Identification of Val117 and Arg372 as critical amino acid residues for the activity difference between human CYP2A6 and CYP2A13 in coumarin 7-hydroxylation

Polymorphic cytochromes P450 in non-human primates

Cynomolgus macaques (Macaca fascicularis, an Old World monkey) are widely used in drug development because of their genetic and physiological similarities to humans, and this trend has continued with the use of common marmosets (Callithrix jacchus, a New World monkey). Information on the major drug-metabolizing cytochrome P450 (CYP, P450) enzymes of these primate species indicates that multiple forms of their P450 enzymes have generally similar substrate selectivities to those of human P450 enzymes; however, some differences in isoform, activity, and substrate specificity account for limited species differences in drug oxidative metabolism. This review provides information on the P450 enzymes of cynomolgus macaques and marmosets, including cDNA, tissue expression, substrate specificity, and genetic variants, along with age differences and induction. Typical examples of important P450s to be considered in drug metabolism studies include cynomolgus CYP2C19, which is expressed abundantly in liver and metabolizes numerous drugs. Moreover, genetic variants of cynomolgus CYP2C19 affect the individual pharmacokinetic data of drugs such as R-warfarin. These findings provide a foundation for understanding each P450 enzyme and the individual pharmacokinetic and toxicological results in cynomolgus macaques and marmosets as preclinical models. In addition, the effects of induction on some drug clearances mediated by P450 enzymes are also described. In summary, this review describes genetic and acquired individual differences in cynomolgus and marmoset P450 enzymes involved in drug oxidation that may be associated with pharmacological and/or toxicological effects.

Genetic and Enzymatic Characteristics of CYP2A13 in Relation to Lung Damage

Cytochrome P450 2A13 is an omitted brother of CYP2A6 that has an important role in the drug metabolism of liver. Due to extrahepatic expression, it has gained less attention than CYP2A6, despite the fact that it plays a significant role in toxicant-induced pulmonary lesions and, therefore, lung cancer. The purpose of this mini-review is to summarize the basic knowledge about this enzyme in relation to the substrates, inhibitors, genetic polymorphisms, and transcriptional regulation that are known so far (September 2021).

Molecular docking and oxidation kinetics of 3-phenyl coumarin derivatives by human CYP2A13

CYP2A13 enzyme is expressed in human extrahepatic tissues, while CYP2A6 is a hepatic enzyme. Reactions catalyzed by CYP2A13 activate tobacco-specific nitrosamines and some other toxic xenobiotics in lungs.To compare oxidation characteristics and substrate-enzyme active site interactions in CYP2A13 vs CYP2A6, we evaluated CYP2A13 mediated oxidation characteristics of 23 coumarin derivatives and modelled their interactions at the enzyme active site.CYP2A13 did not oxidize six coumarin derivatives to corresponding fluorescent 7-hydroxycoumarins. The K_m-values of the other coumarins varied 0.85-97 µM, V_max-values of the oxidation reaction varied 0.25-60 min^-1, and intrinsic clearance varied 26-6190 kL/min*mol CYP2A13). K_m of 6-chloro-3-(3-hydroxyphenyl)-coumarin was 0.85 (0.55-1.15 95% confidence limit) µM and V_max 0.25 (0.23-0.26) min^-1, whereas K_m of 6-hydroxy-3-(3-hydroxyphenyl)-coumarin was 10.9 (9.9-11.8) µM and V_max 60 (58-63) min^-1. Docking analyses demonstrated that 6-chloro or 6-methoxy and 3-(3-hydroxyphenyl) or 3-(4-trifluoromethylphenyl) substituents of coumarin increased affinity to CYP2A13, whereas 3-triazole or 3-(3-acetate phenyl) or 3-(4-acetate phenyl) substituents decreased it.The active site of CYP2A13 accepts more diversified types of coumarin substrates than the hepatic CYP2A6 enzyme. New sensitive and convenient profluorescent CYP2A13 substrates were identified, such as 6-chloro-3-(3-hydroxyphenyl)-coumarin having high affinity and 6-hydroxy-3-(3-hydroxyphenyl)-coumarin with high intrinsic clearance.

The marmoset cytochrome P450 superfamily: Sequence/phylogenetic analyses, genomic structure, and catalytic function

The common marmoset (Callithrix jacchus) is a New World monkey that has attracted much attention as a potentially useful primate model for preclinical testing. A total of 36 marmoset cytochrome P450 (P450) isoforms in the P450 1-51 subfamilies have been identified and characterized by the application of genome analysis and molecular functional characterization. In this mini-review, we provide an overview of the genomic structures, sequence identities, and substrate selectivities of marmoset P450s compared with those of human P450s. Based on the sequence identity, phylogeny, and genomic organization of marmoset P450s, orthologous relationships were established between human and marmoset P450s. Twenty-four members of the marmoset P450 1A, 2A, 2B, 2C, 2D, 2E, 3A, 4A, and 4F subfamilies shared high degrees of homology in terms of cDNA (>89%) and amino acid sequences (>85%) with the corresponding human P450s; P450 2C76 was among the exceptions. Phylogenetic analysis using amino acid sequences revealed that marmoset P450s in the P450 1-51 families were located in the same clades as their human and macaque P450 homologs. This finding underlines the evolutionary closeness of marmoset P450s to their human and macaque homologs. Most marmoset P450 1-4 enzymes catalyzed the typical drug-metabolizing reactions of the corresponding human P450 homologs, except for some differences of P450 2A6 and 2B6. Consequently, it appears that the substrate specificities of enzymes in the P450 1-4 families are generally similar in marmosets and humans. The information presented here supports a better understanding of the functional characteristics of marmoset P450s and their similarities and differences with human P450s. It is hoped that this mini-review will facilitate the successful use of marmosets as primate models in drug metabolism and pharmacokinetic studies.

Betel quid containing safrole enhances metabolic activation of tobacco specific 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)

Cigarette smoking (CS) and betel quid (BQ) chewing are two known risk factors that have synergistic potential for the enhancing the development of oral squamous cell carcinoma (OSCC) in Taiwan. Most mutagens and carcinogens are metabolically activated by cytochrome P450 (CYP450) to exert their mutagenicity or carcinogenicity. Previous studies have shown that metabolic activation of the tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), by CYP2A6 activity determines NNK-induced carcinogenesis. In addition, safrole affects cytochrome P450 activity in rodents. However, the effect of BQ safrole on the metabolism of tobacco-specific NNK and its carcinogenicity remains elusive. This study demonstrates that safrole (1 mg/kg/d) induced CYP2A6 activity, reduced urinary 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) levels, and increased NNK-induced DNA damage, including N7-methylguanine, 8-OH-deoxyguanosine and DNA strand breaks in a Syrian golden hamster model. Furthermore, altered NNK metabolism and increased NNK-induced DNA damage were also observed in healthy subjects with CS and BQ chewing histories compared to healthy subjects with CS histories. In conclusion, BQ containing safrole induced tobacco-specific NNK metabolic activation, resulting in higher NNK-induced genotoxicity. This study provides valuable insight into the synergistic mechanisms of CS- and BQ-induced OSCC.

Coumarins and P450s, Studies Reported to-Date

Cytochrome P450 enzymes (CYPs) are important phase I enzymes involved in the metabolism of endogenous and xenobiotic compounds mainly through mono-oxygenation reactions into more polar and easier to excrete species. In addition to their role in detoxification, they play important roles in the biosynthesis of endogenous compounds and the bioactivation of xenobiotics. Coumarins, phytochemicals abundant in food and commonly used in fragrances and cosmetics, have been shown to interact with P450 enzymes as substrates and/or inhibitors. In this review, these interactions and their significance in pharmacology and toxicology are discussed in detail.

Roles of Human CYP2A6 and Monkey CYP2A24 and 2A26 Cytochrome P450 Enzymes in the Oxidation of 2,5,2',5'-Tetrachlorobiphenyl

2,5,2',5'-Tetrachlorobiphenyl (TCB) induced type I binding spectra with cytochrome P450 (P450) 2A6 and 2A13, with Ks values of 9.4 and 0.51 µM, respectively. However, CYP2A6 oxidized 2,5,2',5'-TCB to form 4-hydroxylated products at a much higher rate (∼1.0 minute-1) than CYP2A13 (∼0.02 minute-1) based on analysis by liquid chromatography-tandem mass spectrometry. Formation of 4-hydroxy-2,5,2',5'-TCB by CYP2A6 was greater than that of 3-hydroxy-2,5,2',5'-TCB and three other hydroxylated products. Several human P450 enzymes, including CYP1A1, 1A2, 1B1, 2B6, 2D6, 2E1, 2C9, and 3A4, did not show any detectable activities in oxidizing 2,5,2',5'-TCB. Cynomolgus monkey CYP2A24, which shows 95% amino acid identity to human CYP2A6, catalyzed 4-hydroxylation of 2,5,2',5'-TCB at a higher rate (∼0.3 minute-1) than CYP2A26 (93% identity to CYP2A6, ∼0.13 minute-1) and CYP2A23 (94% identity to CYP2A13, ∼0.008 minute-1). None of these human and monkey CYP2A enzymes were catalytically active in oxidizing other TCB congeners, such as 2,4,3',4'-, 3,4,3',4'-, and 3,5,3',5'-TCB. Molecular docking analysis suggested that there are different orientations of interaction of 2,5,2',5'-TCB with the active sites (over the heme) of human and monkey CYP2A enzymes, and that ligand interaction energies (U values) of bound protein-ligand complexes show structural relationships of interaction of TCBs and other ligands with active sites of CYP2A enzymes. Catalytic differences in human and monkey CYP2A enzymes in the oxidation of 2,5,2',5'-TCB are suggested to be due to amino acid changes at substrate recognition sites, i.e., V110L, I209S, I300F, V365M, S369G, and R372H, based on the comparison of primary sequences.

Electrochemical detection of human cytochrome P450 2A6 inhibition: a step toward reducing dependence on smoking

Inhibition of human cytochrome P450 2A6 has been demonstrated to play an important role in nicotine metabolism and consequent smoking habits. Here, the "molecular Lego" approach was used to achieve the first reported electrochemical signal of human CYP2A6 and to improve its catalytic efficiency on electrode surfaces. The enzyme was fused at the genetic level to flavodoxin from Desulfovibrio vulgaris (FLD) to create the chimeric CYP2A6-FLD. Electrochemical characterization by cyclic voltammetry shows clearly defined redox transitions of the haem domain in both CYP2A6 and CYP2A6-FLD. Electrocatalysis experiments using coumarin as substrate followed by fluorimetric quantification of the product were performed with immobilized CYP2A6 and CYP2A6-FLD. Comparison of the kinetic parameters showed that coumarin catalysis was carried out with a higher efficiency by the immobilized CYP2A6-FLD, with a calculated kcat value significantly higher (P < 0.005) than that of CYP2A6, whereas the affinity for the substrate (KM) remained unaltered. The chimeric system was also successfully used to demonstrate the inhibition of the electrochemical activity of the immobilized CYP2A6-FLD, toward both coumarin and nicotine substrates, by tranylcypromine, a potent and selective CYP2A6 inhibitor. This work shows that CYP2A6 turnover efficiency is improved when the protein is linked to the FLD redox module, and this strategy can be utilized for the development of new clinically relevant biotechnological approaches suitable for deciphering the metabolic implications of CYP2A6 polymorphism and for the screening of CYP2A6 substrates and inhibitors.

Binding of diverse environmental chemicals with human cytochromes P450 2A13, 2A6, and 1B1 and enzyme inhibition

A total of 68 chemicals including derivatives of naphthalene, phenanthrene, fluoranthene, pyrene, biphenyl, and flavone were examined for their abilities to interact with human P450s 2A13 and 2A6. Fifty-one of these 68 chemicals induced stronger Type I binding spectra (iron low- to high-spin state shift) with P450 2A13 than those seen with P450 2A6, i.e., the spectral binding intensities (ΔAmax/Ks ratio) determined with these chemicals were always higher for P450 2A13. In addition, benzo[c]phenanthrene, fluoranthene, 2,3-dihydroxy-2,3-dihydrofluoranthene, pyrene, 1-hydroxypyrene, 1-nitropyrene, 1-acetylpyrene, 2-acetylpyrene, 2,5,2',5'-tetrachlorobiphenyl, 7-hydroxyflavone, chrysin, and galangin were found to induce a Type I spectral change only with P450 2A13. Coumarin 7-hydroxylation, catalyzed by P450 2A13, was strongly inhibited by 2'-methoxy-5,7-dihydroxyflavone, 2-ethynylnaphthalene, 2'-methoxyflavone, 2-naphththalene propargyl ether, acenaphthene, acenaphthylene, naphthalene, 1-acetylpyrene, flavanone, chrysin, 3-ethynylphenanthrene, flavone, and 7-hydroxyflavone; these chemicals induced Type I spectral changes with low Ks values. On the basis of the intensities of the spectral changes and inhibition of P450 2A13, we classified the 68 chemicals into eight groups based on the order of affinities for these chemicals and inhibition of P450 2A13. The metabolism of chemicals by P450 2A13 during the assays explained why some of the chemicals that bound well were poor inhibitors of P450 2A13. Finally, we compared the 68 chemicals for their abilities to induce Type I spectral changes of P450 2A13 with the Reverse Type I binding spectra observed with P450 1B1: 45 chemicals interacted with both P450s 2A13 and 1B1, indicating that the two enzymes have some similarty of structural features regarding these chemicals. Molecular docking analyses suggest similarities at the active sites of these P450 enzymes. These results indicate that P450 2A13, as well as Family 1 P450 enzymes, is able to catalyze many detoxication and activation reactions with chemicals of environmental interest.

Nicotine and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone binding and access channel in human cytochrome P450 2A6 and 2A13 enzymes

Cytochromes P450 (CYP) from the 2A subfamily are known for their roles in the metabolism of nicotine, the addictive agent in tobacco, and activation of the tobacco procarcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). Although both the hepatic CYP2A6 and respiratory CYP2A13 enzymes metabolize these compounds, CYP2A13 does so with much higher catalytic efficiency, but the structural basis for this has been unclear. X-ray structures of nicotine complexes with CYP2A13 (2.5 Å) and CYP2A6 (2.3 Å) yield a structural rationale for the preferential binding of nicotine to CYP2A13. Additional structures of CYP2A13 with NNK reveal either a single NNK molecule in the active site with orientations corresponding to metabolites known to form DNA adducts and initiate lung cancer (2.35 Å) or with two molecules of NNK bound (2.1 Å): one in the active site and one in a more distal staging site. Finally, in contrast to prior CYP2A structures with enclosed active sites, CYP2A13 conformations were solved that adopt both open and intermediate conformations resulting from an ∼2.5 Å movement of the F to G helices. This channel occurs in the same region where the second, distal NNK molecule is bound, suggesting that the channel may be used for ligand entry and/or exit from the active site. Altogether these structures provide multiple new snapshots of CYP2A13 conformations that assist in understanding the binding and activation of an important human carcinogen, as well as critical comparisons in the binding of nicotine, one of the most widely used and highly addictive drugs in human use.

RNA-Seq quantification of the human small airway epithelium transcriptome

Background

The small airway epithelium (SAE), the cell population that covers the human airway surface from the 6^th generation of airway branching to the alveoli, is the major site of lung disease caused by smoking. The focus of this study is to provide quantitative assessment of the SAE transcriptome in the resting state and in response to chronic cigarette smoking using massive parallel mRNA sequencing (RNA-Seq).

Results

The data demonstrate that 48% of SAE expressed genes are ubiquitous, shared with many tissues, with 52% enriched in this cell population. The most highly expressed gene, SCGB1A1, is characteristic of Clara cells, the cell type unique to the human SAE. Among other genes expressed by the SAE are those related to Clara cell differentiation, secretory mucosal defense, and mucociliary differentiation. The high sensitivity of RNA-Seq permitted quantification of gene expression related to infrequent cell populations such as neuroendocrine cells and epithelial stem/progenitor cells. Quantification of the absolute smoking-induced changes in SAE gene expression revealed that, compared to ubiquitous genes, more SAE-enriched genes responded to smoking with up-regulation, and those with the highest basal expression levels showed most dramatic changes. Smoking had no effect on SAE gene splicing, but was associated with a shift in molecular pattern from Clara cell-associated towards the mucus-secreting cell differentiation pathway with multiple features of cancer-associated molecular phenotype.

Conclusions

These observations provide insights into the unique biology of human SAE by providing quantit-ative assessment of the global transcriptome under physiological conditions and in response to the stress of chronic cigarette smoking.

An investigation of the catalytic activity of CYP2A13*4 with coumarin and polymorphisms of CYP2A13 in a Chinese Han population

CYP2A13 has been identified as an efficient catalyst for the metabolisms of coumarin, aflatoxin B(1) (AFB(1)), and several tobacco-specific carcinogens. The reported CYP2A13 polymorphisms with missense variations have been studied for their functional consequences, and CYP2A13*4 (R101Q) variant was found to be a null enzyme in metabolizing 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), AFB(1), and 5-methoxypsoralen. In the present study, CYP2A13*4 was expressed in Sf9 cells and evaluated for coumarin 7-hydroxylation activity. Our results demonstrated that CYP2A13*4 showed no activity in coumarin 7-hydroxylation. Furthermore, computer modeling studies were conducted to probe the mechanisms underlying the loss of catalytic activity of CYP2A13*4. The results suggested that the R101Q alteration may result in the absence of several hydrogen bonds involved in heme binding and thus lead to the loss of function in CYP2A13*4. In addition, for the first time, the distribution frequencies of all eight known CYP2A13 missense alleles were examined in a Chinese Han population. The distribution frequencies of CYP2A13*3 allele and CYP2A13*4 allele in the Chinese Han population were statistically significantly different from the reported values in Japanese. Considering that the two variants of CYP2A13 are incapable of metabolic activation of NNK and AFB(1), the susceptibility to NNK or AFB(1) exposure between the Chinese Han population and Japanese can be different.

Identification of novel CYP2A6 inhibitors by virtual screening

The human CYP2A6 enzyme metabolises several xenobiotics including nicotine, the addictive component in tobacco. Reduced activity of CYP2A6, either for genetic reasons or by administering inhibitors of CYP2A6, reduces tobacco smoking. The aim was to design novel inhibitors of CYP2A6 using 3D-QSAR analysis combined with virtual screening. A 3D-QSAR model was utilised to identify the most important features of the inhibitors, and this knowledge was used to design inhibitors for CYP2A6. Chemical database screening yielded several potent inhibitor candidates such as alkylamine derivatives (compound no. 5, IC(50)=0.1 μM) and 1-benzothiophene-3-carbaldehyde that can be used as lead compounds in the development of drugs for smoking reduction therapy.

Computational insights into the different catalytic activities of CYP2A13 and CYP2A6 on NNK

The human cytochrome P450 2A13 (CYP2A13) and P450 2A6 (CYP2A6) are 94% identical in amino acid sequence, but they metabolize many substrates with different efficiencies. Previous experimental results have shown that CYP2A13 exhibited catalytic activity that was more than 300-fold higher than CYP2A6 toward 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a carcinogen present in tobacco products. At present, however, the structural determinants accounting for the differential catalytic activities of these two isozymes toward NNK remain unclear. In the present study, molecular docking combined with molecular dynamics simulation and binding free energy calculation was performed to investigate the above issue. The results demonstrate that NNK was able to form a hydrogen bond with Asn297 in either CYP2A13 or CYP2A6. The hydrogen-bond acceptor was the pyridine nitrogen of NNK in the CYP2A13 complex, but it changed to the carbonyl oxygen in the CYP2A6 complex. NNK interacted with the residues in helix I and the K-β2 loop in CYP2A13, whereas it preferred to contact with the phenylalanine cluster in CYP2A6. The residues in helix I and the K-β2 loop of CYP2A13 played a vital role in keeping NNK in a more stable binding state. The binding free energies calculated by MM-GBSA were in agreement with the experimental results.

Effects of heme precursors on CYP1A2 and POR expression in the baculovirus/Spodoptera frugiperda system

OBJECTIVE

CYP1A2 and NADPH-CYP450 oxidoreductase (POR) were expressed in the baculovirus/Spodoptera frugiperda (sf9) system. The aim of this study was to investigate the effects of heme precursors on the expression of CYP1A2 and POR.

METHODS

The heme precursors [δ-Aminolaevulinic Acid (5-ALA), Fe(3+) and hemin] were introduced into the system to evaluate their effects on the expression of CYP1A2, POR and their co-expression. All the proteins were identified using immunoblotting, CO-difference spectroscopy, or cytochrome c assay.

RESULTS

In the present study, functional CYP1A2 and POR were successfully expressed in the baculovirus/sf9 system, and both of them showed high activities. Co-addition of 5-ALA and Fe(3+) significantly improved expression of CYP1A2 by about 50% compared with the addition of 5-ALA, Fe(3+) or hemin alone. Either co-addition of 5-ALA and Fe(3+) or addition of 5-ALA or Fe(3+) alone improved the POR expression level 2 fold and its activity 7-10 fold compared with control (no addition). However, unlike CYP1A2, there was no difference between the co-addition and addition of these heme precursors alone. Different ratios of BvCYP1A2 to BvPOR also affected the co-expression of CYP1A2 and POR, with a 3:1 ratio of BvCYP1A2 / BvPOR significantly increasing their co-expression. Surprisingly, the addition of 0.1 mM 5-ALA or Fe(3+) alone, but not their co-addition, could significantly improve the CYP1A2 and POR co-expression (P < 0.05).

CONCLUSION

5-ALA and Fe(3+) increased the expression of CYP1A2 and POR in a baculovirus/sf9 system, but the pattern of their expression was different between their expression alone and co-expression.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Key residues controlling binding of diverse ligands to human cytochrome P450 2A enzymes

Although the human lung cytochrome P450 2A13 (CYP2A13) and its liver counterpart cytochrome P450 2A6 (CYP2A6) are 94% identical in amino acid sequence, they metabolize a number of substrates with substantially different efficiencies. To determine differences in binding for a diverse set of cytochrome P450 2A ligands, we have measured the spectral binding affinities (K(D)) for nicotine, phenethyl isothiocyanate (PEITC), coumarin, 2'-methoxyacetophenone (MAP), and 8-methoxypsoralen. The differences in the K(D) values for CYP2A6 versus CYP2A13 ranged from 74-fold for 2'-methoxyacetophenone to 1.1-fold for coumarin, with CYP2A13 demonstrating the higher affinity. To identify active site amino acids responsible for the differences in binding of MAP, PEITC, and coumarin, 10 CYP2A13 mutant proteins were generated in which individual amino acids from the CYP2A6 active site were substituted into CYP2A13 at the corresponding position. Titrations revealed that substitutions at positions 208, 300, and 301 individually had the largest effects on ligand binding. The collective relevance of these amino acids to differential ligand selectivity was verified by evaluating binding to CYP2A6 mutant enzymes that incorporate several of the CYP2A13 amino acids at these positions. Inclusion of four CYP2A13 amino acids resulted in a CYP2A6 mutant protein (I208S/I300F/G301A/S369G) with binding affinities for MAP and PEITC much more similar to those observed for CYP2A13 than to those for CYP2A6 without altering coumarin binding. The structure-based quantitative structure-activity relationship analysis using COMBINE successfully modeled the observed mutant-ligand trends and emphasized steric roles for active site residues including four substituted amino acids and an adjacent conserved Leu(370).

Molecular genetics of nicotine metabolism

The molecular genetics of nicotine metabolism involves multiple polymorphic catalytic enzymes. Variation in metabolic pathways results in nicotine disposition kinetics that differ between individuals and ethnic groups. Twin studies indicate that a large part of this variance is genetic in origin, although environmental influences also contribute. The primary aim of this chapter is to review the current knowledge regarding the genetic variability in the enzymes that metabolize nicotine in humans. The focus is on describing the genetic polymorphisms that exist in cytochromes P450 (CYPs), aldehyde oxidase 1 (AOX1), UDP-glucuronosyltransferases (UGTs), and flavin-containing monooxygenase 3 (FMO3). Genetic studies have demonstrated that polymorphisms in CYP2A6, the primary enzyme responsible for nicotine breakdown, make a sizable contribution to the wide range of nicotine metabolic capacity observed in humans. Thus, special attention will be given to CYP2A6, because slower nicotine metabolism requires less frequent self-administration, and accordingly influences smoking behaviors. In addition, the molecular genetics of nicotine metabolism in nonhuman primates, mice, and rats will be reviewed briefly.

Key residues controlling phenacetin metabolism by human cytochrome P450 2A enzymes

Cytochrome P450s (P450s) metabolize a large number of diverse substrates with specific regio- and stereospecificity. A number of compounds, including nicotine, cotinine, and aflatoxin B(1), are metabolites of the 94% identical CYP2A13 and CYP2A6 enzymes but at different rates. Phenacetin and 4-aminobiphenyl were identified as substrates of human cytochromes P450 1A2 and 2A13 but not of CYP2A6. The purpose of this study was to identify active site amino acids that are responsible for CYP2A substrate specificity using phenacetin as a structural probe. Ten amino acid residues that differ in the CYP2A13 and CYP2A6 active sites were exchanged between the two enzymes. Phenacetin binding revealed that the six substitution, CYP2A13 S208I, A213S, F300I, A301G, M365V, and G369S decreased phenacetin affinity. Although incorporation of individual CYP2A13 residues into CYP2A6 had little effect on this enzyme's very low levels of phenacetin metabolism, the combination of double, triple, and quadruple substitutions at positions 208, 300, 301, and 369 increasingly endowed CYP2A6 with the ability to metabolize phenacetin. Enzyme kinetics revealed that the CYP2A6 I208S/I300F/G301A/S369G mutant protein O-deethylated phenacetin with a K(m) of 10.3 muM and a k(cat) of 2.9 min(-1), which compare very favorably with those of CYP2A13 (K(m) of 10.7 muM and k(cat) of 3.8 min(-1)). A 2.15 A crystal structure of the mutant CYP2A6 I208S/I300F/G301A/S369G protein with phenacetin in the active site provided a structural rationale for the differences in phenacetin metabolism between CYP2A6 and CYP2A13.

Characterization of CYP2A13*2, a variant cytochrome P450 allele previously found to be associated with decreased incidences of lung adenocarcinoma in smokers

CYP2A13, a human cytochrome P450 enzyme expressed mainly in the respiratory tract, is believed to play an important role in the initiation of smoking-induced lung cancer. CYP2A13.1 has high efficiency in the metabolic activation of a major tobacco-specific carcinogenic nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). CYP2A13(*)2, a variant allele, was previously found to be associated with decreased incidence of lung adenocarcinoma in smokers. The aim of the present study was to determine whether the CYP2A13.2 protein has decreased enzyme activity and/or expression levels in the lung, compared with CYP2A13.1. CYP2A13.2 has two sequence variations from CYP2A13.1: R25Q and R257C. We compared the activities of heterologously expressed CYP2A13.1 and CYP2A13.2 toward several known CYP2A13.1 substrates: NNK, N-nitrosomethylphenylamine, N,N-dimethylaniline, 2'-methoxyacetophenone, and hexamethylphosphoramide. Our results indicated that CYP2A13.2 was 20 to 40% less active than CYP2A13.1 with the substrates tested. We also determined the levels of the CYP2A13(*)2 mRNA, relative to the level of the CYP2A13(*)1 mRNA, in the lung tissue from (*)1/(*)2 heterozygotes. We found that the CYP2A13(*)2 allele was associated with a level of allelic expression approximately 40% lower than that of the CYP2A13(*)1 allele. Sequence analysis of the promoter region of the CYP2A13(*)2 allele identified a 26-nucleotide deletion. Functional analysis of a 2-kilobase pair CYP2A13-luciferase promoter construct indicated that the 26-nucleotide deletion causes decreases in CYP2A13 promoter activity in the A549 human lung cell line. These findings suggest that the reported association of the CYP2A13(*)2 allele with decreased incidences of lung adenocarcinoma in smokers can be at least partly explained by a decrease in CYP2A13 function.

Functional characterization of premnaspirodiene oxygenase, a cytochrome P450 catalyzing regio- and stereo-specific hydroxylations of diverse sesquiterpene substrates

Solavetivone, a potent antifungal phytoalexin, is derived from a vetispirane-type sesquiterpene, premnaspirodiene, by a putative regio- and stereo-specific hydroxylation, followed by a second oxidation to yield the alpha,beta-unsaturated ketone. Mechanistically, these reactions could occur via a single, multifunctional cytochrome P450 or some combination of cytochrome P450s and a dehydrogenase. We report here the characterization of a single cytochrome P450 enzyme, Hyoscyamus muticus premnaspirodiene oxygenase (HPO), that catalyzes these successive reactions at carbon 2 (C-2) of the spirane substrate. HPO also catalyzes the equivalent regio-specific (C-2) hydroxylation of several eremophilane-type (decalin ring system) sesquiterpenes, such as with 5-epi-aristolochene. Moreover, HPO displays interesting comparisons to other sesquiterpene hydroxylases. 5-Epi-aristolochene di-hydroxylase (EAH) differs catalytically from HPO by introducing hydroxyl groups first at C-1, then C-3 of 5-epi-aristolochene. HPO and EAH also differ from one another by 91-amino acid differences, with four of these differences mapping to putative substrate recognition regions 5 and 6. These four positions were mutagenized alone and in various combinations in both HPO and EAH and the mutant enzymes were characterized for changes in substrate selectivity, reaction product specificity, and kinetic properties. These mutations did not alter the regio- or stereo-specificity of either HPO or EAH, but specific combinations of the mutations did improve the catalytic efficiencies 10-15-fold. Molecular models and comparisons between HPO and EAH provide insights into the catalytic properties of these enzymes of specialized metabolism in plants.

Structure of the human lung cytochrome P450 2A13

The human lung cytochrome P450 2A13 (CYP2A13) activates the nicotine-derived procarcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) into DNA-altering compounds that cause lung cancer. Another cytochrome P450, CYP2A6, is also present in human lung, but at much lower levels. Although these two enzymes are 93.5% identical, CYP2A13 metabolizes NNK with much lower K(m) values than does CYP2A6. To investigate the structural differences between these two enzymes the structure of CYP2A13 was determined to 2.35A by x-ray crystallography and compared with structures of CYP2A6. As expected, the overall CYP2A13 and CYP2A6 structures are very similar with an average root mean square deviation of 0.5A for the Calpha atoms. Like CYP2A6, the CYP2A13 active site cavity is small and highly hydrophobic with a cluster of Phe residues composing the active site roof. Active site residue Asn(297) is positioned to hydrogen bond with an adventitious ligand, identified as indole. Amino acid differences between CYP2A6 and CYP2A13 at positions 117, 300, 301, and 208 relate to different orientations of the ligand plane in the two protein structures and may underlie the significant variations observed in binding and catalysis of many CYP2A ligands. In addition, docking studies suggest that residues 365 and 366 may also contribute to differences in NNK metabolism.

In vivo evaluation of coumarin and nicotine as probe drugs to predict the metabolic capacity of CYP2A6 due to genetic polymorphism in Thais

The association between the distribution characteristics of CYP2A6 catalytic activities toward nicotine and coumarin, and the frequency distribution of CYP2A6 variant alleles reported was estimated in 120 healthy Thais. The distributions of the subjects as classified by the amounts of 7-hydroxycoumarin (7-OHC) excreted in the urine and by cotinine/nicotine ratio in the plasma were clearly bimodal. However, the numbers of apparently poor metabolizers for coumarin and nicotine were different. The inter-individual variability in the in vivo dispositions of coumarin and nicotine closely related to the CYP2A6 genetic polymorphism. There was a close correlation between the rate of 7-OHC excretion in the urine and cotinine/nicotine ratio in the plasma among subjects (R=0.92, p<0.001). The frequency of CYP2A6 allele found in the present study was: CYP2A6*1A=32% (95% CI, 22.1-39.4%), CYP2A6*1B=27% (95% CI, 19.4-33.5%), CYP2A6*9=20% (95% CI, 17.6-23.3%), CYP2A6*4=14% (95% CI, 9.6-17.8%), CYP2A6*7=5% (95% CI, 3.7-9.4%), CYP2A6*10=2% (95% CI, 0.8-5.1%). Subjects having CYP2A6*1A/*1B were found to have a higher rate of 7-OHC excretion, as well as a higher cotinine/nicotine ratio in the plasma compared with those of the other genotypes. In contrast, subjects with CYP2A6*4/*7 and CYP2A6*7/*7 almost lacked any cotinine formation, whereas urinary 7-OHC was still detectable. CYP2A6*9 allele clearly resulted in reduced enzyme activities. Despite the absence of the homozygote for CYP2A6*10 allele, the presence of CYP2A6*10 allele significantly decreased the enzyme activities. The results of the present study demonstrate that in vivo phenotyping of CYP2A6 using nicotine and coumarin are not metabolically equivalent. Nicotine is a better probe according to its specificity, while coumarin is still valuable to be used for a routine CYP2A6 phenotyping since the test employs a non-invasive method.

CYP2A13 metabolizes the substrates of human CYP1A2, phenacetin, and theophylline

Human cytochrome CYP2A13 shows overlapping substrate specificity with CYP2A6, catalyzing the metabolism of coumarin, nicotine, cotinine, and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone. Recently, it was found that CYP2A13 could catalyze the metabolic activations of 4-aminobiphenyl and aflatoxin B(1), which are known to be catalyzed by human CYP1A2. In the present study, we investigated the substrate specificity of CYP2A13. It was shown that CYP2A13 could catalyze ethoxyresorufin O-deethylation, methoxyresorufin O-demethylation, and phenacetin O-deethylation, which are used as marker activities for human CYP1A2. Although the intrinsic clearances (V(max)/K(m)) of the two former reactions by CYP2A13 were much lower than that of CYP1A2, the value of the last reaction by CYP2A13 was 2-fold higher than that of CYP1A2. Of particular interest was that CYP2A13 has higher affinity toward phenacetin than CYP1A2. In contrast, CYP2A6 hardly catalyzed these reactions, although the amino acid identity with CYP2A13 is as high as 93.5%. Furthermore, we found that CYP2A13 can catalyze theophylline 8-hydroxylation and 3-demethylation, which are known to be mainly catalyzed by human CYP1A2, although the intrinsic clearances were approximately one-tenth that of CYP1A2. CYP2A13 would not contribute to the systemic clearance of these drugs because CYP2A13 is hardly expressed in human liver. However, it may play a role in metabolism in local tissues such as lung or trachea. In conclusion, the results of the present study could extend our understanding of the substrate specificity of CYP2A13.

Generation of a homology model for the human cytochrome P450, CYP24A1, and the testing of putative substrate binding residues by site-directed mutagenesis and enzyme activity studies

A systematic analysis of conserved H-bonding patterns and tertiary structural motifs from 13 crystal structures was used to create a homology model for the human multicatalytic cytochrome P450, CYP24A1, involved in catabolism of 1alpha,25-dihydroxyvitamin D3. The substrate was docked in the active site and used to identify potential substrate contact residues in the B' helix, B'/C loop, F-helix and the beta-5 hairpin. Seven CYP24A1 mutants were created and studied by mammalian cell transfection and CYP24A1 activity assay. Mutants showed reduced metabolic rates and altered metabolite patterns compared to wild-type. We conclude that: Ile-131 positions substrate via A-ring and cis-triene contacts; Trp-134 and Gly-499 are determinants of substrate access; Leu-148 contacts the substrate side-chain; Met-246 is important in mediating regioselectivity. Our findings validate the new model of CYP24A1, which can now be used to predict structural modifications for rational vitamin D drug design.

Characterization and comparison of nicotine and cotinine metabolism in vitro and in vivo in DBA/2 and C57BL/6 mice

DBA/2 and C57BL/6 are two commonly used mouse strains that differ in response to nicotine. Previous studies have shown that the nicotine-metabolizing enzyme CYP2A5 differs in coumarin metabolism between these two strains, suggesting differences in nicotine metabolism. Nicotine was metabolized to cotinine in vitro by two enzymatic sites. The high-affinity sites exhibited similar parameters (Km, 10.7 +/- 4.8 versus 11.4 +/- 3.6 microM; Vmax, 0.58 +/- 0.18 versus 0.50 +/- 0.07 nmol/min/mg for DBA/2 and C57BL/6, respectively). In vivo, the elimination half-lives of nicotine (1 mg/kg, s.c.) were also similar between DBA/2 and C57BL/6 mice (8.6 +/- 0.4 versus 9.2 +/- 1.6 min, respectively); however, cotinine levels were much higher in DBA/2 mice. The production and identity of the putative cotinine metabolite 3'-hydroxycotinine in mice was confirmed by liquid chromatography/mass spectrometry/mass spectrometry. The in vivo half-life of cotinine (1 mg/kg, s.c.) was significantly longer in the DBA/2 mice compared with the C57BL/6 mice (50.2 +/- 4.7 versus 37.5 +/- 9.6 min, respectively, p < 0.05). The in vitro metabolism of cotinine to 3'-hydroxycotinine was also less efficient in DBA/2 than C57BL/6 mice (Km, 51.0 +/- 15.6 versus 9.5 +/- 2.1 microM, p < 0.05; Vmax, 0.10 +/- 0.01 versus 0.04 +/- 0.01 nmol/min/mg, p < 0.05, respectively). Inhibitory antibody studies demonstrated that the metabolism of both nicotine and cotinine was mediated by CYP2A5. Genetic differences in Cyp2a5 potentially contributed to similar nicotine but different cotinine metabolism, which may confound the interpretation of nicotine pharmacological studies and studies using cotinine as a biomarker.

Genetic variation of human cytochrome p450 reductase as a potential biomarker for mitomycin C-induced cytotoxicity

The importance of genetic variation in clinical response to various drugs is now well recognized. Identification of genetic biomarkers that can predict efficacy and toxicity of chemotherapeutic drugs in cancer patients holds great promise in treatment improvement and cost reduction. Mitomycin C (MMC) is a common anticancer drug used for the treatment of numerous types of tumors. Metabolism-mediated activation, by either one-electron or two-electron reduction, plays a critical role in the chemotherapeutic action of MMC. NADPH-cytochrome P450 (oxido)reductase (POR) is a major enzyme responsible for MMC activation through the one-electron reductive pathway, which leads to the production of semiquinone anion radicals and subsequent DNA damage in the cells. Recently, a total of six naturally occurring human POR variants with single amino acid changes (Y181D, A287P, R457H, V492E, C569Y, and V608F) have been identified. Although the catalytic efficiency of these variants in reduction of cytochrome c was reported to be altered, their capability in activating MMC, a direct substrate of POR, has not been examined. In the present study, we demonstrated that except for the C569Y variant, MMC-induced toxicity assayed as cell viability and proliferative capability was significantly decreased in the Flp-In Chinese hamster ovary cells stably expressing all the other POR variants in comparison with the cells expressing wild-type human POR. Cells expressing the V608F and Y181D variants had a complete loss of the capability to activate MMC. Our finding suggests that these functional POR genetic variations may serve as a potential biomarker to predict the chemotherapeutic response to MMC.

CYP2A13 in human respiratory tissues and lung cancers: an immunohistochemical study with a new peptide-specific antibody

Human cytochrome P450 2A13 (CYP2A13) is highly efficient in the metabolic activation of a tobacco-specific carcinogen, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), and another potent carcinogen, aflatoxin B1 (AFB1). Although previous studies demonstrated that CYP2A13 mRNA is predominantly expressed in human respiratory tissues, expression of CYP2A13 protein in these tissues and the involved cell types have not been determined because of the lack of CYP2A13-specific antibodies. To explore the toxicological and physiological function of CYP2A13, it is important to understand the tissue/cellular distribution of CYP2A13 protein. In this study, we generated a peptide-specific antibody against human CYP2A13 and demonstrated by immunoblot analysis that this antibody does not cross-react with heterologously expressed human CYP2A6 and mouse CYP2A5 proteins, both sharing a high degree of amino acid sequence similarity with CYP2A13. Nor does the antibody cross-react with heterologously expressed human CYP3A4, CYP2S1, or any of the cytochrome P450 enzymes present in the human liver microsomes. Using this highly specific antibody for immunohistochemical staining, we detected a high level of CYP2A13 protein expression in the epithelial cells of human bronchus and trachea, but a rare distribution in the alveolar cells. There was little expression of CYP2A13 protein in different types of lung cancers. In consideration of the high efficiency of CYP2A13 in NNK metabolic activation, our result is consistent with the reported observations that most smoking-related human lung cancers are bronchogenic and supports that CYP2A13-catalyzed in situ activation may play a critical role in human lung carcinogenesis related to NNK and AFB1 exposure.

Efficient activation of aflatoxin B1 by cytochrome P450 2A13, an enzyme predominantly expressed in human respiratory tract

The worldwide human exposure to aflatoxin B1 (AFB1), particularly in developing countries, remains to be a serious public health concern. Although AFB1 is best known as a hepatocarcinogen, epidemiological studies have shown a positive association between human lung cancer occurrence and inhalation exposure to AFB1. Cytochrome P450 (CYP)-catalyzed metabolic activation is required for AFB1 to exert its carcinogenicity. Previous studies have identified CYP1A2 and CYP3A4 as the major enzymes for AFB1 activation in human liver. However, the key CYP enzymes in human lung that can efficiently activate AFB1 in situ are unknown. In the present study, we demonstrate that CYP2A13, an enzyme predominantly expressed in human respiratory tract, has a significant activity in metabolizing AFB1 to its carcinogenic/toxic AFB1-8,9-epoxide and AFM1-8,9-epoxide at both low (15 microM) and high (150 microM) substrate concentrations. Under the same conditions, there was no detectable AFB1 epoxide formation by CYP2A6, which was also reported to be involved in the metabolic activation of AFB1. Consistent with the activity data, there was an approximately 800-fold difference in LC50 values of AFB1 (48-hr treatment) between Chinese hamster ovary (CHO) cells expressing CYP2A13 and CYP2A6 (50 nM versus 39 microM). We further demonstrate that amino acid residues Ala117 and His372 in CYP2A13 protein are important for AFB1 epoxidation and its related cytotoxicity. Our results suggest that CYP2A13-catalyzed metabolic activation in situ may play a critical role in human lung carcinogenesis related to inhalation exposure to AFB1.

Effect of Genetic Variation on Human Cytochrome P450 Reductase-Mediated Paraquat Cytotoxicity

Paraquat (1,1'-dimethyl-4,4'-bipyridylium dichloride) is a widely used herbicide and is highly toxic to human and animals. The mechanisms of paraquat toxicity involve the generation of superoxide anion through the process of redox cycling. NADPH-cytochrome P450 oxidoreductase (POR) has been reported to be a major enzyme for one-electron reduction of paraquat that initiates the redox cycling. Recently, a total of six missense variants of human POR have been identified in patients with discorded steroidogenesis. However, the effect of these genetic variations on POR-mediated paraquat toxicity is not known. Using the Flp-In Chinese hamster ovary (CHO) cells stably expressing either mouse or human POR and the cells with POR knockdown by siRNA, we confirmed that POR is responsible for paraquat-induced cytotoxicity. We further used this validated system to compare paraquat-induced toxicity among the cells that stably expressed wild-type human POR and its natural variants. While there was no difference in paraquat-induced toxicity between the cells expressing wild-type human POR and the Cys569Tyr variant, the toxicity in cells expressing all the other variants (Tyr181Asp, Ala287Pro, Arg457His, Val492Glu, and Val608Phe) was significantly decreased. Our results provide further evidence on the important role of POR in paraquat-induced toxicity and suggest that individuals carrying the functional variant POR alleles may have an altered susceptibility to paraquat exposure.

Effects of benzyl and phenethyl isothiocyanate on P450s 2A6 and 2A13: potential for chemoprevention in smokers

Isothiocyanates have been shown to be potent inhibitors of carcinogenesis in animals exposed to a number of chemical carcinogens including the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). In this study the effects of benzyl isothiocyanate (BITC) and phenethyl isothiocyanate (PEITC), two naturally occuring isothiocyanates, on P450 2A6 and 2A13 were investigated. P450s 2A6 and 2A13 are thought to be the primary human P450 enzymes responsible for the in vivo metabolism of nicotine and NNK, respectively. In vitro, BITC and PEITC efficiently inhibited P450 2A6- and 2A13-mediated coumarin 7-hydroxylation. The inhibition of P450 2A6 and 2A13 by BITC was non-competitive with KI's of 4.1 and 1.3 microM, respectively. PEITC was a more potent inhibitor of both enzymes than BITC, with a KI of 0.37 microM for P450 2A6 and 0.03 microM for P450 2A13. P450 2A6-mediated metabolism of nicotine and P450 2A13-mediated alpha-hydroxylation of NNK were also inhibited significantly by these two isothiocyanates. Both BITC and PEITC were able to inactivate P450 2A6 and 2A13 in an NADPH-dependent manner potentially through the formation of adducts to the apoprotein. The potent inhibition of P450 2A6- and 2A13-mediated metabolisms together with the ability of BITC and PEITC to inactivate the enzymes suggests the possibility that these isothiocyanates could be developed as chemopreventive agents to protect smokers who are unwilling or unable to quit smoking against lung cancer.

Analysis of coumarin 7-hydroxylation activity of cytochrome P450 2A6 using random mutagenesis

Cytochrome P450 (P450) 2A6 is an important human enzyme involved in the metabolism of many xenobiotic chemicals including coumarin, indole, nicotine, and carcinogenic nitrosamines. A combination of random mutagenesis and high-throughput screening was used in the analysis of P450 2A6, utilizing a fluorescent coumarin 7-hydroxylation assay. The steady-state kinetic parameters (k(cat) and Km) for coumarin 7-hydroxylation by wild-type P450 2A6 and 35 selected mutants were measured and indicated that mutants throughout the coding region can have effects on activity. Five mutants showing decreased catalytic efficiency (k(cat)/Km) were further analyzed for substrate selectivity and binding affinities and showed reduced catalytic activities for 7-methoxycoumarin O-demethylation, tert-butyl methyl ether O-demethylation, and indole 3-hydroxylation. All mutants except one (K476E) showed decreased coumarin binding affinities (and also higher Km values), indicating that this is a major basis for the decreased enzymatic activities. A recent x-ray crystal structure of P450 2A6 bound to coumarin (Yano, J. K., Hsu, M. H., Griffin, K. J., Stout, C. D., and Johnson, E. F. (2005) Nat. Struct. Mol. Biol. 12, 822-823) indicates that the recovered A481T and N297S mutations appear to be close to coumarin, suggesting direct perturbation of substrate interaction. The decreased enzymatic activity of the K476E mutant was associated with decreases both in NADPH oxidation and the reduction rate of the ferric P450 2A6-coumarin complex. The attenuation is caused in part to lower binding affinity for NADPH-P450 reductase, but the K476E mutant did not achieve the wild-type coumarin 7-hydroxylation activity even at high reductase concentrations.

Generation and characterization of a transgenic mouse model with hepatic expression of human CYP2A6

The aim of this study was to prepare and characterize a transgenic mouse model in which CYP2A6, a human P450 enzyme, is expressed specifically in the liver. CYP2A6, which is mainly expressed in human liver, is active toward many xenobiotics. Our transgene construct contained the mouse transthyretin promoter/enhancer, a full-length CYP2A6 cDNA, and a downstream neomycin-resistance gene for positive selection in embryonic stem cells. Hepatic expression of the CYP2A6 transgene was demonstrated by immunoblotting, whereas tissue specificity of CYP2A6 expression was confirmed by RNA-PCR. The transgenic mouse was further characterized after being backcrossed to the B6 strain for six generations. Hepatic microsomes from homozygous transgenic mice had activities significantly higher than those of B6 mice toward coumarin. The in vivo activity of transgenic CYP2A6 was also determined. Systemic clearance of coumarin was significantly higher in the transgenic mice than in B6 controls, consistent with the predicted role of CYP2A6 as the major coumarin hydroxylase in human liver. The CYP2A6-transgenic mouse model should be valuable for studying the in vivo function of this polymorphic human enzyme in drug metabolism and chemical toxicity.

Human cytochrome p450 2s1: lack of activity in the metabolic activation of several cigarette smoke carcinogens and in the metabolism of nicotine

Cytochrome P450 (P450) enzymes play a critical role in the metabolic activation of a wide variety of environmental carcinogens. Recently, a novel human P450 enzyme, CYP2S1, has been identified. It is inducible by dioxin and other classical aryl hydrocarbon receptor ligands. However, little is known regarding the substrates and the functional role of CYP2S1. Since CYP2S1 is predominantly expressed in human lung and trachea, it is reasonable to speculate that CYP2S1 may play an important role in metabolizing the environmental chemicals to which human respiratory tissues are exposed. In the present study, we examined the activity of human CYP2S1 in the metabolism of nicotine and in the activation of three potent carcinogens in cigarette smoke, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), benzo[a]pyrene (BaP), and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). The full-length CYP2S1 cDNA was amplified by nested polymerase chain reaction from a human lung cDNA library and was expressed in both Chinese hamster ovary (CHO) cells and Sf9 insect cells. In contrast to the positive controls, i.e., CHO cells expressing human CYP2A13 (for NNK activation) or human CYP1A1 (for BaP activation), there was no increase in NNK- or BaP-induced toxicity in the CHO cells expressing CYP2S1. The heterologously expressed CYP2S1 proteins showed no detectable activity in metabolizing nicotine and PhIP. These results clearly demonstrate that CYP2S1 does not catalyze the metabolism of nicotine and the metabolic activation of these lung carcinogens.

Progress in cytochrome P450 active site modeling

Models capable of predicting the possible involvement of cytochromes P450 in the metabolism of drugs or drug candidates are important tools in drug discovery and development. Ideally, functional information would be obtained from crystal structures of all the cytochromes P450 of interest. Initially, only crystal structures of distantly related bacterial cytochromes P450 were available-comparative modeling techniques were used to bridge the gap and produce structural models of human cytochromes P450, and thereby obtain some useful functional information. A significant step forward in the reliability of these models came four years ago with the first crystal structure of a mammalian cytochrome P450, rabbit CYP2C5, followed by the structures of two human enzymes, CYP2C8 and CYP2C9, and a second rabbit enzyme, CYP2B4. The evolution of a CYP2D6 model, leading to the validation of the model as an in silico tool for predicting binding and metabolism, is presented as a case study.

Identification of critical amino acid residues of human CYP2A13 for the metabolic activation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, a tobacco-specific carcinogen

Among all the known human cytochrome P450 enzymes, CYP2A13 has the highest efficiency in catalyzing the metabolic activation (keto aldehyde and keto alcohol formation) of the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a potent lung carcinogen in animals and a suspected human lung carcinogen. As part of the structure-activity relationship (SAR) study, the present work was done to identify the key amino acid residues in CYP2A13 that are responsible for this high catalytic efficiency by using a series of mutants (Ala117Val, His164Gly, Ser208Ile, His372Arg, and Pro465Ser). In these CYP2A13 mutants, the amino acid residues were substituted by the residues at the corresponding positions of CYP2A6, which shares 93.5% amino acid sequence identity with CYP2A13 but is significantly less active (<5%) than CYP2A13 in NNK alpha-hydroxylation. We demonstrated that, except for the His164Gly mutant, all the CYP2A13 mutant proteins showed a significant decrease in the catalytic efficiency (Vmax/Km) for NNK alpha-hydroxylation. The His372 to Arg substitution resulted in a 20-fold increase in the Km value and a 7-fold decrease in the Vmax value for keto aldehyde formation as well as a total loss of detectable keto alcohol formation. The Ala117 to Val substitution, however, only caused a selective decrease in the Vmax value for keto aldehyde formation. The role of these amino acid residues in CYP2A13-catalyzed reactions is clearly substrate-dependent, since the same Ala117Val and His372Arg mutants showed a 9-fold increase in the catalytic efficiency for coumarin 7-hydroxylation. Together with the computational substrate docking, our study provides new SAR in formation of human CYP2A13.