N-Nitrosobenzylmethylamine hydroxylation and coumarin 7-hydroxylation: catalysis by rat esophageal microsomes and cytochrome P450 2A3 and 2A6 enzymes

Determining recommended acceptable intake limits for N-nitrosamine impurities in pharmaceuticals: Development and application of the Carcinogenic Potency Categorization Approach (CPCA)

N-Nitrosamine impurities, including nitrosamine drug substance-related impurities (NDSRIs), have challenged pharmaceutical industry and regulators alike and affected the global drug supply over the past 5 years. Nitrosamines are a class of known carcinogens, but NDSRIs have posed additional challenges as many lack empirical data to establish acceptable intake (AI) limits. Read-across analysis from surrogates has been used to identify AI limits in some cases; however, this approach is limited by the availability of robustly-tested surrogates matching the structural features of NDSRIs, which usually contain a diverse array of functional groups. Furthermore, the absence of a surrogate has resulted in conservative AI limits in some cases, posing practical challenges for impurity control. Therefore, a new framework for determining recommended AI limits was urgently needed. Here, the Carcinogenic Potency Categorization Approach (CPCA) and its supporting scientific rationale are presented. The CPCA is a rapidly-applied structure-activity relationship-based method that assigns a nitrosamine to 1 of 5 categories, each with a corresponding AI limit, reflecting predicted carcinogenic potency. The CPCA considers the number and distribution of α-hydrogens at the N-nitroso center and other activating and deactivating structural features of a nitrosamine that affect the α-hydroxylation metabolic activation pathway of carcinogenesis. The CPCA has been adopted internationally by several drug regulatory authorities as a simplified approach and a starting point to determine recommended AI limits for nitrosamines without the need for compound-specific empirical data.

Strategies for Assessing Acceptable Intakes for Novel N-Nitrosamines Derived from Active Pharmaceutical Ingredients

The detection of N-nitrosamines, derived from solvents and reagents and, on occasion, the active pharmaceutical ingredient (API) at higher than acceptable levels in drug products, has led regulators to request a detailed review for their presence in all medicinal products. In the absence of rodent carcinogenicity data for novel N-nitrosamines derived from amine-containing APIs, a conservative class limit of 18 ng/day (based on the most carcinogenic N-nitrosamines) or the derivation of acceptable intakes (AIs) using structurally related surrogates with robust rodent carcinogenicity data is recommended. The guidance has implications for the pharmaceutical industry given the vast number of marketed amine-containing drugs. In this perspective, the rate-limiting step in N-nitrosamine carcinogenicity, involving cytochrome P450-mediated α-carbon hydroxylation to yield DNA-reactive diazonium or carbonium ion intermediates, is discussed with reference to the selection of read-across analogs to derive AIs. Risk-mitigation strategies for managing putative N-nitrosamines in the preclinical discovery setting are also presented.

What Makes a Potent Nitrosamine? Statistical Validation of Expert-Derived Structure-Activity Relationships

The discovery of carcinogenic nitrosamine impurities above the safe limits in pharmaceuticals has led to an urgent need to develop methods for extending structure-activity relationship (SAR) analyses from relatively limited datasets, while the level of confidence required in that SAR indicates that there is significant value in investigating the effect of individual substructural features in a statistically robust manner. This is a challenging exercise to perform on a small dataset, since in practice, compounds contain a mixture of different features, which may confound both expert SAR and statistical quantitative structure-activity relationship (QSAR) methods. Isolating the effects of a single structural feature is made difficult due to the confounding effects of other functionality as well as issues relating to determining statistical significance in cases of concurrent statistical tests of a large number of potential variables with a small dataset; a naïve QSAR model does not predict any features to be significant after correction for multiple testing. We propose a variation on Bayesian multiple linear regression to estimate the effects of each feature simultaneously yet independently, taking into account the combinations of features present in the dataset and reducing the impact of multiple testing, showing that some features have a statistically significant impact. This method can be used to provide statistically robust validation of expert SAR approaches to the differences in potency between different structural groupings of nitrosamines. Structural features that lead to the highest and lowest carcinogenic potency can be isolated using this method, and novel nitrosamine compounds can be assigned into potency categories with high accuracy.

Metformin suppresses the esophageal carcinogenesis in rats treated with NMBzA through inhibiting AMPK/mTOR signaling pathway

Metformin is a widely used antidiabetic drug for the management of type 2 diabetes mellitus. Recently, epidemiological studies demonstrate that metformin has anticancer effects on esophageal squamous cell carcinoma (ESCC) and other cancers. However, the effects and potential mechanisms of metformin on ESCC remain elusive. In this study, we used N-nitroso-N-methylbenzylamine (NMBzA), a special carcinogen for esophagi, to develop a rat ESCC model, in which the carcinogenesis progression of ESCC in rat was induced and promoted. We investigated the effects of metformin on carcinogenesis of ESCC in this model. Our results revealed that metformin significantly decreased the incidence and precancerous lesions of ESCC and inhibited proliferation and promoted apoptosis of esophageal epithelial cells in rat treated with NMBzA. Moreover, metformin also increased apoptosis and inhibited migration, colony formation and tumor sphere formation of human ESCC cells in vitro. Immunohistochemistry and western blotting showed that without interfering the metabolism of NMBzA, metformin inhibited the inflammation of esophagi via reducing the expressions of inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2) and interleukin-6 (IL-6). Treatment of metformin led to activation of AMP-activated protein kinase (AMPK) and attenuated signaling of the downstream molecules such as p-mTOR, p-p70S6K and cyclin D1 expression both in vivo and in vitro. Taken together, our study demonstrated that metformin suppressed the carcinogenesis of ESCC through inhibiting AMPK/mammalian target of the rapamycin (mTOR) signaling pathway, resulting in its chemopreventive effects on the carcinogenesis of ESCC.

The enhancing effect of ethanol on the mutagenic activation of N-nitrosomethylbenzylamine by cytochrome P450 2A in the rat oesophagus

Alcohol consumption is frequently associated with various cancers and the enhancement of the metabolic activation of carcinogens has been proposed as a mechanism underlying this relationship. The ethanol-induced enhancement of N-nitrosodiethylamine (DEN)-mediated carcinogenesis can be attributed to an increase in hepatic activity. However, the mechanism of elevation of N-nitrosomethylbenzylamine (NMBA)-induced tumorigenesis remains unclear. To elucidate the mechanism underlying the role of ethanol in the enhancement of NMBA-induced oesophageal carcinogenesis, we evaluated the hepatic and extrahepatic levels of the cytochrome P450 (CYP) and mutagenic activation of environmental carcinogens by immunoblot analyses and Ames preincubation test, respectively, in F344 rats treated with ethanol. Five weeks of treatment with 10% ethanol added to the drinking water or two intragastric treatments with 50% ethanol, both resulted in elevated levels of CYP2E1 (1.5- to 2.3-fold) and mutagenic activities of DEN, N-nitrosodimethylamine and N-nitrosopyrrolidine in the presence of rat liver S9 (1.5- to 2.4-fold). This was not the case with CYP1A1/2, CYP2A1/2, CYP2B1/2 or CYP3A2, nor with the activities of 2-amino-3-methylimidazo[4,5-f]quinoline, 3-amino-1-methyl-5H-pyrido[4,3-b]indole, aflatoxin B(1) or other N-nitroso compounds (NOCs), including NMBA. Ethanol-induced elevations of CYP2A and CYP2E1 were observed in the oesophagus (up to 1.7- and 2.3-fold) and kidney (up to 1.5- and 1.8-fold), but not in the lung or colon. In oesophagus and kidney, the mutagenic activities of NMBA and four NOCs were markedly increased (1.3- to 2.4-fold) in treated rats. The application of several CYP inhibitors revealed that CYP2A were likely to contribute to the enhancing effect of ethanol on NMBA activation in the rat oesophagus and kidney, but that CYP2E1 failed to do so. These results showed that the enhancing effect of ethanol on NMBA-induced oesophageal carcinogenesis could be attributed to an increase in the metabolic activation of NMBA by oesophageal CYP2A during the initiation phase, and that this occurred independently of CYP2E1.

CYP2A6- and CYP2A13-catalyzed metabolism of the nicotine Δ5'(1')iminium ion

Nicotine, the major addictive agent in tobacco, is metabolized primarily by CYP2A6-catalyzed oxidation. The product of this reaction, 5'-hydroxynicotine, is in equilibrium with the nicotine Δ5'(1')iminium ion and is further metabolized to cotinine. We reported previously that both CYP2A6 and the closely related extrahepatic enzyme CYP2A13 were inactivated during nicotine metabolism; however, inactivation occurred after metabolism was complete. This led to the hypothesis that oxidation of a nicotine metabolite, possibly the nicotine Δ5'(1')iminium ion, was responsible for generating the inactivating species. In the studies presented here, we confirm that the nicotine Δ5'(1')iminium ion is an inactivator of both CYP2A6 and CYP2A13, and inactivation depends on time, concentration, and the presence of NADPH. Inactivation was not reversible and was accompanied by a parallel loss in spectrally active protein, as measured by reduced CO spectra. These data are consistent with the characterization of the nicotine Δ5'(1')iminium ion as a mechanism-based inactivator of both CYP2A13 and CYP2A6. We also confirm that both CYP2A6 and CYP2A13 catalyze the metabolism of the nicotine Δ5'(1')iminium ion to cotinine and provide evidence that both enzymes catalyze the sequential metabolism of the nicotine Δ5'(1')iminium ion. That is, a fraction of the cotinine formed may not be released from the enzyme before further oxidation to 3'-hydroxycotinine.

Construction of a CYP2E1-template system for prediction of the metabolism on both site and preference order

We have constructed an in silico system for the prediction of CYP2E1-mediated reaction using a two-dimensional template derived from substrate structures. Although CYP2E1 prefers small-size molecules for the substrates, the enzyme mediates oxidations of large-size molecules, such as benzo[a]pyrene. Overlays of these substrates, to assemble their sites of oxidation into a specific area, suggested a range of regions frequently occupied. The region, having a benzo[a]pyrene-like shape, was thus used as a CYP2E1 template. In this system, atoms in substrates, except for hydrogen atoms, were placed on corners of honeycomb structures of the template after having expanded the structures. Using published data for the metabolism on more than 80 substrates of CYP2E1, the core template was further refined to verify the adjacent area and to define the relative contribution of template positions for the catalysis. The positions on the template were classified into four different point (0-3) groups, depending on relative usage. In addition, we set independent points (-5 to 3) for specific positions to incorporate three-dimensional or functional information. Total scores from both position-occupancy and -function points were calculated for all the orientations of possible conformers of test substrates, and the scores were found to predict the relative abundance (i.e., order) as well as the regioselectivity of human CYP2E1 reactions with high fidelities.

Inhibition and inactivation of cytochrome P450 2A6 and cytochrome P450 2A13 by menthofuran, β-nicotyrine and menthol

Nicotine is the primary addictive agent in tobacco products and is metabolized in humans by CYP2A6. Decreased CYP2A6 activity has been associated with decreased smoking. The extrahepatic enzyme, CYP2A13 (94% identical to CYP2A6) also catalyzes the metabolism of nicotine, but is most noted for its role in the metabolic activation of the tobacco specific lung carcinogen, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). In this study, the inhibition and potential inactivation of CYP2A6 and CYP2A13 by two tobacco constituents, 1-methyl-4-(3-pyridinyl) pyrrole (β-nicotyrine) and (-)-menthol were characterized and compared to the potent mechanism based inactivator of CYP2A6, menthofuran. The effect of these compounds on CYP2A6 and CYP2A13 activity was significantly different. (-)-Menthol was a more efficient inhibitor of CYP2A13 than of CYP2A6 (KI, 8.2 μM and 110 μM, respectively). β-Nicotyrine was a potent inhibitor of CYP2A13 (KI, 0.17 μM). Neither menthol nor β-nicotyrine was an inactivator of CYP2A13. Whereas, β-nicotyrine was a mechanism based inactivator of CYP2A6 (KI(inact), 106 μM, kinact was 0.61 min(-1)). Similarly, menthofuran, a potent mechanism based inactivator of CYP2A6 did not inactivate CYP2A13. Menthofuran was an inhibitor of CYPA13 (KI, 1.24 μM). The inactivation of CYP2A6 by either β-nicotyrine or menthofuran was not due to modification of the heme and was likely due to modification of the apo-protein. These studies suggest that β-nicotyrine, but not menthol may influence nicotine and NNK metabolism in smokers.

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.

Chronic nicotine consumption does not influence 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone-induced lung tumorigenesis

Nicotine replacement therapy is often used to maintain smoking cessation. However, concerns exist about the safety of long-term nicotine replacement therapy use in ex-smokers and its concurrent use in smokers. In this study, we determined the effect of nicotine administration on 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)-induced lung tumors in A/J mice. Female mice were administered a single dose of NNK (10 μmol) and 0.44 μmol/mL nicotine in the drinking water. Nicotine was administered 2 weeks prior to NNK, 44 weeks after NNK, throughout the experiment, or without NNK treatment. The average weekly consumption of nicotine-containing water was 15 ± 3 mL per mouse, resulting in an estimated daily nicotine dose of 0.9 μmol (0.15 mg) per mouse. Nicotine administration alone for 46 weeks did not increase lung tumor multiplicity (0.32 ± 0.1 vs. 0.53 ± 0.1 tumors per mouse). Lung tumor multiplicity in NNK-treated mice was 18.4 ± 4.5 and was not different for mice consuming nicotine before or after NNK administration, 21.9 ± 5.3 and 20.0 ± 5.4 tumors per mouse, respectively. Lung tumor multiplicity in animals consuming nicotine both before and after NNK administration was 20.4 ± 5.4. Tumor size and progression of adenomas to carcinomas was also not affected by nicotine consumption. In addition, nicotine consumption had no effect on the level of O(6)-methylguanine in the lung of NNK-treated mice. These negative findings in a commonly used model of human lung carcinogenesis should lead us to question the interpretation of the many in vitro studies that find that nicotine stimulates cancer cell growth.

Eukaryotic initiation factor 4E binding protein family of proteins: sentinels at a translational control checkpoint in lung tumor defense

The usurping of translational control by sustained activation of translation initiation factors is oncogenic. Here, we show that the primary negative regulators of these oncogenic initiation factors--the 4E-BP protein family--operate as guardians of a translational control checkpoint in lung tumor defense. When challenged with the tobacco carcinogen 4-(methylnitrosamino)-I-(3-pyridyl)-1-butanone (NNK), 4ebp1(-/-)/4ebp2(-/-) mice showed increased sensitivity to tumorigenesis compared with their wild-type counterparts. The 4E-BP-deficient state per se creates pro-oncogenic, genome-wide skewing of the molecular landscape, with translational activation of genes governing angiogenesis, growth, and proliferation, and translational activation of the precise cytochrome p450 enzyme isoform (CYP2A5) that bioactivates NNK into mutagenic metabolites. Our study provides in vivo proof for a translational control checkpoint in lung tumor defense.

Effects of eleven isothiocyanates on P450 2A6- and 2A13-catalyzed coumarin 7-hydroxylation

Many isothiocyanates (ITCs), both naturally occurring and synthetic, are potent and selective inhibitors of carcinogenesis in animal models and are now viewed as a class of promising chemopreventive agents. We have investigated the ability of 11 ITCs to inhibit and/or inactivate P450 2A6- and 2A13-mediated coumarin 7-hydroxylation. Two of these 11 ITCs, phenylpropyl isothiocyanate (PPITC) and phenylhexyl isothiocyanate (PHITC), were potent inhibitors of P450 2A13. The K I values for the inhibition of P450 2A13-mediated coumarin 7-hydroxylation by PPITC and PHITC were approximately 0.14 and 1.1 microM, respectively. P450 2A6 was also inhibited by these two ITCs; however, the K I values indicated they were approximately 10-20-fold less potent for P450 2A6 than for P450 2A13. Most of the ITCs tested, including PPITC and PHITC, showed some degree of inactivation of both P450s; however, only one compound, tert-butyl isothiocyanate (tBITC), showed significant inactivation of P450 2A13 at a concentration of 10 microM. None of the ITCs caused significant inactivation of P450 2A6 at this concentration. tBITC inactivated P450 2A13 with an apparent K I of 4.3 microM and a k inact of 0.94 min (-1). Inactivation of P450 2A6 by tBITC was observed only at high concentrations and long incubation times. The observed differences in inhibition and/or inactivation of P450 2A6 and 2A13 by a few of the isothiocyanates suggest that these compounds may be useful for structure-function studies.

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.

Inactivation of CYP2A6 and CYP2A13 during nicotine metabolism

Nicotine is the major addictive agent in tobacco. The primary catalyst of nicotine metabolism in humans is CYP2A6. However, the closely related enzyme CYP2A13 is a somewhat better catalyst. CYP2A13 is an extrahepatic enzyme that is an excellent catalyst of the metabolic activation of the tobacco-specific carcinogen 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanone (NNK). Here we report that both CYP2A6 and CYP2A13 were inactivated during nicotine metabolism. Inactivation of both enzymes was dependent on NADPH and increased with time and concentration. Alternate substrates for CYP2A6 and CYP2A13 protected these enzymes from inactivation. Inactivation of CYP2A13 was irreversible upon extensive dialysis and seems to be mechanism-based. The K(I) of CYP2A13 inactivation by nicotine was 17 microM, the rate of inactivation, k(inact), was 0.1 min(-1), and the t(1/2) was 7 min. However, the loss in enzyme activity occurred after nicotine metabolism was complete, suggesting that a secondary or possible tertiary metabolite of nicotine may be responsible. [5-(3)H]Nicotine metabolism by CYP2A13 was monitored by radioflow high-pressure liquid chromatography during the course of enzyme inactivation; the major product was the Delta(1'(5'))iminium ion. However, cotinine was a significant metabolite even at short reaction times. The metabolism of the nicotine Delta(1'(5'))iminium ion to cotinine did not require the addition of aldehyde oxidase. CYP2A13 catalyzed this reaction as well as further metabolism of cotinine to 5'-hydroxycotinine, trans-3'-hydroxycotinine, and N-(hydroxymethyl)-norcotinine as enzyme inactivation occurred. Studies are on-going to identify the metabolite responsible for nicotine-mediated inactivation of CYP2A13.

Nicotine 5'-oxidation and methyl oxidation by P450 2A enzymes

In smokers, the primary pathway of nicotine metabolism is P450 2A6-catalyzed 5'-oxidation. The nicotine Delta(5'(1'))-iminium ion product of this reaction is further metabolized to cotinine by aldehyde oxidase. Previous investigators have reported kinetic parameters for cotinine formation using human liver cytosol as a source of aldehyde oxidase. Using [5-(3)H]nicotine and radioflow high-performance liquid chromatography analysis, we determined kinetic parameters for nicotine 5'-oxidation by P450 2A6 and the closely related human extrahepatic P450 2A13 as well as the rodent P450s 2A3, 2A4, and 2A5. The formation of both cotinine and nicotine Delta(5'(1'))-iminium ion was monitored. The K(m) and V(max) values for P450 2A6 were 144 +/- 15 muM and 1.30 +/- 0.05 pmol/min/pmol, respectively. Previously reported K(m) values for cotinine formation by P450 2A6 in the presence of cytosol were much lower, ranging from 11 to 45 muM. P450 2A13 was a somewhat better catalyst of nicotine Delta(5'(1'))-iminium formation, with 2-fold lower K(m) and 2-fold higher V(max) values than P450 2A6. The rat P450 2A3 and the mouse P450 2A5, which are 85 and 84% identical to P450 2A6, were much more efficient catalysts of nicotine 5'-oxidation. P450 2A4 was not an efficient catalyst of nicotine metabolism. Whereas 5'-oxidation was the major pathway of nicotine metabolism for all five P450 2A enzymes, these enzymes also catalyzed methyl oxidation. Nornicotine, the product of this reaction was detected as 5 to 15% of the total nicotine metabolites. Nornicotine is the amine precursor to the esophageal carcinogen N'-nitrosonornicotine. Therefore, methyl oxidation of nicotine by P450 2A6 or P450 2A13 followed by nitrosation of nornicotine are possible endogenous pathways of N'-nitrosonornicotine formation.

Immunoblot analysis and immunohistochemical characterization of CYP2A expression in human olfactory mucosa

The aim of the present study was to further characterize the expression of the CYP2A genes in human nasal mucosa. Fetal nasal tissues at 12-26 weeks of gestational age and surgical biopsy tissues from various regions of nasal cavity of adult patients were studied to determine whether CYP2A proteins can be detected by immunoblot in adults, whether higher levels of CYP2A proteins are present in adult than in fetal nasal mucosal microsomes, and whether CYP2A13 mRNA is more abundant than CYP2A6 mRNA in fetal nasal mucosa. In adults, immunoblot analysis detected CYP2A proteins in microsomes of the olfactory region from 8 of 10 individuals, but in none of the nasal microsomes of the respiratory region from 47 patients. Quantitative immunoblot analysis confirmed that CYP2A proteins are selectively expressed in the olfactory region in both adult and fetal tissues. Interestingly, the levels of CYP2A proteins in nasal microsomes were generally higher in fetuses than in adults. In the fetus, the level of CYP2A13 mRNA was much higher than that of CYP2A6 mRNA, as has been previously found in adult nasal mucosa. Immunohistochemical studies confirmed that, in the fetus, the CYP2A proteins are expressed in the supporting cells in the olfactory epithelium and in the Bowman's glands in the lamina propria. The prenatal expression of the CYP2A proteins in the olfactory mucosa suggests potential risks of developmental toxicity from maternally derived xenobiotics, since both CYP2A6 and CYP2A13 are known to be efficient in the metabolic activation of tobacco-specific nitrosamines and other respiratory toxicants.

CYP2A13-catalysed coumarin metabolism: comparison with CYP2A5 and CYP2A6

1. We investigated the total metabolism of coumarin by baculovirus (BV)-expressed CYP2A13 and compared it with metabolism by BV-expressed CYP2A6. The major coumarin metabolite formed by CYP2A13 was 7-hydroxycoumarin, which accounted for 43% of the total metabolism. The product of 3,4-epoxidation, o-hydroxyphenylacetaldehyde (o-HPA), accounted for 30% of the total metabolites. 2. The K(m) and V(max) for CYP2A13-mediated coumarin 7-hydroxylation were 0.48+/-0.07 micro m and 0.15+/-0.006 nmol min(-1) nmol(-1) CYP, respectively. The V(max) of coumarin 7-hydroxylation by CYP2A13 was about 16-fold lower than that of CYP2A6, whereas the K(m) was 10-fold lower. 3. In the mouse, there were two orthologues for CYP2A6: CYP2A4 and CYP2A5, which differed by only 11 amino acids. However, CYP2A5 is an efficient coumarin 7-hydroxylase, where as CYP2A4 is not. We report here that BV-expressed CYP2A4 metabolizes coumarin by 3,4-epoxidation. Two products of the 3,4-epoxidation pathway, o-HPA and o-hydroxyphenylacetic acid (o-HPAA), were detected by radioflow HPLC. 4. The K(m) and V(max) for the coumarin 3,4-epoxidation by CYP2A4 were 8.7+/-3.6 micro m and 0.20+/-0.04 nmol min(-1) nmol(-1) CYP, respectively. Coumarin 7-hydroxylation by CYP2A5 was more than 200 times more efficient than 3,4 epoxidation by CYP2A4.

Functional role of cytochrome p-450 2a3 in N-nitrosomethylbenzylamine metabolism in rat esophagus

Previous in vitro studies demonstrated that the rat esophageal carcinogen N-nitrosomethylbenzylamine (NMBA) is metabolically activated by cytochrome P-450s (CYP) 2A3 and 2E1. However, the in vivo role of these P-450s in the metabolism of NMBA has not been fully evaluated. In this study, the effects of single and multiple doses of NMBA were investigated on CYP2A3 and CYP2E1 mRNA expression in the rat esophagus and lung. Seven- to 8-wk old male Fischer 344 rats were administered a single subcutaneous dose of NMBA at either 0.5 mg/kg or 2 mg/kg body weight, after which the rats were sacrificed at 1, 3, 6, 12, 24, 48, and 72 h. In the multiple-dose experiment, 2 groups of rats were dosed with 0.5 mg/kg body weight NMBA 3 times per week for 1 wk or 3 wk. The animals were sacrificed 24 h following the last treatment. Semiquantitative reverse-transcription polymerase chain reaction (RT-PCR) analysis demonstrated a reduction of CYP2A3 mRNA expression in lung and esophagus from NMBA-treated animals compared to dimethyl sulfoxide (DMSO)-treated vehicle controls. This reduction in CYP2A3 mRNA was significant at 48 h in the esophagus and at 24 and 48 h in the lung following a single dose of 2 mg/kg body weight NMBA. In contrast, CYP2E1 mRNA expression remained unchanged in rat lung following NMBA treatment and no consistent pattern of expression could be observed in the esophagus. In the multiple-dose study, a 32% and 25% reduction in esophageal CYP2A3 mRNA expression was observed at 1 and 3 wk, respectively. Similar reductions in CYP2A3 mRNA expression were also observed in the lung. Further, esophageal explants derived from animals pretreated with NMBA in vivo demonstrated a reduced ability to metabolize the carcinogen in vitro as compared to explants from vehicle control animals. Taken together, these data provide further support for a potential role of CYP2A3 in NMBA metabolism in the rat esophagus. Data suggest that CYP2A3 levels in the rat esophagus can be a determinant of its ability to metabolize this carcinogen in vivo.

Activation of the NPTA element of the CYP2A3 gene by NFI-A2, a nasal mucosa-selective nuclear factor 1 isoform

The aim of this study was to determine whether the NPTA element of the olfactory mucosa-predominant CYP2A3 gene can be activated by NFI-A2, a recently identified member of the nuclear factor 1 family of transcription factors. Isoform-specific RNA-PCR confirmed that NFI-A2 is mainly expressed in rat olfactory mucosa. A full-length NFI-A2 cDNA was isolated from a cDNA library of rat olfactory mucosa and was used for preparation of a construct encoding a fusion protein of NFI-A2 with the yeast GAL4 activation domain. Expression of the fusion protein in yeast was detected with an antibody to NFI-A. The fusion protein activated the expression of a LacZ reporter gene in yeast one-hybrid assays with a reporter construct containing the NPTA element, but not with other constructs lacking the NPTA element. These findings suggest that NFI-A2 may be involved in the tissue-selective transcriptional activation of the CYP2A3 gene in the olfactory mucosa.

Characterization of Cytochrome P450 2A4 and 2A5-Catalyzed 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) Metabolism

The tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), is a potent lung carcinogen in the A/J mouse, and is believed to be a causative agent for human lung cancer. NNK requires metabolic activation by alpha-hydroxylation to exert its carcinogenic potential. The human P450, 2A6 is a catalyst of this reaction. There are two closely related enzymes in the mouse, P450 2A4 and 2A5, which differ from each other by only 11 amino acids. In the present study these two mouse P450s were expressed in Spodoptera frugiperda (Sf9) cells using recombinant baculovirus. The catalysis of NNK metabolism by Sf9 microsomal fractions containing either P450 2A4 or 2A5 was determined. Both enzymes catalyzed the alpha-hydroxylation of NNK but with strikingly different efficiencies and specificities. P450 2A5 preferentially catalyzed NNK methyl hydroxylation, while P450 2A4 preferentially catalyzed methylene hydroxylation. The KM and Vmax for the former were 1.5 microM and 4.0 nmol/min/nmol P450, respectively, and for the latter 3.9 mM and 190 nmol/min/nmol P450. The mouse coumarin 7-hydroxylase, P450 2A5 is a significantly better catalyst of NNK alpha-hydroxylation than is the closely related human enzyme, P450 2A6.