Characterization of a phenobarbital-inducible dog liver cytochrome P450 structurally related to rat and human enzymes of the P450IIIA (steroid-inducible) gene subfamily

Four Decades of Cytochrome P450 2B Research: From Protein Adducts to Protein Structures and Beyond

This article features selected findings from the senior author and colleagues dating back to 1978 and covering approximately three-fourths of the 60 years since the discovery of cytochrome P450. Considering the vast number of P450 enzymes in this amazing superfamily and their importance for so many fields of science and medicine, including drug design and development, drug therapy, environmental health, and biotechnology, a comprehensive review of even a single topic is daunting. To make a meaningful contribution to the 50th anniversary of Drug Metabolism and Disposition, we trace the development of the research in a single P450 laboratory through the eyes of seven individuals with different backgrounds, perspectives, and subsequent career trajectories. All co-authors are united in their fascination for the structural basis of mammalian P450 substrate and inhibitor selectivity and using such information to improve drug design and therapy. An underlying theme is how technological advances enable scientific discoveries that were impossible and even inconceivable to prior generations. The work performed spans the continuum from: 1) purification of P450 enzymes from animal tissues to purification of expressed human P450 enzymes and their site-directed mutants from bacteria; 2) inhibition, metabolism, and spectral studies to isothermal titration calorimetry, deuterium exchange mass spectrometry, and NMR; 3) homology models based on bacterial P450 X-ray crystal structures to rabbit and human P450 structures in complex with a wide variety of ligands. Our hope is that humanizing the scientific endeavor will encourage new generations of scientists to make fundamental new discoveries in the P450 field. SIGNIFICANCE STATEMENT: The manuscript summarizes four decades of work from Dr. James Halpert's laboratory, whose investigations have shaped the cytochrome P450 field, and provides insightful perspectives of the co-authors. This work will also inspire future drug metabolism scientists to make critical new discoveries in the cytochrome P450 field.

In vitro metabolism of polychlorinated biphenyls and cytochrome P450 monooxygenase activities in dietary-exposed Greenland sledge dogs

The in vitro metabolism of a polychlorinated biphenyl (PCB) mixture was examined using hepatic microsomes of dietary-exposed Greenland sledge dogs (Canis familiaris) to an organohalogen-rich diet (Greenland minke whale blubber: EXP cohort) or a control diet (pork fat: CON cohort). The associations between in vitro PCB metabolism, activity of oxidative hepatic microsomal cytochrome P450 (CYP) isoenzymes and concentrations of PCBs and hydroxylated metabolites were investigated. The CON dogs exhibited a 2.3-fold higher depletion percentage for the PCB congeners having at least two pairs of vicinal meta-para Cl-unsubstituted carbons (PCB-18 and -33) relative to the EXP dogs. This depletion discrepancy suggests that there exist substrates in liver of the organohalogen-contaminated EXP dogs that can competitively bind and/or interfere with the active sites of CYP isoenzymes, leading to a lower metabolic efficiency for these PCBs. Testosterone (T) hydroxylase activity, determined via the formation of 6beta-OH-T, 16alpha-OH-T, 16beta-OH-T and androstenedione, was strongly correlated with the depletion percentages of PCB-18 and -33 in both cohorts. Based on documented hepatic microsomal CYP isoenzyme substrate specificities in canines, present associations suggest that primarily CYP2B/2C and CYP3A were inducible in sledge dogs and responsible for the in vitro metabolism of PCB-18 and -33.

Engineering and assaying of cytochrome P450 biocatalysts

Cytochrome P450s constitute a highly fascinating superfamily of enzymes which catalyze a broad range of reactions. They are essential for drug metabolism and promise industrial applications in biotechnology and biosensing. The constant search for cytochrome P450 enzymes with enhanced catalytic performances has generated a large body of research. This review will concentrate on two key aspects related to the identification and improvement of cytochrome P450 biocatalysts, namely the engineering and assaying of these enzymes. To this end, recent advances in cytochrome P450 development are reported and commonly used screening methods are surveyed.

Genetic polymorphism of cytochrome P450s in beagles: possible influence of CYP1A2 deficiency on toxicological evaluations

A number of human cytochrome P450 (CYP) isozymes have been shown to be genetically polymorphic, and extensive pharmaceutical studies have been conducted to characterize the clinical relevance of the polymorphism. Although the beagle is extensively used in the safety assessment studies of new drug candidates and agricultural chemicals, only a limited number of studies have been reported on the significance of the CYP isozyme polymorphism in dogs. Recently, a single nucleotide polymorphism that results in a deficiency of canine CYP1A2 was discovered. This deficiency was shown to significantly alter the pharmacokinetic behavior of two drugs, and can be associated with a large inter-individual difference in the kinetic behavior of a third. In this article, the five genetically polymorphic canine CYP isozymes that have been reported so far are reviewed, and the altered pharmacokinetics of the drugs concerned are described. Although little information on toxicological relevance has been reported, it is possible that the modified pharmacokinetics may also cause altered toxic responses as well. This phenomenon may occur only with the types of chemicals that are eliminated mainly through polymorphic-enzyme mediated metabolism. However, it is recommended that genetically pure beagles are used for the toxicity studies and safety assessment of new chemical entities in order to reduce the potential inter-individual differences.

Functional role of residues in the helix B' region of cytochrome P450 2B1

Comparison of several recently determined X-ray crystal structures of mammalian cytochrome P450 family 2 enzymes suggests considerable movement of helix B' when ligands bind. To investigate the functional role of helix B' in P450 2B1, residues 100-109 were substituted with alanine and phenylalanine. Kinetic properties were examined with the typical 2B substrates 7-benzyloxyresorufin, 7-ethoxy-4-trifluoromethylcoumarin, benzphetamine, and testosterone. Several mutants showed 2- to 3-fold changes in k(cat) values and significant differences in catalytic efficiencies among the substrates examined, consistent with structural information suggesting that the helix B' region can adopt multiple conformations with different contact residues depending on the substrate. Homology modeling of P450 2B1 was performed based on an inhibitor-bound P450 2B4 structure, and the docking analyses were consistent with experimental results. The findings suggest that residues in the helix B' region affect regio- and stereoselective oxidation in P450 family 2 enzymes as well as substrate entry.

Substrate specificity and kinetic properties of seven heterologously expressed dog cytochromes p450

Seven dog cytochromes p450 (p450s) were heterologously expressed in baculovirus-Sf21 insect cells. Of all enzymes examined, CYP1A1 exhibited high 7-ethoxyresorufin O-deethylase activity (low Km enzyme, 1 microM). CYP2B11 and CYP3A12 effectively catalyzed the N1-demethylation and C3-hydroxylation of diazepam (and its derivatives), whereas CYP3A12 and CYP2D15 catalyzed exclusively the N- and O-demethylation, respectively, of dextromethorphan. However, no saturation velocity curves for the N-demethylation of dextromethorphan (up to 500 microM) were achieved, suggesting a high Km for CYP3A12. In contrast to CYP3A12, the CYP2D15-dependent O-demethylation of dextromethorphan was a low Km process (Km = 0.7 microM), similar to that in dog liver microsomes (Km = 2.3 microM). CYP2D15 was also capable of metabolizing bufuralol (1'-hydroxylation), with a Km of 3.9 microM, consistent with that obtained with dog liver microsomes. CYP3A12 was shown to primarily oxidize testosterone at 16alpha-, 2alpha/2beta-, and 6beta-positions. Selectivity of CYP3A12 was observed toward testosterone 6beta-(Km = 83 microM) and 2alpha/2beta-hydroxylations (Km = 154 microM). However, the 16alpha-hydroxylation of testosterone was catalyzed by CYP2C21 also (Km = 6.4 microM for CYP2C21). Therefore, the 6beta- and 16alpha-hydroxylation of testosterone can potentially be employed as markers of CYP3A12 and CYP2C21 (at low concentration), respectively. CYP2C21 was also capable of catalyzing diclofenac 4'-hydroxylation, although some activity was detected with CYP2B11. Surprisingly, none of the p450s selectively metabolized (S)-mephenytoin 4'-hydroxylation. The results described herein are a first step toward the systematic evaluation of a panel of dog p450s and the development of dog p450 isoenzyme-selective marker substrates, as well as providing useful information on prediction and extrapolation of the results from in vitro to in vivo and from dog to human.

Effect of oral administration of low doses of pentobarbital on the induction of cytochrome P450 isoforms and cytochrome P450-mediated reactions in immature Beagles

OBJECTIVE

To determine the effect of oral administration of low doses of pentobarbital on cytochrome P450 (CYP) isoforms and CYP-mediated reactions in immature Beagles.

ANIMALS

42 immature (12-week-old) Beagles.

PROCEDURE

Dogs were grouped and treated orally as follows for 8 weeks: low-dose pentobarbital (50 microg/d; 4 males, 4 females), mid-dose pentobarbital (150 microg/d; 4 males, 4 females), high-dose pentobarbital (500 microg/d; 4 males, 4 females), positive-pentobarbital control (10 mg/kg/d; 2 males, 2 females), positive-phenobarbital control (10 mg/kg/d; 2 males, 2 females), and negative control (saline 10.9% NaCl] solution; 5 males, 5 females). Serum biochemical and hematologic values were monitored. On necropsy examination, organ weights were determined, and histologic evaluation of tissue sections of liver, kidney, small intestine, testes, epididymis, and ovaries was performed. Hepatic and intestinal drug-metabolizing enzyme activities were measured, and relative amounts of CYP isoforms were determined by western blot analysis.

RESULTS

The amount of a hepatic CYP2A-related isoform in dogs from the high-dose pentobarbital treatment group was twice that of dogs from the negative control group. CYP2C was not detectable in small intestinal mucosa of dogs from the negative control group; measurable amounts of CYP2C were found in dogs from the various (low-, mid-, and high-dose) pentobarbital treatment groups and from positive-pentobarbital and positive phenobarbital control groups. Several CYP-mediated reactions increased in a dose-dependent manner. The lowest calculated effective dose of pentobarbital ranged from 200 to 450 microg/d.

CONCLUSIONS AND CLINICAL RELEVANCE

Several CYP isoforms and their associated reactions were induced in dogs by oral administration of low amounts of pentobarbital.

A rational approach to Re-engineer cytochrome P450 2B1 regioselectivity based on the crystal structure of cytochrome P450 2C5

The regioselectivity for progesterone hydroxylation by cytochrome P450 2B1 was re-engineered based on the x-ray crystal structure of cytochrome P450 2C5. 2B1 is a high K(m) progesterone 16alpha-hydroxylase, whereas 2C5 is a low K(m) progesterone 21-hydroxylase. Initially, nine individual 2B1 active-site residues were changed to the corresponding 2C5 residues, and the mutants were purified from an Escherichia coli expression system and assayed for progesterone hydroxylation. At 150 microm progesterone, I114A, F297G, and V363L showed 5-15% of the 21-hydroxylase activity of 2C5, whereas F206V showed high activity for an unknown product and a 13-fold decrease in K(m). Therefore, a quadruple mutant, I114A/F206V/F297G/V363L (Q), was constructed that showed 60% of 2C5 progesterone 21-hydroxylase activity and 57% regioselectivity. Based on their 2C5-like testosterone hydroxylation profiles, S294D and I477F alone and in combination were added to the quadruple mutant. All three mutants showed enhanced regioselectivity (70%) for progesterone 21-hydroxylation, whereas only Q/I477F had a higher k(cat). Finally, the remaining three single mutants, V103I, V367L, and G478V, were added to Q/I477F and Q/S294D/I477F, yielding seven additional multiple mutants. Among these, Q/V103I/S294D/I477F showed the highest k(cat) (3-fold higher than that of 2C5) and 80% regioselectivity for progesterone 21-hydroxylation. Docking of progesterone into a three-dimensional model of this mutant indicated that 21-hydroxylation is favored. In conclusion, a systematic approach to convert P450 regioselectivity across subfamilies suggests that active-site residues are mainly responsible for regioselectivity differences between 2B1 and 2C5 and validates the reliability of 2B1 models based on the crystal structure of 2C5.

mRNA and protein expression of dog liver cytochromes P450 in relation to the metabolism of human CYP2C substrates

1. Interpretation of novel drug exposure and toxicology data from the dog is tempered by our limited molecular and functional knowledge of dog cytochromes P450 (CYPs). The aim was to study the mRNA and protein expression of hepatic dog CYPs in relation to the metabolism of substrates of human CYP, particularly those of the CYP2C subfamily. 2. The rate of 7-hydroxylation of S-warfarin (CYP2C9 in humans) by dog liver microsomes (mean +/- SD from 12 (six male and six female) dogs = 10.8 +/- 1.9 fmol mg(-1) protein min(-1)) was 1.5-2 orders of magnitude lower than that in humans. 3. The rate of 4'-hydroxylation of S-mephenytoin, catalysed in humans by CYP2C19, was also low in dog liver (4.6 +/-1.5 pmol mg(-1) protein min(-1)) compared with human liver. In contrast, the rate of 4'-hydroxylation of the R-enantiomer of mephenytoin by dog liver was much higher. The kinetics of this reaction (range of K(m) or K(0.5) 15-22 micro M, V(max) 35-59 pmol mg(-1) protein min(-1), n = 4 livers) were consistent with the involvement of a single enzyme. 4. In contrast to our findings for S-mephenytoin, dog liver microsomes 5'-hydroxylated omeprazole (also catalysed by CYP2C19 in humans) at considerably higher rates (range of K(m) 42-64 micro M, V(max) 22-46 pmol mg(-1) protein min(-1), n = 4 livers). 5. For all the substrates except omeprazole, a sex difference in their metabolism was observed in the dog (dextromethorphan N-demethylation: female range = 0.7-0.9, male = 0.4-0.8 nmol mg(-1) protein min(-1) (p < 0.02); S-warfarin 7-hydroxylation: female = 9-15.5, male = 8-12 fmol mg(-1) protein min(-1) (p < 0.02); R-mephenytoin 4'-hydroxylation: female = 16-35, male = 11.5-19 pmol mg(-1) protein min(-1) (p < 0.01); omeprazole 5'-hydroxylation: female = 15-20, male 13-22 pmol mg(-1) protein min(-1) (p < 0.2)). 6. All dog livers expressed mRNA and CYP3A12, CYP2B11, CYP2C21 proteins, with no sex differences being found. Expression of CYP2C41 mRNA was undetectable in the livers of six of 11 dogs. 7. Correlation analysis suggested that CYP2B11 catalyses the N-demethylation of dextromethorphan (mediated in humans by CYP3A) and the 4'-hydroxylation of mephenytoin (mediated in humans by CYP2C19) in the dog, and that this enzyme and CYP3A12 contribute to S-warfarin 7-hydroxylation (mediated in humans by CYP2C9). 8. In conclusion, we have identified a distinct pattern of hepatic expression of the CYP2C41 gene in the Alderley Park beagle dog. Furthermore, marked differences in the metabolism of human CYP2C substrates were observed in this dog strain compared with humans with respect to rate of reaction, stereoselectivity and CYP enzyme selectivity.

In vivo and in vitro induction of cytochrome P450 enzymes in beagle dogs

The aim of this study was to determine the in vitro and in vivo effects of several prototypical inducers, namely beta-naphthoflavone, 3-methylcholanthrene, phenobarbital, isoniazid, rifampin, and clofibric acid, on the expression of cytochrome P450 (P450) enzymes in beagle dogs. For the in vitro induction study, primary cultures of dog hepatocytes were treated with enzyme inducers for 3 days, after which microsomes were prepared and analyzed for P450 activities. For the in vivo induction study, male and female beagle dogs were treated with enzyme inducers for 4 days (with the exception of phenobarbital, which was given for 14 days), after which the livers were removed and microsomal P450 activities were determined ex vivo. Treatment of male beagle dog hepatocyte cultures (n = 3) with beta-naphthoflavone or 3-methlychloranthrene resulted in up to a 75-fold increase in microsomal 7-ethoxyresorufin O-dealkylase (CYP1A1/2) activity, whereas in vivo treatment of male and female beagle dogs with beta-naphthoflavone followed by ex vivo analysis resulted in up to a 24-fold increase. Phenobarbital caused a 13-fold increase in 7-benzyloxyresorufin O-dealkylase (CYP2B11) activity in vitro and up to a 9.9-fold increase in vivo. Isoniazid had little or no effect on 4-nitrophenol hydroxylase activity in vitro. Rifampin caused a 13-fold induction of testosterone 6beta-hydroxylase (CYP3A12) activity in vitro and up to a 4.5-fold increase in vivo. Treatment of dogs in vivo or dog hepatocytes in vitro with clofibric acid appeared to have no effect on CYP4A activity as determined by the 12-hydroxylation of lauric acid. In general, the absolute rates (picomoles per minute per milligram of microsomal protein) of P450 reactions catalyzed by microsomes from cultured hepatocytes (i.e., in vitro rates) were considerably lower than those catalyzed by microsomes from dog liver (i.e., ex vivo rates). These results suggest that beagle dogs have CYP1A, CYP2B, CYP2E, and CYP3A enzymes and that the induction profile resembles the profile observed in humans more than in rats.

Substrate routes to the buried active site may vary among cytochromes P450: mutagenesis of the F-G region in P450 2B1

Until recently, all known structures of bacterial cytochromes P450 suggested that substrate access to the buried active site occurred via the F-G region, a surface loop distal to the heme cavity. However, the structure of P450 51 indicates a large opening from the protein surface along the I helix N-terminus, at right angles to the F-G channel. The single available microsomal P450 structure (2C5) does not obviously favor one potential access route over the other. To determine whether the F-G region forms part of the substrate access channel in the microsomal cytochrome P450 2B1, 11 residues between positions 208 and 230 were substituted with smaller and larger side chains in a highly expressed truncated form of the enzyme. Steady-state kinetic parameters were determined with the substrates testosterone, 7-ethoxy-4-trifluoromethylcoumarin (7-EFC), and 7-benzyoxyresorufin (7-BR). The largest changes, 2-6-fold increases in k(cat) with testosterone and 7-EFC, were observed for L209A, which also exhibits an altered testosterone metabolite profile and probably forms part of the active site roof. F219W demonstrated little or no activity with any of the three substrates examined, although the K(s) value for benzphetamine binding was unaltered. S221F showed little activity with 7-BR. No significant changes were observed in K(m)(testosterone) or S(50)(7-EFC) values for any of the mutants, in stark contrast to the 10-fold and 100-fold changes in K(m) observed for mutants in this region of other cytochromes P450. The minimal changes in 2B1 do not support access via the F-G region of 2B1 and suggest the alternate access route identified in P450 51.

Disposition and clinical use of bromide in cats

OBJECTIVE

To establish a dosing regimen for potassium bromide and evaluate use of bromide to treat spontaneous seizures in cats.

DESIGN

Prospective and retrospective studies.

ANIMALS

7 healthy adult male cats and records of 17 cats with seizures.

PROCEDURE

Seven healthy cats were administered potassium bromide (15 mg/kg [6.8 mg/lb], p.o., q 12 h) until steady-state concentrations were reached. Serum samples for pharmacokinetic analysis were obtained weekly until bromide concentrations were not detectable. Clinical data were obtained from records of 17 treated cats.

RESULTS

In the prospective study, maximum serum bromide concentration was 1.1 +/- 0.2 mg/mL at 8 weeks. Mean disappearance half-life was 1.6 +/- 0.2 weeks. Steady state was achieved at a mean of 5.3 +/-1.1 weeks. No adverse effects were detected and bromide was well tolerated. In the retrospective study, administration of bromide (n = 4) or bromide and phenobarbital (3) was associated with eradication of seizures in 7 of 15 cats (serum bromide concentration range, 1.0 to 1.6 mg/mL); however, bromide administration was associated with adverse effects in 8 of 16 cats. Coughing developed in 6 of these cats, leading to euthanasia in 1 cat and discontinuation of bromide administration in 2 cats.

CONCLUSIONS AND CLINICAL RELEVANCE

Therapeutic concentrations of bromide are attained within 2 weeks in cats that receive 30 mg/kg/d (13.6 mg/lb/d) orally. Although somewhat effective in seizure control, the incidence of adverse effects may not warrant routine use of bromide for control of seizures in cats.

The genetic determinants of the CYP3A5 polymorphism

CYP3A proteins comprise a significant portion of the hepatic cytochrome P450 (CYP) protein and they metabolize around 50% of drugs currently in use. The dissection of the individual contributions of the four CYP3A genes identified in humans to overall hepatic CYP3A activity has been hampered by sequence and functional similarities. We have investigated the expression of CYP3A5 and its genetic determinants in a panel of 183 Caucasian liver samples. CYP3A5 expression is increased in 10% of livers in this ethnic group. Using a high density map of CYP3A5 variants, we searched for genetic markers of the increased CYP3A5 expression. In agreement with an independent, recent study, we report that a SNP within intron 3 (g.6986G>A) is the primary cause of the CYP3A5 protein polymorphism. The frequencies of the g.6986A variant which allow for normal splicing of CYP3A5 transcripts are 5% in Caucasians, 29% in Japanese, 27% in Chinese, 30% in Koreans and 73% in African-Americans. In the last ethnic group, the expression of CYP3A5 in some individuals who carry the g.6986A variant is affected adversely by a frame shift mutation (CYP3A5*7, D348., q = 0.10). In summary, these results should add to efforts to identify clinically relevant, CYP3A5-specific reactions and to further elucidate traits responsible for variable expression of the entire CYP3A family.

A truncation of 2B subfamily cytochromes P450 yields increased expression levels, increased solubility, and decreased aggregation while retaining function

The hydrophobic membrane-spanning domain in four cytochromes P450 2B was removed (Delta3-21) and several positive charges were substituted at the N-terminus to increase expression and solubility. Histidine residues were appended to the C-terminus to simplify purification. The truncated proteins were highly expressed in Escherichia coli, could be released from the membrane using high salt conditions, and were purified from this fraction to specific contents up to 19 nmol P450/mg protein using a single Ni(2+)-agarose column. Gel filtration revealed that truncated P450 2B1 forms a monodisperse solution of hexamers in the absence of detergent and >95% monomers in 0.25% sodium cholate. All truncated proteins, including human 2B6, were active with 7-ethoxy-4-trifluoromethylcoumarin, and truncated 2B1 was shown to retain the native regio- and stereospecificity of testosterone hydroxylation. These data demonstrate that modification of the N-terminus yields high levels of properly folded P450s 2B with increased solubility, which are suitable for functional and structural analysis.

The relationship among microsomal enzyme induction, liver weight and histological change in beagle dog toxicology studies

The present study represents a retrospective analysis of hepatic microsomal enzyme induction data collected over a period of years for the beagle dog. Comparisons were completed for up to six enzyme activities and P450 content versus histopathological examination of the liver for hepatic changes and serum chemistry data analysis for markers indicative of hepatic injury. In addition, qualitative comparisons were made for these compounds to data reported in the rat by the same authors. In this analysis of canine study data for nine different compounds comprising five different pharmacological classes, significant elevations in several microsomal enzyme activities were observed under study conditions that did not result in liver weight increases, histological changes or serum chemistry changes that would be indicative of hepatocellular or hepatobiliary damage. Despite some species differences in cytochrome P450 homologues, for this compound set, there was clearly a general association between the response in dog liver and that of the rat liver. Compounds that elicited significant increases in more than one canine P450 endpoints were also likely to produce an inductive response in rat liver; however, the magnitude of the response and the P450 endpoint involved were not always identical. We conclude that hepatic drug-metabolizing enzyme induction in the beagle dog liver is typically a benign adaptive response, which parallels that reported previously in the rat.

Identification and functional characterization of eight CYP3A4 protein variants

The genetic component of the inter-individual variability in CYP3A4 activity has been estimated to be between 60% and 90%, but the underlying genetic factors remain largely unknown. A study of 213 Middle and Western European DNA samples resulted in the identification of 18 new CYP3A4 variants, including eight protein variants. A total of 7.5% of the population studied was found to be heterozygous for one of these variants. In a bacterial heterologous expression system, two mutants, R130Q and P416L, did not result in detectable P450 holoprotein. One mutant, T363M, expressed at significantly lower levels than wild-type CYP3A4. G56D, V170I, D174H and M445T were not significantly different when compared with wild-type CYP3A4 in expression or steroid hydroxylase activity. L373F displayed a significantly altered testosterone metabolite profile and a four-fold increase in the Km value for 1'-OH midazolam formation. The results suggest a limited contribution of CYP3A4 protein variants to the inter-individual variability of CYP3A4 activity in Caucasians. Some variants may, however, play a role in the atypical response to drugs or altered sensitivity to carcinogens.

cDNA cloning and initial characterization of CYP3A43, a novel human cytochrome P450

The RACE amplification technology was used on a novel CYP3A-like exon 1 sequence detected during the reverse transcriptase/polymerase chain reaction analysis of human CYP3A gene expression. This resulted in the identification of cDNAs encompassing the complete coding sequence of a new member of the CYP3A gene subfamily, CYP3A43. Interestingly, the majority of the cDNAs identified were characterized by alternative splicing events such as exon skipping and complete or partial intron inclusion. CYP3A43 expression was detected in liver, kidney, pancreas, and prostate. The amino acid sequence is 75% identical to that of CYP3A4 and CYP3A5 and 71% identical to CYP3A7. CYP3A43 differs from CYP3A4 at six amino acid residues, found within the putative substrate recognition sites of CYP3A4, that are known to be determinants of substrate selectivity. The N terminus of CYP3A43 was modified for efficient expression of the protein in Escherichia coli, and a 6X histidine tag was added at the C terminus to facilitate purification. CYP3A43 gave a reduced carbon monoxide difference spectra with an absorbance maximum at 450 nm. The level of heterologous expression was significantly lower than that observed for CYP3A4 and CYP3A5. Immunoblot analyses revealed that CYP3A43 comigrates with CYP3A4 in polyacrylamide gel electrophoresis but does separate from CYP3A5. Monooxygenase assays were performed under a variety of conditions, several of which yielded reproducible, albeit low, testosterone hydroxylase activity. The findings from this study demonstrate that there is a novel CYP3A member expressed in human tissues, although its relative contribution to drug metabolism has yet to be ascertained.

Cytochrome P450 3A9 catalyzes the metabolism of progesterone and other steroid hormones

The catalytic requirements and the role of P450 3A9, a female-specific isoform of CYP3A from rat brain, in the metabolism of several steroid hormones were studied using recombinant P450 3A9 protein. The optimal steroid hormone hydroxylase activities of P450 3A9 required cholate but not cytochrome b5. P450 3A9 was active in the hydroxylation reactions of testosterone, androstenedione, progesterone and dehydroepiandrosterone (DHEA). No activity of P450 3A9 toward cortisol was detectable under our reconstitution conditions. Among all the steroid hormones examined, female-specific P450 3A9 seemed to catalyze most efficiently the metabolism of progesterone, one of the major female hormones, to form three mono-hydroxylated products, 6beta-, 16alpha-, and 21-hydroxyprogesterone. Our data also showed that P450 3A9 can catalyze the formation of a dihydroxy product, 4-pregnen-6beta, 21-diol-3, 20-dione, from progesterone with a turnover number, 1.3 nmol/min/nmol P450. Based on the Vmax/Km values for P450 3A9 using either 21-hydroxprogesterone or 6beta-hydroxyprogesterone as a substrate, 4-pregnen-6beta, 21-diol-3, 20-dione may be formed either by 6beta-hydroxylation of 21-hydroxprogesterone or 21-hydroxylation of 6beta-hydroxyprogesterone. As a major isoform of CYP3A expressed in rat brain, the activities of P450 3A9 toward two major neurosteroids, progesterone and DHEA suggested a possible role for P450 3A9 in the metabolism of neurosteroids.

Evidence for propofol hydroxylation by cytochrome P4502B11 in canine liver microsomes: breed and gender differences

1. The study aimed to ascertain the enzyme kinetic basis for breed differences in the biotransformation of propofol in dog and to identify the responsible canine cytochrome P450 (CYP) isoenzymes. 2. The NADPH-dependent formation of 4-hydroxypropofol (the rate-limiting biotransformation in dog) was assayed using hepatic microsomes from the male greyhound and beagle, and from both sexes in mixed-breed dogs (five of each). 3. Enzyme kinetic analysis revealed that whereas there were no significant differences in Km, Vmax averaged > 3-fold lower in greyhound compared with beagle (p = 0.032). Although average Vmax was > 3-fold higher in the male compared with female mixed-breed dogs, this difference did not achieve statistical significance (p = 0.095), probably because of the high variability of data from mixed-breed dogs. 4. Chloramphenicol (a specific CYP2B11 inhibitor) and diethyldithiocarbamate (a non-specific CYP2 inhibitor) inhibited propofol hydroxylation in all microsomes. Quinine (a CYP2D15 inhibitor) was also inhibitory, but only in one-half of the microsomes examined. Immuno-inhibition by anti-CYP2B1 sera resulted in > 50% reduction in metabolite formation in all dogs except mixed-breed females, which showed a 30% reduction. Differences in propofol hydroxylase activity between microsomal preparations were primarily attributed to a component that was sensitive to inhibition by chloramphenicol and anti-CYP2B1 sera. 5. The results indicate that propofol hydroxylation in dog is primarily mediated by CYP2B11 and that breed (and possibly gender) differences in propofol metabolism may result from differences in the liver content of this CYP.

The importance of SRS-1 residues in catalytic specificity of human cytochrome P450 3A4

The structural basis for the regioselective hydroxylation of Delta-4-3-ketosteroids by human CYP3A4 was investigated. Prior studies had suggested that the chemical reactivity of the allylic 6beta-position might have a greater influence than steric constraints by the enzyme. Six highly conserved CYP3A residues from substrate recognition site 1 were examined by site-directed mutagenesis. F102A and A117L showed no spectrally detectable P450. V101G and T103A exhibited a wild-type progesterone metabolite profile. Of five mutants at residue N104, only N104D yielded holoenzyme and exhibited the same steroid metabolite profile as wild-type. Of four mutants at position S119 (A, L, T, V), the three hydrophobic ones produced 2beta-OH rather than 6beta-OH progesterone or testosterone as the major metabolite. Kinetic analysis showed S(50) values similar to wild-type for S119A (progesterone) and S119V (testosterone), whereas the V(max) values for 2beta-hydroxysteroid formation were increased in both cases. All four mutants exhibited an altered product profile for 7-hexoxycoumarin side-chain hydroxylation, whereas the stimulation of steroid hydroxylation by alpha-naphthoflavone was similar to the wild-type. The results indicate that the highly conserved residue S119 is a key determinant of CYP3A4 specificity and reveal an important role of the active site topology in steroid 6beta-hydroxylation.

Cloning and functional expression of a first inducible avian cytochrome P450 of the CYP3A subfamily (CYP3A37)

CYP3As represent a family of cytochromes P450 involved in the metabolism of both endogenous and exogenous natural and synthetic compounds. Well described in mammals, none have yet been cloned and characterized in avian species. In this paper, we report the cloning and analysis of an avian CYP3A (CYP3A37). Using an RNA differential display approach, an 80-bp phenobarbital-inducible cDNA fragment was amplified from chicken embryo liver. Based on its homology with mammalian CYP3As, this fragment was used to clone a full-length cDNA consisting of 1638 bp encoding a putative protein of 509 amino acids. The sequence shares between 57.4 and 62% identity at the amino acid level with CYP3As of other species. This cDNA was designated CYP3A37 according to the current cytochrome P450 nomenclature. When expressed in COS1 cells, the CYP3A37 cDNA produced a protein of congruent with55 kDa, which was recognized by polyclonal anti-rat CYP3A1 antiserum. In a bacterial expression system, the CYP3A37 cDNA produced a protein capable of steroid 6beta-hydroxylation. At a substrate concentration of 100 microM, progesterone, testosterone, and androstenedione were found to be 6beta-hydroxylated at a rate of 15.4, 11.7, 12.2 nmol/min/nmol P450, respectively. Used as control, the human CYP3A4 gave similar hydroxylation rates. Finally, in both chicken embryo liver and chicken hepatoma cells (LMH), CYP3A37 mRNA was increased after treatment with typical CYP3A inducers, such as metyrapone, phenobarbital, dexamethasone, and pregnenolone 16alpha-carbonitrile, but not rifampicin. CYP2H1, a well-characterized inducible chicken cytochrome P450, also was induced by the same compounds, suggesting similar regulation of CYP3 and CYP2 genes in this species.

Influence of urethane and ketamine on rat hepatic cytochrome P450 in vivo

The aim of this study was to identify the effects of widely used laboratory anaesthetics on cytochrome (CYP) activity in male Sprague Dawley rats in vivo. The anaesthetics used were urethane and ketamine. 7-Ethoxyresorufin (EROD), 7-pentoxyresorufin (PROD), aniline and ethylmorphine were used as substrates for CYP 1A, CYP 2B, CYP 2E1 and CYP 3A, respectively. Urethane increased EROD (CYP 1A) activity by 40 % (p < 0.01), and hydroxylation of aniline (CYP 2E1) by 14 % in the early phase of anaesthesia and by 60 % (p < 0.01) in the later one. Urethane also reduced the demethylation of ethylmorphine by 37 % (p < 0.01), but did not affect CYP 2B activity significantly. Ketamine did not significantly affect CYP 1A, 2B or 2E1. However, it reduced the demethylation of ethylmorphine (i.e. CYP 3A) by 32 % (p < 0.01). From these data, we concluded that a single dose of urethane inhibits CYP 3A but increases CYP 2E1 and CYP 1A, and that a single dose of ketamine inhibits the activity of CYP 3A.

Human, rat and crocodile liver microsomal monooxygenase activities measured using diazepam and nifedipine: effects of CYP3A inhibitors and relationship to immunochemically detected CYP3A apoprotein

Nifedipine oxidase and diazepam C3-hydroxylase were tested as activities for selectively measuring CYP3A enzymes using liver microsomes from male and female human organ donors, male and female Wistar rats and male and female estuarine crocodiles. The association between CYP3A enzymes and these monooxygenations was confirmed for the human samples. Male rat samples had lower specific contents of CYP3A apoprotein than the human samples but had equivalent (nifedipine) or higher (diazepam) monooxygenase specific activities. CYP3A apoprotein was undetectable in female rat samples which had very low activities towards both substrates. Enzyme inhibition studies showed that diazepam C3-hydroxylase of male rat liver was attributable to CYP3A but corresponding results for female rats suggested a contribution from non-CYP3A enzyme. Western blotting with immunochemical detection using anti-CYP3A4 IgG suggested the presence of putative CYP3A apoprotein in male and female crocodile liver samples and inhibition studies with diazepam as substrate suggested the presence of CYP3A subfamily monooxygenase activity in these enzyme preparations. Results for nifedipine oxidase with male and female rat liver and male crocodile liver suggested major contributions to catalysis from non-CYP3A enzymes. Inhibition studies suggested that a higher proportion of nifedipine oxidase in female crocodile liver may be attributable to the putative CYP3A enzyme(s) than in male crocodile liver. These results show the need for care in the assessment of CYP3A activity of fractionated tissues when using these substrates in cross-species studies and where gender is a variable.

Stereoselective metabolism of bisoprolol enantiomers in dogs and humans

To clarify the mechanism of the species difference in the metabolism of bisoprolol enantiomers, in vitro metabolic studies were performed using dog liver microsomes and human cytochrome P450 (CYP) isoforms. The O-deisopropylation of bisoprolol enantiomers showed biphasic kinetics in dog liver microsomes. The intrinsic clearance (Vmax/Km) for O-deisopropylation of R(+)-bisoprolol was higher than S(-)-isomer in both high-affinity and low-affinity components. The R/S ratio of the intrinsic clearance in high- and low-affinity components was 1.34 and 1.65, respectively. The inhibition studies in dog liver microsomes using CYP isoform-selective inhibitors indicated that the O-deisopropylation of both bisoprolol enantiomers was mediated via the CYP2D and CYP3A subfamily, and suggested that high-affinity oxidation was dependent on CYP2D. The kinds of CYP subfamilies in dogs, which contribute to the metabolism of bisoprolol enantiomers, were the same as those in humans. The intrinsic clearance for O-deisopropylation of R(+)bisoprolol by human recombinant CYP2D6 was also different from that of S(-)-enantiomers (R/S:1.50). However, unlike the dog microsomes, the intrinsic clearance by the human recombinant CYP3A4 did not show a stereoselective difference. Therefore, the species difference in the R/S ratio of metabolic clearance for the oxidation of bisoprolol enantiomers (dog > human) is mainly due to the species difference in the stereoselectivity of one of the cytochrome P450 subfamilies (CYP3A).

Expression and characterization of canine cytochrome P450 2D15

CYP2D15 is the canine ortholog of human CYP2D6, the human CYP2D isoform involved in the metabolism of drugs such as antiarhythmics, adrenoceptor antagonists, and tricyclic antidepressants. Similar to human, canine CYP2D15 is expressed in the liver, with detectable levels in several other tissues. Three different CYP2D15 cDNA clones were obtained by RT-PCR from dog liver RNA. Two clones corresponded to variant full-length CYP2D15 cDNAs (termed CYP2D15 WT2 and CYP2D15 V1); the third was identified as a splicing variant missing exon 3 (termed CYP2D15 V2). Recombinant baculoviruses were constructed containing full-length cDNAs and used to express CYP2D15 WT2 and CYP2D15 V1 in Spodoptera frugiperda (Sf9) cells with expression levels of up to 0.14 nmol/mg cell protein. As with human CYP2D6, the recombinant CYP2D15 enzymes exhibited bufuralol 1'-hydroxylaseand dextromethorphan O-demethylase activities whencoexpressed with rabbit NADPH:P450 oxidoreductase. For bufuralol 1'-hydroxylase, apparent Km values were 4.9, 3.7, and 2.5 microM and the Vmax values were 0.14, 0.034, and 0.60 nmol/min/mg protein for dog liver microsomes, CYP2D15 WT2, and the variant CYP2D15 V1, respectively. For dextromethorphan O-demethylase, apparent Km values were 0.6, 0.6, and 2.0 microM and the Vmax values were 0.18, 0.034, and 0.057 nmol/min/mg protein for dog liver microsomes, CYP2D15 WT2, and the variant CYP2D15 V1, respectively. The human CYP2D6-specific inhibitor quinidine and the rat CYP2D1-specific inhibitor quinine were both shown to be inhibitors of bufuralol 1'-hydroxylase activity for dog liver microsomes, CYP2D15 WT2, and the CYP2D15 V1 variant with nearly equal potency. Thus, the dog expresses a CYP2D ortholog possessing enzymatic activities similar to human CYP2D6, but is affected by the inhibitors quinine and quinidine in a manner closer to that of rat CYP2D1.

Changes in the enzymatic activities of beagle liver during maturation as assessed both in vitro and in vivo

1. We have examined changes in caffeine and trimethadione (TMO) metabolism in vivo, agents which are used as probe drugs. In this study the total body clearance (Cl) of caffeine and TMO was low 1 week after birth (week 1), increased rapidly from week 3, peaked and then decreased gradually until reaching the level for the mature, adult dog. The elimination half-life (t1/2) of caffeine and TMO was prolonged during week 1; however, it then gradually became shorter. Gradually it became longer and reached the level for the adult dog. The apparent volume of distribution (Vd) of caffeine did not change throughout the study. However, the Vd of TMO was only high during week 1. 2. The in vitro changes in a variety of typical substrates for seven different cytochrome P450 (CYP) isozymes were examined. In this study three different patterns of metabolism can be identified: (1) activity is low immediately after birth, increases, peaks and then decreases to the adult dog level (p-nitroanisole; CYP1A1, caffeine; CYP1A2, benzphetamine; CYP3A/2B(?), aniline; 2E1 and TMO; CYP2C9/2E1/3A4); (2) activity generally increases rapidly soon after birth, continues to increase, peaks and then gradually decreases to the adult level (phenytoin; CYP2C9); and (3) activity is high (about the same level as the adult) immediately after birth, decreases and then gradually increases to the adult level (erythromycin; CYP3A4/5). 3. The results of these in vivo and in vitro studies suggest that changes in enzyme activity are due to differences in P450 isoenzymes during development.

Down-regulation of cytochrome P450 mRNAs and proteins in mice lacking a functional NOS2 gene

Endotoxemia results in both the down-regulation of multiple cytochrome P450 genes and the induction of inducible nitric oxide synthase (NOS2). The nitric oxide (NO) released during inflammation has been implicated as the mediator of the decreased catalytic activity and expression of several cytochrome P450 isozymes. We examined the role of NO in the decreases in both gene expression and activity of three P450s in endotoxemic parental and NOS2 knockout mice. Twenty-four hours of endotoxin (LPS) treatment significantly suppressed CYP2C29 and CYP3A11 mRNA expression in both the parental and NOS2 knockout strains. Microsomal CYP2E1, CYP2C-like, and CYP3A-like protein levels were also decreased in both strains of mouse. Similar results were obtained in parental strain endotoxemic mice co-administered the NOS inhibitor aminoguanidine. Six hours after LPS treatment, there was an NO-dependent decrease in testosterone 6beta-hydroxylase activity, because no decreases in activity were observed in the NOS2 knockout mice or in mice co-administered aminoguanidine. LPS also evoked decreases in testosterone 15alpha- and 16beta-hydroxylase activity after 24 hr that were observed in the parental strain and not in NOS2 knockout mice. Our results demonstrate that the down-regulation of CYP2C-like, CYP3A-like and CYP2E1 proteins and mRNAs, in the endotoxemic mouse can occur independently of NO production. We do, however, show that the NO released during endotoxemia is capable of causing decreases in some cytochrome P450 catalytic activities.

Characterization of cytochrome P450 (CYP3A12) induction by rifampicin in dog liver

1. Effects of rifampicin (Rif) on the contents of cytochrome P450 (P450) enzymes (CYP1A1/2, 2B11, 2C21 and 3A12) assessed by enzyme-linked immunosorbent assay and catalytic activities (ethoxyresorufin O-deethylase, and testosterone 6 beta-, 16 alpha- and 16 beta-hydroxylase; 6 beta-, 16 alpha- and 16 beta-OHT) in dog liver microsomes were compared between liver lobes of both the male and female dogs. 2. In the control dogs, the contents of individual P450 enzymes and their activities showed no significant differences between individual liver lobes and between the sexes. 3. Rif treatment (10 mg/kg/day, p.o. for 7 days) induced substantial increases in the content of CYP3A12 and 6 beta- and 16 beta-OHT activities, and slight increases in the content of CYP2B11 and 16 alpha-OHT activity, and their elevated levels were virtually the same between liver lobes. The magnitudes of the elevation of the CYP3A12 level and 6 beta- and 16 beta-OHT activities compared with control levels appeared to be greater in the female dogs. However, the ratios of their magnitudes (CYP3A content/6 beta-OHT activity and CYP3A content/16 beta-OHT activity) showed no differences between the sexes. 4. In both the control and Rif-treated dogs, the activities of 6 beta- and 16 beta-OHT were specifically inhibited by anti-CYP3A12 antiserum, and 16 alpha-OHT activity was specifically inhibited by anti-CYP2B11 and anti-CYP2C21 antiserum. 5. These results indicate that Rif treatment induces the expression of CYP3A12 protein, and correlates well with the elevation of its catalytic activity (6 beta- and 16 beta-OHT), and that the female dog is more responsive to Rif treatment as compared with the male.

Anticonvulsant therapy in small animals

Successful control of seizures with anticonvulsant drugs reflects a balance in achieving seizure control while minimizing undesirable drug side effects. Variability in the disposition of anticonvulsants and interaction among them are important confounders of successful therapy. This article will provide a review of selected anticonvulsants, focusing on drugs most likely to control seizures in small animals. The proper use of anticonvulsants will be discussed, with emphasis on differences in individual drug disposition, detection of these differences, and rational approaches to responding to these differences by dose modification. The primary target of discussion will be treatment of generalized, tonic clonic seizures, the most common type afflicting small animals. Opinions regarding anticonvulsive therapy vary among clinicians. Most of the comments and recommendations offered in this discussion reflect observations made from our therapeutic drug monitoring service and completed and ongoing clinical trials that focus on the use of anticonvulsants used either alone or in combination with phenobarbital.

Analysis of four residues within substrate recognition site 4 of human cytochrome P450 3A4: role in steroid hydroxylase activity and alpha-naphthoflavone stimulation

Sequence alignment of human cytochrome P450 3A4 with bacterial enzymes of known structure has provided a basis from which to predict residues involved in substrate oxidation. Substitutions were made at four residues (I301, F304, A305, and T309) predicted to be located within the highly conserved substrate recognition site 4. Site-directed mutants engineered to contain carboxy-terminal histidine tags were expressed in Escherichia coli and purified on a metal affinity column. The integrity of each protein was assessed by SDS-polyacrylamide gel electrophoresis and immunoblotting. Functional analysis was performed using progesterone and testosterone as substrates and alpha-naphthoflavone as an activator. In testosterone hydroxylase assays, all of the mutants displayed rates of total product formation similar to wild-type 3A4, with several mutants showing small differences in specific products formed. However, with progesterone as the substrate, mutants F304A, A305V, and T309A exhibited altered product ratios and/or changes in the rates of product formation. F304A and A305V also displayed altered flavonoid stimulation that resulted in product ratios dramatically different from wild-type 3A4. Therefore, the kinetics of progesterone hydroxylation of these mutants and the wild-type enzyme were further assessed, and the data were analyzed with the Hill equation. Results with wild-type 3A4 and F304A indicated that at high progesterone concentrations, hydroxylation rates and product ratios are independent of the presence of alpha-NF. This suggests that progesterone may be equivalent to alpha-NF as an activator. In contrast, A305V exhibited autoactivation by progesterone but inhibition by alpha-NF.

Structural determinants of progesterone hydroxylation by cytochrome P450 2B5: the role of nonsubstrate recognition site residues

The highly related rabbit cytochromes P450 2B4 and 2B5 differ in only 12 amino acid positions, but only 2B5 has activity toward progesterone. Previously, simultaneous site-directed mutagenesis of four key substrate recognition site (SRS) residues (114, 294, 363, and 367) was shown to result in interconversion of the androstenedione hydroxylase specificities of cytochrome P450 2B4 and 2B5. However, the progesterone metabolite profiles of the 2B4 quadruple mutant or of a quintuple mutant in which residue 370 was also mutated to the 2B5 residue were not identical to that of P450 2B5. Therefore, single mutants of P450 2B5 at the remaining seven positions were constructed, expressed in Escherichia coli, and studied with progesterone as the substrate. The single mutants at positions 120 and 221, which are outside any known SRS, exhibited a significant alteration in progesterone hydroxylation. Based on these results, Ile-114, Arg-120, Ser-221, Ser-294, Ile-363, and Val-367 in cytochrome P450 2B4 were replaced simultaneously with Phe, His, Pro, Thr, Val, and Ala, respectively, from 2B5. This yielded a mutant with a very similar progesterone metabolite profile to that of 2B5, although the total activity was lower. An additional substitution at residue 370 produced a multiple mutant P450 2B4 I114F-R120H-S221P-S294T-I363V-V367A- T370M with very similar or identical substrate specificity, regio- and stereospecificity and kinetic properties to that of P450 2B5 wild type.

Quantitation of cytochrome P450 enzymes (CYP1A1/2, 2B11, 2C21 and 3A12) in dog liver microsomes by enzyme-linked immunosorbent assay

1. An enzyme-linked immunosorbent assay (ELISA) using specific antisera has been developed to quantify individual cytochrome P450 (P450) enzymes (1A1/2, 2B11, 2C21 and 3A12) in dog liver microsomes. 2. The specific contents of CYP1A1/2, 2B11, 2C21 and 3A12 in untreated male dog liver microsomes determined by the ELISA were 17, 48, 160 and 69 pmol/mg protein respectively, corresponding to 4, 10, 34 and 15% of total optically determined P450 respectively. These P450 enzymes in untreated female dog liver microsomes showed almost similar amounts and relative proportions to those observed in male dog liver microsomes. 3. The oral treatment of male dogs with phenobarbital (PB), rifampicin (Rif) or beta-naphthoflavone (beta-NF) induced significant increases in the contents of CYP1A1/2 (12-fold by beta-NF), 2B11 (16-fold by PB), 2C21 (2-fold by PB) and 3A12 (5-fold by PB and Rif), resulting in marked proportional alterations of the P450 enzymes in dog liver microsomes. 4. This ELISA method will be a useful tool for investigating possible influences (induct on/suppression) of xenobiotics on the expression of P450 enzymes in dog liver.

Influence of partial hepatectomy in dogs on trimethadione metabolism and microsomal monooxygenases

1. The recovery of trimethadione (TMO) metabolism and its association with liver weight and the activity of TMO N-demethylase have been reported in rat following partial (68%) hepatectomy. In the present study, we examined the effect of liver regeneration on hepatic P450 isozymes and TMO metabolism in dog. 2. The ratio of dimethadione (DMO), being the only TMO metabolite, to TMO at 2 h after i.v. injection of TMO (4 mg/kg) fell to 80% of that in the preoperative animals by 24 h after hepatectomy. The DMO/TMO ratio gradually recovered from days 7 to 14, and by day 21 after hepatectomy it had increased to about 25%. At 28 days post-hepatectomy the ratio had returned to preoperative levels. 3. The activity of benzphetamine N-demethylase, TMO N-demethylase, p-nitro-anisole O-demethylase and aniline hydroxylase increased 3 days post-hepatectomy, exhibiting levels 4.77, 3.45, 1.51 and 1.91 times greater respectively than that of the preoperative liver in the same animal. Two weeks post-hepatectomy these activities had returned to normal. The activity of the 16 beta- and 2 beta-hydroxylation of testosterone was unchanged. However, the activity of 6 beta-hydroxylase decreased 7 days post-hepatectomy, while 16 alpha-hydroxylation had increased at 3 and 7 days post-hepatectomy compared with controls. 4. The changes in liver weight were nearly restored to preoperative levels 7 days post-hepatectomy. 5. Although the P450 content was unchanged from days 1 to 7 post-hepatectomy, it had decreased by 30% at day 14 and by 20% at day 28. The P4502B11 content 3, 7 and 14 days post-hepatectomy had increased 8, 10 and 2 times respectively, while the P4503A12 content at 7 and 14 days decreased by 30 approximately 50% compared with that of the pre-operative liver. 6. The data presented above do not reveal any relationship between P4502B11 induction and liver regeneration. The reason for such a change is unknown, therefore further investigation needs to be carried out.

Hepatic microsomal induction profile of carbamic acid [[2,6-bis(1- methylethyl)phenoxy] sulfonyl]-2,6-bis(1-methylethyl) phenyl ester, monosodium salt (PD138142-15), a novel lipid regulating agent

Induction of hepatic microsomal cytochrome P450 produced by carbamic acid [2,6-bis(1-methylethyl)phenoxy]sulfonyl]-2,6-bis(1-methylethyl) phenyl ester, monosodium salt (PD138142-15), a novel water-soluble inhibitor of acyl-CoA: cholesterol acyltransferase, was examined in male and female rats, dogs, and monkeys, and in male guinea pigs. Relative to control, PD138142-15 increased hepatic microsomal total spectral P450 in all species examined. Hepatic microsomal ethoxyresorufin-O-deethylase, pentoxyresorufin-O-dealkylase, and peroxisomal carnitine acetyltransferase activities and cyanide-insensitive Beta-oxidation were affected only marginally. Erythromycin-N-demethylase activity was increased (2- to 6-fold) in all three species in which it was examined (rat, dog and pig). Marked increases in immunoreactive P450 3A were noted in the rats and dogs, while slight increases were seen in monkeys. Pharmacokinetic studies of PD138142-15 in rats and dogs revealed pronounced decreases (80-90%) in plasma Cmax and AUC within 2 weeks of initiation of daily dosing. In spite of the marked decline in plasma drug levels, efficacy in dogs, as determined by serum cholesterol levels, was maintained for up to 6 weeks with continued dosing. Potential acid (gastric) breakdown products of PD 138142-15 were examined for their hepatic cytochrome P450 induction profiles in rats adn were found to differ both quantitatively and qualitatively from profiles produced by the parent compound. This suggested that induction observed in rats was due to parent PD138142-15 and not to any of the known potential acid breakdown products. The cumulative data establish that PD 138142-15 is an inducer of P450 3A in rats and dogs. The results also suggest that P450 3A is induced in monkeys and pigs as well, although the data are less definitive. Decreases in plasma drug levels imply that the compound may be an autoinducer in dogs and rats. The maintenance of efficacy in spite of decreased drugs levels in dogs suggests that the effects on serum cholesterol are due to a metabolite or that cholesterol lowering effects occur before the compound is metabolized by the liver.

Structural determinants of cytochrome P450 2B1 specificity: evidence for five substrate recognition sites

Twelve site-directed mutants of rat cytochrome P450 2B1 distributed over seven positions and four putative substrate recognition sites (SRS) were constructed and expressed in COS cells. Function was examined using androstenedione and testosterone as substrates. Substitutions at positions 303, 360, and 473 did not markedly affect the regio- or stereoselectivity of androgen metabolism, whereas mutants in positions 206 (SRS-2), 302 (SRS-4), and 363 and 367 (SRS-5) exhibited markedly different steroid metabolite profiles compared with parental P450 2B1. In particular, the Phe-206-->Leu substitution conferred androgen 6 alpha- and testosterone 7 alpha-hydroxylase activities, and the Thr-302-->Ser substitution suppressed androgen 16 beta-hydroxylation in favor of androstenedione 16 alpha- and testosterone 15 alpha-hydroxylation. Replacement of Val-363 or Val-367 with Ala conferred androgen 15 alpha-hydroxylase and 6 beta-hydroxylase activities, respectively, and suppressed susceptibility to mechanism-based inactivation by the P450 2B1-selective chloramphenicol analog N-(2-p-nitrophenethyl)chlorofluoroacetamide. The Val-367-->Ala mutant was also resistant to chloramphenicol itself. The Leu mutant at position 363 exhibited increased specificity for androstenedione and testosterone 16 beta-hydroxylation, whereas the Leu mutant at position 367 exhibited decreased stereospecificity. Most interestingly, the size of key residues identified plays a critical role in governing steroid hydroxylation from the alpha-face or beta-face and hydroxylation on the D-ring or the B-ring.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of phenobarbital and other model inducers on cytochrome P450 isoenzymes in primary culture of dog hepatocytes

1. The effects of phenobarbital (PB), beta-naphthoflavone (beta-NF), omeprazole (Omep) and rifampicin (Rif) on drug-metabolizing activities in dog hepatocytes, cultured with William's medium E, were examined. 2. The drug metabolizing activities of the hepatocytes decreased during culture; 7-ethoxycoumarin O-deethylase (ECOD) activity was nearly 70% of initial value at 72 h, but 7-methoxycoumarin O-demethylase (MCOD), 7-propoxycoumarin O-depropylase (PCOD), progesterone 6 beta-hydroxylase (6 beta-OH-P), progesterone 16 alpha-hydroxylase (16 alpha-OH-P), progesterone 21-hydroxylase (21-OH-P), 7-ethoxyresorufin O-deethylase (EROD) activities and total cytochrome P450 content were approx. 50%. 3. When the hepatocytes were cultured with PB, the enzyme activities increased time- and dose-dependently. MCOD, ECOD and PCOD activities increased 5-8 fold with 2 mM PB in 96 h. Similar results were obtained for 6 beta-OH-P, 16 alpha-OH-P and 21-OH-P activities, and total cytochrome P450. The effect of PB was abolished when 2.5 microM cycloheximide or 0.1 microM actinomycin D was included in the culture. 4. Treatment of hepatocytes with 40 microM beta-NF for 72 h resulted in 25-fold elevation of EROD activity. beta-NF enhanced PCOD activity approx. six-fold, while ECOD increased only slightly, and 7-MCOD negligibly. 5. Omep (100 microM) increased EROD activity nearly 10-fold, and 25 microM Rif increased 6 beta-OH-P activity approx. 8-fold, but ECOD only slightly. 6. Western blot analysis of microsomes from cultured dog hepatocytes with anti-rat CYP 2B1 antibodies indicated that PB increased an immunochemically-reactive protein. The protein showed the same mobility as the major dog P450 isozyme (cytochrome P450 PBD-2 or CYP 2B11) purified from liver microsomes of PB-treated male beagle dog. In a similar manner, induction of cytochrome P450 PBD-1 (CYP 3A12) by PB was confirmed.

Picrotoxin as a potent inducer of rat hepatic cytochrome P450, CYP2B1 and CYP2B2

The induction by the central stimulant picrotoxin of hepatic drug-metabolizing enzymes was studied in rats. The hepatic content of P450 and the activity of benzphetamine N-demethylation increased gradually after administration of picrotoxin dissolved in drinking water (2 mg/mL), to three-times higher levels than the initial values at the third day of treatment. The increase in benzphetamine N-demethylase activity by picrotoxin was somewhat higher than the increase produced by phenobarbital. Supporting these results, immunoblot analysis showed that CYP2B1 and 2B2 proteins in the liver microsomes were increased by picrotoxin Picrotoxinin and picrotin, which are components of the picrotoxin molecule, had the same ability to induce the hepatic activity of benzphetamine N-demethylation. The liver microsomal activities of testosterone 16 alpha- and 16 beta-hydroxylation were enhanced significantly after treatment with picrotoxinin and picrotin. However, benzo[a]pyrene 3-hydroxylation, aniline 4-hydroxylation, and testosterone hydroxylations at the 2 alpha- and 7 alpha-positions were not increased by picrotoxinin and picrotin treatment. In addition to monooxygenase, significant induction of glutathione S-transferase activity for 1-chloro-2,4-dinitrobenzene and UDP-glucuronyltransferase activity for 4-hydroxybiphenyl and 4-nitrophenol was also observed by pretreatment of picrotoxin. These results clearly indicate that picrotoxin is an inducer of phenobarbital-inducible liver enzymes.

The effect of enzyme induction on the cytochrome P450-mediated bioactivation of carbamazepine by mouse liver microsomes

Predisposition to idiosyncratic toxicity with carbamazepine is thought to be due to a deficiency of the detoxication enzyme, microsomal epoxide hydrolase, although in some cases, concurrent administration of enzyme inducers might be a contributory risk factor, by altering the critical balance between bioactivation and detoxication. In this study, a mouse model has been used to determine the factors affecting carbamazepine bioactivation, using covalent binding and cytotoxicity as markers of bioactivation in vitro. Microsomes prepared from mice pre-treated with phenobarbitone increased (relative to the control microsomes) the formation of cytotoxic (12.3% vs 3.2%), protein-reactive (3.0% vs 2.0%) and stable (33.8% vs 18.1%) metabolites of carbamazepine. Similarly, pre-treatment with dexamethasone also increased the formation of the cytotoxic (24.8% vs 6.7%), protein-reactive (2.8% vs 1.5%) and stable (38% vs 19.8%) metabolites of carbamazepine, while beta-naphthoflavone pretreatment did not increase the formation of either the toxic or stable metabolites of carbamazepine when compared with its control microsomes. Co-incubation with gestodene (10-250 microM) resulted in a dose-dependent inhibition of both the bioactivation of carbamazepine and the formation of its stable 10,11-epoxide. SDS-PAGE and immunoblotting of the microsomes with anti-CYP3A antibody revealed the presence of a 52 kDa protein band in each preparation of microsomes, but the relative intensities of the bands, as measured by laser densitometry, were highest with the phenobarbitone and dexamethasone microsomes. The microsomal oxidation of cortisol to 6 beta-hydroxycortisol was also enhanced by pretreatment of mice with phenobarbitone (6.5% vs 2.7%) and dexamethasone (8.2% vs 4.3%), but not beta-naphthoflavone (2.2% vs 1.6%), when compared with their respective control microsomes, and was inhibited (range 25-68% inhibition), with all the microsomes by gestodene (50 microM). Taken collectively, the data in this study demonstrate that in the mouse, induction of the CYP3A subfamily significantly increases carbamazepine bioactivation. It is likely that in humans inducers of the orthologous form of this enzyme, most notably anticonvulsants, may increase the bioactivation of carbamazepine.

Metabolism of 2,4,5,2',4',5'-hexachlorobiphenyl with liver microsomes of phenobarbital-treated dog; the possible formation of PCB 2,3-arene oxide intermediate

1. Metabolism of 2,4,5,2',4',5'-hexachlorobiphenyl (HCB) was investigated in vitro using liver microsomes of one male beagle dog after phenobarbital treatment. 2. Three major metabolites were isolated and identified as 3-hydroxy-2,4,5,2',4',5'-HCB, 2-hydroxy-4,5,2',4',5'-pentachlorobiphenyl (PenCB), and 2-hydroxy-3,4,5,2',4',5'-HCB, by comparison of g.l.c.-mass spectrometry and 1H-n.m.r. data with those of authentic samples. 3. 2-Hydroxy-3,4,5,2',4',5'-HCB was found as a metabolite of 2,4,5,2',4',5'-HCB for the first time using dog liver microsomes. Present result indicate that this metabolite and the dechlorinated PenCB are derived from a metabolic intermediate, namely, 2,3-epoxy-2,4,5,2',4',5'-HCB. 2,3-Epoxide formation is a new metabolic pathway of PCB.

Role of residue 478 as a determinant of the substrate specificity of cytochrome P450 2B1

Two allelic variants and eight site-directed mutants of cytochrome P450 2B1 differing at residue 478 have been expressed in COS cells and assayed for androstenedione hydroxylase activities. The 478Gly and 478Ala variants and five mutants (Ser, Thr, Val, Ile, and Leu) exhibited 16 beta-OH:16 alpha-OH ratios ranging from 0.7 to 9.3, whereas the Pro, Glu, and Arg mutants were expressed but inactive. The seven samples active toward androstenedione also exhibited testosterone 16 beta-OH:16 alpha-OH ratios ranging from 0.4 to 2.3. With both steroids, the Gly variant had the highest 16 beta-hydroxylase activity, and the 16 beta-OH:16 alpha-OH ratio increased with the size of aliphatic size chains (Ala, Val, and Ile/Leu). The highest ratio of androgen 15 alpha:16-hydroxylation was observed with the Ser mutant. On the basis of previous work indicating decreased susceptibility of the 478Ala variant in liver microsomal and reconstituted systems to inactivation by chloramphenicol analogs, methodology was refined for monitoring enzyme inactivation in COS cell microsomes. The Gly and Ala variants were inactivated by chloramphenicol with similar rate constants, whereas the Ser and Val mutants were inactivated more slowly, and the Leu mutant was refractory. Only the Gly variant was inactivated by the chloramphenicol analog N-(2-p-nitrophenethyl)chlorofluoroacetamide. Thus, the side chain of residue 478 appears to be a major determinant of enzyme inactivation as well as of androgen hydroxylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Toxicological significance of dog liver cytochrome P-450: examination with the enzyme expressed in Saccharomyces cerevisiae using recombinant expression plasmid

A complementary DNA (cDNA) coding for a form of beagle dog cytochrome P-450 (Dah1), which is the orthologue to the CYP1A1 cDNA of rat, mouse and human, was inserted between the alcohol dehydrogenase (ADH) promoter and terminator regions of the yeast expression vector pAAH5. On introduction of the resulting recombinant plasmid pDC-1, Saccharomyces cerevisiae strain AH22 cells synthesized up to 1.5 x 10(5) molecules per cell of cytochrome P-450 protein (P-450(Dah1)). The carbon monoxide-bound reduced form of P-450(Dah1) showed an absorption peak at 447 nm and specific content of P-450(Dah1) was about 0.1 nmole P-450 per mg of microsomal protein. P-450(Dah1) cross-reacted with antibodies to rat P-448-H (CYP1A2) and dog P-450-D2 (CYP1A2). P-450(Dah1) activated 2-amino-3-methyl-imidazo[4,5-f]quinoline (IQ) and 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ) most efficiently in the umu test and exhibited a high activity of aryl hydrocarbon hydroxylase toward benzo[a]pyrene.

Microsomal cytochrome P450-dependent steroid metabolism in male sheep liver. Quantitative importance of 6 beta-hydroxylation and evidence for the involvement of a P450 from the IIIA subfamily in the pathway

The metabolism of testosterone (TEST), androstenedione (AD) and progesterone (PROG) was assessed in hepatic microsomal fractions from male sheep. Rates of total hydroxylation of each steroid were lower in sheep liver than in microsomes isolated from untreated male rat, guinea pig or human liver, 6 beta-Hydroxylation was the most important pathway of biotransformation of each of the three steroids (0.80, 0.89 and 0.43 nmol/min/mg protein for TEST, AD and PROG, respectively). Significant minor metabolites from TEST were the 2 beta-, 15 beta- and 15 alpha-alcohols (0.19, 0.22 and 0.17 nmol/min/mg microsomal protein, respectively). Apart from the 6 beta-hydroxysteroid, only the 21-hydroxy derivative was formed from PROG at a significant rate (0.27 nmol/min/mg protein). The 6 beta-alcohol was the only metabolite formed from AD at a rate greater than 0.1 nmol/min/mg protein. Antisera raised in rabbits to several rat hepatic microsomal P450s were assessed for their capacity to modulate sheep microsomal TEST hydroxylation. Anti-P450 IIIA isolated from phenobarbital-induced rat liver effectively inhibited TEST hydroxylation at the 2 beta-, 6 beta-, 15 alpha- and 15 beta-positions (by 31-56% when incubated with microsomes at a ratio of 5 mg IgG/mg protein). IgG raised against rat P450 IIC11 and IIB1 inhibited the formation of some of the minor hydroxysteroid metabolites but did not decrease the rate of TEST 6 beta-hydroxylation. Western immunoblot analysis confirmed the cross-reactivity of anti-rat P450 IIIA with an antigen in sheep hepatic microsomes; anti-IIC11 and anti-IIB1 exhibited only weak immunoreactivity with proteins in these fractions. Considered together, the present findings indicate that, as is the case in many mammalian species, 6 beta-hydroxylation is the principal steroid biotransformation pathway of male sheep liver. Evidence from immunoinhibition and Western immunoblot experiments strongly implicate the involvement of a P450 from the IIIA subfamily in ovine steroid 6 beta-hydroxylation.

cDNA and deduced amino acid sequences of a dog liver cytochrome P-450 of the IIIA gene subfamily

A 1.96 kbp cDNA encoding a male Beagle dog liver cytochrome P-450 of 503 amino acid residues (Mr 57,636) has been isolated and sequenced. The deduced amino acid sequence is 79.8%, 69.3% and 74.1% identical to the P450IIIA forms human NF25, rat PCN1 and rabbit LM3c, respectively. The amino terminal sequence is identical to the first 28 residues of the dog P450IIIA form PBD-1. Southern blot analysis yields restriction patterns consistent with IIIA gene subfamily multiplicity.