Cytochrome P450 2A1, 2E1, and 2C9 cDNA-expression by insect cells and partial purification using hydrophobic chromatography

Identification of CYP 2A6 inhibitors in an effort to mitigate the harmful effects of the phytochemical nicotine

AIM

In this study, our goal was to study the inhibition of nicotine metabolism by P450 2A6, as a means for reduction in tobacco use and consequently the prevention of smoking-related cancers. Nicotine, a phytochemical, is an addictive stimulant, responsible for the tobacco-dependence in smokers. Many of the other phytochemicals in tobacco, including polycyclic aromatic hydrocarbons, N-nitrosamines, and aromatic amines, are potent systemic carcinogens. Tobacco smoking causes about one of every five deaths in the United States annually. Nicotine plasma concentration is maintained by the smokers' smoking behavior within a small range. Nicotine is metabolized by cytochrome P450s 2A6 and 2A13 to cotinine. This metabolism causes a decrease in nicotine plasma levels, which in turn leads to increased tobacco smoking, and increased exposure to the tobacco carcinogens.

METHODS

Using the phytochemical nicotine as a lead structure, and taking its interactions with the P450 2A6 binding pocket into consideration, new pyridine derivatives were designed and synthesized as potential selective mechanism-based inhibitors for this enzyme.

RESULTS

The design and synthesis of two series of novel pyridine-based compounds, with varying substituents and substitution locations on the pyridine ring, as well as their inhibitory activities on cytochrome P450 2A6 and their interactions with its active site are discussed here. Substitutions at position 3 of the pyridine ring with an imidazole or propargyl ether containing group showed the most optimal interactions with the P4502A6 active site.

CONCLUSION

The pyridine compounds with an imidazole or propargyl ether containing substituent on position 3 were found to be promising lead compounds for further development. Hydrogen-bonding interactions were determined to be crucial for effective binding of these molecules within the P450 2A6 active site.

Effect of CYP2A6 genetic polymorphism on the metabolic conversion of tegafur to 5-fluorouracil and its enantioselectivity

Tegafur (FT), a prodrug of 5-fluorouracil, is a chiral molecule, a racemate of R- and S-isomers, and CYP2A6 plays an important role in the enantioselective metabolism of FT in human liver microsomes (R-FT >> S-FT). This study examined the enantioselective metabolism of FT by microsomes prepared from Sf9 cells expressing wild-type CYP2A6 and its variants (CYP2A6*7, *8, *10, and *11) that are highly prevalent in the Asian population. We also investigated the metabolism of coumarin and nicotine, both CYP2A6 probe drugs, in these variants. Enzyme kinetic analyses showed that CYP2A6.7 (I471T) and CYP2A6.10 (I471T and R485L) had markedly lower Vmax values for both enantiomers than wild-type enzyme (CYP2A6.1) and other variant enzymes, whereas Km values were higher in most of the variant enzymes for both enantiomers than CYP2A6.1. The ratios of Vmax and Km values for R-FT to corresponding values for S-FT (R/S ratio) were similar among enzymes, indicating little difference in enantioselectivity among the wild-type and variant enzymes. Similarly, both CYP2A6.7 and CYP2A6.10 had markedly lower Vmax values for coumarin 7-hydroxylase and nicotine C-oxidase activities than CYP2A6.1 and other variant enzymes, whereas Km values were higher in most of the variant enzymes for both activities than CYP2A6.1. In conclusion, the amino acid substitutions in CYP2A6 variants generally resulted in lower affinity for substrates, while Vmax values were selectively reduced in CYP2A6.7 and CYP2A6.10. Consistent R/S ratios among CYP2A6.1 and variant enzymes indicated that the amino acid substitutions had little effect on enantioselectivity in the metabolism of FT.

Expression and functional characterization of cytochrome P450 26A1, a retinoic acid hydroxylase

Retinoic acid (RA) is a critical signaling molecule that performs multiple functions required to maintain cellular viability. It is also used in the treatment of some cancers. Enzymes in the CYP26 family are thought to be responsible for the elimination of RA, and CYP26A1 appears to serve the most critical functions in this family. In spite of its importance, CYP26A1 has neither been heterologously expressed nor characterized kinetically. We expressed the rCYP26A1 in baculovirus-infected insect cells and purified the hexahistidine tagged protein to homogeneity. Heme incorporation was determined by carbon monoxide difference spectrum and a type 1 spectrum was observed with RA binding to CYP26A1. We found that RA is a tight binding ligand of CYP26A1 with low nM binding affinity. CYP26A1 oxidized RA efficiently (depletion K(m) 9.4+/-3.3nM and V(max) 11.3+/-4.3pmolesmin(-1)pmoleP450(-1)) when supplemented with P450 oxidoreductase and NADPH but was independent of cytochrome b5. 4-Hydroxy-RA (4-OH-RA) was the major metabolite produced by rCYP26A1 but two other primary products were also formed. 4-OH-RA was further metabolized by CYP26A1 to more polar metabolites and this sequential metabolism of RA occurred in part without 4-OH-RA leaving the active site of CYP26A1. The high efficiency of CYP26A1 in eliminating both RA and its potentially active metabolites supports the major role of this enzyme in regulating RA clearance in vivo. These results provide a biochemical framework for CYP26A1 function and offer insight into the role of CYP26A1 as a drug target as well as in fetal development and cell cycle regulation.

Characterization of Anopheles minimus CYP6AA3 expressed in a recombinant baculovirus system

Metabolism by cytochrome P450 monooxygenases is a major mechanism implicated in resistance of insects to insecticides, including pyrethroids. We previously isolated the cytochrome P450 CYP6AA3 from deltamethrin-selected resistant strain of Anopheles minimus mosquito, a major malaria vector in Thailand. In the present study, we further investigated the role of CYP6AA3 enzyme in deltamethrin metabolism in vitro. The CYP6AA3 was expressed in Spodoptera frugiperda (Sf9) insect cells via baculovirus-mediated expression system. The enzymatic activity of CYP6AA3 in deltamethrin metabolism was characterized after being reconstituted with An. minimus NADPH-cytochrome P450 reductase and a NADPH-regenerating system. The contribution of CYP6AA3 responsible for deltamethrin metabolism was determined by measurement of deltamethrin disappearance following the incubation period and deltamethrin-derived compounds were detected using combined gas chromatography mass spectrometry analysis. 3-Phenoxybenzaldehyde was a major product of CYP6AA3-mediated deltamethrin metabolism. Deltamethrin degradation and formation of metabolites were NADPH-dependent and inhibited by piperonyl butoxide. Deltamethrin was catalyzed by CYP6AA3 with an apparent K(m) of 80.0 +/- 2.0 and V(max) of 60.2 +/- 3.6 pmol/min/pmol P450. Furthermore, deltamethrin cytotoxicity assays by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and trypan blue dye exclusion were examined in Sf9 insect cells, with and without expression of CYP6AA3. Results revealed that CYP6AA3 could play a role in detoxifying deltamethrin in the cells. Thus, the results of this study support the role of CYP6AA3 in deltamethrin metabolism.

Functional characterization of medaka CYP3A38 and CYP3A40: kinetics and catalysis by expression in a recombinant baculovirus system

Phylogenic analysis of the teleost genomic lineages has demonstrated the precedent for multiple genome duplications. Among many of the genes duplicated, cytochrome P450 genes have undergone independent diversification, which can be traced to a single ancestral gene. In teleosts, cytochrome P450s, from all major families, have been identified. Among these, the CYP3A family has been cloned in several teleost species and demonstrated to contain multiple paralogs differing in gene expression patterns and tissue distribution. Herein we characterized the catalytic and kinetic activities of two medaka CYP3A paralogs (CYP3A38 and CYP3A40) with benzyloxyresorufin (BFC), a fluorescent 3A-selective substrate, and testosterone, a known metabolic substrate for CYP3A enzymes. Recombinant CYP3A was produced using the baculovirus expression vector system in Spodoptera frugiperda (Sf9) and Trichoplusia ni (Tn5) insect cells and accounted for up to 24% of total cellular protein. Following addition of a heme-albumin conjugate to log phase cells, spectral P450 content reached a maximum of 560 and 2350 pmol/mg microsomal protein for CYP3A38 and CYP3A40, respectively. Incubations containing recombinant CYP3A, human NADPH-cytochrome P-450 oxidoreductase reductase, human cytochrome b5, and a NADPH generation system catalyzed the dealkylation of BFC and hydroxylation of testosterone with a high degree of stereoselectivity. However, efficiencies and specificities were significantly different between the two isoforms. Km and Vmax activities based on BFC-catalysis were 0.116 and 0.363 muM, and 7.95 and 7.77 nmol/min/nmol P450 for CYP3A38 and CYP3A40, respectively. CYP3A38 preferentially catalyzed testosterone hydroxylation at the 6beta-, 2beta- and 16beta-positions with minor hydroxylation at other positions within the steroid nucleus. Testosterone catalysis with CYP3A40 was limited predominantly to the 6beta- and 2beta-positions. Putative identification of CYP3A substrate recognition sites (SRS) 1-6 indicates that 12 of the 49 amino acid differences between CYP3A38 and CYP3A40 OFRs occur in SRS regions previously known to be associated with steroid hydroxylation. We suggest that differences in kinetics and catalytic activities are a result of amino acid substitutions in SRS regions 1, 3 and 5 within the CYP3A38 and CYP3A40 protein sequence.

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

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

Purification of cytochrome P-450 enzymes

Among the liver P-450 xenobiotic-metabolizing enzymes, P450-2E1 is of interest because of its activation of potent carcinogens, and P-450 1A2 is of interest because of its role in oxidation of drugs and carcinogens. This unit describes column chromatography protocols for purification of recombinant forms of these enzymes expressed in a bacterial expression system.

CYP2M1: cloning, sequencing, and expression of a new cytochrome P450 from rainbow trout liver with fatty acid (omega-6)-hydroxylation activity

A cDNA clone was isolated from a female rainbow trout liver lambda g tau 11 library using polyclonal antibodies raised against rainbow trout cytochrome P450 LMC1. This 2149-nucleotide clone contained an open reading frame encoding a protein of 499 amino acids with a calculated M(r) of 56,850 Da. On the basis of cytochrome P450 (P450) amino acid sequence comparisons, this rainbow trout P450 was assigned by the P450 Nomenclature Committee to a new P450 subfamily designated as CYP2M1. Northern blot results suggest that the expression of CYP2M1 at the transcriptional level was generally sex, tissue, and age specific. By use of a full-length CYP2M1 cDNA probe, it was observed that this cDNA hybridized strongly to a single 2.2-kb transcript in juvenile female rainbow trout trunk kidney and in liver from juvenile and sexually mature trout from both sexes. Negligible amounts of mRNA hybridizable to CYP2M1 cDNA were found in the juvenile and sexually mature male trunk kidney. cDNA-directed expression in COS-7 cells and of recombinant baculovirus in insect cells produced a protein that was reactive with rabbit anti-trout P450 LMC1 polyclonal antibody and exhibited the unique (omega-6)-hydroxylation toward lauric acid previously observed with rainbow trout P450 LMC1.

Effect of guaiazulene on some cytochrome P450 activities. Implication in the metabolic activation and hepatotoxicity of paracetamol

The in vitro and in vivo effect of guaiazulene, a natural azulene derivative, on rat hepatic cytochrome P450 (CYP) is investigated. Furthermore, paracetamol hepatotoxicity is induced in rats and the activity of specific cytochrome P450 forms, involved in the metabolic activation of paracetamol to the toxic metabolite N-acetyl-p-benzoquinone imine (NAPQI) is examined, after the administration of guaiazulene, using diagnostic cytochrome P450 substrates. It is found that guaiazulene inhibited considerably CYP1A2 and CYP2B1 and had a weak effect on CYP1A1 in rat hepatic microsomal fractions. Guaiazulene administered to rats did not produce any macroscopic toxic effect and caused no change of liver weight, microsomal protein and total cytochrome P450 content. Guaiazulene inhibited CYP1A2 activity in rats with or without paracetamol intoxication. Considering that CYP1A2 participates in the formation of NAPQI, as well as in the metabolic activation of several toxic and carcinogenic compounds, these results, in combination with the antioxidant activity of guaiazulene that we have found in previous investigations, indicate potential useful applications of guaiazulene.

Specificity of cDNA-expressed human and rodent cytochrome P450s in the oxidative metabolism of the potent carcinogen 7,12-dimethylbenz[a]anthracene

7,12-Dimethylbenz[a]anthracene (DMBA), a potent carcinogen, requires metabolic activation by cytochrome P450s (P450s) to electrophilic metabolites that result in DNA modification, mutagenicity, and carcinogenicity. In this study, we used eight human forms, four rodent forms, and one rabbit form of P450 expressed from recombinant vaccinia or baculovirus vectors to define their specificity for metabolizing DMBA. Of the eight human P450s, 1A1 was the most active (specific activity = 14.7 nmol/min/nmol of P450) in total metabolism of DMBA and showed approximately 6- to 33-fold more activity than other P450s, 2B6, 2C9, and 1A2 were also capable of metabolizing DMBA (2.0-2.5 nmol/min/nmol of P450), whereas 2C8, 2E1, 3A4, and 3A5 exhibited relatively low activities. Among animal P450s, mouse 1A1 exhibited activity similar to that of human 1A1 and had 5.0- to 37-fold more activity than other rodent and rabbit P450s. In regard to enzyme regioselectivity, most human and rodent P450s predominantly formed the 8,9-diol, but human 2B6 and rat 2B1 preferentially formed the 5,6-diol. In the production of monohydroxymethyl metabolites, all the enzymes yielded more 7-hydroxymethyl-12-methylbenz[a]anthracene (7HOM12MBA) than 12-hydroxymethyl-7-methylbenz[a]anthracene (7M12HOMBA), except for human 1A1, which presented the reverse selectivity. Human liver microsomes from 10 organ donors were shown to metabolize DMBA and in most circumstances generated the metabolic profile DMBA trans-8,9-dihydrodiol > 7HOM12MBA > or = DMBA trans-5,6-dihydrodiol > or = 7,12-dihydroxymethylbenz[a]anthracene > 7M12HOMBA > DMBA trans-3,4-dihydrodiol. Thus, the combined activity of hepatic microsomal 2C9, 1A2, and 2B6 may contribute to the metabolic activation and the metabolism of DMBA in normal human liver.

Inhibitory and non-inhibitory monoclonal antibodies to human cytochrome P450 3A3/4

Cytochromes P450 3A3/4 are inordinately important P450 enzymes catalyzing the metabolism of a large variety of clinically useful drugs, steroids, and carcinogens. Two monoclonal antibodies, MAb 3-29-9 and MAb 275-1-2, were prepared to human P450 3A4 from mice immunized with baculovirus-expressed human P450 3A4. MAb 3-29-9 was a powerful inhibitor of the enzymatic activity of P450 3A3/4/5. MAb 3-29-9 inhibited the P450 3A3, 3A4, and 3A5 catalyzed metabolism of substrates of divergent molecular weights, e.g., p-nitroanisole, phenanthrene, diazepam, testosterone, taxol, and cyclosporin. However, MAb 3-29-9 did not give a western blot with P450 3A3 or 3A4. MAb 275-1-2 was non-inhibitory but yielded a strong western blot with P450 3A3 and 3A4 but not with 3A5, and thus distinguished between 3A3/4 and 3A5. The two MAbs did not cross-react with human 2E1, 1A2, 2B6, 2C8, and 2C9; rat 2A1, 3A1/2, 4A1, 4A3, and 2B1; and mouse 1A1 and 1A2. MAb 3-29-9 has been used successfully to measure the quantitative contribution of P450 3A3 and 3A4 to the metabolism of the above-designated substrates in human adult liver. MAb 3-29-9 and MAb 275-1-2 are precise and sensitive reagents for P450 3A studies.