Monooxygenase activity of Saccharomyces cerevisiae cells transformed with expression plasmids carrying rat cytochrome P-450MC cDNA

Serum derived from ulcerative colitis mouse changes the metabolism of the fluorescent substrate by P450 depending on the degree of disease progression

Ulcerative colitis (UC) is characterized by erosions of the intestinal mucosa. The number of patients with UC has recently been increasing rapidly. Since the diagnosis of UC is complex and difficult, a simple, rapid, noninvasive technique for diagnosing UC is needed urgently. The expression of cytochrome P450 (P450 or CYP) species in mouse liver is known to be changed dependent on the disease. Various components such as P450 substrates and P450 metabolites in the blood may possibly change with the UC-specific way in mouse. In this study, in order to evaluate UC-specific components in UC mouse serum, we analyzed the influence of serum derived from UC mice on the results of fluorescent P450 inhibition assays based on 12 human P450 enzymes, such as CYP1A1, CYP2C8, CYP2E1,CYP3A4, CYP1A2, CYP2D6, CYP2A13, CYP2B6, CYP2C9, CYP2C18, CYP2C19, and CYP3A5. At first, in order to induce UC, mice received 4,4-dimethyl-4-silapentane-1-sulfonic acid (DSS) dissolved in their drinking water for 7 days. Next, these 12 human P450 enzymes were expressed in E. coli cells. Then, P450 fluorescent competition reaction was performed using these P450 enzymes and serum of UC mice. We found that the metabolism of fluorescent substrates by CYP2B6, CYP2C19, CYP2E1, and CYP1A1 in the presence of serum obtained from DSS-treated mice was activated by 42%, 37%, 37%, and 23%, respectively, relative to that associated with sera from control mice. A receiver operating characteristic (ROC) curve analysis was carried out with the 31 samples of UC mice and healthy mice. Area under the ROC curve (AUC) value was calculated from ROC curve. AUC value of CYP2E1 and CYP2C19 showed 0.921 and 0.892, respectively. Therefore, it was shown that CYP2C19 and CYP2E1 could be used as biomarkers for evaluating ulcerative colitis. From these results, it is suggested that these simple fluorescent P450 inhibition assays have potential as a new diagnostic procedure for UC in mouse. This study is the first report on a simple non-invasive method for evaluating UC using P450 enzyme and serum interaction.

Mammalian cytochrome P450-dependent metabolism of polychlorinated dibenzo-p-dioxins and coplanar polychlorinated biphenyls

Polychlorinated dibenzo-p-dioxins (PCDDs) and coplanar polychlorinated biphenyls (PCBs) contribute to dioxin toxicity in humans and wildlife after bioaccumulation through the food chain from the environment. The authors examined human and rat cytochrome P450 (CYP)-dependent metabolism of PCDDs and PCBs. A number of human CYP isoforms belonging to the CYP1 and CYP2 families showed remarkable activities toward low-chlorinated PCDDs. In particular, human CYP1A1, CYP1A2, and CYP1B1 showed high activities toward monoCDDs, diCDDs, and triCDDs but no detectable activity toward 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-tetraCDD). Large amino acids located at putative substrate-recognition sites and the F-G loop in rat CYP1A1 contributed to the successful metabolism of 2,3,7,8-tetraCDD. Rat, but not human, CYP1A1 metabolized 3,3',4,4',5-pentachlorobiphenyl (CB126) to two hydroxylated metabolites. These metabolites are probably less toxic than is CB126, due to their higher solubility. Homology models of human and rat CYP1A1s and CB126 docking studies indicated that two amino acid differences in the CB126-binding cavity were important for CB126 metabolism. In this review, the importance of CYPs in the metabolism of dioxins and PCBs in mammals and the species-based differences between humans and rats are described. In addition, the authors reveal the molecular mechanism behind the binding modes of dioxins and PCBs in the heme pocket of CYPs.

Practical application of mammalian cytochrome P450

Heterologous expression systems play an important role in the analysis of structure-function relationships of mammalian P450s. In addition, these expression systems allow practical application of mammalian P450s. Genetically engineered fused enzymes between mammalian P450 and yeast NADPH-P450 reductase have possible applications in bioconversion processes. Combined use of techniques reported thus far could produce steroid hormones in the recombinant yeast cells harboring four P450 species, CYP11A1, CYP17A1, CYP21B1 and CYP11B1. In an Escherichia coli expression system, the technology of the construction of the mitochondrial P450 electron transport chain has been established. The recombinant E. coli cells expressing CYP27B1, adrenodoxin and NADPH-adrenodoxin reductase would be applicable to a bioconversion process to produce 1alpha,25-dihydroxyvitamin D3. We also demonstrated the usefulness of heterologous expression systems for human liver microsomal P450s for the prediction of drug metabolism in the human body. Microsomal fractions prepared from recombinant yeast, insect and mammalian cells are commercially available and play an important role in preclinical drug development. Application of mammalian P450 to bioremediation with genetic engineering has also been developed. Thus, mammalian P450s appear to have great potential for a wide range of practical applications.

Expression of drug-metabolizing enzymes

A principal advance in the production of drug-metabolizing enzymes has been the development of catalytically self-sufficient cytochrome P450 systems, including additional P450-reductase fusion proteins and Escherichia coli and baculovirus coexpression constructs. Continuing work with glutathione transferases has resulted in the identification of important residues by random mutagenesis screening techniques, as well as in the engineering of model Salmonella typhimurium strains for genotoxicity analysis.

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.

Identification of retained N-formylmethionine in bacterial recombinant mammalian cytochrome P450 proteins with the N-terminal sequence MALLLAVFL...: roles of residues 3-5 in retention and membrane topology

An N-terminal block to Edman degradation was observed when any of five different mammalian cytochrome P450 (P450) proteins was expressed in Escherichia coli using the N-terminal sequence MALLLAVFL... This block was also seen in Salmonella typhimurium. With all proteins examined, the block could be removed by mild acid hydrolysis (0.6--6 N HCl, 23 degrees C) to expose Met as the N-terminus, suggesting N-formylMet retention. The N-terminal peptide of a modified P450 1A2 ("mutant 1", containing a thrombin-sensitive site inserted at residue 25) was released with thrombin and analyzed by electrospray mass spectrometry and found to yield the M(r) expected for the N-formyl derivative (+/- 0.8 amu). The region of positions 3--5 was altered by random mutagenesis, and three P450 1A2-expressing clones were analyzed for nucleotide and amino acid sequences. The changes from LLL were to RER (P450 1A2a), VDS (P450 1A2b), and WRH (P450 1A2c); these all show slightly dissimilar hydropathy plots compared to the MALLLAVFL... sequence. Mutant P450 1A2a had the N-terminal Met removed to yield N-terminal Ala; P450 1A2b contained an unmodified Met at the N-terminus; P450 1A2c had an approximately 80% block of the N-terminal Met. Experiments with bacterial membranes containing expressed P450 1A2 mutant 1 and P450 1A2 mutant 2 (thrombin-sensitive site inserted at residue 46) suggest that thrombin site 2, but not 1, is sequestered in the membrane. Spheroplasts of bacteria expressing P450 1A2 and the mutants at positions 3--5 were treated with proteinase K; amino acid analysis indicated that no cleavage occurred. These results are interpreted in a model in which most of the mammalian P450 expressed in the bacterium is located in the cytosol, the region near residue 46 is in the inner membrane, the region near residue 25 is in the cytosol, and the N-terminus is either imbedded in the membrane or free in the cytosolic space, depending upon the sequence. However, the possibility that the differences in N-terminal processing are the result of direct changes in interactions with the deformylase and Met aminopeptidase cannot be excluded.

Overexpression of integral membrane proteins for structural studies

Determination of the structure of integral membrane proteins is a challenging task that is essential to understand how fundamental biological processes (such as photosynthesis, respiration and solute translocation) function at the atomic level. Crystallisation of membrane proteins in 3D has led to the determination of four atomic resolution structures [photosynthetic reaction centres (Allenet al. 1987; Changet al. 1991; Deisenhofer & Michel, 1989; Ermleret al. 1994); porins (Cowanet al. 1992; Schirmeret al. 1995; Weisset al. 1991); prostaglandin H2synthase (Picotet al. 1994); light harvesting complex (McDermottet al. 1995)], and crystals of membrane proteins formed in the plane of the lipid bilayer (2D crystals) have produced two more structures [bacteriorhodopsin (Hendersonet al. 1990); light harvesting complex (Kühlbrandtet al. 1994)].

Formation of 4,4'-methylene-bis(2-chloroaniline)-DNA adducts in yeast expressing recombinant cytochrome P450s

N-Oxidation of 4,4'-methylene-bis(2-chloroaniline) (MBOCA) may lead to formation of DNA adducts. To determine if cytochrome P450s are involved in the formation of MBOCA derived-DNA adducts, yeast strains expressing rodent P450s were exposed to MBOCA, and 32P-postlabelling of nucleotides from yeast genomic DNA was done. Chromatographic analysis on PEI cellulose showed that, upon exposure to MBOCA for 1 h, nine DNA adducts were formed in yeast expressing phenobarbital-inducible rabbit P450 2B5. With a 4-h-exposure, all adducts increased in parallel. In cell-free experiments, the incubation of MBOCA with phenobarbital-induced rat microsomal fraction followed by incubation with thymus DNA, led to the formation of more than ten DNA adducts. When yeast expressing 3-methylcholanthrene-inducible rat P450 1A1 was exposed to MBOCA, one major and two minor adducts were formed. No adducts were detected in control yeast. These results show that recombinant rabbit P450 2B5 exhibits a potential activation of MBOCA and that rat P450 1A1 has some effect. The use of yeast expressing recombinant P450s and the technique of 32P-postlabelling facilitates a simple search for chemicals with carcinogenic potential.

Artificial P450/reductase fusion enzymes: what can we learn from their structures?

Cytochromes P450 constitute a superfamily of hemoproteins which have evolved from a common ancestor. Recent advances in molecular biology have offered a powerful approach to the research on the multiplicity of P450. Now, every mammalian P450 species can be characterized by heterologous expression of its cDNA. In addition, a heterogous expression system is also useful for analysis of structure-function relationships of P450 monooxygenases. Comparison of enzymatic activity and expression level in yeast of a series of artificial genetic fusion enzymes of P450 with P450 reductase has revealed the strategy of how to construct a most suitable fusion. The P450/reductase fused enzyme is a simplified monooxygenase as compared with the two enzyme systems in nature. The P450 domain of the fused enzyme is bound to the microsomes, while the reductase domain lies on the cytoplasmic side, moving flexibly. This structural feature seems to reflect the topology of both enzymes in mammalian microsomes, and will be used as a model to analyze in vivo protein-protein interactions of microsomal P450 monooxygenases. Combined with the discovery of some naturally occurring P450/reductase fusions from bacteria to mammals, comparison of these natural enzymes with artificial ones will be discussed.

Optimization of Candida tropicalis cytochrome P450alk gene expression in Saccharomyces cerevisiae with continuous cultures

The cytochrome P450alk gene (P45alk) from Candida tropicalis ATCC 750 was expressed in Saccharomyces cerevisiae GRF18 under control of the alcohol dehydrogenase I (ADHI) promoter. To achieve stable expression over long time periods, a 2-microns derived replicative and an integrative expression system were tested in continuous culture. The 2-microns derived replicative system could not be maintained in cells over high generation numbers. In continuous culture, the instability was more pronounced at high dilution rates (D) and high histidine concentration, for which the yeast is auxotrophic. The nature of the instability was probably due to a gene conversion event between the plasmid and the yeast chromosome. In contrast, the integrative expression system was stably maintained in cells over prolonged cultivation times. Since this work focused on the production of large quantities of P450 by heterologous expression in yeast over prolonged time periods, the integrant was used to optimize P450alk expression by varying continuous culture parameters. The P450alk expression was shown to be dependent on the D applied to the culture. The highest P450alk expression levels were obtained at high D, when cell metabolism was shifted to partial glucose oxidation, yielding ethanol as a major metabolite in the culture supernatant. In contrast, when glucose was completely oxidized at low D, the ADHI-dependent P450alk expression was reduced and followed by a corresponding decrease in heterologous protein.

Functional expression of mammalian cytochromes P450IIB in the yeast Saccharomyces cerevisiae

Three mammalian cytochromes P450 from the IIB subfamily, P450IIB11 from canine and P450IIB4 and P450IIB5 from rabbit, have been expressed in the yeast Saccharomyces cerevisiae by use of an autonomously replicating vector containing the galactose-inducible gal10 promoter. Cytochromes P450IIB4 and P450IIB5 are closely related proteins, with only 11 amino acid substitutions between them. P450IIB11 is a homologous protein, likely orthologous with IIB4 or IIB5, with 102 amino acid substitutions compared with the P450IIB4 protein and 106 compared with the P450IIB5 protein. The expressed proteins are functional in yeast microsomes, exhibiting activity toward androstenedione, 7-ethoxycoumarin, and, in some cases, progesterone. Expressed cytochromes P450IIB4 and P450IIB11 hydroxylate androstenedione with regio- and stereoselectivity characteristic of the purified, reconstituted proteins. A striking difference in the androstenedione metabolite profiles of IIB4 and IIB5 was observed, with IIB4 producing almost exclusively the 16 beta-hydroxy metabolite and IIB5 producing the 16 alpha-hydroxy and 15 alpha-hydroxy products. This is the first time that 15 alpha-hydroxylase activity has been associated with IIB4/IIB5. This activity has also been detected in liver microsomes from some, but not all, individual phenobarbital-induced rabbits tested and is largely inhibited by anti-rabbit P450IIB immunoglobulin G. These studies illustrate the utility of the yeast expression system for defining catalytic activities of individual mammalian cytochromes P450 and identifying new marker activities that can be utilized in liver microsomes.

Expression of bovine cytochrome P450c21 and its fused enzymes with yeast NADPH-cytochrome P450 reductase in Saccharomyces cerevisiae

Recombinant plasmids for expression of bovine cytochrome P450c21 (pA gamma 2), both P450c21 and yeast NADPH-cytochrome P450 reductase (pAR gamma 1), P450c21/yeast reductase fused enzymes (pAF gamma R1, pAF gamma R2, and pAF gamma R20), and yeast reductase/P450c21 fused enzymes (pAFR gamma 1 and pAFR gamma 2) were constructed by using expression vector pAAH5. The plasmids were each introduced into the yeast Saccharomyces cerevisiae AH22 cells. The recombinant yeast strains AH22/pA gamma 2 (Y21) and AH22/pAR gamma 1 (Y21R) produced 2-3 X 10(3) molecules of P450c21 per cell. The cultures of both strains converted progesterone and 17 alpha-hydroxyprogesterone into 11-deoxycorticosterone and 11-deoxycortisol, respectively. The 21-hydroxylase activity per cell of the strain Y21R was about three times higher than that of the strain Y21, probably due to overproduction of yeast reductase. The recombinant yeast strains AH22/pAF gamma R1 (Y21RF1), AH22/pAF gamma R2 (Y21RF2), and AH22/pAF gamma R20 (Y21RF20) produced about 1.1-2.0 X 10(4) molecules per cell of the corresponding P450c21/yeast reductase fused enzymes. The specific 21-hydroxylase activity toward 17 alpha-hydroxyprogesterone per cell of the strains Y21RF1, Y21RF2, and Y21RF20 was about 21, 28, and 49 times higher than that of the strain Y21, respectively. Thus, the fused enzymes were superior to P450c21 in the specific activity and in the expression level in the yeast. The Km values for 17 alpha-hydroxyprogesterone of P450c21 in the strains Y21 and Y21R, and of the fused enzymes in the strains Y21RF1 and Y21RF2 were 0.29, 0.30, 0.67, and 0.65 microM, respectively. The Vmax values of P450c21 in the strains Y21 and Y21R, and of the fused enzymes in the strains Y21RF1 and Y21RF2 were 28, 124, 151, and 222 moles/min.mole P450c21 or fused enzyme, respectively. These results indicated that the fused enzymes showed lower affinity for the substrate, probably due to structural modification and higher reaction rates through efficient intramolecular electron transfer as compared with those of P450c21. While the strain AH22/pAFR gamma 2 (YR21F2) produced about 3 X 10(4) molecules per cell of the reductase/P450c21 fused enzyme, the specific 21-hydroxylase activity of the fused enzyme toward 17 alpha-hydroxyprogesterone was extremely low, suggesting that the structure of the fused enzyme might not be suited for electron transfer in yeast microsomes.

Expression of alcohol-inducible rabbit liver cytochrome P-450 3a (P-450IIE1) in Saccharomyces cerevisiae with the copper-inducible CUP1 promoter

The expression of the cDNA for alcohol-inducible rabbit liver microsomal cytochrome P-450 form 3a (P450IIE1) in Saccharomyces cerevisiae, with the use of the copper-inducible yeast metallothionein (CUP1) promoter and the ADH1 promoter, is described. Strains 50.L4 and PP1002 were compared for optimal levels of expressed protein. Immunoblot analysis showed that a much higher level of expression of cytochrome P-450 3a is obtained with strain 50.L4, and that the uninduced levels of expressed protein are similar with the two promoters. With the CUP1 promoter, transcription of the cDNA is strongly induced in the presence of cupric ions, and the amount of immunoreactive protein expressed in increased 20-fold in strain 50.L4, such that it constitutes 0.8% of the total cellular protein. The cytochrome P-450 holoenzyme content of these cells, calculated from the reduced CO difference spectrum, is about 0.02 nmole/mg of protein, or 0.1% of the total cellular protein. The holoenzyme content of microsomes prepared from these cells is up to 0.06 nmole/mg of protein, or 0.4% of the microsomal protein. Microsomal assays for ethylene formation from N-nitrosodiethylamine and for aniline p-hydroxylation, two reactions typical of purified rabbit cytochrome P-450 form 3a, showed that the cytochrome synthesized in yeast catalyzes both reactions. Furthermore, polyclonal anti-3a IgG completely inhibits the reactions with both substrates in yeast microsomes. A comparison of the product ratios from these substrates showed that the cytochrome P-450 3a expressed in yeast has catalytic activities similar to those of the authentic rabbit protein.

Genetically engineered modification of P450 monooxygenases: functional analysis of the amino-terminal hydrophobic region and hinge region of the P450/reductase fused enzyme

Modified constructions of a microsomal cytochrome P450, of NADPH-cytochrome P450 reductase, and of a P450/reductase fused enzyme were prepared to analyze the function of the amino-terminal hydrophobic regions of these enzymes and the hinge region of the fused enzyme. Expression plasmids for delta P450c, delta reductase, and the delta P450/reductase fused enzyme, all of which lacked their amino-terminal hydrophobic regions, were constructed by inserting each of the corresponding cDNAs between the yeast alcohol dehydrogenase I promoter and the terminator of the expression vector pAAH5. Yeast transformed with plasmids encoding delta P450 and the delta P450/reductase fused enzyme produced smaller amounts of the respective enzymes and showed lower monooxygenase activity toward 7-ethoxycoumarin than did yeast transformed with plasmids encoding the complete enzymes. Both delta P450 and delta P450/reductase were found in the microsomal fraction of the yeast cells. Yeast transformed with the expression plasmid for delta reductase produced 20 times more enzyme than did yeast transformed with the plasmid for the complete enzyme. delta Reductase was present in the soluble fraction and was 33 times more active in reducing cytochrome c than was the complete enzyme. The results suggest that the amino-terminal hydrophobic regions of P450c and the P450/reductase fused enzyme play an important role in their stability and function in the yeast microsomes. By contrast, the amino-terminal-containing P450 reductase appears to be unstable in yeast cells. Altering the size of the hinge regions does not affect the activity of the P450/reductase fused enzyme significantly, but some amino acid changes in this region increase the stability of the fused enzyme slightly.

A genetically engineered P450 monooxygenase: construction of the functional fused enzyme between rat cytochrome P450c and NADPH-cytochrome P450 reductase

A hybrid cDNA encoding a fused enzyme consisting of rat cytochrome P450c and rat NADPH-cytochrome P450 reductase was constructed by combining the cytochrome P450c cDNA with the cDNA fragment encoding the protease-solubilized moiety of the NADPH-cytochrome P450 reductase. The hybrid cDNA was inserted between the yeast alcohol dehydrogenase I promoter and terminator of the expression vector pAAH5 to yield expression plasmid pAMP19. Saccharomyces cerevisiae AH22 cells transformed with the expression plasmid pAMP19 produced a 130-kD protein reactive with both anti-cytochrome P450c Ig and antireductase Ig. The yeast cells containing the fused enzyme exhibited about four times higher monooxygenase activity toward 7-ethoxycoumarin than those containing rat cytochrome P450c alone. The fused enzyme was purified from the yeast microsomal fraction by sequential chromatography with DEAE-cellulose and 2',5'-ADP Sepharose 4B columns. The preparation had an apparent molecular weight of 130 kD and the same sequence of the 10 amino-terminal amino acids as that of rat cytochrome P450c. Spectral properties of the fused enzyme indicated the presence of a protoheme, flavin adenine dinucleotide, and flavin mononucleotide in the molecule. The reaction mechanism of the fused enzyme followed first-order kinetics. These results clearly indicate that the fused enzyme is a new self-catalytic P450 monooxygenase. Trypsin treatment of yeast microsomes containing the fused enzyme suggested that the P450 moiety is embedded in the microsomal membrane with the reductase moiety lying on the cytoplasmic side.

Expression in Saccharomyces cerevisiae of chimeric cytochrome P450 cDNAs constructed from cDNAs for rat cytochrome P450c and P450d

Three chimeric cytochrome P450 cDNAs were constructed by replacing the central region, carboxy-terminal region, or both central and carboxy-terminal regions of cytochrome P450c cDNA with the corresponding regions of cytochrome P450d cDNA. These were inserted between the alcohol dehydrogenase I promoter and terminator of yeast expression vector pAAH5 to form expression plasmids pACDC2, pACCD1, and pACDD2. On introduction of each of these plasmids into Saccharomyces cerevisiae AH22 cells, chimeric cytochrome P450 proteins were expressed in AH22/pACDC2, AH22/pACCD1, and AH22/pACDD2 cells at the level of at least 10(5), 4 X 10(5) molecules per cell, respectively. The reduced CO-difference spectra showed that AH22/pACCD1 and AH22/pACDD2 cells contained 4 X 10(5) and 10(5) molecules per cell of the corresponding chimeric cytochrome P450 hemoproteins, designated as cytochrome P450ccd and cytochrome P450cdd, respectively. Cytochrome P450ccd exhibited higher monooxygenase activities toward 7-ethoxycoumarin, acetanilide, and benzo[alpha]pyrene than cytochrome P450c, although the substrate specificity of cytochrome P450ccd seemed to be the same as that of cytochrome P450c. Cytochrome P450cdd exhibited lower activities toward 7-ethoxycoumarin and benzo[alpha]pyrene, and a higher activity toward acetanilide as compared with those of cytochrome P450c and cytochrome P450ccd. Therefore, the substrate specificity of cytochrome P450cdd seemed to be the same as that of cytochrome P450d. These results suggest that the central one-third region of cytochrome P450c and cytochrome P450d is responsible for substrate-binding, and that the carboxy-terminal third of both cytochromes P450 plays an important role in electron transport.