Characterization of rat cytochrome P-450MC synthesized in Saccharomyces cerevisiae

Whole-cell dependent biosynthesis of N- and S-oxides using human flavin containing monooxygenases expressing budding yeast

Flavin containing monooxygenases (FMOs) represent one of the predominant types of phase I drug metabolizing enzymes (DMEs), and thus play an important role in the metabolism of xeno- and endobiotics for the generation of their corresponding oxides. These oxides often display biological activities, however they are difficult to study since their chemical or biological synthesis is generally challenging even though only small amounts are required to evaluate their efficacy and safety. Previously, we constructed a DME expression system for cytochrome P450, UDP-glucuronosyltransferase (UGT), and sulfotransferase (SULT) using yeast cells, and successfully produced xenobiotic metabolites in a whole-cell dependent manner. In this study, we developed a heterologous expression system for human FMOs, including FMO1-FMO5, in Saccharomyces cerevisiae and examined its N- and S-oxide productivity. The recombinant yeast cells expressed each of the FMO successfully, and the FMO4 transformant produced N- and S-oxide metabolites at several milligrams per liter within 24 h. This whole-cell dependent biosynthesis enabled the production of N- and S-oxides without the use of the expensive cofactor NADPH. Such novel yeast expression system could be a powerful tool for the production of oxide metabolites.

Whole-cell-dependent biosynthesis of sulfo-conjugate using human sulfotransferase expressing budding yeast

Cytosolic sulfotransferases (SULTs), one of the predominant phase II drug metabolizing enzymes (DME), play important roles in metabolism of xeno- and endobiotics to generate their sulfo-conjugates. These sulfo-conjugates often have biological activities but are difficult to study, because even though only small amounts are required to evaluate their efficacy and safety, chemical or biological synthesis of sulfo-conjugatesis is often challenging. Previously, we constructed a DME expression system for cytochrome P450 and UGT, using yeast cells, and successfully produced xenobiotic metabolites in a whole-cell-dependent manner. In this study, we developed a yeast expression system for human SULTs, including SULT1A1, 1A3, 1B1, 1C4, 1E1, and 2A1, in Saccharomyces cerevisiae and examined its sulfo-conjugate productivity. The recombinant yeast cells expressing each of the SULTs successfully produced several hundred milligram per liter of xeno- or endobioticsulfo-conjugates within 6 h. This whole-cell-dependent biosynthesis enabled us to produce sulfo-conjugates without the use of 3'-phosphoadenosine-5'-phosphosulfate, an expensive cofactor. Additionally, the production of regiospecific sulfo-conjugates of several polyphenols was possible with this method, making this novel yeast expression system a powerful tool for uncovering the metabolic pathways and biological actions of sulfo-conjugates.

cDNA-directed expression of a functional zebrafish CYP1A in yeast

A cytochrome P450 1A (CYP1A) cDNA was isolated from an adult zebrafish (Danio rerio) library. The 2580-bp clone (GenBank Accession No. AF210727) contained a 62-bp 5'-unstranslated region (UTR), 1557-bp coding region and 962-bp 3'-UTR. The deduced 519-residue protein (calculated molecular weight 58,556, pI = 7.58) shared 74% identity with rainbow trout CYP1A and 57 and 54% identities with mouse and human CYP1A1s, respectively. The zebrafish CYP1A protein coding region was cloned into the pDONR201 entry vector and then transferred to a yeast expression vector pYES-DEST52. Expression of zebrafish CYP1A in Saccharomyces cerevisiae transformants was induced by galactose to a maximum level of 493 pmol CYP1A per mg microsomal protein or about 8 nmol/l of culture. Recombinant CYP1A protein expressed in yeast was mainly in the denatured P420 form under normal microsomal preparation conditions but when the oxygen concentration was reduced in the buffer by degassing and the yeast cells were maintained at less than 10 degrees C, the integrity of the CYP1A was preserved and it exhibited a characteristic reduced CO-difference spectrum maximum at 448 nm. The recombinant zebrafish CYP1A demonstrated 7-ethoxyresorufin O-deethylase (EROD) activity with an apparent Km (Km(app)) and Vmax values at 30 degrees C of 0.31 +/- 0.04 microM and 0.70 +/- 0.10 nmol/min/nmol CYP, respectively. The recombinant protein also metabolized benzo(a)pyrene with a Km(app) and Vmax values of 5.34 +/- 0.58 microM and 1.16 +/- 0.13 nmol/min/nmol CYP, respectively. These results show the recombinant expression of a functional zebrafish CYP in yeast and validated yeast as a host for heterologous expression of zebrafish CYP1A and potentially for other zebrafish CYPs.

Comparison in metabolic activity of cytochrome P450 1A1 on heterocyclic amines between human and rat

In mutagenicity and antimutagenicity tests, the toxicants have been activated to the ultimate mutagenic forms usually with rat cytochrome P450 (CYP) enzymes. An understanding is important of whether these data can be available for human. In this paper are compared the activating abilities of CYP1A1 between human and rat using recombinant yeast cells that express respective CYP1A1 and yeast NADPH-CYP-oxidoreductase simultaneously. Three different types of dietary procarcinogens, heterocyclic amines, were tested by two methods: a bioassay with Salmonella mutagenicity test and a chemical determination of N-hydroxyls as the ultimate mutagenic forms. Compared with ED(50) values, saturation levels, and V(max)/K(m) values at an initial stage of the enzyme activity, human and rat CYP1A1 showed almost similar abilities for the metabolic activation on heterocyclic amines. The two enzymes also had the same preference for the tested procarcinogens and the same affinities to the specific inhibitors such as flavonoids.

Dynamic mobility of genetically expressed fusion protein between cytochrome P4501A1 and NADPH-cytochrome P450 reductase in yeast microsomes

A fusion protein of rat liver CYP1A1 with NADPH-cytochrome P450 reductase was expressed genetically in yeast microsomal membranes. This flavo-cytochrome is active in 6-hydroxylation of zoxazolamine. Rotational diffusion of the fusion protein was examined by observing the flash-induced absorption anisotropy r(t) of the P450.CO complex. Theoretical analysis of r(t) was performed based on a "rotation-about-membrane normal" model. The absorption anisotropy decayed within 2 ms to a time-independent value r(3). Forty percent of the fusion protein rotated with a rotational relaxation time phi of 1.35 ms. Treatment with high salt increased the mobile population of the fusion protein to 62% with phi = 0.96 ms. The mobile population of the fusion protein is close to that of CYP1A1 coexpressed with the P450 reductase and greater than that of CYP1A1 alone [Iwase et al. (1991) Biochemistry 30, 8347-8351]. The large mobile population of the fusion protein provides evidence that CYP1A1 is mobilized by forming associations with P450 reductase in microsomal membranes.

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.

Expression of human cytochrome P450 enzymes in yeast and bacteria and relevance to studies on catalytic specificity

Heterologous expression systems can be utilized to great advantage in the study of cytochrome P450 (P450) and other enzymes involved in the biotransformation of drugs and other xenobiotics. The list of studies made possible with the technology includes discernment of catalytic specificity, elucidation of structure-activity relationships, and various biophysical measurements. There are advantages and disadvantages to each of the vector systems and choices must be made on the basis of needs. Yeast expression systems were used to establish that different P450 2C enzymes are involved in the hydroxylations of tolbutamide and (S)-mephenytion. P450 3A4 was also expressed in yeast and its very broad catalytic specificity was confirmed. Recently, it has been possible to express P450 3A4 as well as other human and animal P450s in bacteria after slight modification of their 5'-coding sequences.

Cytochrome P450 4A4: expression in Escherichia coli, purification, and characterization of catalytic properties

Rabbit lung prostaglandin omega-hydroxylase (P450 4A4) was expressed in Escherichia coli using the isopropyl beta-D-thiogalactopyranoside (IPTG) inducible expression vector pCWori+, containing the full-length cDNA encoding the P450 4A4. The first seven codons were changed to reflect E. coli codon bias [a modification of the method of Barnes et al. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 5597-5601]; only the second residue of P450 4A4 was altered (Ser to Ala), while the remaining mutations were silent. This strategy was adopted in order to minimize changes in the structure of the expressed enzyme. Induction by IPTG of the apoprotein peaked after 6 h, and by including the heme precursor delta-aminolevulinic acid, enzymatic activity peaked 12 h after addition of IPTG. The isolated membrane fraction, free of cell debris, contained 12-15 nmol of P450/L of media. The expressed enzyme was purified to electrophoretic homogeneity, and kinetic and spectrophotometric data indicate that this expressed, purified enzyme is equivalent to the enzyme purified from rabbit lung. The Km for PGE1 was determined to be 3.0 microM, which is the same as that obtained for the enzyme purified from lung [Williams et al. (1984) J. Biol. Chem. 259, 14600-14608]. The CO-reduced difference spectrum of purified P450 4A4 exhibited a lambda max at 450 nm, and the absolute absorbance spectrum of the pyridine hemochromogen revealed a typical b type heme. To characterize P450 4A4 further, the catalytic activities with prostaglandin E1 (PGE1), arachidonate, 15-hydroxyeicosatetraenoic acid (15-HETE), and palmitate were investigated.(ABSTRACT TRUNCATED AT 250 WORDS)

Recombinant DNA approaches for the development of metabolic systems used in in vitro toxicology

In the past few years there has been considerable progress in the development of mammalian cell systems for use in genetic toxicology by the stable transfer of genes/cDNAs coding for drug metabolizing enzymes directly into the target cell. Alternative approaches have also been developed in which mammalian cells are transiently transfected with cDNAs coding for drug-metabolizing enzymes and S9 preparations expressing a single metabolizing enzyme isolated and used for metabolic activation. Progress in these areas is reviewed here and the relative merits of the different approaches are discussed. Work to date has focused primarily on the cytochrome P450 family of enzymes, although other enzyme systems involved in xenobiotic metabolism have been used. The central theme of this review is the transfer of genetic information to improve the metabolic capability of cell systems used in genetic toxicology. However, a basic philosophy of the review is that genetic manipulation of cultured mammalian cells has the potential for developing systems to be used to better understand chemically induced toxicological effects.

The expression of human cytochrome P450IA1 in the yeast Saccharomyces cerevisiae

Data from animal studies suggest that cytochrome P450IA1 catalyses the metabolic activation of several procarcinogenic compounds. In the present study, we have expressed human cytochrome P450IA1 in yeast cells. A 1.70 kb BclI/BamHI fragment containing a full-length human cytochrome P450IA1 cDNA was inserted into the BglII expression site of the yeast expression plasmid pMA91 thereby allowing the ATG initiation codon to be located adjacent to the PGK (phosphoglycerate kinase) promoter. The resulting recombinant plasmid, pCK-1, was introduced into Saccharomyces cerevisiae strains ATCC 44773 and AH22. Microsomes prepared from yeast transformatants of strain ATCC 44773 contained undetectable levels of cytochrome P450. In contrast, microsomes from strain AH22 contained cytochrome P450 with a specific content of 33.3 +/- 10.8 pmol/mg of microsomal protein and showed a reduced carbon monoxide difference spectrum with a peak at 448 nm. Control yeast cells transformed with pMA91 showed no cytochrome P450. Western blots were carried out using an antibody that reacts against rat cytochrome P450IA1 and an antibody that reacts against a synthetic peptide representing a short sequence of human cytochrome P450IA1. A band with a molecular weight of 54 kD was observed in microsomes of yeast transformed with pCK-1, but not with pMA91. When microsomes from yeast transformed with pCK-1 were incubated with benzo(a)pyrene (10 min, 10-160 microM), an estimated Km value of 7 microM was obtained. The availability of yeast cells with functionally active human cytochrome P450IA1 will facilitate molecular structure-activity studies of procarcinogen and drug metabolism by this enzyme in man.

The heterologous expression of the cytochromes P450: a new approach for the study of enzyme activities and regulation

The superfamily of cytochrome P450s encompasses a vast arena of biologically important reactions. The ever-increasing numbers of P450s and the diversity of their enzymatic properties dictate the need to develop new approaches for studying their chemical, physical and catalytic properties. The heterologous expression of P450s in various cell systems (e.g., COS cells, yeast, E. coli, etc.) now provides a means of producing recombinant proteins for such studies. The example is presented of the expression of P450(17)alpha in COS cells and the use of this technique for the comparison of the enzymatic properties of the rat, bovine and human enzymes. Further, studies are described whereby cotransfection results in the simultaneous expression of more than one P450 permitting the construction of 'designer membranes' for assessing protein-protein interactions and the reconstruction of complex pathways of metabolism. Recent advances with genetically engineered systems point to the power of the transfection technique for the study of structure-function relationships with this class of important hemoproteins.

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.

Expression of bovine cytochrome P450c17 cDNA in Saccharomyces cerevisiae

We constructed expression plasmids for bovine adrenal cytochrome P450c17 (P450c17) by inserting the corresponding cDNA between the yeast alcohol dehydrogenase I promoter and terminator of the expression vector pAAH5. Plasmids pA alpha 1 and pA alpha 2 contained the entire coding region for bovine P450c17, whereas pAC alpha 1 included the cDNA coding for chimeric P450c alpha consisting of the amino-terminal 45 amino acid residues of rat P450c and the carboxy-terminal 482 amino acid residues of bovine P450c17. The transformed Saccharomyces cerevisiae AH22/pA alpha 1, AH22/pA alpha 2, and AH22/pAC alpha 1 cells produced about 1 x 10(5), 1 x 10(5), and 2 x 10(4) molecules per cell of the corresponding P450 hemoproteins, respectively. On incubation with the cultures of each of the three strains, progesterone was specifically converted into 17 alpha-hydroxyprogesterone, which was not further converted into androstenedione, indicating that the three strains showed 17 alpha-hydroxylase activity, but almost no C17,20-lyase activity. The microsomal fraction prepared from the AH22/pA alpha 1 cells showed 17 alpha-hydroxylase activity toward progesterone and pregnenolone to higher extents, and exhibited C17,20-lyase activity toward 17 alpha-hydroxypregnenolone to a lesser extent and almost no C17,20-lyase activity toward 17 alpha-hydroxyprogesterone. These results indicated that bovine P450c17 synthesized in S. cerevisiae cells manifests the 17 alpha-hydroxylase activity, but not the C17,20-lyase activity.

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.

Cytochromes P-450

1. The cytochromes P-450 are of interest in fields as diverse as biochemistry, pharmacology, mechanistic and synthetic chemistry, carcinogenesis, toxicology, endocrinology, and nutrition. 2. Cytochrome P-450 enzymes are coded by a multi-gene family. 3. The many forms have some elements of similarity in their primary sequences and catalyze chemically equivalent reactions, either mixed-function oxidations or reductions. 4. The catalytic specificity of the individual forms of the enzyme is related to three-dimensional interactions between individual substrates and their binding sites, and the reactions are related in their chemistry. 5. Regulation of enzyme levels appears to involve transcriptional and post-transcriptional aspects.

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.

3-(Trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine photolabels a substrate-binding site of rat hepatic cytochrome P-450 form PB-4

Hepatic microsomes isolated from untreated male rats or from rats pretreated with phenobarbital (PB) or 3-methylcholanthrene (3-MC) were labeled with the hydrophobic, photoactivated reagent 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine ([125I]TID). [125I]TID incorporation into 3-MC- and PB-induced liver microsomal protein was enhanced 5- and 8-fold, respectively, relative to the incorporation of [125I]TID into uninduced liver microsomes. The major hepatic microsomal cytochrome P-450 forms inducible by PB and 3-MC, respectively designated P-450s PB-4 and BNF-B, were shown to be the principal polypeptides labeled by [125I]TID in the correspondingly induced microsomes. Trypsin cleavage of [125I]TID-labeled microsomal P-450 PB-4 yielded several radiolabeled fragments, with a single labeled peptide of Mr approximately 4000 resistant to extensive proteolytic digestion. The following experiments suggested that TID binds to the substrate-binding site of P-450 PB-4. [125I]TID incorporation into microsomal P-450 PB-4 was inhibited in a dose-dependent manner by the P-450 PB-4 substrate benzphetamine. In the absence of photoactivation, TID inhibited competitively about 80% of the cytochrome P-450-dependent 7-ethoxycoumarin O-deethylation catalyzed by PB-induced microsomes with a Ki of 10 microM; TID was a markedly less effective inhibitor of the corresponding activity catalyzed by microsomes isolated from uninduced or beta-naphthoflavone-induced livers.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of a cytochrome P-450 structural gene from Saccharomyces cerevisiae

We have transformed a Saccharomyces cerevisiae host with an S. cerevisiae genomic library contained in the shuttle vector YEp24 and screened the resultant transformants for resistance to ketoconazole (Kc), an inhibitor of the cytochrome P-450 (P-450) enzyme lanosterol 14 alpha-demethylase. Two plasmids were isolated which transformed yeast to both increased resistance to Kc and increased levels of total P-450. Hybrid-selection and immunoprecipitation experiments showed that these plasmids, pVK1 and pVK2, contained the structural gene for an S. cerevisiae P-450. This conclusion was confirmed by the nucleotide sequence of a portion of pVK2, which revealed an open reading frame encoding a characteristic P-450 heme-binding region.