Purification and characterization of the dog hepatic cytochrome P-450 isozyme responsible for the metabolism of 2,2',4,4',5,5'-hexachlorobiphenyl

Pharmacogenomics of poor drug metabolism in greyhounds: Canine P450 oxidoreductase genetic variation, breed heterogeneity, and functional characterization

Greyhounds metabolize cytochrome P450 (CYP) 2B11 substrates more slowly than other dog breeds. However, CYP2B11 gene variants associated with decreased CYP2B11 expression do not fully explain reduced CYP2B11 activity in this breed. P450 oxidoreductase (POR) is an essential redox partner for all CYPs. POR protein variants can enhance or repress CYP enzyme function in a CYP isoform and substrate dependent manner. The study objectives were to identify POR protein variants in greyhounds and determine their effect on coexpressed CYP2B11 and CYP2D15 enzyme function. Gene sequencing identified two missense variants (Glu315Gln and Asp570Glu) forming four alleles, POR-H1 (reference), POR-H2 (570Glu), POR-H3 (315Gln, 570Glu) and POR-H4 (315Gln). Out of 68 dog breeds surveyed, POR-H2 was widely distributed across multiple breeds, while POR-H3 was largely restricted to greyhounds and Scottish deerhounds (35% allele frequencies), and POR-H4 was rare. Three-dimensional protein structure modelling indicated significant effects of Glu315Gln (but not Asp570Glu) on protein flexibility through loss of a salt bridge between Glu315 and Arg519. Recombinant POR-H1 (reference) and each POR variant (H2-H4) were expressed alone or with CYP2B11 or CYP2D15 in insect cells. No substantial effects on POR protein expression or enzyme activity (cytochrome c reduction) were observed for any POR variant (versus POR-H1) when expressed alone or with CYP2B11 or CYP2D15. Furthermore, there were no effects on CYP2B11 or CYP2D15 protein expression, or on CYP2D15 enzyme kinetics by any POR variant (versus POR-H1). However, Vmax values for 7-benzyloxyresorufin, propofol and bupropion oxidation by CYP2B11 were significantly reduced by coexpression with POR-H3 (by 34-37%) and POR-H4 (by 65-72%) compared with POR-H1. Km values were unaffected. Our results indicate that the Glu315Gln mutation (common to POR-H3 and POR-H4) reduces CYP2B11 enzyme function without affecting at least one other major canine hepatic P450 (CYP2D15). Additional in vivo studies are warranted to confirm these findings.

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.

Mammalian cytochrome P450 biodiversity: Physiological importance, function, and protein and genomic structures of cytochromes P4502B in multiple species of woodrats with different dietary preferences

The vast diversity of cytochrome P450 enzymes in mammals has been proposed to result in large measure from plant-animal warfare, whereby evolution of chemical defenses such as phenolics and terpenoids in plants led to duplication and divergence of P450 genes in herbivores. Over evolutionary time, natural selection is predicted to have produced P450s with high affinity and enhanced metabolism of substrates that are ingested regularly by herbivores. Interestingly, however, almost all knowledge of the interactions of mammalian P450 enzymes with substrates stems from studies of the metabolism of drugs and model compounds rather than studies on wild mammalian herbivores and their respective PSMs. A question of particular interest centers on the role of individual P450 enzymes in the ability of certain herbivores to specialize on plants that are lethal to most other species, including those from the same genus as the specialists. We tackled this intricate problem using a tractable natural system (herbivorous woodrats, genus Neotoma) focusing on comparisons of the specialist N. stephensi, the facultative specialist N. lepida, and the generalist N. albigula, and employing a cross-disciplinary approach involving ecology, biochemistry, pharmacology, structural biology, and genomics. Based on multiple findings suggesting the importance of CYP2B enzymes for ingestion of juniper and a major constituent, α-pinene, we characterized the structure, function and activity of several CYP2B enzymes in woodrats with different dietary habits. Results to date suggest that differences in CYP2B gene copy number may contribute to differential tolerance of PSMs among woodrat species, although additional work is warranted to firmly link gene copy number to juniper tolerance.

Strategies in herbivory by mammals revisited: The role of liver metabolism in a juniper specialist (Neotoma stephensi) and a generalist (Neotoma albigula)

Although herbivory is widespread among mammals, few species have adopted a strategy of dietary specialization. Feeding on a single plant species often exposes herbivores to high doses of plant secondary metabolites (PSMs), which may exceed the animal's detoxification capacities. Theory predicts that specialists will have unique detoxification mechanisms to process high levels of dietary toxins. To evaluate this hypothesis, we compared liver microsomal metabolism of a juniper specialist, Neotoma stephensi (diet >85% juniper), to a generalist, N. albigula (diet ≤30% juniper). Specifically, we quantified the concentration of a key detoxification enzyme, cytochrome P450 2B (CYP2B) in liver microsomes, and the metabolism of α-pinene, the most abundant terpene in the juniper species consumed by the specialist woodrat. In both species, a 30% juniper diet increased the total CYP2B concentration (2-3×) in microsomes and microsomal α-pinene metabolism rates (4-fold). In N. stephensi, higher levels of dietary juniper (60% and 100%) further induced CYP2B and increased metabolism rates of α-pinene. Although no species-specific differences in metabolism rates were observed at 30% dietary juniper, total microsomal CYP2B concentration was 1.7× higher in N. stephensi than in N. albigula (p < .01), suggesting N. stephensi produces one or more variant of CYP2B that is less efficient at processing α-pinene. In N. stephensi, the rates of α-pinene metabolism increased with dietary juniper and were positively correlated with CYP2B concentration. The ability of N. stephensi to elevate CYP2B concentration and rate of α-pinene metabolism with increasing levels of juniper in the diet may facilitate juniper specialization in this species.

PCB levels in adipose tissue of dogs from illegal dumping sites in Campania region (Italy)

The aim of the study is to investigate the potential relationship between exposure to PCBs and cancer. In doing so we relied on a sample of dogs coming from a peculiar area of the Campania region (Italy), that has been suffering for illegal waste dumping and open air burning of plastic waste for many years. The latter determined the release of organic and inorganic pollutants, such as the PCBs. By comparing dogs with cancer and healthy dogs, we found much higher PCB concentrations in the former, with a significant difference (p < 0.05) for the non-indicator ∑10NDL-PCB and the DL-PCBs. A regression analysis, controlling for three potentially confounding factors, that are sex, age and weight, confirmed the higher ∑10NDL-PCB concentration in dogs with cancer. Hence, our evidence suggests a potential health hazard for animals and likewise people living in a risky area due to the presence of environmental organic pollutants.

So many roads traveled: A career in science and administration

I have traveled many roads during my career. After spending my first 19 years in Los Angeles, I became somewhat of an academic nomad, studying and/or working in six universities in the United States and three in Sweden. In chronological order, I have a B.A. in Scandinavian languages and literature from UCLA, a Ph.D. in biochemistry from Uppsala University, and an M.S. in toxicology from the Karolinska Institute. I have been in schools of natural science, pharmacy, and medicine and have worked in multiple basic science departments and one clinical department. I have served as a research-track and tenured faculty member, department chair, associate dean, and dean. My research has spanned toxinology, biochemistry, toxicology, and pharmacology. Through all the moves, I have gained much and lost some. For the past 40 years, my interest has been cytochrome P450 structure-function and structure-activity relationships. My lab has focused on CYP2B enzymes using X-ray crystallography, site-directed mutagenesis, deuterium-exchange MS, isothermal titration calorimetry, and computational methods in conjunction with a variety of functional assays. This comprehensive approach has enabled detailed understanding of the structural basis of the remarkable substrate promiscuity of CYP2B enzymes. We also have investigated the mechanisms of CYP3A4 allostery using biophysical and advanced spectroscopic techniques, and discovered a pivotal role of P450-P450 interactions and of multiple-ligand binding. A major goal of this article is to provide lessons that may be useful to scientists in the early and middle stages of their careers and those more senior scientists contemplating an administrative move.

Pharmacogenomics of poor drug metabolism in Greyhounds: Cytochrome P450 (CYP) 2B11 genetic variation, breed distribution, and functional characterization

Greyhounds recover more slowly from certain injectable anesthetics than other dog breeds. Previous studies implicate cytochrome P450 (CYP) 2B11 as an important clearance mechanism for these drugs and suggest Greyhounds are deficient in CYP2B11. However, no CYP2B11 gene mutations have been identified that explain this deficiency in Greyhounds. The objectives of this study were to provide additional evidence for CYP2B11 deficiency in Greyhounds, determine the mechanisms underlying this deficiency, and identify CYP2B11 mutations that contribute to this phenotype in Greyhounds. Greyhound livers metabolized CYP2B11 substrates slower, possessed lower CYP2B11 protein abundance, but had similar or higher mRNA expression than other breeds. Gene resequencing identified three CYP2B11 haplotypes, H1 (reference), H2, and H3 that were differentiated by mutations in the gene 3'-untranslated region (3'-UTR). Compared with 63 other dog breeds, Greyhounds had the highest CYP2B11-H3 allele frequency, while CYP2B11-H2 was widely distributed across most breeds. Using 3'-UTR luciferase reporter constructs, CYP2B11-H3 showed markedly lower gene expression (over 70%) compared to CYP2B11-H1 while CYP2B11-H2 expression was intermediate. Truncated mRNA transcripts were observed in CYP2B11-H2 and CYP2B11-H3 but not CYP2B11-H1 transfected cells. Our results implicate CYP2B11 3'-UTR mutations as a cause of decreased CYP2B11 enzyme expression in Greyhounds through reduced translational efficiency.

Absolute Quantitation of Drug-Metabolizing Cytochrome P450 Enzymes and Accessory Proteins in Dog Liver Microsomes Using Label-Free Standard-Free Analysis Reveals Interbreed Variability

Dogs are commonly used in human and veterinary pharmaceutical development. Physiologically based pharmacokinetic modeling using recombinant cytochrome P450 (CYP) enzymes requires accurate estimates of CYP abundance, particularly in liver. However, such estimates are currently available for only seven CYPs, which were determined in a limited number of livers from one dog breed (beagle). In this study, we used a label-free shotgun proteomics method to quantitate 11 CYPs (including four CYPs not previously measured), cytochrome P450 oxidoreductase, and cytochrome b5 in liver microsomes from 59 dogs representing four different breeds and mixed-breed dogs. Validation included showing correlation with CYP marker activities, immunoquantified protein, as well as CYP1A2 and CYP2C41 null allele genotypes. Abundance values largely agreed with those previously published. Average CYP abundance was highest (>120 pmol/mg protein) for CYP2D15 and CYP3A12; intermediate (40-89 pmol/mg) for CYP1A2, CYP2B11, CYP2E1, and CYP2C21; and lowest (<12 pmol/mg) for CYP2A13, CYP2A25, CYP2C41, CYP3A26, and CYP1A1. The CYP2C41 gene was detected in 12 of 58 (21%) livers. CYP2C41 protein abundance averaged 8.2 pmol/mg in those livers, and was highest (19 pmol/mg) in the only liver with two CYP2C41 gene copies. CYP1A2 protein was not detected in the only liver homozygous for the CYP1A2 stop codon mutation. Large breed-associated differences were observed for CYP2B11 (P < 0.0001; ANOVA) but not for other CYPs. Research hounds and Beagles had the highest CYP2B11 abundance; mixed-breed dogs and Chihuahua were intermediate; whereas greyhounds had the lowest abundance. These results provide the most comprehensive estimates to date of CYP abundance and variability in canine liver. SIGNIFICANCE STATEMENT: This work provides the most comprehensive quantitative analysis to date of the drug-metabolizing cytochrome P450 proteome in dogs that will serve as a valuable reference for physiologically based scaling and modeling used in drug development and research. This study also revealed high interindividual variation and dog breed-associated differences in drug-metabolizing cytochrome P450 expression that may be important for predicting drug disposition variability among a genetically diverse canine population.

Long-term exposure to chemicals in sewage sludge fertilizer alters liver lipid content in females and cancer marker expression in males

BACKGROUND

The increased incidence of diseases, including metabolic syndrome and infertility, may be related to exposure to the mixture of chemicals, which are ubiquitous in the modern environment (environmental chemicals, ECs). Xeno-detoxification occurs within the liver which is also the source of many plasma proteins and growth factors and plays an important role in the regulation of homeostasis.

OBJECTIVES

The objective of this study was to investigate the effects of ECs on aspects of liver function, in a well characterized ovine model of exposure to a real-life EC mixture.

METHODS

Four groups of sheep (n = 10-12/sex/treatment) were maintained long-term on control or sewage sludge-fertilized pastures: from conception to culling at 19 months of age in females and from conception to 7 months of age and thereafter in control plots until culling at 19 months of age in males. Environmental chemicals were measured in sheep livers and RNA and protein extracts were assessed for exposure markers. Liver proteins were resolved using 2D differential in-gel electrophoresis and differentially expressed protein spots were identified by liquid chromatography/tandem mass spectroscopy.

RESULTS

Higher levels of polycyclic aromatic hydrocarbons (PAHs) and lower levels of polychlorinated biphenyls (PCBs) in the livers of control males compared to control females indicated sexually dimorphic EC body burdens. Increased levels of the PAHs Benzo[a]anthracene and chrysene and reduced levels of PCB 153 and PCB 180 were observed in the livers of continuously exposed females. EC exposure affected xenobiotic and detoxification responses and the liver proteome in both sexes and included major plasma-secreted and blood proteins, and metabolic enzymes whose pathway analysis predicted dysregulation of cancer-related pathways and altered lipid dynamics. The latter were confirmed by a reduction in total lipids in female livers and up-regulation of cancer-related transcript markers in male livers respectively by sewage sludge exposure.

CONCLUSIONS

Our results demonstrate that chronic exposure to ECs causes major physiological changes in the liver, likely to affect multiple systems in the body and which may predispose individuals to increased disease risks.

Use of Phenoxyaniline Analogues To Generate Biochemical Insights into the Interactio n of Polybrominated Diphenyl Ether with CYP2B Enzymes

Human hepatic cytochromes P450 (CYP) are integral to xenobiotic metabolism. CYP2B6 is a major catalyst of biotransformation of environmental toxicants, including polybrominated diphenyl ethers (PBDEs). CYP2B substrates tend to contain halogen atoms, but the biochemical basis for this selectivity and for species specific determinants of metabolism has not been identified. Spectral binding titrations and inhibition studies were performed to investigate interactions of rat CYP2B1, rabbit CYP2B4, and CYP2B6 with a series of phenoxyaniline (POA) congeners that are analogues of PBDEs. For most congeners, there was a <3-fold difference between the spectral binding constants (K_S) and IC₅₀ values. In contrast, large discrepancies between these values were observed for POA and 3-chloro-4-phenoxyaniline. CYP2B1 was the enzyme most sensitive to POA congeners, so the Val-363 residue from that enzyme was introduced into CYP2B4 or CYP2B6. This substitution partially altered the protein-ligand interaction profiles to make them more similar to that of CYP2B1. Addition of cytochrome P450 oxidoreductase (POR) to titrations of CYP2B6 with POA or 2'4'5'TCPOA decreased the affinity of both ligands for the enzyme. Addition of cytochrome b₅ to a recombinant enzyme system containing POR and CYP2B6 increased the POA IC₅₀ value and decreased the 2'4'5'TCPOA IC₅₀ value. Overall, the inconsistency between K_S and IC₅₀ values for POA versus 2'4'5'TCPOA is largely due to the effects of redox partner binding. These results provide insight into the biochemical basis of binding of diphenyl ethers to human CYP2B6 and changes in CYP2B6-mediated metabolism that are dependent on POA congener and redox partner identity.

Tramadol metabolism to O-desmethyl tramadol (M1) and N-desmethyl tramadol (M2) by dog liver microsomes: Species comparison and identification of responsible canine cytochrome P-450s (CYPs)

Tramadol is widely used to manage mild to moderately painful conditions in dogs. However, this use is controversial since clinical efficacy studies in dogs showed conflicting results, while pharmacokinetic studies demonstrated relatively low circulating concentrations of O-desmethyltramadol (M1). Analgesia has been attributed to the opioid effects of M1, while tramadol and the other major metabolite (N-desmethyltramadol, M2) are considered inactive at opioid receptors. The aims of this study were to determine whether cytochrome P450 (CYP) dependent M1 formation by dog liver microsomes is slower compared with cat and human liver microsomes; and identify the CYPs responsible for M1 and M2 formation in canine liver. Since tramadol is used as a racemic mixture of (+)- and (-)-stereoisomers, both (+)-tramadol and (-)- tramadol were evaluated as substrates. M1 formation from tramadol by liver microsomes from dogs was slower than from cats (3.9-fold), but faster than humans (7-fold). However, M2 formation by liver microsomes from dogs was faster than from cats (4.8-fold) and humans (19-fold). Recombinant canine CYP activities indicated that M1 was formed by CYP2D15, while M2 was largely formed by CYP2B11 and CYP3A12. This was confirmed by dog liver microsomes studies that showed selective inhibition of M1 formation by quinidine and M2 formation by chloramphenicol and CYP2B11 antiserum, and induction of M2 formation by phenobarbital. Findings were similar for both (+)-tramadol and (-)-tramadol. In conclusion, low circulating M1 concentrations in dogs is explained in part by low M1 formation and high M2 formation, which are mediated by CYP2D15 and CYP2B11/CYP3A12, respectively.

The assessment of daily dietary intake reveals the existence of a different pattern of bioaccumulation of chlorinated pollutants between domestic dogs and cats

Pet dogs and cats have been proposed as sentinel species to assess environmental contamination and human exposure to a variety of pollutants, including POPs. However, some authors have reported that dogs but not cats exhibit intriguingly low levels of some of the most commonly detected POPs, such as DDT and its metabolites. This research was designed to explore these differences between dogs and cats. Thus, we first determined the concentrations of 53 persistent and semi-persistent pollutants (16 polycyclic aromatic hydrocarbons (PAHs), 18 polychlorinated biphenyls (PCBs) and 19 organochlorine pesticides (OCPs)) in samples of the most consumed brands of commercial feed for dogs and cats, and we calculated the daily dietary intake of these pollutants in both species. Higher levels of pollutants were found in dog food and our results showed that the median values of intake were about twice higher in dogs than in cats for all the three groups of pollutants (ΣPAHs: 274.8 vs. 141.8; ΣOCPs: 233.1 vs. 83; ΣPCBs: 101.8 vs. 43.8 (ng/kg bw/day); respectively). Additionally, we determined the plasma levels of the same pollutants in 42 and 35 pet dogs and cats, respectively. All these animals lived indoors and were fed on the commercial brands of feed analyzed. As expected (considering the intake), the plasma levels of PAHs were higher in dogs than in cats. However, for organochlorines (OCPs and PCBs) the plasma levels were much higher in cats than in dogs (as much as 23 times higher for DDTs), in spite of the higher intake in dogs. This reveals a lower capacity of bioaccumulation of some pollutants in dogs, which is probably related with higher metabolizing capabilities in this species.

Interspecies comparison of chlorinated contaminant concentrations and profiles in wild terrestrial mammals from northwest Poland

The aim of this study was to determine residual polychlorinated biphenyls (PCBs) and organochlorine pesticides in the adipose tissue of wild terrestrial mammals coming from the same area in northwest Poland and to compare the organochlorine content and profile between species. The study was performed on five mammalian species, including omnivores (badger, wild boar, raccoon dog) and herbivores (roe deer, red deer). The obtained results show that the levels of residues of most of the analyzed compounds were greater in omnivorous mammals than herbivorous mammals. We found differences in the pattern of accumulation of organochlorines. In the raccoon dog organochlorines accumulated in the following descending order: DDTs > PCBs > HCHs > endrin > dieldrin; in the badger, wild boar, and roe deer, the order was as follows: DDTs > HCHs > PCBs > endrin > dieldrin; and in red deer, the order was as follows: HCHs > DDTs > PCBs > endrin > dieldrin. PCB 153 was dominant in herbivores and in the wild boar, whereas in the raccoon dog it was lower than PCB 138 and in badger lower than PCB 180. These differences in the tested species may reflect differences in metabolic capacity and/or feeding habits and an uneven distribution of organochlorines in the area where the animals lived. Maximum residue levels (MRLs) were exceeded in single samples from animals whose meat and fat can be consumed by humans. The greatest number of cases where MRLs were exceeded was observed in the adipose tissue of the badger (HCHs, DDTs, endrin, non-dioxin-like PCBs) and the fewest in the adipose tissue of roe deer (only lindane).

Organohalogenated contaminants (OHCs) in the serum and hair of pet cats and dogs: biosentinels of indoor pollution

Concentrations of different classes of organohalogenated contaminants (OHCs) viz., polybrominated diphenyl ethers (PBDEs), novel brominated flame retardants (NBFRs), bromophenols (BPs), polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs) and their metabolites were determined in cat and dog serum and hair samples from Pakistan. The major DDT metabolite, p,p'-DDE, was the major OHC in cat serum (N=20) and ranged between 1 and 2150 ng/g lipid weight (lw). p,p'-DDE was not detected in dog serum (N=16). In contrary to other OHCs, levels of ∑HO-PCBs were significantly higher in dog serum (median=6.0 ng/g lw) than cat serum (median=2.2 ng/g lw). Levels of most OHCs were significantly higher (p<0.05) in cat serum than those found in human serum from the same region, in particular for ∑PBDEs (ranged 1-1280 ng/g lw). Significantly lower levels of OCPs (p<0.05) were detected in dog serum than in human serum. The concentrations of ∑BPs were seven times higher in cat serum (median 112 ng/g lw) than dog serum (median 16 ng/g lw). To the best of our knowledge, this is the first time that NBFRs, e.g. 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPE), decabromodiphenylethane (DBDPE), and bis(2-ethylhexyl)-3,4,5,6-tetrabromophthalate (TBPH), were detected in cat and dog's hair. BTBPE had the highest detection frequency (30%) in the serum samples. In cat and dog hair samples, the order of importance of OHCs was ∑OCPs>∑NBFRs>∑PBDEs>∑PCB, with the highest concentrations being around 38 ng/g hair. In paired hair-serum cat samples (N=12), ∑DDTs (r=0.65, p=0.001) were significantly correlated, while for all other OHCs no significant correlations (p<0.001) were observed in both cats and dogs. Our findings on both hair and serum samples suggested that pet dogs do not bioaccumulate DDTs. Our results are also in agreement with the hypothesis that pets may serve as biosentinels for indoor pollution. This is the first study to document the presence of OHCs in pets from Pakistan and provides baseline information for future monitoring of OHCs in pets.

Assigning atropisomer elution orders using atropisomerically enriched polychlorinated biphenyl fractions generated by microsomal metabolism

Changes in the enantiomeric fraction of chiral polychlorinated biphenyls (PCBs) are a powerful tool to investigate the movement of PCBs in the environment, for example as part of source apportionment and ecological studies. Environmental studies typically employ a series of cyclodextrin-based gas chromatography columns to separate all environmentally relevant PCB congeners. The elution order of most PCB atropisomers has not been established on different enantioselective columns due to the unavailability of analytical standards. To overcome this limitation, the current study generated atropisomerically enriched fractions of chiral PCBs with rat liver microsomes. Subsequently, the enrichment profile of the enriched PCB fractions was used to determine the elution order of PCB atropisomers on selected enantioselective gas chromatography columns. While the elution order of PCB 95, 131, 132, 136, 149 and 176 atropisomers was identical on all enantioselective columns investigated, an inversion of the elution order was observed for PCB 45, 84, 91 and 174 atropisomers on a few columns. These results demonstrate that atropisomerically enriched fractions obtained from microsomal metabolism can be used to unambiguously establish the relative elution order of the atropisomers of PCBs and potentially other environmental pollutant, especially if pure enantiomers are not available.

Gas chromatographic analysis with chiral cyclodextrin phases reveals the enantioselective formation of hydroxylated polychlorinated biphenyls by rat liver microsomes

Chiral PCB congeners are major components of PCB mixtures and undergo enantioselective biotransformation to hydroxylated (OH-)PCBs by cytochrome P450 enzymes. While it is known that biotransformation results in an enantiomeric enrichment of the parent PCB, it is currently unknown if OH-PCBs are formed enantioselectively. The present study screened seven commercial capillary gas chromatography columns containing modified β- or γ-cyclodextrins for their potential to separate the atropisomers of methylated derivatives of OH-PCB. The atropisomers of 3-, 4- and 5-methoxy derivatives were at least partially separated on one or more columns. A subsequent biotransformation study was performed with rat liver microsomes to assess if hydroxylated metabolites are formed enantioselectively from PCBs 91, 95, 132, and 149. The OH-PCBs were extracted from the microsomal incubations, derivatized with diazomethane and analyzed as the respective methoxylated (MeO-)PCB derivatives using selected columns. The 5-hydroxylated metabolites of PCBs 91, 95, 132, and 149 were the major metabolites, which is consistent with PCB's biotransformation by cytochrome P450 2B enzymes. All 5-hydroxylated metabolites displayed a clear, congener-specific enantiomeric enrichment. Overall, this study demonstrates for the first time that chiral PCBs, such as PCB 91, 95, 132, and 149, are enantioselectively metabolized to OH-PCBs by cytochrome P450 enzymes.

Structure and function of cytochromes P450 2B: from mechanism-based inactivators to X-ray crystal structures and back

This article reviews work from the author dating back to 1978 and focuses on the structural basis of cytochrome P450 (P450) function using available contemporary techniques. Early studies used mechanism-based inactivators that bound to the protein moiety of hepatic P450s to try to localize the active site. Subsequent studies used cDNA cloning, heterologous expression, site-directed mutagenesis, and homology modeling based on multiple bacterial P450 X-ray crystal structures to predict the active sites of CYP2B enzymes with considerable accuracy. Breakthroughs in engineering and expression of mammalian P450s enabled us to determine our first X-ray crystal structure of ligand-free rabbit CYP2B4. To date, we have solved 11 CYP2B4 and three human CYP2B6 structures, which represent four significantly different conformations. The plasticity of CYP2B4 has been confirmed by deuterium exchange mass spectrometry and is substantiated by molecular dynamics simulations. In addition to major movement of secondary structure elements, more subtle reorientation of active site side chains, especially Phe206, Phe297, and Glu301, contributes to the ability of CYP2B enzymes to bind various ligands. Isothermal titration calorimetry has proven to be a useful tool for studying the thermodynamics of ligand binding to CYP2B4 and CYP2B6, and NMR has enabled study of ligand binding orientation in solution as an adjunct to X-ray crystallography. A major challenge remains to harness the power of the various approaches to facilitate prediction of CYP2B specificity and inhibition.

Accumulation of polychlorinated biphenyls and organochlorine pesticide in pet cats and dogs: assessment of toxicological status

PCB and DDT concentrations were determined in the adipose tissue of cats and dogs from Southern Italy. In cats p,p'-DDE was the most abundant DDT component (95.0%), while in dogs these compounds were absent, except in two specimens. PCB concentrations were higher in cats (199.02 ng g(-1) lipid weight) than in dogs (41.61 ng g(-1) lipid weight). Also there were inter-specific differences in the contribution of the different congeners to PCBs, although PCB 138, PCB 153 and PCB 180 were the most representative congeners in both species. Animals from one location, Taranto City, had significantly elevated concentrations of dioxin-like PCBs compared to the other locations. Consequently the estimated mean 2,3,7,8-tetrachlorodibenzo-p-dioxin toxic equivalents (TEQs) of coplanar PCBs were higher in these animals (cats: 0.65 pg g(-1) lipid weight; dogs 0.29 pg g(-1) lipid weight) than in the other ones (cats: 0.12 pg g(-1) lipid weight; dogs: 0.001 pg g(-1) lipid weight).

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.

Hydroxylated polychlorinated biphenyls (OH-PCBs) in the blood of mammals and birds from Japan: lower chlorinated OH-PCBs and profiles

An analytical method was developed to measure tri- to octa-chlorinated OH-PCBs and pentachlorophenol (PCP) in the whole blood. Further, baseline data on the levels and profiles of these phenolic compounds in Japanese mammals (human, cat, dog, raccoon dog, and northern fur seal) and birds (black-tailed gull, common cormorant, and jungle crow) were obtained. Eighteen identifiable and fifty unknown peaks of OH-PCBs were detected and the major congeners identified were 4'OH-CB101/120, 4OH-CB107/4'OH-CB108, 4OH-CB146, 4OH-CB178, 4OH-CB187, 4'OH-CB172, 4OH-CB202, and 4'OH-CB199. Relatively higher concentrations of OH-PCBs were found in animal species than humans; OH-PCB levels in dog, raccoon dog, black-tailed gull, and common cormorant blood were one order of magnitude higher than in humans. Penta- to hepta-chlorinated OH-PCB congeners were predominant in human blood, but profiles of OH-PCBs in other animals widely varied by species. Elevated composition of tri- and tetra-chlorinated OH-PCBs in cat blood and octa-chlorinated OH-PCBs in dog and raccoon dog blood were observed. In cat blood, elevated PCP concentration was also found. When concentration ratios of OH-PCBs to PCBs were calculated in all the animal blood, the ratios in dog, raccoon dog, and cat were notably higher than in other species. These results indicate that animals other than humans, especially cat and canine species such as dog and raccoon dog, might be at risk from OH-PCBs.

2,2',3,3',6,6'-Hexachlorobiphenyl (PCB 136) atropisomers interact enantioselectively with hepatic microsomal cytochrome P450 enzymes

2,2',3,3',6,6'-Hexachlorobiphenyl (PCB 136) is a chiral and highly neurotoxic PCB congener of environmental relevance. (+)-PCB 136 was previously shown to be enriched in tissues from mice treated with racemic PCB 136. We investigated the spectral interactions of (+)-, (-)-, and (+/-)-PCB 136 with mouse and rat hepatic microsomal cytochrome P450 (P450) enzymes to test the hypothesis that enantioselective binding to specific P450 enzymes causes the enrichment of (+)-PCB 136 in vivo. Hepatic microsomes prepared from C57BL/6 mice or Long Evans rats treated with beta-naphthoflavone or 3-methylcholanthrene, phenobarbital, and dexamethasone (prototypical inducers of CYP1A, CYP2B, and CYP3A, respectively) were used to determine first, whether the (+)-PCB 136 atropisomer binds to hepatic microsomal P450 enzymes to a greater extent than does the (-)-PCB 136 atropisomer and second, whether P450 enzymes of one subfamily bind the two PCB 136 atropisomers more efficiently than do P450 enzymes of other subfamilies. Increasing concentrations of (+)-, (-)-, or (+/-)-PCB 136 were added to hepatic microsomes, and the difference spectrum and maximal absorbance change, a measure of PCB binding to P450 enzymes, were measured. A significantly larger absorbance change was observed with (+)-PCB 136 than with (-)-PCB 136 with all four hepatic microsomal preparations in mice and rats, indicating that (+)-PCB 136 interacted with microsomal P450 enzymes to a greater degree than did (-)-PCB 136. In addition, binding of the PCB 136 atropisomers was greatest in microsomes from PB-treated mice and rats and was inhibited by CYP2B antibodies, indicating the involvement of CYP2B enzymes. Together, these results suggest preferential binding of (+)-PCB 136 to P450 enzymes (such as CYP2B and CYP3A) in hepatic microsomes, an observation that may explain the enantioselective enrichment of the (+)-PCB 136 atropisomer in tissues of mice.

Effects of Food Natural Products on the Biotransformation of PCBs

Many food products, particularly fruits and vegetables, contain natural products that affect biotransformation enzymes. These may be expected to affect the rate of biotransformation of PCBs that are metabolized by the affected enzymes. The first step in PCB metabolism is cytochrome P450-dependent monooxygenation. Natural products present in cruciferous vegetables have been shown to selectively up-regulate CYP1A1 and CYP1A2 isozymes on chronic ingestion, and may lead to increased metabolism of those PCB congeners that are substrates for the induced P450s. On the other hand, several natural products selectively inhibit monooxygenation, especially in the intestine, and may lead to increased bioavailability and reduced metabolism of dietary PCBs. Food natural products are known to affect phase II pathways important in the detoxication of hydroxylated PCBs, namely UDP-glucuronosyltransferase and PAPS-sulfotransferase. Continual dietary exposure to chrysin and quercetin, found in fruits and vegetables, induces UGT1A1 and may reduce exposure to hydroxylated PCBs through increased glucuronidation. These and other natural products are also inhibitors of glucuronidation and sulfonation, potentially leading to transient decreases in the elimination of hydroxylated PCBs. In summary, the expected effects of food natural products on PCB biotransformation are complex and may be biphasic, with initial inhibition followed by enhanced biotransformation through monooxygenation and conjugation pathways.

Biomagnification of PBDEs in three small terrestrial food chains

Eight polybrominated diphenyl ethers (PBDE) congeners (IUPAC nos. 28, 47, 99, 100, 153, 154, 183, and 209) were measured in passerines (great tits--Parus major), wood mice (Apodemus sylvaticus), and bank voles (Clethrionomys glareolus). These data were combined with previously obtained data on PBDEs in common buzzards (Buteo buteo), sparrowhawks (Accipiter nisus), and red fox (Vulpes vulpes). This enabled estimation of the biomagnification potential of PBDEs in the following three terrestrial food chains: great tit--sparrowhawk, small rodents--buzzard, and small rodents--fox. Biomagnification of BDE 209 could not be assessed because levels of this congener were below the LOQ in the prey species. All other congeners, except BDE 28, were biomagnified in both predatory bird species. Biomagnification of BDE 28 could not be observed from rodents to buzzard. Biomagnification factors (BMFs) were calculated as the ratio between the lipid-normalized concentrations in the predator and in the prey. BMFs ranged from 2 to 34 for the sum of PBDEs in predatory bird food chain. Although the fox is a top-predator, this is not reflected in the PBDE concentrations that were measured in its tissues. In the small rodent-fox food chain, no biomagnification could be observed. This observation is most likely related to the high metabolic capacity of the fox with regard to organohalogens and should be taken into account when selecting species for environmental monitoring purposes. Not all top-predators will give a representative reflection of the pollution of their habitat, but confounding factors, such as metabolism, can influence the results to a great extent and can therefore lead to misinterpretations.

Remarkable findings concerning PBDEs in the terrestrial top-predator red fox (Vulpes vulpes)

In the present study, we have analyzed muscle, liver, and adipose tissue of 33 red foxes from Belgium for their content of polybrominated diphenyl ethers (PBDEs). Median sums of seven tri- to hepta-BDEs (BDE 28, BDE 47, BDE 99, BDE 100, BDE 153, BDE 154, and BDE 183) were 2.2, 2.4, and 3.4 ng/g lipid weight in adipose tissue, liver, and muscle, respectively. These levels were lower than those found in various species of voles and mice, the main prey species of the red fox. This is probably related to the high capacity of the foxes to metabolize and eliminate lower brominated congeners. BDE 209 generally dominated the PBDE congener profiles in the red fox samples. In samples containing BDE 209, this congener contributed, on the average, approximately 70% to the total PBDE content. BDE 209 was measured in concentrations as high as 760 ng/g lipid weight in the liver, but the detection frequency was not more than 40%. In animals with the highest BDE 209 levels, this congener was detected in muscle, liver, as well as in adipose tissue. Other abundant congeners were BDE 153 and BDE 47, which prevail in other terrestrial species. The particular PBDE congener profile observed in the red fox resembles that seen in grizzly bears from Canada, but differs from those previously reported for terrestrial avian species. Our data confirms unambiguously that BDE 209 does bioaccumulate in terrestrial top predators, such as the red fox.

In vitro metabolism of 2,2',3,4',5,5',6-heptachlorobiphenyl (CB187) by liver microsomes from rats, hamsters and guinea pigs

The metabolism of 2,2',3,4',5,5',6-heptachlorobiphenyl (heptaCB) (CB187) was studied using liver microsomes of rats, hamsters and guinea pigs, and the effect of cytochrome P450 (CYP) inducers, phenobarbital (PB) and 3-methylcholanthrene (MC), was also investigated. In untreated animals, guinea pig liver microsomes formed three metabolites which were deduced to be 4'-hydroxy-2,2',3,5,5',6-hexachlorobiphenyl (M-1), 4'-hydroxy-2,2',3,3',5,5',6-heptaCB (M-2) and 4-OH-CB187 (M-3) from the comparison of GC/MS data with some synthetic authentic samples. The formation rate of M-1, M-2 and M-3 was 18.1, 36.6, 14.7 pmol h-1 mg protein-1, respectively. Liver microsomes of untreated rats and hamsters did not form CB187 metabolites. In guinea pigs, PB-treatment increased M-1 and M-2 significantly to 1.9- and 3.4-fold of untreated animals but did not affect the formation of M-3. In rats, PB-treatment resulted in the appearance of M-2 and M-3 with formation rates of 87.1 and 13.7 pmol h-1 mg protein-1, respectively, but M-1 was not observed. In hamsters, PB-treatment formed only M-2 at a rate of 29.4 pmol h-1 mg protein-1. On the other hand, MC-treatment of guinea pigs decreased the formation of M-1 and M-2 to less than 50% of untreated animals. MC-microsomes of rats and hamsters produced no metabolites. Preincubation of antiserum (300 microl) against guinea pig CYP2B18 with liver microsomes of PB-treated guinea pigs produced 80% inhibition of M-1 and the complete inhibition of M-2 and M-3. These results suggest that PB-inducible CYP forms, especially guinea pig CYP2B18, rat CYP2B1 and hamster CYP2B, are important in CB187 metabolism and that CB187 metabolism in guinea pigs may proceed via the formation of 3,4- or 3',4'-oxide and subsequent NIH-shift or dechlorination.

Contamination status and accumulation features of persistent organochlorines in pet dogs and cats from Japan

Concentrations of persistent organochlorines (OCs) such as polychlorinated dibenzo-p-dioxin (PCDDs), dibenzofurans (PCDFs), biphenyls, dichlorodiphenyltrichloroethane and their metabolites (DDTs), hexachlorocyclohexane isomers, hexachlorobenzene, and chlordane compounds were determined in genital organs of pet dogs and cats and pet foods from Japan. Levels of OCs in dogs were relatively lower than those in cats, while residue levels in their diets were almost similar, implying that accumulation and elimination mechanisms of these contaminants are different between dogs and cats. When bioconcentration factors (BCFs) were estimated from concentrations of OCs in dogs, cats, and their diets, BCFs of all the OCs except PCDD/DFs exceeded 1.0 in cats. On the other hand, in all the dogs, BCFs of DDTs were below 1.0, suggesting that dogs do not bioconcentrate DDTs. Furthermore, BCFs of all the OCs except PCDD/DFs in dogs were notably lower than those in cats, suggesting that dogs have higher metabolic and elimination capacity for these contaminants than cats. When residue levels of OCs in livers, adipose tissue, and genital organs of two pet dogs were examined, hepatic sequestration of PCDD/DFs and oxychlordane was observed.

Activation of the anticancer prodrugs cyclophosphamide and ifosfamide: identification of cytochrome P450 2B enzymes and site-specific mutants with improved enzyme kinetics

Cyclophosphamide (CPA) and ifosfamide (IFA) are oxazaphosphorine anticancer prodrugs metabolized by two alternative cytochrome P450 (P450) pathways, drug activation by 4-hydroxylation and drug inactivation by N-dechloroethylation, which generates the neurotoxic and nephrotoxic byproduct chloroacetaldehyde. CPA and IFA metabolism catalyzed by P450s 2B1, 2B4, 2B5, and seven site-specific 2B1 mutants was studied in a reconstituted Escherichia coli expression system to identify residues that contribute to the unique activities and substrate specificities of these enzymes. The catalytic efficiency of CPA 4-hydroxylation by rat P450 2B1 was 10- to 35-fold higher than that of rabbit P450 2B4 or 2B5. With IFA, approximately 50% of metabolism proceeded via N-dechloroethylation for 2B1 and 2B4, whereas CPA N-dechloroethylation corresponded to only approximately 3% of total metabolism (2B1) or was absent (2B4, 2B5). Improved catalytic efficiency of CPA and IFA 4-hydroxylation was obtained upon substitution of 2B1 Ile-114 by Val, and replacement of Val-363 by Leu or Ile selectively suppressed CPA N-dechloroethylation >or=90%. P450 2B1-V367A, containing the Ala replacement found in 2B5, exhibited only approximately 10% of wild-type 2B1 activity for both substrates. Canine P450 2B11, which has Val-114, Leu-363, and Val-367, was therefore predicted to be a regioselective CPA 4-hydroxylase with high catalytic efficiency. Indeed, P450 2B11 was 7- to 8-fold more active as a CPA and IFA 4-hydroxylase than 2B1, exhibited a highly desirable low K(m) (80-160 microM), and catalyzed no CPA N-dechloroethylation. These findings provide insight into the role of specific P450 2B residues in oxazaphosphorine metabolism and pave the way for gene therapeutic applications using P450 enzymes with improved catalytic activity toward these anticancer prodrug substrates.

Spectral interactions of tetrachlorobiphenyls with hepatic microsomal cytochrome p450 enzymes

This study investigated the spectral interactions of hepatic microsomal cytochrome p450 (CYP) enzymes with four symmetrical polychlorinated biphenyls (PCBs): 2,2',4,4'-tetrachlorobiphenyl (PCB 47); 2,2',5,5'-tetrachlorobiphenyl (PCB 52); 2,2',6,6'-tetrachlorobiphenyl (PCB 54); and 3,3',4,4'-tetrachlorobiphenyl (PCB 77). The PCBs were selected to explore structure-activity relationships and the effect of the chlorination pattern on PCB-CYP spectral interactions. To examine CYP enzyme specificity, difference spectra were measured with hepatic microsomes prepared from control, phenobarbital (PB)-, 3-methylcholanthrene (MC)-, and dexamethasone (DEX)-treated rats in the absence and presence of CYP-specific antibodies. The four PCB congeners elicited a type I spectral change with all hepatic microsomal preparations. The binding efficiency of the PCBs was highest with microsomes from PB-treated rats. The largest absorbance change and highest binding efficiency were observed with PCB 54, the most non-coplanar congener tested. Antibody inhibition and CYP immunoquantitation data showed that the PCBs bind to CYP1A, CYP2B, CYP2C and CYP3A enzymes to varying degrees. For example, PCB 47, 52, and 54 bind preferentially to CYP2B and to a lesser extent to CYP3A enzymes in microsomes from PB-treated male rats; PCB 52 binds primarily to CYP3A enzymes in microsomes from DEX-treated female rats; and PCB 54 binds to CYP3A and to CYP2C enzymes in microsomes from control male rats. The study demonstrated that the extent of PCB-CYP binding interaction was dependent on the chlorination pattern of the PCB and on the relative abundance of individual CYP enzymes in hepatic microsomes.

Organochlorine contaminant and stable isotope profiles in Arctic fox (Alopex lagopus) from the Alaskan and Canadian Arctic

Arctic fox (Alopex lagopus) is a circumpolar species distributed across northern Canada and Alaska. Arctic fox muscle and liver were collected at Barrow, AK, USA (n=18), Holman, NT, Canada (n=20), and Arviat, NU, Canada (n=20) to elucidate the feeding ecology of this species and relate these findings to body residue patterns of organochlorine contaminants (OCs). Stable carbon (delta 13C) and nitrogen (delta 15N) isotope analyses of Arctic fox muscle indicated that trophic position (estimated by delta 15N) is positively correlated with increasing delta 13C values, suggesting that Arctic fox with a predominantly marine-based foraging strategy occupy a higher trophic level than individuals mostly feeding from a terrestrial-based carbon source. At all sites, the rank order for OC groups in muscle was polychlorinated biphenyls (Sigma PCB) > chlordane-related compounds (Sigma CHLOR) > hexachlorocyclohexane (Sigma HCH) > total toxaphene (TOX) > or = chlorobenzenes (Sigma ClBz) > DDT-related isomers (Sigma DDT). In liver, Sigma CHLOR was the most abundant OC group, followed by Sigma PCB > TOX > Sigma HCH > Sigma ClBz > Sigma DDT. The most abundant OC analytes detected from Arctic fox muscle and liver were oxychlordane, PCB-153, and PCB-180. The comparison of delta 15N with OC concentrations indicated that relative trophic position might not accurately predict OC bioaccumulation in Arctic fox. The bioaccumulation pattern of OCs in the Arctic fox is similar to the polar bear. While Sigma PCB concentrations were highly variable, concentrations in the Arctic fox were generally below those associated with the toxicological endpoints for adverse effects on mammalian reproduction. Further research is required to properly elucidate the potential health impacts to this species from exposure to OCs.

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.

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.

Role of nitric oxide in down-regulation of CYP2B1 protein, but not RNA, in primary cultures of rat hepatocytes

There are conflicting reports about the role of nitric oxide in the down-regulation of cytochrome P450 that occurs when animals or cultured hepatocytes are exposed to inflammatory stimuli. Here, we investigated the participation of NO in the down-regulation of CYP2B1 by bacterial endotoxin (LPS) in rat hepatocytes cultured on Matrigel. LPS caused the down-regulation of CYP2B1 mRNA to 20% of control values within 12 h of treatment, and this was not reversed by concentrations of NO synthase inhibitors that completely blocked NO production. LPS was several orders of magnitude more potent in the down-regulation of CYP2B1 mRNA than in induction of NO production. In contrast, concentrations of LPS in the 1 to 100 ng/ml range induced NO production and produced a rapid down-regulation of CYP2B1 protein to 30% and <5% of control at 6 and 24 h, respectively, that could be completely prevented both by inhibitors of NO synthase and by LY83583, which prevents NO synthase-2 induction. The blockade of CYP2B1 down-regulation by NO synthase inhibitors was reversed by arginine, and the NO donors S-nitrosoglutathione and S-nitroso-N-acetylpenicillamine mimicked CYP2B1 protein suppression. Taken together, these experiments demonstrate two independent mechanisms of CYP2B1 down-regulation by LPS: a rapid, NO-dependent suppression of the protein occurring at high concentrations of LPS and a slower, NO-independent pretranslational suppression occurring at low concentrations of LPS.

In vitro metabolism of polychlorinated biphenyl congeners by beluga whale (Delphinapterus leucas) and pilot whale (Globicephala melas) and relationship to cytochrome P450 expression

We measured rates of oxidative metabolism of two tetrachlorobiphenyl (TCB) congeners by hepatic microsomes of two marine mammal species, beluga whale and pilot whale, as related to content of selected cytochrome P450 (CYP) forms. Beluga liver microsomes oxidized 3,3',4,4'-TCB at rates averaging 21 and 5 pmol/min per mg for males and females, respectively, while pilot whale samples oxidized this congener at 0.3 pmol/min per mg or less. However, rates of 3,3',4,4'-TCB metabolism correlated with immunodetected CYP1A1 protein content in liver microsomes of both species. The CYP1A inhibitor alpha-naphthoflavone inhibited 3,3',4,4'-TCB metabolism by 40% in beluga, supporting a role for a cetacean CYP1A as a catalyst of this activity. Major metabolites of 3,3',4,4'-TCB generated by beluga liver microsomes were 4-OH-3,3',4',5-TCB and 5-OH-3,3',4,4'-TCB (98% of total), similar to metabolites formed by other species CYP1A1, and suggesting a 4,5-epoxide-TCB intermediate. Liver microsomes of both species metabolized 2,2',5,5'-TCB at rates of 0.2-1.5 pmol/min per mg. Both species also expressed microsomal proteins cross-reactive with antibodies raised against some mammalian CYP2Bs (rabbit; dog), but not others (rat; scup). Whether CYP2B homologues occur and function in cetaceans is uncertain. This study demonstrates that PCBs are metabolized to aqueous-soluble products by cetacean liver enzymes, and that in beluga, rates of metabolism of 3,3',4,4'-TCB are substantially greater than those of 2,2',5,5'-TCB. These directly measured rates generally support the view that PCB metabolism plays a role in shaping the distribution patterns of PCB residues found in cetacean tissue.

Metabolism of 2,3',4',5-tetrachlorobiphenyl by cytochrome P450 from rats, guinea pigs and hamsters

The metabolism of 2,3',4',5-tetrachlorobiphenyl (TCB) was compared using liver microsomes and six isoforms of cytochrome P450 purified from rats, guinea pigs and hamsters. In microsomal study, the following species differences were observed: 1) Untreated guinea pigs and hamsters but not rats can metabolize this TCB to 3-hydroxy- or 4-hydroxy-2,3',4',5-TCB, 2) Guinea pig microsomes showed only 3-hydroxylating activity, whereas hamster microsomes showed higher activity of 4-hydroxylation than that of 3-hydroxylation. In common with three species, the 3-hydroxylation was accelerated by phenobarbital. The 4-hydroxylation in rats and hamsters was increased by pretreatment with 3-methylcholanthrene and 3,3',4,4',5-pentachlorobiphenyl. The hydroxylation activities of liver microsomes from the three species could be explained by an involvement of different isoforms of cytochrome P450. In addition, it is apparent that hamster CYP1A2 as well as hamster CYP2A8 is involved in the 4-hydroxylation of 2,3',4',5-TCB although it has no activity for 2,2',5,5'-TCB.

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.

Organochlorine pesticides and polychlorinated biphenyl congeners in wild terrestrial mammals and birds from Chubu region, Japan: interspecies comparison of the residue levels and compositions

In order to understand the residue levels of organochlorine compounds (OCs) and their accumulation patterns in wildlife inhabiting Chubu region, Japan, the concentrations of polychlorinated biphenyls (PCBs), hexachlorocyclohexane isomers (HCHs), DDT compounds (DDTs) and hexachlorobenzene (HCB) were measured in 8 species of terrestrial mammals and 10 species of birds. In view of feeding habits, the contamination levels of OCs were found to be higher in omnivorous mammals than in herbivorous ones, and in fish-eating ones and raptores than in omnivorous birds. In fox and dog, PCB-180 (2, 2', 3, 4, 4', 5, 5'-heptachlorobiphenyl) was the most dominant PCB congener, while in the other species PCB-153 (2, 2', 4, 4', 5, 5'-hexachlorobiphenyl) was the most persistent. The ratios of lower chlorinated PCB congeners (tri- to tetra-) to total PCBs were larger in fish-eating birds than in the other birds. The results indicate that the compositions of PCB congeners would reflect the differences of feeding habits and xenobiotic metabolizing systems among each species.

Metabolism of 2,4,5,2',4',5'-hexachlorobiphenyl (PCB153) in guinea pig

1. The in vitro and in vivo metabolism of 2,4,5,2',4',5'-hexachlorobiphenyl (PCB153) in guinea pig has been studied. 2. Seven metabolites were detected in the faeces of PCB153-treated animals and three were identical to those produced by dog liver microsomes. The detection of a metabolite where a chlorine atom was shifted from the 2- to 3-position strongly suggested the involvement of 2,3-arene oxide intermediate, and evidence for the concomitant formation of a 3,4-arene oxide intermediate was provided by identifying other two minor metabolites which were dechlorinated at the 4-position. 3. In vitro studies using liver microsomes from guinea pigs revealed that the 2,3-arene oxide and 3-hydroxylation pathways are the predominant metabolic routes compared with the 3,4-arene oxide pathway. Although the guinea pig is an another species that can metabolize PCB153 mainly to the 2,3-arene oxide intermediate, the rate of formation was only about one-tenth of the dog. 4. These results indicate that the ability to form this unusual 2,3-arene oxide intermediate may not be responsible for high excretion rate of this congener. Our data also suggest that the cytochrome P450-catalysed metabolism of PCB153 in the guinea pig and dog are similar, whereas for post-cytochrome P450 metabolism, the guinea pig resembles the rabbit.

Role of cytochrome b5 in the oxidative metabolism of polychlorinated biphenyls catalyzed by cytochrome P450

1. The role of cytochrome b5 in the cytochrome P450-dependent hydroxylation of tetrachlorobiphenyl (TCB) isomers was examined using a reconstituted system consisting of CYP2B1 and CYP1A1 and rat liver microsomes. 2. By addition of cytochrome b5 to the reconstituted system containing CYP2B1, the 3-hydroxylation of 2,5,2,'5'- and 2,5,3',4'-TCB was increased about six-fold, but the 3- and 5-hydroxylation of 2,4,3',4'-TCB was decreased by about 50% 3. All hydroxylations of 3 ,4,3',4'-,2,5, 3,4'- and 2,4,3',4'-TCBs were decreased by addition of cytochrome b5 to the reconstituted system containing CYPlA1. 4. In stoichiometry measurements, changes in NADPH oxidation and coupling efficiency by addition of cytochrome b5 was observed and these differed according to the position of chlorine atoms of TCBs and cytochrome P450 isoforms used in the systems.

Involvement of cytochrome b5 in the metabolism of tetrachlorobiphenyls catalyzed by CYP2B1 and CYP1A1

The role of cytochrome b5 in the cytochrome P450 (CYP)-dependent hydroxylation of tetrachlorobiphenyl (TCB) isomers was examined using a reconstituted mixed function oxygenase (MFO) system containing purified CYP2B1 or 1A1, and rat liver microsomes. Hydroxylations of 2,2',5,5'- and 3,3',4,4'-TCBs were catalyzed mainly by CYP2B1 and 1A1, respectively, in the reconstituted MFO system and those of 2,3',4',5- and 2,3',4,4'-TCBs were mediated by both cytochrome P450 systems. The activity toward 2,2',5,5'- and 2,3',4',5-TCB was significantly increased 6.5- and 5.5-fold, respectively, by addition of cytochrome b5 in the reconstituted MFO system containing of CYP2B1. Either hydroxylation activity toward 2,3',4,4'-TCB with the CYP2B1 system was very low or decreased by addition of cytochrome b5. These results suggest that the involvement of cytochrome b5 to the hydroxylation of TCBs is dependent on the TCB congener being metabolized, and the cytochrome P450 isoform involved in its metabolism.

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.

Metabolism of 3,5,3',5'-tetrachlorobiphenyl by rat liver microsomes and purified P4501A1

1. The metabolism of 3,5,3',5'-tetrachlorobiphenyl (TCB) was investigated with liver microsomes and purified P450 from the male Wistar rat. 2. One novel metabolite was produced after incubation with liver microsomes derived from the 3-methylcholanthrene (MC)- and 3,4,5,3',4'-pentachlorobiphenyl-pretreated rat, but not after incubation with those from the untreated or phenobarbital (PB)-pretreated rat. These results suggest that P450 isozyme(s) induced by MC-type inducers is involved in 3,5,3',5'-TCB metabolism. 3. The chemical structure of this metabolite was identified to be 4-hydroxy-3,5,3',5'-TCB by comparison of its retention time in glc and the ms with those of a synthetic sample. 4. Purified rat P4501A1, a major MC-inducible P450 isozyme, catalyzed the 4-hydroxylation of 3,5,3',5'-TCB, but P4502B1, a major PB-inducible isozyme, was inactive. 5. Reduced glutathione completely inhibited the formation of the hydroxylated metabolite, suggesting that 4-hydroxylation of 3,5,3',5'-TCB proceeded via the 3,4-epoxide.

Polychlorinated biphenyl congeners in foxes in Germany from 1983 to 1991

Red foxes served as a biological indicator for the temporal development of environmental contamination with polychlorinated biphenyls (PCB). The concentration of PCB congeners nos. 28, 49, 52, 101, 138, 153, and 180 were analyzed in the body fat of 80 foxes (Canis vulpes) from Germany. The samples were from animals that had been submitted for examination in 1983, 1987, and 1991. Throughout this time period, a reduction was seen in the concentration of the highly chlorinated biphenyls 138, 153, and 180, whereas the concentration of the low-chlorinated congeners PCB nos. 28, 49, and 52 increased. No change in contamination with congener 101 was observed. These results show a trend toward reduction of environmental contamination with highly-chlorinated biphenyls since 1983, while contamination with low-chlorinated congeners is apparently increasing. An interesting observation is the disproportionately higher amount of 2,2',3,4,4',5,5'-hepatachlorobiphenyl (PCB 180) over that of 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153) in body-fat samples from all foxes analyzed. This phenomenon was also observed in 10 dogs examined as controls. Based on evidence from other authors of experimental toxicological studies on beagles, it appears that the fox possesses a similar cytochrome P-450 isoenzyme that can degrade 2,4,5-trichloro-substituted aromatic compounds. As a consequence, in canines, PCB 180 which is additionally meta-chloro substituted is accumulated to a greater degree than is PCB 153.

Hybrid enzymes for structure-function analysis of cytochrome P-450 2B11

Previous work has shown that P-450 2B11 is responsible for the unique ability of dogs to metabolize and eliminate certain highly-chlorinated biphenyls such as 2,2',4,4',5,5'-hexachlorobiphenyl (245-HCB), whereas the related P-450 2B forms in rat and rabbit are unable to metabolize the compound to any significant degree. To determine the structural basis for this functional diversity, hybrid enzymes were generated. Success with this approach required a careful choice of second enzyme and common substrate with which to assess the functional integrity of the hybrid proteins. The choices of P-450 2B5 from rabbit as the second enzyme and androstenedione as the substrate were based in part on the finding that P-450 2B11 and P-450 2B5 hydroxylate androstenedione with similar overall activities but distinct profiles. Enzymatic studies with eight hybrid enzymes provided evidence for two regions of P-450 2B11 and 2B5, between residues 95-239 and 240-370, that appear to be involved in defining substrate specificity for androstenedione, and three regions of P-450 2B11, between residues 95-239, 240-370, and 371-494, that contain amino acids necessary for metabolism of 245-HCB. This deliberate approach to the creation of hybrid cytochromes P-450 has generated a series of enzymes that will be central to further structure-function studies of the cytochromes P-450 2B.

Species differences of testosterone 16-hydroxylases in liver microsomes of guinea pig, rat and dog

1. In hepatic microsomes, remarkable species differences in the activity of testosterone 16-hydroxylase was observed in guinea pig, dog, and rat. The activity of testosterone 16 beta-hydroxylase was higher than that of 16 alpha-hydroxylase in guinea pig, whereas 16 alpha-hydroxylated testosterone was predominant as the metabolite in dog and rat. 2. Since P4502B isoenzyme has been shown to be a catalyst for testosterone 16-hydroxylations, we compared the catalytic properties of the P4502B subfamily (P450GP-1, P450b and P450PBD-2) purified from liver microsomes of guinea pig, dog, and rat, respectively. P450GP-1, P450b and P450PBD-2 showed different stereoselectivities for hydroxylation of testosterone at the 16-position. 3. P450GP-1, P450b and P450PBD-2 together comprised 47, < 0.1 and 23% of total P450 in liver microsomes of untreated guinea pig, rat and dog, respectively, indicating that the amounts of the P4502B isoenzyme in untreated animals were clearly different in these three animal species. Both 16 alpha- and 16 beta-hydroxylations of testosterone in liver microsomes of phenobarbital-treated guinea pig, rat and dog were inhibited by anti-P450GP-1, anti-P450b and anti-P450PBD-2 antibodies, respectively. 4. These and other results indicate that the species difference observed in testosterone 16-hydroxylation may be, in part, due to differences in the amounts of P450 of the P4502B subfamily, and their stereoselectivities for 16-hydroxylation.

Tissue-specific expression, induction, and inhibition through metabolic intermediate-complex formation of guinea pig cytochrome P450 belonging to the CYP2B subfamily

The tissue-specific expression and induction of P450GP-1, a constitutive form of cytochrome P450 of the guinea pig classified into the CYP2B subfamily, were studied. Prior to these studies, a P450 form (P450GP-1 PB) was purified from phenobarbital-treated guinea pigs and the properties were compared with those of the P450GP-1. This form was judged to be the same as P450GP-1 existing in untreated animals by comparisons of their N-terminal amino acid sequences, peptide maps, and affinities toward anti-P450GP-1 antibody. Immunostaining of P450GP-1 revealed that the lung and small intestine as well as the liver of untreated guinea pigs contain P450GP-1, while none or only small amounts of this P450 form were observed in the kidney, heart, spleen, urinary bladder, and testis. The amount of liver P450GP-1 protein expressed in untreated guinea pigs was estimated to be 19.4% of the total cytochrome P450 and this form was increased 1.7-fold by phenobarbital treatment. Similarly, intestinal P450GP-1 was increased by phenobarbital treatment. However, lung P450GP-1 was not increased by the treatment. It was also observed that the liver P450GP-1 is induced with SKF-525A to the same extent as with phenobarbital. On the other hand, dexamethsone, p,p'-dichlorodiphenyltrichloroethane, and isosafrole showed no or only a weak ability to increase the liver P450GP-1 content. The drug-metabolizing activities in the liver microsomes of SKF-525A-pretreated guinea pigs were lower than those in phenobarbital-treated animals, although the P450GP-1 protein was induced equally by these treatments. The low activities of SKF-525A-treated animals in the drug metabolisms were attributed to the formation of the metabolic-intermediate complex between P450GP-1 and SKF-525A metabolite. These results permitted us to conclude that the tissue specificity in the expression of guinea pig P450 belonging to the CYP2B subfamily and the inducibility with chemicals are similar to those of rat CYP2B1, although the constitutive expression of guinea pig liver P450GP-1 is much higher than that of CYP2B1.