450 isozymes, P450 MC1 and P450 MC4. Immunoblots using specific antibodies against P450 MC1 and P450 MC4 demonstrated that these two P450s were present in very low levels in control hamster livers and were greatly induced by 3-methylcholanthrene treatment. P450 MC1 was immunochemically different from P450 MC4, rat P450c and P450d, and rabbit LM4. The immunorelated polypeptide to P450 MC1 was not present in the control or the 3-methylcholanthrene-treated rat liver microsomes, whereas it was present in two human liver microsomal preparations. On the other hand, P450 MC4 was immunochemically related to rat P450d and rabbit LM4. The immunorelated polypeptide to P450 MC4 was present in the human and 3-methylcholanthrene-treated rat liver microsomes. We also isolated full-length cDNA clones encoding P450 MC1 and P450 MC4 mRNAs from a 3-methylcholanthrene-induced hamster liver cDNA library. The full-length cDNA clones of P450 MC1 and P450 MC4 contained 1771 and 1868 base pairs, which encoded polypeptides of 494 and 513 amino acids, respectively. RNA blot analysis revealed that the mRNAs for P450 MC1 and P450 MC4 were 2100 and 2600 bases in length, respectively. 3-Methylcholanthrene pretreatment increased the P450 MC1 mRNA level by 16-fold and the P450 MC4 mRNA level by 11-fold in the hamster livers. A comparison of the deduced amino acid sequences with other cytochrome P450s revealed that P450 MC1 was most similar to the mouse P450(15) alpha with 75% sequence identity, whereas P450 MC4 shared 87% identity with the rat P450d or mouse P3(450). These results indicated that P450 MC1 was a unique member (CYP2A8) in the P450IIA subfamily, whereas P450 MC4 was the hamster P450IA2.

Induction of hepatic cytochrome P-450 mediated alkoxyresorufin O-dealkylase activities in different species by prototype P-450 inducers

The induction of cytochrome P-450-mediated alkoxyresorufin O-dealkylase activities by various xenobiotics was examined in liver from a variety of animal species in order to gain insights into the substrate specificities of the induced P-450s. We found that forms of cytochrome P-450 capable of mediating the O-dealkylation of the short-chain phenoxazone ethers methoxy-, ethoxy- and propoxyresorufin were highly induced by 3-methylcholanthrene-type inducers and by Aroclor-1254 in all species tested, although there were species differences in the relative turnover rates for the various substrates. For example, in hamster liver the turnover rates for the short-chain resorufin ethers decreased in the following order: methoxy greater than ethoxy much greater than propoxy, while in the rat liver almost the exact opposite order was observed: ethoxy = propoxy much greater than methoxy. In contrast, the degree of induction by phenobarbital-type inducers of isozymes catalyzing the O-dealkylation of pentoxy- or benzyloxyresorufin was highly species-dependent. Thus, F344/NCr rats, B6C3F1 mice and NZB rabbits showed the greatest (greater than 20-fold) induction of these activities, either by phenobarbital or Aroclor-1254, while Mongolian gerbils showed intermediate levels of induction and Syrian golden hamsters exhibited very low induction. In the Japanese quail, phenobarbital- or DDT-treatment resulted in minimal induction of pentoxy- or benzyloxyresorufin O-dealkylase activity, although significant induction of the latter activity occurred following treatment with 5,6-benzoflavone or with Aroclor-1254. Since substrate specificities of most enzymes can be rationalized based upon differences in the steric requirements at the enzyme active site, we employed molecular modeling techniques to calculate the molecular dimensions of the alkoxyresorufins. Surprisingly, the minimal energy conformations in vacuo of each of the resorufin ethers examined are essentially planar. However, alternative configurations, especially for the pentoxy- and benxyloxy-ethers, having greater three-dimensional bulk are also energetically possible.

Aflatoxin B1 metabolism by 3-methylcholanthrene-induced hamster hepatic cytochrome P-450s

We have studied the activation of aflatoxin B1 by hamster liver microsomes and purified hamster cytochrome P-450 isozymes using a umu mutagen test. The hamster liver microsomes or S-9 fractions were much more active than rat liver microsomes or S-9 fractions in the activation of umu gene expression by aflatoxin B1 metabolites. 3-Methyl-cholanthrene treatment increased aflatoxin B1 activation by hamster liver microsomes. Two major 3-methylcholanthrene-inducible cytochrome P-450 isozymes, P-450 MC1 (IIA) and P-450 MC4 (IA2), were purified from 3-methylcholanthrene-treated hamster liver microsomes, and the metabolism of aflatoxin B1 by these two cytochromes was studied. In the reconstituted enzyme system, both P-450 MC1 and P-450 MC4 were highly active in the activation of aflatoxin B1, and antibodies against these P-450s specifically inhibited these activities. Antibody against P-450 MC1 inhibited the activation of aflatoxin B1 by 20% in the presence of 3-methyl-cholanthrene-treated hamster liver microsomes. In contrast, antibody against P-450 MC4 stimulated the activity by 175%. These results indicated that hamster P-450 MC1 might convert aflatoxin B1 to more toxic metabolite(s), whereas P-450 MC4 might convert aflatoxin B1 to less toxic metabolite(s), than aflatoxin B1 in liver microsomes. The metabolite(s) produced by both hamster cytochrome P-450 MC1 and MC4 were genotoxic in the umu mutagen test.

Effect of beta-naphthoflavone on the metabolism of aflatoxin B1 in hamsters

The effect of beta-naphthoflavone (BNF) pretreatment of hamsters on the hepatic metabolism of aflatoxin B1 (AFB1) has been examined in studies in vitro and in vivo. Pretreatment with BNF not only increased microsomal cytochrome P-450 by 50-80% but also increased microsome-mediated AFB1 epoxidation as measured by AFB1-DNA binding 2.6 fold without significantly affecting other hydroxylations. Neither cytosolic GSH S-transferases' activities nor AFB1-GSH (AFB1-SG) conjugation were affected. In vivo, hepatic AFB1-DNA binding was also increased about 3-4-fold. These results in contrast to those observed in the rat indicate that induced species of cytochrome P-450 are primarily responsible for higher epoxidation of AFB1 in the hamster.

Olfactory cytochrome P-450. Studies with suicide substrates of the haemoprotein

1. The olfactory epithelium of male hamsters has been found to be extremely active in the cumene hydroperoxide-supported oxidation of tetramethylphenylenediamine, and this peroxidase activity has been shown to be cytochrome P-450-dependent. 2. The interaction of a series of suicide substrates of cytochrome P-450 with the hepatic and olfactory mono-oxygenase systems has been assessed by determination of peroxidase, 7-ethoxycoumarin O-de-ethylase (ECOD) and 7-ethoxyresorufin O-de-ethylase (EROD) activities after treatment in vivo with these compounds. Chloramphenicol, OOS-trimethylphosphorothiolate and two dihydropyridines [DDC (3,5-diethoxycarbonyl-1,4-dihydrocollidine) and 4-ethyl DDC (3,5-diethoxycarbonyl-4-ethyl-1,4-dihydro-2,6-dimethylpyridine)] all caused similar percentage inhibitions of hepatic and olfactory activities, but the absolute amounts of enzymic activity lost were considerably greater in the latter tissue. In contrast, halothane had little effect upon hepatic cytochrome P-450-dependent reactions, whereas it severely inhibited those of the olfactory epithelium. 3. The time course of loss and recovery of hepatic and olfactory peroxidase, ECOD and EROD activities after a single dose of 4-ethyl DDC was studied. The rates of loss of activity observed were very similar, irrespective of tissue or reaction examined. In the olfactory epithelium, all three activities recovered concurrently and at a rate similar to that of the hepatic peroxidase activity. In contrast, the hepatic de-ethylation of 7-ethoxycoumarin and 7-ethoxy-resorufin recovered significantly more rapidly. 4. It is suggested that this behaviour is due to 4-ethyl DDC acting not only as a suicidal inhibitor but also as an inducer of certain forms of cytochrome P-450 in the liver; in the olfactory epithelium, however, inactivation, but not induction, occurs. Classical inducing agents were reported to have no effect upon olfactory cytochrome P-450, and in the present study neither phenobarbitone nor beta-naphthoflavone treatment had any effect upon olfactory cytochrome P-450-dependent reactions, although it induced those of the liver.

Qualitative and quantitative differences in the induction and inhibition of hepatic benzo[a]pyrene metabolism in the rat and hamster

The present study compared the induction and inhibition of the metabolism of the prototype polycyclic aromatic hydrocarbon, benzo[a]pyrene (BaP), in rat and hamster liver microsomes. The production of total polar metabolites was quantitated by separating 3H-metabolites from [3H]-BaP using reverse-phase thin-layer chromatography. The rate of hepatic microsomal BaP metabolism was similar in the rat and hamster (0.81 vs 0.72 nmol/min/nmol cytochrome P-450 respectively). In the rat, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 5 micrograms/kg, i.p.) and 3-methylcholanthrene (3-MC; 50 mg/kg, i.p., X 3 days) pretreatments doubled the rate of BaP metabolism, whereas phenobarbital pretreatment (PB; 80 mg/kg, i.p., X 3 days) had no effect. In contrast, hamster hepatic microsomal BaP metabolism was elevated 2.3-fold by PB pretreatment, whereas TCDD and 3-MC pretreatments had no effect. Isosafrole pretreatment (ISO; 150 mg/kg, i.p., X 3 days) elevated the rate by almost 2-fold in each species. Another cytochrome P-448-mediated activity, 7-ethoxyresorufin O-deethylase (EROD), was induced by the same compounds that induced BaP metabolism in the rat. In hamster liver microsomes, in contrast to BaP metabolism, EROD was induced by TCDD and 3-MC but not PB or ISO pretreatments. The results suggest differences in the substrate specificity of the cytochromes P-448-450 induced by TCDD, 3-MC and PB in these species. This was supported by the different selectivity of the in vitro inhibitors, metyrapone and 7,8-benzoflavone, towards BaP metabolism and EROD in hepatic microsomes from TCDD- or PB-pretreated rats and hamsters. Reverse-phase HPLC analysis indicated that, while 3-hydroxy-BaP was the major metabolite formed by the untreated rat, untreated hamster liver microsomes formed predominantly BaP-4,5-diol. Microsomes from TCDD-treated rats generated elevated levels of all BaP-diols, diones and 3-hydroxy-BaP, with the major metabolites being BaP-9,10- and BaP-7,8-diols. In contrast, the metabolite profile from TCDD-pretreated hamsters was unchanged from the control. PB-treated hamster microsomes produced elevated levels of BaP-diones and 3-hydroxy-BaP. However, the major hepatic metabolite formed by PB-pretreated hamsters was BaP-4,5-diol, while BaP-9,10- and BaP-7,8-diols were not detected. The results of the study indicate differences in the induced cytochrome P-450s and the generation of toxic BaP metabolites in the liver of the rat and hamster.

Expression of hepatic microsomal cholesterol 7 alpha-hydroxylase activity in lean and obese Zucker rats

The activity of hepatic microsomal cholesterol 7 alpha-hydroxylase was studied in genetically obese and lean Zucker rats. The liver microsomal cholesterol 7 alpha-hydroxylase activity in fatty Zucker rats (fa/fa) is about 50% to 70% lower than that of the lean (Fa/-) rats of the same sex, when animals were sacrificed at the middle of the dark cycle. When rats were sacrificed at the middle of the light cycle, cholesterol 7 alpha-hydroxylase activity was the same as in the dark cycle in obese rats of both sexes, but was 65% lower in lean rats. However, cholesterol 7 alpha-hydroxylase activity was stimulated by the treatment with cholestyramine in both obese and lean rats. Our results suggested that the diurnal regulation of cholesterol 7 alpha-hydroxylase activity is lost in obese rats but was present under cholestyramine treatment in the genetically obese strain of rats.

Induction of P-450 isoenzyme activities in Syrian golden hamster liver compared to rat liver as probed by the rate of 7-alkoxyresorufin-O-dealkylation

The activities of 7-ethoxyresorufin-O-deethylase (EROD), 7-pentoxyresorufin-O-deethylase (PROD), 7-ethoxycoumarin-O-deethylase (ECOD) and aromatic hydrocarbon hydroxylase (AHH) were measured in hepatic microsomes from male and female Wistar rats and Syrian golden hamsters in order to probe the basal activity and the inducibility by phenobarbital (PB) and 3-methylcholanthrene (MC) of different P-450 isoenzymes. The basal activities of EROD and ECOD, but not PROD and AHH, were higher in male hamsters than in male rats. No sex-related difference in enzyme activities was observed with hamsters, whereas male rats had a higher ECOD and AHH activity than female rats. Induction by PB led to a 450-fold and 250-fold increase in PROD activity in male and female rat liver microsomes, respectively, while MC had a more pronounced inductive effect on EROD activity in this species. In hamsters, EROD activity was induced by MC but not by PB. Unexpectedly PROD activity in male and female hamster liver microsomes was only moderately induced by PB, the extent being lower than on induction by MC. Therefore, the activity of PROD, which is useful as a specific enzymatic assay for P-450 IIB in the rat liver, cannot be used to probe PB-like inducers in the hamster liver.

Characterization of the cytochrome P-450 monooxygenase system of hamster liver microsomes. Effects of prior treatment with ethanol and other xenobiotics

The cytochrome P-450 monooxygenase system of hamster liver microsomes and its response to prior treatment with ethanol and other xenobiotics have been examined. Male Syrian golden hamsters were administered ethanol (ETOH), phenobarbital (PB), 5,6-benzoflavone (BF) or isoniazid (INH). Each treatment resulted in a moderate increase (20-60%) in the specific content of liver microsomal cytochrome P-450 along with a unique hemeprotein ferrous carbonyl Soret maximum. Sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis of liver microsomes revealed distinctive changes in protein banding patterns in the cytochrome P-450 (45-60 kDa) region with each treatment. NADPH: cytochrome c reductase activity was increased by both PB and INH, whereas cytochrome b5 content was increased by INH only. Microsomal oxidation of ETOH and aniline p-hydroxylation (expressed per nmol cytochrome P-450) were enhanced dramatically by ETOH and INH, whereas PB and BF had no effect on these enzymatic activities. Both ETOH and INH also increased zoxazolamine 6-hydroxylation but, in contrast to other rodent species, this drug-metabolizing activity was decreased in hamster liver microsomes after treatment with either PB or BF. Microsomal benzphetamine N-demethylation was decreased by ETOH, INH and BF administration and was only modestly enhanced after treatment with PB. ETOH and INH had no effect on the O-deethylation of 7-ethoxycoumarin, and enzymatic activity increased by BF but decreased by PB. These results demonstrate that the cytochrome P-450-dependent monooxygenase system of hamster liver microsomes responds to treatment with ETOH and other xenobiotics in a manner that is quantitatively and, in certain respects, qualitatively different from that reported for the rat, rabbit, and mouse.

Characterization of hepatic and pulmonary cytochromes P-450 in 3-methylcholanthrene-treated hamsters

Two major forms of hepatic cytochrome P-450 (hepatic P-450MCI and P-450MCII) were purified approximately 5-fold from liver microsomes in Syrian golden hamsters treated with 3-methylcholanthrene (MC). The purified preparations of hepatic P-450MCI and P-450MCII contained 9.6 and 8.3 nmol cytochrome P-450 (P-450) per mg protein, respectively, and were essentially free from NADPH-cytochrome c (P-450) reductase (fpT), NADH-cytochrome b5 reductase and cytochrome b5. By sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE), the molecular weights of hepatic P-450MCI and P-450MCII were estimated to be 56,000 and 53,500. Further, a major form of pulmonary P-450 (P-450MC) were purified from lung microsomes of MC-treated hamster, and contained 14.2 nmol P-450 per mg protein, and estimated to be 56,000 in monomeric molecular weight, indicating the similar molecular weight to hepatic P-450MCI in the hamster. From the absorption spectra the oxidized forms of hepatic P-450MCI and P-450MCII were high- and low-spin ferric hemoproteins, respectively, and pulmonary P-450MC was similar to hepatic P-450MCII in their hemoprotein spin state. No difference, however, was observed in the CO-reduced forms among hepatic P-450MCI, P-450MCII and pulmonary P-450MC, all exhibiting 446.5 nm Soret bands. In a reconstituted system containing fpT and dilauroylphosphatidylcholine (DLPC), pulmonary P-450MC efficiently catalyzed benzo[a]pyrene (BP) hydroxylation at a rate of 11.4 mol formed per min per mol P-450, but hepatic P-450MCI and P-450MCII both exhibited lower levels, e.g., 0.49 and 0.54, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of hepatic oxidative and conjugative drug metabolism in the hamster by N-substituted imidazoles

Three antimycotic N-substituted imidazoles, clotrimazole, tioconazole and miconazole, were able to induce hepatic microsomal cytochrome P-450 and monooxygenase reactions in both male and female hamsters to an extent similar to that seen with phenobarbital treatment. Imidazole treatment did not alter the cytochrome P-450 concentration, and ketoconazole treatment decreased it. Cytosolic sulfo- and glutathione transferases were not significantly altered by any imidazole. Induction of microsomal morphine glucuronosyltransferase activity by each compound generally paralleled the effect on cytochrome P-450 in females but induction was not evident in males. Clotrimazole treatment, in contrast to phenobarbital treatment, also caused a large induction of l-naphthol glucuronosyltransferase in females. The potential for antimycotic imidazoles to alter the hepatotoxicity of compounds will require consideration of the inductive changes in both Phase I and Phase II drug metabolizing enzymes in addition to their known inhibitory effects on Phase I oxidations. The inductive effects differ for each imidazole, and in the hamster model, depend upon the sex of the animal.

Purification and characterization of a form of cytochrome P-450 with high specificity for aflatoxin B1 from 3-methylcholanthrene-treated hamster liver

A form of cytochrome P-450 highly active in inducing mutagenicity of aflatoxin B1 was purified to a specific content of 15.1 nmol/mg of protein from 3-methylcholanthrene-treated hamster liver. This species of cytochrome P-450, having its absorption maximum at 448.5 nm in carbon monoxide-complex of reduced form and low spin ferric ion, is of molecular weight of 56,000 and distinctly different in physicochemical and catalytic properties from major forms of cytochrome P-450 purified from phenobarbital- or 3-methylcholanthrene-treated rat liver. In the induction of aflatoxin B1 mutagenicity, this hamster cytochrome P-450 is 50 times more potent than those from rat liver.

Changes in hamster hepatic cytochrome P-450, ethoxycoumarin O-deethylase, and reduced NAD(P): menadione oxidoreductase following treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin. Partial dissociation of temporal and dose-response relationships from elicited toxicity

The temporal and dose-related characteristics of hepatic enzymes induced in the hamster by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were examined. Male Syrian golden hamsters received a single intraperitoneal injection of TCDD at a dose of 0-500 micrograms/kg. At various times up to 35 days, a number of variables were determined and compared: whole body, liver, and thymus weights; hepatic concentrations of cytochrome P-450 (P-450); and activities of 7-ethoxycoumarin O-deethylase (ECOD) and reduced NAD(P): menadione oxidoreductase (NMOR). Increased liver weights and decreased thymus weights were observed to be dose related. At day 7 following treatment, the approximate ED50 values for these responses were 15 and 100 micrograms/kg respectively. The ED50 values for the increase in hepatic P-450 concentrations and activities of ECOD and NMOR ranged from 0.5 to 2.0 micrograms/kg. At 10 and 500 micrograms/kg, NMOR activity remained maximally induced for up to 35 days. This was also the case for P-450 and ECOD activity at a dose of 10 micrograms/kg. At 500 micrograms/kg, both P-450 and ECOD demonstrated an induction up to day 4 followed by a decrease to near control levels by day 14. This decrease appeared to correlate with changes in hepatic morphology. These results demonstrate a dissociation of the induction of these hepatic enzymes from TCDD-induced lethality, in this species.

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.

The biologic and toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin in chickens

The administration of 2,3,7,8-TCDD to 2-week-old White Leghorn cockerels produced a dose-dependent induction of hepatic microsomal ethoxyresorufin O-deethylase (EROD) and benzo[alpha]pyrene (B[alpha]P) hydroxylase activities with induction EC50 values of 778 and 302 ng/kg, respectively. In addition, 2,3,7,8-TCDD also induced 4-dimethylaminoantipyrine (DMAP) N-demethylase (EC50 = 561 ng/kg), this result contrasting with studies reported for other animal species in which 2,3,7,8-TCDD either does not induce or inhibits this cytochrome P-450 dependent monooxygenase enzyme activity. However, the reduced cytochrome P-450:CO binding difference spectral absorption maxima for the 2,3,7,8-TCDD induced microsomes was observed at 448 nm which was similar to that reported for most animals which have been investigated. Electrophoresis of control and 2,3,7,8-TCDD induced microsomal proteins using SDS polyacrylamide slab gels showed intensification of 3 protein staining bands at Mr 53 000, 56 000 and 58 000. Incubation of the 2,3,7,8-TCDD-induced microsomes with SKF-525A and alpha-naphthoflavone showed that both compounds inhibited DMAP N-demethylase, EROD and AHH and that alpha-naphthoflavone was the more potent inhibitor of all 3 microsomal monooxygenases. 2,3,7,8-TCDD treatment caused significant involution of the Bursa of Fabricius when administered at a dose level of 10 micrograms/kg for 3 days and this result confirmed the extreme sensitivity of the immature White Leghorn cockerel to the biologic and toxic effects elicited by 2,3,7,8-TCDD. However, in contrast to other sensitive species, no high affinity cytosolic receptor protein for [3H]2,3,7,8-TCDD could be detected in the liver of chick embryos or 2-week-old birds.

Hamster hepatic cytochrome b5: purifications, immunochemical properties, and in vitro synthesis

Cytochrome b5 has been purified from hamster liver microsomes. Both Ouchterlony double-diffusion and rocket immunoelectrophoresis experiments indicate that no immuno-cross-reactivity exists between guinea-pig anti-rabbit cytochrome b5 antibody and hamster cytochrome b5. However, anti-rabbit b5 IgG inhibited both hamster microsomal NADH-cytochrome c reductase and NADPH-dependent 7-ethoxycoumarin-O-deethylase activities. Hamster cytochrome b5 stimulated several reconstituted hamster cytochrome P-450-dependent monooxygenase activities and this stimulatory effect could be inhibited by antibody against rabbit cytochrome b5. Two-dimensional iodinated tryptic peptide mapping experiments provided evidence that the polypeptide fingerprint of hamster cytochrome b5 is substantially different from the fingerprints of cytochrome b5 isolated from rabbit, rat and bovine. We also studied the in vitro synthesis of hamster cytochrome b5 from liver mRNA using a wheat germ lysate system. A 16 kDa polypeptide, which is the same size as hamster cytochrome b5, was immunoprecipitated by antibody against rabbit b5. This experiment suggested that in vitro synthesized hamster cytochrome b5 is recognized by a heterologous antibody. Thus, hamster and rabbit cytochrome b5 do share some common immuno-determinants which may be located close to the heme-binding active site.

Effects of chronic ethanol consumption on male Syrian hamster hepatic, microsomal mixed-function oxidases

Chronic alcohol consumption significantly increases the risk of drug interactions. We have described its effects on hamster microsomal monooxygenases. Male Syrian hamsters (85 g) were given 10% ethanol in water and food ad lib for up to 6 weeks. Microsomal electron transport components and metabolism of ethylmorphine, benzphetamine, aniline, and acetaminophen were measured. At 4 weeks, SDS-PAGE of ethanol microsomes showed an induced band with an Mr of 53,900 daltons and there was a 2-3 fold stimulation of aniline and acetaminophen metabolism. Cytochrome P-450 increase was not significant. For the six week period, Caloric intake (3 weeks, p less than 0.001), liquid consumption (3 weeks, p less than 0.05) and body weights (6 weeks, p less than 0.05) of ethanol animals were significantly greater than controls; kidney weights were significantly less (p less than 0.05). Ethanol consumption increased from 20% of the daily caloric intake (week 1) to 31% (week 6). Induction of specific substrate metabolism without apparent deleterious physiological changes establishes hamsters fed 10% ethanol in drinking water as a biochemical model for the study of chronic alcohol consumption and specific drug interactions.