Human cytochromes P450 1A1 and 1A2 participate in detoxication of carcinogenic aristolochic acid.

OBJECTIVES: A carcinogenic and nephrotoxic plant alkaloid, aristolochic acid (AA), causes the development of aristolochic acid nephropathy, which is characterized by chronic renal failure, tubulointerstitial fibrosis and urothelial cancer. AA may also cause a similar type of kidney fibrosis with malignant transformation of the urothelium, the Balkan endemic nephropathy. The aim of the study was to resolve which cytochromes P450 (CYP) detoxicate the major component of AA, aristolochic acid I (AAI), to its O-demethylated metabolite, aristolochic acid Ia (AAIa). METHODS: High performance liquid chromatography (HPLC) was employed for separation and characterization of AAI metabolites generated by CYPs. RESULTS: Human, rat and mouse hepatic CYPs oxidize AAI into its detoxication metabolite AAIa. Most of the detoxication of AAI in human hepatic microsomes is mediated by CYP1A2 and 1A1, while other CYPs play a minor role. CONCLUSIONS: The data are the first report on identification of human CYP enzymes detoxicating AAI.

Purification, molecular cloning, heterologous expression and characterization of pig CYP1A2.

Porcine cytochrome P450 (CYP) 1A2 was purified to electrophoretic homogeneity from the hepatic microsomes of beta-naphthoflavone-treated male pigs. In a reconstituted system, this enzyme showed a good catalytic activity towards caffeine, acetanilide, and methoxyresorufin, all known markers of mammalian CYP1A2. Using 3'- and 5'-rapid amplification of coding DNA (cDNA) ends (RACE), we amplified from the liver RNA of control pigs a full-length 1827 bp cDNA containing an open reading frame of 1548 bp which encoded a putative CYP1A2 protein of 516 amino acids and an estimated Mr of 58 380 Da. Reverse transcriptase-polymerase chain reaction (RT-PCR) experiments showed that the messenger RNA (mRNA) of CYP1A2 was expressed in liver, heart and nasal mucosa but not in lung, small intestine, kidney and brain. Using the pCW vector containing a N-terminal modified cDNA, pig CYP1A2 was expressed in Escherichia coli. 3-[(3-Chloroamidopropyl)dimethylmmonio]-1-propane-sulfonate (CHAPS)-solubilized E. coli preparations expressing CYP1A2 produced a functionally isoform which, in a reconstituted system, was catalytically active toward ethoxyresorufin and methoxyresorufin showing K(m)'s similar to those obtained with CYP1A2 purified from pig liver or human recombinant CYP1A2. Taken together, these results demonstrate that domestic pigs have a functionally active CYP1A2 gene well expressed in the liver with biochemical properties quite similar to those corresponding to the human enzyme.

Heterologous expression and characterization of wild-type human cytochrome P450 1A2 without conventional N-terminal modification in Escherichia coli.

In this study, wild-type human CYP1A2 without the conventional N-terminal modification (second codon GCT) or the truncation of the N-terminal hydrophobic region was functionally expressed in Escherichia coli. Its enzymatic properties were compared with N-terminally modified CYP1A2. Although modified CYP1A2 is almost all high-spin, some wild-type CYP1A2 shifted to low-spin. Spectral binding titrations with several ligands could be performed with wild-type enzyme, but not with modified enzyme. Kinetic parameters for several substrates were similar for the two CYP1A2 enzymes. However, the oxidation rates of phenacetin by modified enzyme were approximately 2-fold higher than those by wild-type enzyme. The intermolecular isotope effects were approximately 2 for phenacetin O-deethylation catalyzed by both enzymes. However, the wild-type enzyme, but not the modified enzyme, increased C-hydroxylation when O-deethylation rates were lowered by deuterium substitution. Molecular switching indicates that phenacetin rotates within the active site of wild-type enzyme and suggests a looser conformation in the active site of the wild-type enzyme than of the modified enzyme. These results reveal that the overall enzymatic properties of wild-type CYP1A2 enzyme are quite similar to those of modified CYP1A2, although its active site environment seems to differ from that of the modified enzyme.

High-level expression of human cytochrome P450 1A2 by co-expression with human molecular chaperone HDJ-1(Hsp40).

Human cytochrome P450 (CYP) 1A2 is of great interest because of its important roles in the oxidation of numerous drugs and carcinogens. HDJ-1, a molecular chaperone in human, is known to assist the correct folding of unfolded proteins. To achieve high yield of recombinant human CYP1A2 in Escherichia coli, the CYP1A2 encoding gene was co-expressed with the chaperone HDJ-1 under the control of an inducible tac promoter in bicistronic format. Expression level of CYP1A2 in the bicistronic construct reached up to 520 nmol/liter culture within 16 h at 37 degrees C, which is 3.4-fold increase compared to the expression yield of CYP1A2 alone without HDJ-1. By co-expression with HDJ-1, the catalytic activity of CYP1A2 was also increased 5.5-fold. The activity increase seems to be associated with the increase of CYP production at whole cell level. The present over-expression system may be useful for rapid production of large amounts of active human CYP1A2 in E. coli.

Isolation of two cytochrome P450 cDNAs, CYP1A1 and CYP1A2, from harp seal (Phoca groenlandica) and grey seal (Halichoerus grypus).

Two cytochrome P450 (CYP), CYP1A1 and CYP1A2, cDNA sequences have been isolated and cloned from harp seal (Phoca groenlandica) and grey seal (Halichoerus grypus). EROD, a model substrate for CYP1A, and heterologous antibodies have been employed as a biomarker in marine mammals, however the CYP1A sequences have not been characterised in these two seal species. mRNA was used as the template in RT-PCR, rather than DNA as this indicates transcription of the CYP1A gene in these seal species exposed to environmental contaminants. Harp and grey seal CYP1A1 amino acid sequences exhibited >99% identity and the CYP1A2 sequences were >98% identical. Phylogenetic analyses of the two seal species with other mammalian, and avian CYP1A sequences, showed the CYP1A1 and CYP1A2 sequences clustered with corresponding sequences in other mammalian species. The closest sequences to the seal CYP1As was dog CYP1A. The CYP1A sequence information presented in this study has provided the necessary data for the future production of species-specific probes for the use as biomarkers of environmental contaminant exposure.

Establishment of a human hepatocyte line (OUMS-29) having CYP 1A1 and 1A2 activities from fetal liver tissue by transfection of SV40 LT.

Immortalized human hepatocytes that can retain functions of drug-metabolizing enzymes would be useful for medical and pharmacological studies and for constructing an artificial liver. The aim of this study was to establish immortalized human hepatocyte lines having differentiated liver-specific functions. pSVneo deoxyribonucleic acid, which contains large and small T genes in the early region of simian virus 40, was introduced into hepatocytes that had been obtained from the liver of a 21-wk-old fetus. Neomycin-resistant immortalized colonies were cloned and expanded to mass cultures to examine hepatic functions. Cells were cultured in a chemically defined serum-free medium, ASF104, which contains no peptides other than recombinant human transferrin and insulin. As a result, an immortal human hepatocyte cell line (OUMS-29) having liver-specific functions was established from one of the 13 clones. Expression of CYP 1A1 and 1A2 messenger ribonucleic acid by the cells was induced by treatment with benz[a]pyrene, 3-methylcholanthrene, and benz[a]anthracene. OUMS-29 cells had both the polycyclic aromatic hydrocarbon receptor (AhR) and AhR nuclear translocator. Consequently 7-ethoxyresorufin deethylase activity of the cells was induced time- and dose-dependently by these polycyclic aromatic hydrocarbons. This cell line is expected to be instrumental as an alternative method in animal experiments for studying hepatocarcinogenesis, drug metabolisms of liver cells, and hepatic toxicology.

Influence of ligand binding to human cytochrome P-450 1A2: conformational activation and stabilization by alpha-naphthoflavone.

Human cytochrome P-450 (P-450) 1A2 expressed in Escherichia coli is readily converted into non-native cytochrome P-420 (P-420) in the presence of detergents. alpha-Naphthoflavone (ANF) has been used to prevent P-450 1A2 inactivation to P-420 during purification. However, the mechanism by which ANF modulates P-450 1A2 is not clearly understood. We observed that recombinant human P-450 1A2 prepared in the absence of ANF has an approx. 5 times higher maximum catalytic activity in the O-deethylation of 7-ethoxycoumarin than that in the presence of ANF, with the same K(m) values. The results revealed that the enzyme purified with ANF is not catalytically fully active, indicating that ANF tightly binds to the enzyme, only to be dissociated by heat denaturation. Furthermore, the inactive P-420 form of the enzyme could be reconverted to P-450 by ANF in high concentrations of detergents. The reconversion was concentration-dependent, confirming ANF-induced regeneration of active P-450 1A2. The reconversion coincided with the conformational change of the enzyme including increased alpha-helix content. The conformation of P-450 1A2 was also stabilized by ANF, resulting in an approx. 5 degrees C increase in thermal stability.

Rate-determining steps in phenacetin oxidations by human cytochrome P450 1A2 and selected mutants.

Mutants with altered activities were obtained from random libraries of human cytochrome P450 (P450) 1A2 with the putative substrate recognition sequences (SRS) mutated [Parikh, A., Josephy, P. D., and Guengerich, F. P. (1999) Biochemistry 38, 5283-5289]. Six mutants from SRS 2 (E225I, E225N, F226I, and F226Y) and 4 (D320A and V322A) regions were expressed as oligohistidine-tagged proteins, purified to homogeneity, and used to analyze kinetics of individual steps in the catalytic cycle, to determine which reaction steps have been altered. When the wild-type, E225I, E225N, F226I, F226Y, D320A, and V322A proteins were reconstituted with NADPH-P450 reductase, rates of 7-ethoxyresorufin O-deethylation and phenacetin O-deethylation were in accord with those expected from membrane preparations. Within each assay, the values of k(cat)/K(m) varied by 2-3 orders of magnitude, and in the case of E225I and E225N, these parameters were 7-8-fold higher than for the wild-type enzyme. The coupling efficiency obtained from the rates of product formation and NADPH oxidation was low (<20%) in all enzymes. No correlation was found between activities and several individual steps in the catalytic cycle examined, including substrate binding, reduction kinetics, NADPH oxidation, and H(2)O(2) formation. Quench reactions did not show a burst for either phenacetin O-deethylation or formation of the acetol, a minor product, indicating that rate-determining steps occur prior to product formation. Inter- and intramolecular kinetic deuterium isotope effects for phenacetin O-deethylation were 2-3. In the case of phenacetin acetyl hydroxylation (acetol formation), large isotope effects [(D)k(cat) or (D)(k(cat)/K(m)) > 10] were observed, providing evidence for rate-limiting C-H bond cleavage. We suggest that the very high isotope effect for acetol formation reflects rate-limiting hydrogen atom abstraction; the lower isotope effect for O-deethylation may be a consequence of a 1-electron transfer pathway resulting from the low oxidation potential of the substrate phenacetin. These pre-steady-state, steady-state, and kinetic hydrogen isotope effect studies indicate that the rate-limiting steps are relatively unchanged over an 800-fold range of catalytic activity. We hypothesize that these SRS mutations alter steps leading to the formation of the activated Michaelis complex following the introduction of the first electron.

Cytochrome P450 1A2: oligomers in proteoliposomes.

The presence of oligomers of cytochrome P450 1A2 in membranes of proteoliposomes produced by the cholate-dialysis technique was demonstrated by cross-linking of protein molecules with bifunctional reagents followed by electrophoretic analysis of the modified proteins. A hexameric organization of cytochrome P450 1A2 in the membrane of proteoliposomes is suggested with high probability based on the comparison of the purified hemoprotein oligomeric structure in an aqueous medium and that in the proteoliposomes. The comparison was carried out using the same method.

Orientation of caffeine within the active site of human cytochrome P450 1A2 based on NMR longitudinal (T1) relaxation measurements.

Longitudinal (T1) relaxation studies were performed in order to examine the interaction of caffeine with the heme of human P450 1A2. Addition of caffeine to this P450 resulted in a small, incomplete conversion of the heme from high spin to low spin, as shown by changes in the optical spectrum. Determination of a relatively large dissociation constant (Ks = 2.6 mM) as well as the relative instability of the P450 after 2 h at room temperature necessitated the performance of these experiments at high concentrations (25 mM) of caffeine. The relaxation measurements on the three sets of methyl hydrogens led to the determination of the corresponding distances between the iron and the methyl groups on the bound caffeine as well as the position and orientation of caffeine within the active site of P450 1A2. The three methyl groups were found to be nearly equidistant from the iron (> or = 4.79-4.89 A), with slight preference for the N-3 position, and thus, the average position of caffeine was parallel to the heme. In vitro incubations with P450 1A2 and 5 mM caffeine led primarily to paraxanthine formation (N-3 demethylation), as expected. However, with 25 mM substrate, the overall extent of oxidation was doubled and there was more equivalent oxidation at each of the four potential sites on caffeine. This latter observation was consistent with the lack of selective positioning of the N-3 methyl group of caffeine relative to the heme.

Identification of cytochrome P-450 isoform(s) responsible for the metabolism of pimobendan in human liver microsomes.

Pimobendan, 4, 5-dihydro-6-(2-(4-methoxyphenyl)-1H-benzimidazol-5-yl)-5-methyl-3( 2-H )-pyridazinone, is a new inotropic drug that augments Ca(2+) sensitivity and inhibits phosphodiesterase in cardiomyocytes. Pimobendan is well absorbed after oral administration and is metabolized in the liver to the O-demethyl metabolite, which is also active. This study was conducted to identify the cytochrome P-450 (CYP) isoform(s) responsible for the pimobendan O-demethylation in human liver microsomes. Pimobendan O-demethylase activity in human liver microsomes was significantly correlated with phenacetin O-deethylase activity. CYP1A2 antibody and specific inhibitors of CYP1A2 strongly inhibited the metabolism of pimobendan. CYP1A2 was the only one of 10 recombinant human CYP isoforms tested that catalyzed pimobendan O-demethylation at the substrate concentration of 1 microM. At a high substrate concentration (100 microM), recombinant CYP3A4 also catalyzed the reaction, and antibody to CYP3A4 partially inhibited the activity in human liver microsomes. The contribution of CYP1A2 to pimobendan O-demethylation in human liver microsomes varied in the range of 18 to 76%, whereas CYP3A4 accounted for less than 10%, as calculated using the relative activity factor method. We conclude that CYP1A2 is one of the major enzymes responsible for the O-demethylation of pimobendan and CYP3A may make a minor contribution at clinically relevant concentrations of the drug.

High yield purification and characterization of engineered human P450 1A2 and generation of immuno-inhibitor antibodies.

P450 S12, an engineered human P450 1A2 containing the 88-first amino-acids of the P450 1A1, demonstrates particularly high expression level in yeast while exhibiting catalytic properties very similar to the moderately expressed natural human P450 1A2. To facilitate P450 purification by nickel chelate chromatography, C-terminal extensions including histidine tags were tested. The -G(H)4 extension was found to be particularly efficient for permitting high expression levels without any catalytic alteration. This engineered P450 was purified to electrophoretic homogeneity (18 nmol/mg of protein) at a very high yield (87%) without any detectable formation of P420. P450 S12 activities were reconstituted in the presence of yeast and Arabidopsis thaliana (ATR1) NADPH-P450 reductases. The plant reductase supported better ethoxyresorufin-, methoxyresorufin- and phenacetin-O-dealkylase activities than the yeast reductase in reconstituted systems. Interestingly, polyclonal antibodies raised against purified P450 S12 selectively recognized in Western blot and fully immuno-inhibited the natural or recombinant P450 1A2 with very limited or no cross-reaction with P450 1A1 and other isoenzymes.

Expression, purification, and characterization of a catalytically active human cytochrome P450 1A2:rat NADPH-cytochrome P450 reductase fusion protein.

An enzymatically active human cytochrome P450 (P450) 1A2:rat NADPH-P450 reductase fusion protein was purified and partially characterized following heterologous expression in Escherichia coli. A cDNA was engineered to include the coding sequence for human P450 1A2 at its 5' end (up to but not including the stop codon) fused in-frame to the coding sequence for a truncated (soluble) rat NADPH-P450 reductase at its 3' end via an oligonucleotide sequence encoding the hydrophilic dipeptide Ser-Thr. This fusion plasmid was expressed in E. coli and the recombinant protein was purified from the detergent-solubilized membrane fraction via sequential DEAE, ADP-agarose, and hydroxylapatite chromatographies. The purified protein has the spectral characteristics of human P450 1A2 and cytochrome c reduction activity comparable to rabbit NADPH-P450 reductase. The fusion protein catalyzed 7-ethoxyresorufin O-deethylation and phenacetin O-deethylation to appreciable levels in the presence of NADPH and phospholipid. While these activities were comparable to those of other such P450:NADPH-P450 reductase fusion proteins, they were lower than those of the system reconstituted from its individual hemoprotein and flavoprotein components. Nevertheless, the production of a functional, catalytically self-sufficient monooxygenase in E. coli enhances the prospect of using bacterial systems for production and characterization of human P450 drug metabolites as well as for biodegradation of chemicals in the environment.

Intermolecular interactions of antimicrobial fluoroquinolones with purified rat liver CYP1A2 studied by proton nuclear magnetic resonance spectroscopy.

1. Binding and inhibition of antimicrobial fluoroquinolones towards liver CYP1A2 purified from 3-methylcholanthrene-treated rats were investigated using proton nuclear magnetic resonance (nmr) and phenacetin metabolism. 2. The proton nmr longitudinal relaxation rate study indicated that the paramagnetic effects of the haem iron of CYP1A2 were observed in protons of enoxacin with a 1,8-naphthyridine skeleton and its 4'-nitrogen atom on the 7-piperazine ring probably participated in specific binding to the haem iron. These data suggest a facile accessibility and strong binding of enoxacin to the active site of the enzyme. On the contrary, the binding region of norfloxacin with a quinoline skeleton could not be specified, and an 8-fluorinated derivative (AT-3970) had much lower paramagnetic effects and no specific binding region. 3. In a reconstituted CYP1A2 system, enoxacin exhibited the most potent inhibition of phenacetin O-deethylation. The metabolism was less inhibited by norfloxacin, and AT-3970 had a weak inhibitory activity. 4. The binding ability of the fluoroquinolones to the CYP1A2 active site is likely to determine their inhibitory activity against phenacetin metabolism.