Heterologous expression of human cytochromes P450 2D6 and CYP3A4 in Escherichia coli and their functional characterization

This study aimed to express two major drug-metabolizing human hepatic cytochromes P450 (CYPs), CYP2D6 and CYP3A4, together with NADPH-cytochrome P450 oxidoreductase (OxR) in Escherichia coli and to evaluate their catalytic activities. Full length cDNA clones of both isoforms in which the N-terminus was modified to incorporate bovine CYP17α sequence were inserted into a pCWori(+) vector. The modified CYP cDNAs were subsequently expressed individually, each together with OxR by means of separate, compatible plasmids with different antibiotic selection markers. The expressed proteins were evaluated by immunoblotting and reduced CO difference spectral scanning. Enzyme activities were examined using high performance liquid chromatography (HPLC) assays with probe substrates dextromethorphan and testosterone for CYP2D6 and CYP3A4, respectively. Results from immunoblotting demonstrated the presence of both CYP proteins in bacterial membranes and reduced CO difference spectra of the cell preparations exhibited the characteristic absorbance peak at 450 nm. Co-expressed OxR also demonstrated an activity level comparable to literature values. Kinetic parameters, K(m) and V(max) values determined from the HPLC assays also agreed well with literature values. As a conclusion, the procedures described in this study provide a relatively convenient and reliable means of producing catalytically active CYP isoforms suitable for drug metabolism and interaction studies.

Expression, purification, and characterization of mouse CYP2d22

Metabolism of the prototype human CYP2D6 substrates debrisoquine and bufuralol proceeds at a much slower rate in mice; therefore, the mouse has been proposed as an animal model for the human CYP2D6 genetic deficiency. To interpret the molecular mechanism of this deficiency, a cDNA belonging to the CYP2D gene subfamily (Cyp2d22) has been cloned and sequenced from a mouse mammary tumor-derived cell line. In the current study, Cyp2d22 enzyme was overexpressed and purified from insect cells using a baculovirus-mediated system. The activity of this purified enzyme was directly compared with purified human CYP2D6 toward codeine, dextromethorphan, and methadone as substrates. Purified Cyp2d22 was found to catalyze the O-demethylation of dextromethorphan with significantly higher K(m) values (250 microM) than that (4.2 microM) exhibited by purified human CYP2D6. The K(m) for dextromethorphan N-demethylation by Cyp2d22 was found to be 418 microM, much lower than that observed with human CYP2D6 and near the K(m) for dextromethorphan N-demethylation catalyzed by CYP3A4. CYP2D6 catalyzed codeine O-demethylation, whereas Cyp2d22 and CYP3A4 mediated codeine N-demethylation. Furthermore, methadone, a known CYP3A4 substrate and CYP2D6 inhibitor, was N-demethylated by Cyp2d22 with a K(m) of 517 microM and V(max) of 4.9 pmol/pmol/min. Quinidine and ketoconazole, potent inhibitors to CYP2D6 and CYP3A4, respectively, did not show strong inhibition toward Cyp2d22-mediated dextromethorphan O- or N-demethylation. These results suggest that mouse Cyp2d22 has its own substrate specificity beyond CYP2D6-like-deficient activity.

IDENTIFICATION OF HUMAN CYTOCHROME P450 2D6 AS MAJOR ENZYME INVOLVED IN THEO-DEMETHYLATION OF THE DESIGNER DRUGP-METHOXYMETHAMPHETAMINE

p-Methoxymethamphetamine (PMMA) is a new designer drug, listed in many countries as a controlled substance. Several fatalities have been attributed to the abuse of this designer drug. Previous in vivo studies using Wistar rats had shown that PMMA was metabolized mainly by O-demethylation. The aim of the study presented here was to identify the human hepatic cytochrome P450 (P450) enzymes involved in the biotransformation of PMMA to p-hydroxymethamphetamine. Baculovirus-infected insect cell microsomes, pooled human liver microsomes (pHLMs), and CYP2D6 poor-metabolizer genotype human liver microsomes (PM HLMs) were used for this purpose. Only CYP2D6 catalyzed O-demethylation. The apparent K(m) and V(max) values in baculovirus-infected insect cell microsomes were 4.6 +/- 1.0 microM and 92.0 +/- 3.7 pmol/min/pmol P450, respectively, and 42.0 +/- 4.0 microM and 412.5 +/- 10.8 pmol/min/mg protein in pHLMs. Inhibition studies with 1 microM quinidine showed significant inhibition of the metabolite formation (67.2 +/- 0.6%; p < 0.0001), and comparison of the metabolite formation between pHLMs and PM HLMs revealed significantly lower metabolite formation in the incubations with PM HLMs (87.3 +/- 1.1%; p < 0.0001). According to these studies, CYP2D6 is the major P450 involved in O-demethylation of PMMA.

Expression, purification, biochemical characterization, and comparative function of human cytochrome P450 2D6.1, 2D6.2, 2D6.10, and 2D6.17 allelic isoforms

Polymorphism at the cytochrome P450 2D6 (CYP2D6) locus is one of the most widely known causes of pharmacogenetic variability in humans. Our goal is to investigate the intrinsic enzymatic differences that exist among active CYP2D6 isoforms to test the hypothesis that these enzymatic differences are substrate-dependent. Active CYP2D6.1, 2, 10, and 17 holo-enzymes were expressed in vitro and purified to a high degree of homogeneity as confirmed with SDS-polyacrylamide gel electrophoresis, CO-difference spectroscopy, and mass spectral analysis. Purified enzyme was reconstituted with lipid and cytochrome P450 reductase in a 2:1 ratio before kinetic analysis. The reaction rate for dextromethorphan (DXM) O-demethylation, DXM N-demethylation, codeine O-demethylation, and fluoxetine N-demethylation catalyzed by each of the variants was determined. The CYP2D6.10 enzyme was the most impaired, exhibiting an estimated enzyme efficiency (as V(max)/K(m)) 50-fold lower for DXM O-demethylation and 100-fold lower for fluoxetine N-demethylation when compared with CYP2D6.1, whereas no measurable catalytic activity was observed for this variant toward codeine. The atypical DXM N-demethylation pathway catalyzed by this variant decreased only 2-fold in comparison. In the case of CYPD6.17, estimated clearances for each metabolite were decreased 6 to 33%. Likewise, the intrinsic clearance of CYP2D6.2 enzyme was consistently decreased for each reaction examined, indicating that the ultra-rapid metabolizer phenotype sometimes associated with this genotype is not a function of the underlying amino acid substitutions. Overall enzyme efficiencies for the metabolism of each substrate therefore decreased in the order of 2D6.1 > 2D6.2 > 2D6.17 > 2D6.10.

Diversity in the oxidation of substrates by cytochrome P450 2D6: lack of an obligatory role of aspartate 301-substrate electrostatic bonding

Cytochrome P450 (P450) 2D6 was first identified as the polymorphic human debrisoquine hydroxylase and subsequently shown to catalyze the oxidation of a variety of drugs containing a basic nitrogen. Residue Asp301 has been characterized as being involved in electrostatic interactions with substrates on the basis of homology modeling and site-directed mutagenesis experiments [Ellis, S. W., Hayhurst, G. P., Smith, G., Lightfoot, T., Wong, M. M. S., Simula, A. P., Ackland, M. J., Sternberg, M. J. E., Lennard, M. S., Tucker, G. T., and Wolf, C. R. (1995) J. Biol. Chem. 270, 29055-29058]. However, pharmacophore models based on the role of Asp301 in substrate binding are compromised by reports of catalytic activity toward substrates devoid of a basic nitrogen, which have generally been ignored. We characterized a high-affinity ligand for P450 2D6, also devoid of a basic nitrogen atom, spirosulfonamide [4-[3-(4-fluorophenyl)-2-oxo-1-oxaspiro[4.4]non-3-en-4-yl]benzenesulfonamide], with K(s) 1.6 microM. Spirosulfonamide is a substrate for P450 2D6 (k(cat) 6.5 min(-)(1) for the formation of a syn spiromethylene carbinol, K(m) 7 microM). Mutation of Asp301 to neutral residues (Asn, Ser, Gly) did not substantially affect the binding of spirosulfonamide (K(s) 2.5-3.5 microM). However, the hydroxylation of spirosulfonamide was attenuated in these mutants to the same extent (90%) as for the classic nitrogenous substrate bufuralol, and the effect of the D301N substitution was manifested on k(cat) but not K(m). Analogues of spirosulfonamide were also evaluated as ligands and substrates. Analogues in which the sulfonamide moiety was modified to an amide, thioamide, methyl sulfone, or hydrogen were ligands with K(s) values of 1.7-32 microM. All were substrates, and the methyl sulfone analogue was oxidized to the syn spiromethylene carbinol analogue of the major spirosulfonamide product. The D301N mutation produced varying changes in the oxidation patterns of the spirosulfonamide analogues. The peptidometic ritonavir and the steroids progesterone and testosterone had been reported to be substrates for P450 2D6, but the affinities (K(s)) were unknown; these were estimated to be 1.2, 1.5, and 15 microM, respectively (cf. 6 microM for the classic substrate bufuralol). The results are consistent with a role of Asp301 other than electrostatic interaction with a positively charged ligand. H-Bonding or electrostatic interactions probably enhance binding of some substrates, but our results show that it is not required for all substrates and explain why predictive models fail to recognize the proclivity for many substrates, especially those containing no basic nitrogen.

Characterization of dextromethorphan O- and N-demethylation catalyzed by highly purified recombinant human CYP2D6

The O-demethylation of dextromethorphan to dextrorphan in humans is catalyzed primarily by cytochrome P450 2D6 (CYP2D6). However, contrary to conventional wisdom, preparations of recombinant cytochrome P450 (P450) expressed from CYP2D6*1 cDNA also appear to produce significant amounts of 3-methoxymorphinan, the N-demethylated metabolite of dextromethorphan, when assayed in vitro. We hypothesized that both pathways were intrinsic to 2D6 and here further examine the kinetics of formation using a highly purified preparation of CYP2D6 in a reconstituted lipid system. Purified CYP2D6 protein with a measured molecular weight of 55772.0 (55769.6 Da predicted) was reconstituted into an active, lipid-vesicle environment with purified rat cytochrome P450 reductase before the addition of substrate and NADPH. Reaction kinetics were followed, and apparent Michaelis-Menten constants were determined for the appearance of each metabolite by high-pressure liquid chromatography, using both UV and fluorescence detection. In a 2-min assay, purified 2D6 catalyzed the formation of dextrorphan with an apparent K(m) value of 1.9 +/- 0.2 microM and a V(max) value of 8.5 +/- 0.2 nmol/nmol of P450/min and measured simultaneously the formation of 3-methoxymorphinan with an apparent K(m) value of 5000 +/- 700 microM and V(max) value of 176 +/- 12 nmol (nmol of P450)(-1) min(-1). These results indicate that at least two distinct binding orientations exist for dextromethorphan within the active site of CYP2D6.

Coumarin Substrates for Cytochrome P450 2D6 Fluorescence Assays

A set of nine 4-aminomethyl-7-alkoxycoumarin derivatives was synthesized and characterized as substrates for O-dealkylation by recombinant cytochrome P450 2D6, a major human enzyme involved in drug metabolism. Enzymatic O-dealkylation yields 7-hydroxycoumarins, which have useful fluorescence properties. The substrates, which differed in substitution at the amino and 7-hydroxy positions, varied in terms of catalytic efficiency of O-dealkylation and in their selectivity as substrates for cytochrome P450 2D6 in human liver microsomes. Several of the compounds are useful as cytochrome P450 2D6 substrates in single-phase, rapid-throughput assays.

Involvement of human cytochrome P450 2D6 in the bioactivation of acetaminophen

Acetaminophen (APAP), a widely used analgesic and antipyretic agent, can cause acute hepatic necrosis in both humans and experimental animals when consumed in large doses. It is generally accepted that N-acetyl-p-benzoquinone imine (NAPQI) is the toxic, reactive intermediate whose formation from APAP is mediated by cytochrome P450. Several forms of P450 in humans, including 2E1, 1A2, 2A6, 3A4, have been shown to catalyze the oxidation of APAP to NAPQI. We now present evidence which demonstrates that human cytochrome P450 2D6 (CYP2D6) is also involved in the bioactivation of APAP. The formation of NAPQI from APAP by cDNA-expressed CYP2D6 was examined. K(m) and V(max) values were 1.76 mM and 3.02 nmol/min/nmol of P450, respectively, such that the efficiency of CYP2D6 in the conversion of APAP to NAPQI is approximately one-third of that of CYP2E1. The contribution of CYP2D6 to the total formation of NAPQI from APAP (1 mM) in human liver was investigated using quinidine (1 microm) as a CYP2D6-specific inhibitor, and varied from 4.5 to 22.4% among 10 livers, with an average at 12.6%. The correlation between the contribution of CYP2D6 to NAPQI formation in human liver microsomes and the CYP2D6 activity probed by the O-demethylation of dextromethorphan was studied, and found to be strong (r(2) = 0.85), and significant (P <.0001). Our findings indicate that CYP2D6, one of the major P450 isoforms in humans and also one of the pharmacogenetically important isoforms, may contribute significantly to the formation of the cytotoxic metabolite NAPQI, especially in CYP2D6 ultra-rapid and extensive metabolizers and at toxic doses of APAP when plasma APAP concentrations reach 2 mM or more.