Stereoselective epoxidation and hydration at the K-region of polycyclic aromatic hydrocarbons by cDNA-expressed cytochromes P450 1A1, 1A2, and epoxide hydrolase

The unique biodegradation pathway of benzo[a]pyrene in moderately halophilic Pontibacillus chungwhensis HN14

Benzo[a]pyrene (BaP), as the typical representative of polycyclic aromatic hydrocarbons (PAHs), is a serious hazard to human health and natural environments. Though the study of microbial degradation of PAHs has persisted for decades, the degradation pathway of BaP is still unclear. Previously, Pontibacillus chungwhensis HN14 was isolated from high salinity environment exhibiting a high BaP degradation ability. Here, based on the intermediates identified, BaP was found to be transformed to 4,5-epoxide-BaP, BaP-trans-4,5-dihydrodiol, 1,2-dihydroxy-phenanthrene, 2-carboxy-1-naphthol, and 4,5-dimethoxybenzo[a]pyrene by the strain HN14. Furthermore, functional genes involved in degradation of BaP were identified using genome and transcriptome data. Heterogeneous co-expression of monooxygenase CYP102(HN14) and epoxide hydrolase EH(HN14) suggested that CYP102(HN14) could transform BaP to 4,5-epoxide-BaP, which was further transformed to BaP-trans-4,5-dihydrodiol by EH(HN14). Moreover, gene cyp102(HN14) knockout was performed using CRISPR/Cas9 gene-editing system which confirmed that CYP102(HN14) play a key role in the initial conversion of BaP. Finally, a novel BaP degradation pathway was constructed in bacteria, which showed BaP could be converted into chrysene, phenanthrene, naphthalene pathways for the first time. These findings enhanced our understanding of microbial degradation process for BaP and suggested the potential of using P. chungwhensis HN14 for bioremediation in PAH-contaminated environments.

Oxidization of benzo[a]pyrene by CYP102 in a novel PAHs-degrader Pontibacillus sp. HN14 with potential application in high salinity environment

Benzo [a]pyrene (BaP) is a type of high-molecular-weight polycyclic aromatic hydrocarbons (PAHs) with potent carcinogenicity; however, there are limited studies on its degradation mechanism. Here, a strain of Pontibacillus sp. HN14 with BaP degradation ability was isolated from mangrove sediments in Dongzhai Port, Hainan Province. Our study showed that biodegradation efficiencies reached 42.15% after Pontibacillus sp. HN14 was cultured with 20 mg L^-1 BaP as the sole carbon source for 25 days and still had degradability of BaP at a 25% high salinity level. Moreover, 9,10-dihydrobenzo [a]pyrene-7(8H)-one, an intermediate metabolite, was detected during BaP degradation in the HN14 strain. Genome analysis identified a gene encoding the CYP102(HN14) enzyme. The results showed that the E. coli strain with CYP102(HN14) overexpression could transfer BaP to 9,10-dihydrobenzo [a]pyrene-7(8H)-one with a conversion rate of 43.5%, indicating that CYP102(HN14) played an essential role in BaP degradation in Pontibacillus sp. HN14. Thus, our results provide a novel BaP biodegradation molecule, which could be used in BaP bioremediation in high salinity conditions. This study is the first to show that CYP102(HN14) had the BaP oxidization ability in bacteria. CYP102(HN14) could be essential in removing PAHs in saline-alkali soil and other high salt environments through enzyme immobilization.

Benzo[a]pyrene exposure and overcrowding stress impacts anxiety-like behavior and impairs learning and memory in adult zebrafish, Danio rerio

Benzo[a]pyrene (B[a]P), a prototype of polycyclic aromatic hydrocarbons and ubiquitous environmental pollutant, alters neurobehavioral responses in aquatic organisms like zebrafish. Increasing organic load on water bodies causes population explosion leading to overcrowding (OC) stress. The effect of OC stress on neurobehavioral alterations remains unclear. The objective of our study is to elucidate the impact of OC stress on behavioral alterations and neurodegenerative phenotypes on exposure to B[a]P in zebrafish. We demonstrate the effects of OC stress (12 fish/L) on acute waterborne exposure to B[a]P (0.2 mg L^-1 ) in adult wild zebrafish. Anxiety-like behavior, learning, and memory impairment were assayed by novel tank diving test, light/dark preference test, and T-maze test. Oxidative stress bio-markers were assayed along with histopathological changes in zebrafish brain. OC stress significantly impaired the learning ability and mood behavior by increasing the number of transition and time spent in the alter zones. Increased lipid peroxidation and protein carbonyl formation with significant decreased catalase activity and reduced glutathione level showed oxidative stress on exposure to OC stress and B[a]P. Pyknotic neuronal counts dramatically increased in periventricular grey zone of optic tectum brain region of zebrafish. Our findings showed that OC stress modulates the B[a]P-induced behavioral alterations causing learning and memory deficiency with histopathological changes in adult zebrafish brain. OC stress may act as an early risk factor for the eventual development of cognitive impairments and B[a]P exposure plays a key role in mediating both the facilitating and impairing actions of OC stress in memory processes.

Human Family 1-4 cytochrome P450 enzymes involved in the metabolic activation of xenobiotic and physiological chemicals: an update

This is an overview of the metabolic activation of drugs, natural products, physiological compounds, and general chemicals by the catalytic activity of cytochrome P450 enzymes belonging to Families 1-4. The data were collected from > 5152 references. The total number of data entries of reactions catalyzed by P450s Families 1-4 was 7696 of which 1121 (~ 15%) were defined as bioactivation reactions of different degrees. The data were divided into groups of General Chemicals, Drugs, Natural Products, and Physiological Compounds, presented in tabular form. The metabolism and bioactivation of selected examples of each group are discussed. In most of the cases, the metabolites are directly toxic chemicals reacting with cell macromolecules, but in some cases the metabolites formed are not direct toxicants but participate as substrates in succeeding metabolic reactions (e.g., conjugation reactions), the products of which are final toxicants. We identified a high level of activation for three groups of compounds (General Chemicals, Drugs, and Natural Products) yielding activated metabolites and the generally low participation of Physiological Compounds in bioactivation reactions. In the group of General Chemicals, P450 enzymes 1A1, 1A2, and 1B1 dominate in the formation of activated metabolites. Drugs are mostly activated by the enzyme P450 3A4, and Natural Products by P450s 1A2, 2E1, and 3A4. Physiological Compounds showed no clearly dominant enzyme, but the highest numbers of activations are attributed to P450 1A, 1B1, and 3A enzymes. The results thus show, perhaps not surprisingly, that Physiological Compounds are infrequent substrates in bioactivation reactions catalyzed by P450 enzyme Families 1-4, with the exception of estrogens and arachidonic acid. The results thus provide information on the enzymes that activate specific groups of chemicals to toxic metabolites.

The Multifaceted Role of Epoxide Hydrolases in Human Health and Disease

Epoxide hydrolases (EHs) are key enzymes involved in the detoxification of xenobiotics and biotransformation of endogenous epoxides. They catalyze the hydrolysis of highly reactive epoxides to less reactive diols. EHs thereby orchestrate crucial signaling pathways for cell homeostasis. The EH family comprises 5 proteins and 2 candidate members, for which the corresponding genes are not yet identified. Although the first EHs were identified more than 30 years ago, the full spectrum of their substrates and associated biological functions remain partly unknown. The two best-known EHs are EPHX1 and EPHX2. Their wide expression pattern and multiple functions led to the development of specific inhibitors. This review summarizes the most important points regarding the current knowledge on this protein family and highlights the particularities of each EH. These different enzymes can be distinguished by their expression pattern, spectrum of associated substrates, sub-cellular localization, and enzymatic characteristics. We also reevaluated the pathogenicity of previously reported variants in genes that encode EHs and are involved in multiple disorders, in light of large datasets that were made available due to the broad development of next generation sequencing. Although association studies underline the pleiotropic and crucial role of EHs, no data on high-effect variants are confirmed to date.

Tumor suppressor gene glycine N-methyltransferase and its potential in liver disorders and hepatocellular carcinoma

Glycine N-methyltransferase is a protein with many functions. In addition to catalyzing the production of sarcosine in the one carbon metabolism pathway, it plays a role in the detoxification of environmental carcinogens such as benzo[a]pyrene, aflatoxin B1, and aristocholic acid. There is also increasing evidence suggesting a role of GNMT deficiency in liver carcinogenesis. In this review, we discuss the role of GNMT in the detoxification of xenobiotics and the mechanism of GNMT suppression during liver tumorigenesis. The protective role of GNMT in the liver allows GNMT to not only serve as a marker of liver disease, but also potentially be applied in the treatment of liver disorders and hepatocellular carcinoma. We describe the potential use of GNMT in gene therapy and we introduce the development of a GNMT promoter reporter assay that can be used to screen medicinal drugs and herbal libraries for natural compounds with anti-cancer properties.

Association of Activity Altering Genotypes - Tyr113His and His139Arg in Microsomal Epoxide Hydrolase Enzyme with Esophageal Squamous Cell Carcinoma

The study aimed to explore the relationship of microsomal epoxide hydrolase (mEH) exon 3 (Tyr113His) and exon 4 (His139Arg) polymorphisms and predicted mEH activity with esophageal squamous cell carcinoma (ESCC) risk. 482 histologically confirmed cases and equal number of matched controls were analyzed by polymerase chain reaction-restriction length polymorphism (PCR-RFLP). Conditional logistic regression models were used to examine the association of polymorphisms with ESCC. We noted exon 3 slow genotype (OR = 6.57; CI 3.43-12.57) as well as predicted low mEH activity (OR = 3.99; CI 2.32-6.85) was associated with the ESCC risk. Elevated ESCC risk estimates were seen in smokers independent of genotypes but the association was stronger among smokers with exon 3 variant (OR = 6.67; 3.29-13.53) and low activity (OR = 7.52; CI 3.46-16.37) genotypes. Positive family history of cancer synergistically increased ESCC risk in the individuals who harbored exon 3 (OR = 13.59; CI 5.63-32.81) or altered mEH activity genotypes (OR = 13.35; CI 5.10-34.94). Significant interaction was seen between mEH exon 3 and exon 4 genotypes (P = 0.006) and between predicted mEH activity and positive family history of cancer (P = 0.018). These findings suggest association of ESCC risk with mEH polymorphisms which get modified by tobacco smoking and positive family history of cancer.

Development and Uses of Offline and Web-Searchable Metabolism Databases - The Case of Benzo[a]pyrene

BACKGROUND

The present work describes development of offline and web-searchable metabolism databases for drugs, other chemicals, and physiological compounds using human and model species, prompted by the large amount of data published after year 1990. The intent was to provide a rapid and accurate approach to published data to be applied both in science and to assist therapy.

METHODS

Searches for the data were done using the Pub Med database, accessing the Medline database of references and abstracts. In addition, data presented at scientific conferences (e.g., ISSX conferences) are included covering the publishing period beginning with the year 1976.

RESULTS

Application of the data is illustrated by the properties of benzo[a]pyrene (B[a]P) and its metabolites. Analysis show higher activity of P450 1A1 for activation of the (-)- isomer of trans-B[a]P-7,8-diol, while P4501B1 exerts higher activity for the (+)- isomer. P450 1A2 showed equally low activity in the metabolic activation of both isomers.

CONCLUSION

The information collected in the databases is applicable in prediction of metabolic drug-drug and/or drug-chemical interactions in clinical and environmental studies. The data on the metabolism of searched compound (exemplified by benzo[a]pyrene and its metabolites) also indicate toxicological properties of the products of specific reactions. The offline and web-searchable databases had wide range of applications (e.g. computer assisted drug design and development, optimization of clinical therapy, toxicological applications) and adjustment in everyday life styles.

Ligand Access Channels in Cytochrome P450 Enzymes: A Review

Quantitative structure-activity relationships may bring invaluable information on structural elements of both enzymes and substrates that, together, govern substrate specificity. Buried active sites in cytochrome P450 enzymes are connected to the solvent by a network of channels exiting at the distal surface of the protein. This review presents different in silico tools that were developed to uncover such channels in P450 crystal structures. It also lists some of the experimental evidence that actually suggest that these predicted channels might indeed play a critical role in modulating P450 functions. Amino acid residues at the entrance of the channels may participate to a first global ligand recognition of ligands by P450 enzymes before they reach the buried active site. Moreover, different P450 enzymes show different networks of predicted channels. The plasticity of P450 structures is also important to take into account when looking at how channels might play their role.

High Conservation in Transcriptomic and Proteomic Response of White Sturgeon to Equipotent Concentrations of 2,3,7,8-TCDD, PCB 77, and Benzo[a]pyrene

Adverse effects associated with exposure to dioxin-like compounds (DLCs) are mediated primarily through activation of the aryl hydrocarbon receptor (AHR). However, little is known about the cascades of events that link activation of the AHR to apical adverse effects. Therefore, this study used high-throughput, next-generation molecular tools to investigate similarities and differences in whole transcriptome and whole proteome responses to equipotent concentrations of three agonists of the AHR, 2,3,7,8-TCDD, PCB 77, and benzo[a]pyrene, in livers of a nonmodel fish, the white sturgeon (Acipenser transmontanus). A total of 926 and 658 unique transcripts were up- and down-regulated, respectively, by one or more of the three chemicals. Of the transcripts shared by responses to all three chemicals, 85% of up-regulated transcripts and 75% of down-regulated transcripts had the same magnitude of response. A total of 290 and 110 unique proteins were up- and down-regulated, respectively, by one or more of the three chemicals. Of the proteins shared by responses to all three chemicals, 70% of up-regulated proteins and 48% of down-regulated proteins had the same magnitude of response. Among treatments there was 68% similarity between the global transcriptome and global proteome. Pathway analysis revealed that perturbed physiological processes were indistinguishable between equipotent concentrations of the three chemicals. The results of this study contribute toward more completely describing adverse outcome pathways associated with activation of the AHR.

Microsomal epoxide hydrolase 1 (EPHX1): Gene, structure, function, and role in human disease

Microsomal epoxide hydrolase (EPHX1) is an evolutionarily highly conserved biotransformation enzyme for converting epoxides to diols. Notably, the enzyme is able to either detoxify or bioactivate a wide range of substrates. Mutations and polymorphic variants in the EPHX1 gene have been associated with susceptibility to several human diseases including cancer. This review summarizes the key knowledge concerning EPHX1 gene and protein structure, expression pattern and regulation, and substrate specificity. The relevance of EPHX1 for human pathology is especially discussed.

Caffeine induces high expression of cyp-35A family genes and inhibits the early larval development in Caenorhabditis elegans

Intake of caffeine during pregnancy can cause retardation of fetal development. Although the significant influence of caffeine on animal development is widely recognized, much remains unknown about its mode of action because of its pleiotropic effects on living organisms. In the present study, by using Caenorhabditis elegans as a model organism, the effects of caffeine on development were examined. Brood size, embryonic lethality, and percent larval development were investigated, and caffeine was found to inhibit the development of C. elegans at most of the stages in a dosage-dependent fashion. Upon treatment with 30 mM caffeine, the majority (86.1 ± 3.4%) of the L1 larvae were irreversibly arrested without further development. In contrast, many of the late-stage larvae survived and grew to adults when exposed to the same 30 mM caffeine. These results suggest that early-stage larvae are more susceptible to caffeine than later-stage larvae. To understand the metabolic responses to caffeine treatment, the levels of expression of cytochrome P450 (cyp) genes were examined with or without caffeine treatment using comparative micro-array, and it was found that the expression of 24 cyp genes was increased by more than 2-fold (p < 0.05). Among them, induction of the cyp-35A gene family was the most prominent. Interestingly, depletion of the cyp-35A family genes one-by-one or in combination through RNA interference resulted in partial rescue from early larval developmental arrest caused by caffeine treatment, suggesting that the high-level induction of cyp-35A family genes can be fatal to the development of early-stage larvae.

The role of epoxide hydrolases in health and disease

Epoxide hydrolases (EH) are ubiquitously expressed in all living organisms and in almost all organs and tissues. They are mainly subdivided into microsomal and soluble EH and catalyze the hydration of epoxides, three-membered-cyclic ethers, to their corresponding dihydrodiols. Owning to the high chemical reactivity of xenobiotic epoxides, microsomal EH is considered protective enzyme against mutagenic and carcinogenic initiation. Nevertheless, several endogenously produced epoxides of fatty acids function as important regulatory mediators. By mediating the formation of cytotoxic dihydrodiol fatty acids on the expense of cytoprotective epoxides of fatty acids, soluble EH is considered to have cytotoxic activity. Indeed, the attenuation of microsomal EH, achieved by chemical inhibitors or preexists due to specific genetic polymorphisms, is linked to the aggravation of the toxicity of xenobiotics, as well as the risk of cancer and inflammatory diseases, whereas soluble EH inhibition has been emerged as a promising intervention against several diseases, most importantly cardiovascular, lung and metabolic diseases. However, there is reportedly a significant overlap in substrate selectivity between microsomal and soluble EH. In addition, microsomal and soluble EH were found to have the same catalytic triad and identical molecular mechanism. Consequently, the physiological functions of microsomal and soluble EH are also overlapped. Thus, studying the biological effects of microsomal or soluble EH alterations needs to include the effects on both the metabolism of reactive metabolites, as well as epoxides of fatty acids. This review focuses on the multifaceted role of EH in the metabolism of xenobiotic and endogenous epoxides and the impact of EH modulations.

Association of microsomal epoxide hydrolase exon 3 Tyr113His and exon 4 His139Arg polymorphisms with gastric cancer in India

BACKGROUND

Microsomal epoxide hydrolase, an important phase II xenobiotic enzyme, exhibits polymorphisms at exon 3 (Tyr113His [T/C]) and exon 4 (His139Arg [A/G]), which modulate enzyme activity; this may affect susceptibility to cancers. We studied association between these polymorphisms and gastric cancer (GC).

METHODS

In a prospective study, 77 patients with GC, 50 with peptic ulcer, and 160 healthy controls (HC) were genotyped for exon 3 (PCR-RFLP followed by sequencing) and exon 4 (PCR-RFLP). Helicobacter pylori was considered to be present if two of three tests (histology, rapid urease test, and IgG antibody) were positive.

RESULTS

Tyr113His and His139Arg genotypes and haplotypes were comparable among groups. 113His carriers were commoner among H. pylori-negative patients with GC than HC (p-value = 0.019, odds ratio (OR) = 2.5, 95 % confidence interval (CI) = 1.2-5.4). Haplotype combination of exons 3 and 4 113Tyr-139Arg (TA) were associated with higher and reduced risk in patients with GC than HC in presence and absence of H. pylori (25 % vs. 11 %; p-value = 0.033, OR = 2.61, 95 % CI = 1.08-6.3 and 11.6 % vs. 28.7 %; p-value = 0.004, OR = 0.33, 95 % CI = 0.15-0.7, respectively).

CONCLUSIONS

Though 113Tyr-139Arg was associated with GC in presence of H. pylori, in its absence, it appeared to be protective. Exon 3, 113His, however, was associated with GC even in absence of H. pylori infection.

mEH Tyr113His polymorphism and the risk of ovarian cancer development

Background

The causes of ovarian cancer are complex and may be influenced by many factors, including polymorphism in the microsomal epoxide hydrolase (mEH) gene. Previous work suggests an association between the Tyr113His mEH polymorphism rs1051740 and susceptibility to ovarian cancer, but the results have been inconsistent.

Methods

PubMed, EMBASE, Google Scholar, and Chinese National Knowledge Infrastructure databases were systematically searched to identify relevant studies. A meta-analysis was performed to examine the association between Tyr113His mEH polymorphism and susceptibility to ovarian cancer. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated.

Results

Five studies involving 2,566 cases and 2,839 controls were included. Although the polymorphism did not affect ovarian cancer risk in the allelic contrast model (OR = 0.99, 95% CI = 0.83-1.17, P = 0.86), the mutant CC genotype was significantly associated with increased risk in the homozygote comparison (OR = 1.20, 95% CI = 1.01-1.43, P = 0.04) and recessive genetic models (OR = 1.20, 95% CI = 1.01-1.41, P = 0.03). The wild-type TT genotype was not associated with higher or lower ovarian cancer risk in the dominant genetic model (OR = 1.04, 95% CI = 0.83-1.29, P = 0.74). These results were robust to sensitivity analysis.

Conclusions

The CC genotype of Tyr113His mEH may confer increased risk of ovarian cancer. These conclusions should be verified in large and well-designed studies.

Meta-analysis of microsomal epoxide hydrolase gene polymorphism and risk of hepatocellular carcinoma

Background

Hepatocarcinogenesis is a complex process that may be influenced by many factors, including polymorphism in microsomal epoxide hydrolase (mEH). Previous work suggests an association between the Tyr113His and His139Arg mEH polymorphisms and susceptibility to hepatocellular carcinoma (HCC), but the results have been inconsistent.

Methods

PubMed, EMBASE, Google Scholar and the Chinese National Knowledge Infrastructure databases were systematically searched to identify relevant studies. A meta-analysis was performed to examine the association between Tyr113His and His139Arg mEH polymorphism and susceptibility to HCC. Odds ratios (ORs) and 95% confidence intervals (95% CIs) were calculated.

Results

Eleven studies were included in the meta-analysis, involving 1,696 HCC cases and 3,600 controls. The 113His- mEH allele was significantly associated with increased risk of HCC based on allelic contrast (OR = 1.35, 95% CI = 1.04–1.75, p = 0.02), homozygote comparison (OR = 1.65, 95% CI = 1.07–2.54, p = 0.02) and a recessive genetic model (OR = 1.54, 95% CI = 1.21–1.96, p<0.001), while individuals carrying the Arg139Arg mEH genotype had no association with increased or decreased risk of HCC.

Conclusion

The 113His- allele polymorphism in mEH may be a risk factor for hepatocarcinogenesis, while the mEH 139Arg- allele may not be a risk or protective factor. There is substantial evidence that mEH polymorphisms interact synergistically with other genes and the environment to modulate risk of HCC. Further large and well-designed studies are needed to confirm these conclusions.

Preferred binding orientations of phenacetin in CYP1A1 and CYP1A2 are associated with isoform-selective metabolism

Human cytochromes P450 1A1 and 1A2 play important roles in drug metabolism and chemical carcinogenesis. Although these two enzymes share high sequence identity, they display different substrate specificities and inhibitor susceptibilities. In the present studies, we investigated the structural basis for these differences with phenacetin as a probe using a number of complementary approaches, such as enzyme kinetics, stoichiometric assays, NMR, and molecular modeling. Kinetic and stoichiometric analyses revealed that substrate specificity (k(cat)/K(m)) of CYP1A2 was approximately 18-fold greater than that of CYP1A1, as expected. Moreover, despite higher H₂O₂ production, the coupling efficiency of reducing equivalents to acetaminophen formation in CYP1A2 was tighter than that in CYP1A1. CYP1A1, in contrast to CYP1A2, displayed much higher uncoupling, producing more water. The subsequent NMR longitudinal (T₁) relaxation studies with the substrate phenacetin and its product acetaminophen showed that both compounds displayed similar binding orientations within the active site of CYP1A1 and CYP1A2. However, the distance between the OCH₂ protons of the ethoxy group (site of phenacetin O-deethylation) and the heme iron was 1.5 Å shorter in CYP1A2 than in CYP1A1. The NMR findings are thus consistent with our kinetic and stoichiometric results, providing a likely molecular basis for more efficient metabolism of phenacetin by CYP1A2.

Microsomal Epoxide Hydrolase Polymorphisms and Haplotypes as Determinants of Hepatitis B Virusand Hepatitis C Virus-related Liver Disease in Indian Population

BACKGROUND

Hepatocellular carcinoma (HCC) is the fifth most common cancer and third leading cause of death worldwide. Main causes of HCC are hepatitis B virus (HBV) and hepatitis C virus (HCV) infections. mEPHX, a xenobiotic metabolizing enzyme, exhibits a dual role of procarcinogen detoxification and activation, hence considered as a cancer risk factor as well as a protective factor. Two known polymorphic forms of mEPHX, exon in exon 3 and 4, are associated with the development of HCC.

OBJECTIVE

To determine the association of genotypes and haplotypes of mEPHX with risk of HCC developments separately in HBV- and HCV-infected carriers and patients with hepatitis.

METHODS

Polymerase chain reactions (PCR) were carried out using primers to amplify exon 3 (113 Tyr→His variant) and exon 4 (139 His→Arg) polymorphic sites. To distinguish the wild and variant forms, PCR amplification products were digested with restriction endonucleases EcoRV and Rsa1 for exons 3 and 4, respectively.

RESULT

Exon 3 genotypes, Y113H and H113H, shared a protective association with HBV-chronic hepatitis infection (P < 0.001 and P< 0.01, respectively) as well as HBV-HCC development (P < 0.001) among HBV-carrier group, while Y113H acts as a risk factor for HCV-chronic hepatitis development (P < 0.001) as well as for HCC development (P < 0.01) with HCV-carrier group as reference. Both H139R and R139R, exon 4 genotypes, acted as a risk factor for HBV/HCV-chronic hepatitis infection and for HBV/HCV-HCC development (P ranges from < 0.05 to < 0.001) with HBV/HCV carriers as reference. 113His-139His and 113His-139Arg haplotypes shared a significant negative and positive association, respectively, with HBV hepatitis and HBV-HCC risk. 113Tyr-139Arg haplotype acted as a risk for HCV-HCC development.

CONCLUSION

Polymorphic and haplotypic variant forms of mEPHX exon 3 and 4 variably determine the susceptibility to develop HCC in HBV- and HCV-carrier subjects.

Structure, function, regulation and polymorphism and the clinical significance of human cytochrome P450 1A2

Human CYP1A2 is one of the major CYPs in human liver and metabolizes a number of clinical drugs (e.g., clozapine, tacrine, tizanidine, and theophylline; n > 110), a number of procarcinogens (e.g., benzo[a]pyrene and aromatic amines), and several important endogenous compounds (e.g., steroids). CYP1A2 is subject to reversible and/or irreversible inhibition by a number of drugs, natural substances, and other compounds. The CYP1A gene cluster has been mapped on to chromosome 15q24.1, with close link between CYP1A1 and 1A2 sharing a common 5'-flanking region. The human CYP1A2 gene spans almost 7.8 kb comprising seven exons and six introns and codes a 515-residue protein with a molecular mass of 58,294 Da. The recently resolved CYP1A2 structure has a relatively compact, planar active site cavity that is highly adapted for the size and shape of its substrates. The architecture of the active site of 1A2 is characterized by multiple residues on helices F and I that constitutes two parallel substrate binding platforms on either side of the cavity. A large interindividual variability in the expression and activity of CYP1A2 has been observed, which is largely caused by genetic, epigenetic and environmental factors (e.g., smoking). CYP1A2 is primarily regulated by the aromatic hydrocarbon receptor (AhR) and CYP1A2 is induced through AhR-mediated transactivation following ligand binding and nuclear translocation. Induction or inhibition of CYP1A2 may provide partial explanation for some clinical drug interactions. To date, more than 15 variant alleles and a series of subvariants of the CYP1A2 gene have been identified and some of them have been associated with altered drug clearance and response and disease susceptibility. Further studies are warranted to explore the clinical and toxicological significance of altered CYP1A2 expression and activity caused by genetic, epigenetic, and environmental factors.

Mammalian epoxide hydrolases in xenobiotic metabolism and signalling

Epoxide hydrolases catalyse the hydrolysis of electrophilic--and therefore potentially genotoxic--epoxides to the corresponding less reactive vicinal diols, which explains the classification of epoxide hydrolases as typical detoxifying enzymes. The best example is mammalian microsomal epoxide hydrolase (mEH)-an enzyme prone to detoxification-due to a high expression level in the liver, a broad substrate selectivity, as well as inducibility by foreign compounds. The mEH is capable of inactivating a large number of structurally different, highly reactive epoxides and hence is an important part of the enzymatic defence of our organism against adverse effects of foreign compounds. Furthermore, evidence is accumulating that mammalian epoxide hydrolases play physiological roles other than detoxification, particularly through involvement in signalling processes. This certainly holds true for soluble epoxide hydrolase (sEH) whose main function seems to be the turnover of lipid derived epoxides, which are signalling lipids with diverse functions in regulatory processes, such as control of blood pressure, inflammatory processes, cell proliferation and nociception. In recent years, the sEH has attracted attention as a promising target for pharmacological inhibition to treat hypertension and possibly other diseases. Recently, new hitherto uncharacterised epoxide hydrolases could be identified in mammals by genome analysis. The expression pattern and substrate selectivity of these new epoxide hydrolases suggests their participation in signalling processes rather than a role in detoxification. Taken together, epoxide hydrolases (1) play a central role in the detoxification of genotoxic epoxides and (2) have an important function in the regulation of physiological processes by the control of signalling molecules with an epoxide structure.

Glutathione-S-transferase and microsomal epoxide hydrolase polymorphism and viral-related hepatocellular carcinoma risk in India

Hepatocellular carcinoma (HCC) is the fourth most common cancer worldwide, the main etiological factors being chronic infections with hepatitis B and C viruses. Genetic polymorphic forms of glutathione-S-transferase (GST) and microsomal epoxide hydrolase (mEPHX) have been associated with risk for various malignancies. The present study was undertaken to evaluate the association of GSTT1 and GSTM1 null genotypes and mEPHX polymorphisms with hepatitis virus-related HCC risk in an Indian population. Three groups of subjects were considered, control (n = 169), chronic viral hepatitis (n = 174), and HCC (n = 63). Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) was used for this polymorphic study. Genotype distributions between categories were compared using the chi2 test; odds ratios (ORs) and 95% confidence interval were calculated to express the relative risk. GSTT1 null genotype was associated with 2.23-fold (p < 0.05) increased risk for HCC development as compared to the control group. However, GSTM1 null genotype was found to have a protective effect when hepatitis patients were considered. In case of mEPHX, R139R imposed a risk factor for HCC with both control (OR = 1.81) and chronic hepatitis-infected (OR = 2.06) subjects. Combination of heterozygous mutant genotypes at mEPHX exons 3 and 4 revealed a twofold risk (nonsignificant) for HCC. Further, combination of GSTM1 and T1 genotypes with either of exon 3 or 4 polymorphism of mEPHX displayed synergistic associations (risk or protective) for HCC development. GST and mEPHX variants share a positive association with viral-related HCC risk in Indian population, although a larger sample size is still required to confirm the results.

The Endoplasmic Reticulum in Xenobiotic Toxicity

The endoplasmic reticulum (ER) is involved in an array of cellular functions that play important roles in xenobiotic toxicity. The ER contains the majority of cytochrome P450 enzymes involved in xenobiotic metabolism, as well as a number of conjugating enzymes. In addition to its role in drug bioactivation and detoxification, the ER can be a target for damage by reactive intermediates leading to cell death or immune-mediated toxicity. The ER contains a set of luminal proteins referred to as ER stress proteins (including GRP78, GRP94, protein disulfide isomerase, and calreticulin). These proteins help regulate protein processing and folding of membrane and secretory proteins in the ER, calcium homeostasis, and ER-associated apoptotic pathways. They are induced in response to ER stress. This review discusses the importance of the ER in molecular events leading to cell death following xenobiotic exposure. Data showing that the ER is important in both renal and hepatic toxicity will be discussed.

CYP1A1, mEH, and GSTM1 Polymophisms and Risk of Oral and Pharyngeal Cancer: A Spanish Case-Control Study

Background. Genetic polymorphisms of drug metabolizing enzymes involved in the detoxification pathways of carcinogenic substances may influence cancer risk. Methods. Case-control study that investigates the relationship between CYP1A1 Ile/Val, exon 4 mEH, and GSTM1 null genetic polymorphism and the risk of oral and pharyngeal cancer examining the interaction between these genes, tobacco, and alcohol. 92 incident cases and 130 consecutive hospital-based controls have been included. Results. No significant associations were found for any of the genotypes assessed. The estimated risk was slightly elevated in subjects with the wild type of the mEH gene and the null GSTM1 genotype. For exon 4 mEH heterozygous polymorphism, the risk was slightly lower for heavy smokers than for light smokers. The inverse association was observed for the GSTM1 null genotype. Conclusions. The results suggest that exon 4 mEH and GSTM1 null polymorphisms might influence oral and pharyngeal cancer.

Interaction between CYP1A2-T2467DELT polymorphism and smoking in adenocarcinoma and squamous cell carcinoma of the lung

This study aimed to identify new genetic characteristics contributing to individual susceptibility to smoke-induced lung cancer. Despite functional evidence of a possible role of cytochrome P450 1A2 (CYP1A2) in lung cancer susceptibility, no studies have evaluated the influence of CYP1A2 genotypes on lung cancer risk. We investigated the interaction between CYP1A2-T2467delT (allele*1D) polymorphism and smoking in Tunisian lung cancer cases (n=101 male smokers) separately for the histological types squamous cell carcinoma (SCC) (n=60) and adenocarcinoma (n=41), and in controls (n=98 male smokers) using a case-only study design. A significant interaction between CYP1A2-T/delT or delT/delT genotypes and tobacco consumption (pack-years) adjusted for age was evident (OR (95% CI) 7.78 (1.52-42.8)) in the SCC cases who smoked relatively less (< or =33 pack-years, I quartile value), but not in adenocarcinoma and controls. Our results suggest that CYP1A2-T2467delT polymorphism has an important role in lung carcinogenesis, especially SCC, among smokers.

EPHX1 gene polymorphisms and individual susceptibility to lung cancer

We investigated the roles of EPHX1 Tyr113His and His139Arg polymorphisms in lung cancer susceptibility in a Finnish study population comprising of 230 lung cancer cases and a large control group (n=2105). The controls were distributed into five age strata, which enabled us to examine the potential age-related changes in the putative EPHX1 at-risk genotypes in the cancer free population. Although the exon 3 slow activity associated allele (His113) containing genotypes posed a decreased lung cancer risk compared with the homozygous wild-type Tyr113/Tyr113 genotype (OR, 0.68; 95% CI, 0.49-0.94), no association was seen for the EPHX1 phenotypes interpreted from the combined exons 3 and 4 genotype data. Neither was any difference seen in the prevalence of the EPHX1 Tyr113His genotypes or interpreted EPHX1 phenotypes in the different age groups.

On the species-specific biotransformation of dibenzo[a,l]pyrene

We were aimed at investigating the activation of the carcinogenic polycyclic aromatic hydrocarbon (PAH) dibenzo[a,l]pyrene (DB[a,l]P) in Chinese hamster V79 cells that express single human, rat or fish cytochrome P450 (CYP) enzymes. DB[a,l]P is detectable in environmental samples and has been characterized as the most potent carcinogenic species among all PAHs as yet tested in rodent bioassays. Metabolite profiles and metabolite-dependent cytotoxic and clastogenic activities were monitored. The total turnover of CYP-mediated transformation of DB[a,l]P was as follows: human CYP1B1>fish CYP1A1 approximately human CYP1A1>>rat CYP1A2>rat CYP1A1. By contrast, enzyme forms that are not classified as being members of family CYP1, such as CYP2A6, 2E1, 2B1, and 3A4, failed to catalyze any detectable conversion of this substrate. All CYP1A1 enzymes tested formed both the K-region trans-8,9- and the trans-11,12-dihydrodiol, whereas human CYP1B1 failed to catalyze K-region activation. In cells expressing human or fish CYP1A1, human CYP1B1, and rat CYP1A2, the (-)-trans-11,12-dihydrodiol was formed enantiospecifically. DB[a,l]P-dependent cytotoxicities (EC(50)) were found in the following order: human CYP1A1 (12 nM)>fish CYP1A1 (30 nM)>human CYP1B1 (45 nM)>>other forms. In addition, an appreciable micronuclei formation was detected in human CYP1A1- and 1B1-expressing cells during exposure to DB[a,l]P. Our study demonstrates that human CYP1A1, 1B1 and fish CYP1A1 are able to transform DB[a,l]P into genotoxic derivatives in appreciable amounts. In contrast, CYP enzymes from rat predominantly target the K-region of DB[a,l]P and thus are serving more a rather protective route of biotransformation. Together our data suggest that humans might be more susceptible to DB[a,l]P-induced carcinogenicity than rats.

Influence of the genetic polymorphism in the 5′-noncoding region of the CYP1A2 gene on CYP1A2 phenotype and urinary mutagenicity in smokers

The functional significance of genetic polymorphisms on tobacco smoke-induced CYP1A2 activity was examined. The influence of three polymorphisms of the cytochrome P450 1A2 gene (CYP1A2) (-3860 G-->A (allele *1C), -2467 T-->delT (allele *1D), -163C-->A (allele *1F)), located in the 5'-noncoding promoter region of the gene, on CYP1A2 activity (measured as caffeine metabolic ratio, CMR), was studied in Caucasian current smokers (n=95). Tobacco smoke intake was calculated from the number of cigarettes/day. Also, studied was the influence of these CYP1A2 genotypes on smoking-associated urinary mutagenicity, detected in Salmonella typhimurium strain YG1024 with S9 mix, considering the urinary excretion of nicotine plus its metabolites as an internal indicator of tobacco smoke exposure. Smokers with at least one of the variant alleles CYP1A2 -3860A and -2467 delT showed a significantly increased CYP1A2 CMR (-3860 G/A versus G/G, p<0.05; -2467 delT/delT versus T/delT and T/T, p<0.01). Multiple regression analysis showed that the increase in CYP1A2 CMR (ln values) was again significantly related to the presence of CYP1A2 variants -2467delT and also to variant -163A (p<0.05), but moderately to -3860A (p=0.084). No influence of the number of cigarettes smoked per day by each subject was found. Heavy smokers (n=48, with urinary nicotine plus its metabolites>or=0.69 mg/mmol creatinine) with variant allele -2467delT or -163A had significantly increased urinary mutagenicity (p<0.01 and <0.05). CYP1A2 genetic polymorphisms are shown to influence the CYP1A2 phenotype in smokers, -2467 T-->delT having the main effect. This information is of interest for future studies assessing the possible role of tobacco smoke-inducible CYP1A2 genotypes as individual susceptibility factors in exposure to carcinogens.

Genetic polymorphism of CYP1A2 increases the risk of myocardial infarction

BACKGROUND

There is growing evidence that DNA damage caused by mutagens found in tobacco smoke may contribute to the development of coronary heart disease (CHD). In order to bind to DNA many mutagens require metabolic activation by cytochrome P450 (CYP) 1A1 or CYP1A2. The objective of this study was to determine the effects of CYP1A1 and CYP1A2 genotypes on risk of myocardial infarction (MI) and whether smoking interacts with genotype to modify risk.

METHODS

Subjects (n = 873) with a first acute non-fatal MI and population based controls (n = 932) living in Costa Rica, matched for age, sex, and area of residence, were genotyped for CYP1A1*2A and CYP1A2*1F by restriction-fragment length polymorphism (RFLP)-PCR, and smoking status was determined by questionnaire.

RESULTS

After adjusting for matching variables and potential confounders, no association was observed between CYP1A1 genotype and risk of MI. Compared to individuals with the high inducibility CYP1A2*1A/*1A genotype, the adjusted odds ratio and 95% confidence intervals for risk of MI were 1.19 (0.97 to 1.47) for the *1A/*1F genotype and 1.55 (1.10 to 2.18) for the *1F/*1F genotype. No significant interactions were observed between smoking and either CYP1A1 or CYP1A2 genotype.

CONCLUSIONS

The low inducibility genotype for CYP1A2 was associated with an increased risk of MI. This effect was independent of smoking status and suggests that a substrate of CYP1A2 that is detoxified rather than activated may play a role in CHD.

Functional analysis of human microsomal epoxide hydrolase genetic variants

Human microsomal epoxide hydrolase (EPHX1) is active in the metabolism of many potentially carcinogenic or otherwise genotoxic epoxides, such as those derived from the oxidation of polyaromatic hydrocarbons. EPHX1 is polymorphic and encodes allelic variation at least two amino acid positions, Y113H and H139R. In a number of recent molecular epidemiological investigations, EPHX1 polymorphism has been suggested as a susceptibility factor for several human diseases. To better evaluate the functional contribution of EPHX1 genetic polymorphism, we characterized the enzymatic properties associated with each of the respective variant proteins. Enzymatic profiles were evaluated with cis-stilbene oxide (cSO) and benzo[a]pyrene-4,5-epoxide (BaPO), two prototypical substrates for the hydrolase. In one series of experiments, activities of recombinant EPHX1 proteins were analyzed subsequent to their expression using the pFastbac baculovirus vector in Spodoptera frugiperda-9 (Sf9) insect cells, and purification by column chromatography. In parallel studies, EPHX1 activities were evaluated with human liver microsomes derived from individuals of known EPHX1 genotype. Using the purified protein preparations, rates of cSO and BaPO hydrolysis for the reference protein, Y113/H139, were approximately 2-fold greater than those measured with the other EPHX1 allelic variants. However, when activities were analyzed using human liver microsomal fractions, no major differences were evident in the reaction rates generated among preparations representing the different EPHX1 alleles. Collectively, these results suggest that the structural differences encoded by the Y113H and H139R variant alleles exert only modest impact on EPHX1-specific enzymatic activities in vivo.

Metabolic fate of the Ah receptor ligand 6-formylindolo[3,2-b]carbazole

The physiological role of the aryl hydrocarbon receptor (AhR), a member of the basic helix-loop-helix PER-ARNT-SIM (PAS) transcription factor family is not known. We have suggested that the AhR is involved in light signaling through binding of photoproducts with high AhR affinity. This suggestion is based on (i) the high AhR affinity of the tryptophan photoproduct formylindolo[3,2-b]carbazole (FICZ), (ii) the induction of rapid and transient expression of AhR-regulated genes by FICZ and by extracts of UV-irradiated tryptophan as well as (iii) the fact that light induces the AhR-regulated cytochrome P450s CYP1A1, CYP1B1 and CYP2S1. The transient mRNA expression caused by light and tryptophan photoproducts suggests that the biotransformation enzymes induced by AhR activation take part in a metabolic degradation of the natural AhR ligand. This study aimed at identifying the involvement of phase I and phase II enzymes in the metabolic degradation of FICZ. A cytochrome P450-dependent metabolism of FICZ giving rise to preferentially mono- and di-hydroxylated derivatives has earlier been reported. In the present study, rat and human hepatic S9 mixes were employed together with specific enzyme inhibitors and cofactors. Compared to the Aroclor-induced rat liver S9, the non-induced rat liver S9 and the human liver S9 caused a more complex metabolite profile of FICZ. The CYP1A1 enzyme was confirmed to be the most important enzyme for the first step in the metabolism. CYP1A2 was found to have overlapping specificity with CYP1A1 being able to form the same major metabolites although with different kinetics. CYP1B1 turned out to be preferentially involved in the further metabolism of dihydroxylated metabolites. Microsomal epoxide hydrolase, and as yet not identified forms of sulphotransferases and glucuronosyltransferases were also found to take part in the metabolic degradation of FICZ. Thus, tryptophan photoproducts fit into a model in which the ligand-activated AhR signaling is autoregulated by the induced metabolic enzymes.

The effect of reciprocal active site mutations in human cytochromes P450 1A1 and 1A2 on alkoxyresorufin metabolism

Five reciprocal active site mutants of P450 1A1 and 1A2 and an additional mutant, Val/Leu-382 --> Ala, were constructed, expressed in Escherichia coli, and purified by Ni-NTA affinity chromatography. In nearly every case, the residue replacement led to loss of 7-methoxy- and 7-ethoxyresorufin O-dealkylase activity compared to the wild-type enzymes, except for the P450 1A1 S122T mutation which increased both activities. Mutations at position 382 in both P450 1A1 and 1A2 shifted substrate specificity from one enzyme to another, confirming the importance of this residue. Changes in activity of P450 1A enzymes upon amino acid replacement were, in general, consistent with molecular dynamics analyses of substrate motion in the active site of homology models.

Insulin and glucagon signaling in regulation of microsomal epoxide hydrolase expression in primary cultured rat hepatocytes

Microsomal epoxide hydrolase (mEH) plays an important role in the detoxification of a broad range of epoxide intermediates and has been reported to be decreased during diabetes and fasting. The signaling pathways involved in the regulation of mEH expression in response to insulin and glucagon were examined in primary cultured rat hepatocytes. mEH protein levels were increased 2- to 6-fold in hepatocytes cultured for 1 to 4 days, respectively, in the presence of insulin. Concentration-response studies revealed that insulin concentrations >or=1 nM resulted in increased mEH protein levels. The phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin or LY294002 [2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one], and rapamycin, an inhibitor of p70 S6 kinase phosphorylation, ameliorated the insulin-mediated increase in mEH protein levels. The p38 mitogen-activated protein (MAP) kinase inhibitors SB203580 and SB202190 also abrogated the insulin-mediated increase in mEH protein. Treatment of cells with glucagon, 8-bromo-cAMP, or dibutyryl-cAMP for 3 days resulted in decreased mEH protein levels. Pretreatment with the protein kinase A (PKA) inhibitor H89 (N-[2-(4-bromocinnamylamino)ethyl]-5-isoquinoline) prior to glucagon addition markedly attenuated the glucagon effect, implicating PKA signaling in the regulation of mEH expression. These data demonstrate that insulin and glucagon regulate, in an opposing manner, the expression of mEH in primary cultured rat hepatocytes. Furthermore, these data suggest that PI3K and p70 S6 kinase are active in the regulation of insulin-mediated mEH expression. We also provide data implicating p38 MAP kinase in the insulin-mediated increase in mEH levels. Moreover, cAMP and PKA are implicated in mediating the inhibitory effect of glucagon on mEH expression.

Association of microsomal epoxide hydrolase polymorphisms and lung cancer risk

Microsomal epoxide hydrolase (mEH) plays a dual role in the detoxification and activation of tobacco procarcinogens. Two polymorphisms affecting enzyme activity have been described in the exons 3 and 4 of the mEH gene, which result in the substitution of amino acids histidine to tyrosine at residue 113, and arginine to histidine at residue 139, respectively. We performed a hospital-based case-control study consisting of 277 newly diagnosed lung cancer patients and 496 control subjects to investigate a possible association between these two polymorphisms and lung cancer risk. The polymorphisms were determined by polymerase chain reaction/restriction fragment length polymorphism and TaqMan assay using DNA from peripheral white blood cells. Logistic regression was performed to calculate odds ratios (ORs), confidence limits (CL) and to control for possible confounders. The exon 3 polymorphism of the mEH gene was associated with a significantly decreased risk of lung cancer. The adjusted OR, calculated relative to subjects with the Tyr113/Tyr113 wild type, for the His113/His113 genotype was 0.38 (95% CL 0.20-0.75). An analysis according to histological subtypes revealed a statistically significant association for adenocarcinomas; the adjusted OR for the His113/His113 genotype was 0.40 (95% CL 0.17-0.94). In contrast, no relationship between the exon 4 polymorphism and lung cancer risk was found. The adjusted OR, calculated relative to the His139/His139 wild type, was for the Arg139/Arg139 genotype 1.83 (0.76-4.44). Our results support the hypothesis that genetically reduced mEH activity may be protective against lung cancer.

[Functional analysis of drug metabolizing enzymes using gene knockout animals]

Several gene knockout mice have been widely used to analyze the role of drug-metabolizing enzymes in pharmacologic and physiologic responses. The metabolic shift of endogenous and exogenous compounds causes pharmacologic and physiologic alterations. Microsomal epoxide hydrolase (mEH)-null mice are less susceptible to the skin tumorigenesis, splenic immunotoxicity, and embryonic toxicity of 7,12-dimethylbenz[a] anthracene (DMBA). The production of DMBA-3,4-diol is detected in the target organs of wild-type mice, but not in those of mEH-null mice. Soluble epoxide hydrolase (sEH)-null mice exhibit markedly reduced rates of epoxyeicosatrienoic acid conversion to dihydroxyei-cosatrienoic acid in the liver and kidney. Furthermore, sEH-null male mice have a lower blood pressure phenotype compared with male wild-type mice, suggesting the importance of sEH in blood pressure regulation. Nuclear bile acid receptor, farnesoid X receptor (FXR)-null mice are distinguished from wild-type mice by elevated bile acid levels in the liver and serum. However, hepatic lithocholic acid (LCA) levels are lower in LCA-fed FXR-null female mice compared to those in wild-type female mice. Furthermore, FXR-null female mice are less susceptible to liver damage by LCA compared with female wild-type mice. Marked increases in hepattic LCA-sulfating activity and hepatic hydroxysteroid sulfotransferase and biliary sulfated bile acid levels are detected in FXR-null female mice, suggesting the protective role of hydroxysteroid sulfotransferase in LCA-induced liver damage. These and other studies indicate that mice null for drug-metabolizing enzymes and nuclear receptors are of great value in the study of the role of drug-metabolizing enzymes in pharmacologic and physiologic responses.

Characterization of substrate binding to cytochrome P450 1A1 using molecular modeling and kinetic analyses: case of residue 382

Key residue Val-382 in P450 1A1 has been predicted to interact with the alkoxy chain of resorufin derivatives. Therefore, we undertook a detailed analysis of substrate mobility in the active site of the P450 1A1 homology model and assessed the effect of mutations at position 382. Dynamic trajectories of 7-methoxy-, 7-ethoxy-, and 7-pentoxyresorufin indicated that 7-ethoxyresorufin would be oxidized most efficiently by the wild-type enzyme. The Val-382-->Ala mutation would increase the O-dealkylation of 7-pentoxyresorufin but decrease the oxidation of other substrates. In the case of the V382L mutant, the large bulk of Leu would block alkoxyresorufins from productive binding orientations leading to lowered activities. Binding free energy calculations for three substrates with 1A1 WT and two mutants indicated that binding constants would be similar for all enzyme-substrate combinations. Modeling predictions were tested experimentally. The plasmid containing the cDNA for human P450 1A1 modified for bacterial expression was altered to include a C-terminal PCR-generated six histidine domain to facilitate enzyme purification. The V382A and V382L mutants were constructed by site-directed mutagenesis and Escherichia coli-expressed enzymes purified using Ni-NTA affinity chromatography. The activity of the WT 1A1 was highest toward 7-ethoxyresorufin and lowest toward 7-pentoxyresorufin. Both mutants displayed a decrease in V(max) with 7-methoxy- and 7-ethoxyresorufin, whereas for the V382A mutant, V(max) with 7-pentoxyresorufin was increased. No significant changes in K(m) were observed relative to the wild-type enzyme. The experimental results are thus in good agreement with modeling predictions.

A QSAR model of PAHs carcinogenesis based on thermodynamic stabilities of biactive sites

Polycyclic aromatic hydrocarbons (PAHs) have been adopted to study the carcinogenesis of chemicals experimentally and theoretically. A model of carcinogenic activity of 48 PAHs was obtained based on the calculated relative thermodynamic stabilities of epoxide and carbonium intermediates of the PAHs. Using both epoxyl-energy and cationized-energy of two active sites, the model reasonably predicts the carcinogenic activity of these PAHs and shows a good ability to distinguish between carcinogenic and noncarcinogenic PAHs. Furthermore, the model suggests that double active sites and their distance characteristics are important factors in the chemical carcinogenesis of PAHs. The physical meaning of the energies corresponding addition reactions with DNA is also discussed.

Biotransformations using prokaryotic P450 monooxygenases

Recent studies on microbial cytochrome P450 enzymes have covered several new areas. Advances have been made in structure-function analysis and new non-enzymatic/electrochemical systems for the replacement of NAD(P)H in biocatalysis have been developed. Furthermore, the properties of some enzymes have been re-engineered by site-directed mutagenesis or by methods of directed evolution and new P450s have been functionally expressed and characterized. It is thought that a combination of these approaches will facilitate the use of isolated P450 monooxygenases in biocatalysis.

Enantiomer-specific accumulation of PCB atropisomers in the bowhead whale (Balaena mysticetus)

Blubber (n = 40) and liver (n = 20) samples from the bowhead whale (Balaena mysticetus) were collected during the 1997-1998 Native (Inuit) subsistence harvests in Barrow, AK. Bowhead tissues and zooplankton were analyzed for polychlorinated biphenyl (PCB) concentrations and the enantiomeric fractions (EFs) of eight chiral PCB congeners (PCB-91, 95, 135, 136, 149, 174, 176, and 183) to quantify the enantiomer-specific accumulation of PCBs in this cetacean. PCB concentrations in bowhead blubber were low (mean +/- 1 SE: 610 +/- 54 ng g(-1) lipid) relative to other cetaceans. The accumulation of several chiral PCBs (PCB-91, 135, 149, 174, 176, and 183) in bowhead blubber was enantiomer-specific relative to bowhead liver and zooplankton, suggesting that biotransformation processes within the bowhead whale are enantioselective. The EFs for PCB-95 and 149 were significantly correlated with body length in male and female whales, while EFs for PCB-91 correlated with length in males only. Despite evidence for enantioselective biotransformation, all three congeners bioaccumulated in the bowhead relative to PCB-153. Results suggest that enantioselective accumulation of PCB-91, 95, and 149 is influenced by PCB concentrations, age, and/or the modification of an uncharacterized stereoselective process (or processes) during sexual maturity.

Microsomal epoxide hydrolase polymorphisms and lung cancer risk in non-Hispanic whites

Microsomal epoxide hydrolase (mEPHX) is a critical metabolic enzyme involved in the activation and subsequent detoxification of specific tobacco carcinogens. mEPHX harbors polymorphisms in exon 3 and exon 4 that modulate enzymatic activity. The exon 3 polymorphism decreases mEPHX metabolic activity, whereas the exon 4 polymorphism increases activity. We hypothesized that the mEPHX polymorphisms modulate lung cancer risk. Using a case-control study design and restriction fragment length polymorphism polymerase chain reaction assay, we determined the mEPHX polymorphic genotypes of 181 lung cancer cases among non-Hispanic whites and 163 controls (matched for age, sex, ethnicity, and smoking history). Our results showed that the variant allele of mEPHX exon 4 increased the overall lung cancer risk by 56% (odds ratio [OR]=1.56, 95% confidence interval [CI]=0.99-2.46). Additionally, the risk estimates were elevated significantly for younger people (<64 yr) (OR=2.27, 95% CI=1.15-4.50) and current smokers (OR=2.22, 95% CI=1.06-4.65). The variant allele of mEPHX exon 3 had no effect overall (OR=0.88, 95% CI=0.56-1.38), but there was a 53% protective effect (OR=0.47, 95% CI=0.22-0.99) in younger people. When we analyzed the exon 3 and exon 4 polymorphisms together, those people with the high enzymatic activity genotype had an elevated lung cancer risk of 1.72 (95% CI=0.90-3.29). This elevated risk was also evident only in younger people. These findings suggest that these variant alleles of exon 3 and exon 4 of mEPHX modulates lung cancer risk.