Expression of a recombinant derivative of human aromatase P450 in insect cells utilizing the baculovirus vector system

Engineering Electron Transfer Pathway of Cytochrome P450s

Cytochrome P450s (P450s), a superfamily of heme-containing enzymes, existed in animals, plants, and microorganisms. P450s can catalyze various regional and stereoselective oxidation reactions, which are widely used in natural product biosynthesis, drug metabolism, and biotechnology. In a typical catalytic cycle, P450s use redox proteins or domains to mediate electron transfer from NAD(P)H to heme iron. Therefore, the main factors determining the catalytic efficiency of P450s include not only the P450s themselves but also their redox-partners and electron transfer pathways. In this review, the electron transfer pathway engineering strategies of the P450s catalytic system are reviewed from four aspects: cofactor regeneration, selection of redox-partners, P450s and redox-partner engineering, and electrochemically or photochemically driven electron transfer.

Computational method for aromatase-related proteins using machine learning approach

Human aromatase enzyme is a microsomal cytochrome P450 and catalyzes aromatization of androgens into estrogens during steroidogenesis. For breast cancer therapy, third-generation aromatase inhibitors (AIs) have proven to be effective; however patients acquire resistance to current AIs. Thus there is a need to predict aromatase-related proteins to develop efficacious AIs. A machine learning method was established to identify aromatase-related proteins using a five-fold cross validation technique. In this study, different SVM approach-based models were built using the following approaches like amino acid, dipeptide composition, hybrid and evolutionary profiles in the form of position-specific scoring matrix (PSSM); with maximum accuracy of 87.42%, 84.05%, 85.12%, and 92.02% respectively. Based on the primary sequence, the developed method is highly accurate to predict the aromatase-related proteins. Prediction scores graphs were developed using the known dataset to check the performance of the method. Based on the approach described above, a webserver for predicting aromatase-related proteins from primary sequence data was developed and implemented at https://bioinfo.imtech.res.in/servers/muthu/aromatase/home.html. We hope that the developed method will be useful for aromatase protein related research.

Human aromatase: perspectives in biochemistry and biotechnology

Aromatase (CYP19) is involved in steroidogenesis, catalyzing the conversion of androgens into estrogens through a unique reaction that causes the aromatization of the A ring of the steroid. The enzyme is widely distributed and well conserved among species as it plays a crucial role in physiological processes such as control of reproduction and neuroprotection. It has also been a subject of intense research both at the biotechnological level in drug development due to its involvement in estrogen-dependent tumors and at a fundamental biochemical level because there are numerous questions regarding its reaction mechanism. This review will report the great progress made in this area.

Kinetic solvent isotope effect in steady-state turnover by CYP19A1 suggests involvement of Compound 1 for both hydroxylation and aromatization steps

CYP19A1, or human aromatase catalyzes the conversion of androgens to estrogens in a three-step reaction through the formation of 19-hydroxy and 19-aldehyde intermediates. While the first two steps of hydroxylation are thought to proceed through a high-valent iron-oxo species, controversy exists surrounding the identity of the reaction intermediate that catalyzes the lyase and aromatization reaction. We investigated the kinetic isotope effect on the steady-state turnover of Nanodisc-incorporated human CYP19A1 to explore the mechanisms of this reaction. Our experiments reveal a significant (∼ 2.5) kinetic solvent isotope effect for the C10-C19 lyase reaction, similar to that of the first two hydroxylation steps (2.7 and 1.2). These data implicate the involvement of Compound 1 as a reactive intermediate in the final aromatization step of CYP19A1.

Luciferin IPA-based higher throughput human hepatocyte screening assays for CYP3A4 inhibition and induction

The authors report here higher throughput screening (HTS) assays for the evaluation of CYP3A4 inhibition and CYP3A4 induction in human hepatocytes using a novel CYP3A4 substrate, luciferin IPA (LIPA). Using human recombinant CYP450 isoforms, LIPA was found to be metabolized extensively by CYP3A4 but not by CYP1A2, CYP2C9, CYP2C19, CYP2D6, or CYP2E1. In the 384-well plate CYP3A4 inhibition assay, the known inhibitors 1-aminobenzotriazole, erythromycin, ketoconazole, and verapamil were found to cause extensive (maximum inhibition of >80%), dose-dependent, statistically significant inhibition of LIPA metabolism. The non-CYP3A4 inhibitors diethyldithiocarbamate, quercetin, quinidine, sulfaphenazole, ticlopidine, and tranylcypromine were found to have substantially lower (maximum inhibition of <50%) or no apparent inhibitory effects in the HTS assay. In the 96-well plate induction assay, the CYP3A4 inducers rifampin, phenobarbital, carbamazepine, phenytoin, troglitazone, rosiglitazone, and pioglitazone yielded dose-dependent induction of LIPA metabolism, whereas the CYP1A2 inducers omeprazole and 3-methylcholanthrene did not display any induction in the CYP3A4 activity. The high sensitivity and specificity of the assays, the relative ease of execution, and reduced cost, time, and test material requirements suggest that the HTS assays may be applied routinely for screening a large number of chemicals in the drug discovery phase for CYP3A4 inhibitory and inducing potential.

Kinetic analysis of the three-step steroid aromatase reaction of human cytochrome P450 19A1

Cytochrome P450 19A1 (P450 19A1), the aromatase, catalyzes the conversion of androgens to estrogens through a sequential three-step reaction, generating 19-hydroxy and 19-aldehyde intermediates en route to the product estrogen. A procedure for the heterologous expression and purification of P450 19A1 in Escherichia coli was developed (k(cat) of 0.06 s(-1) for the conversion of androstenedione to estrone). Binding of the substrate and intermediates show low micromolar dissociation constants and are at least two-step processes. Rates of reduction of the iron were fast in the presence of substrate, either intermediate, or product. P450 19A1 is a distributive rather than a processive enzyme, with the sequential reaction allowing free dissociation of the intermediates as revealed by pulse-chase experiments. Conversion of androstenedione to estrone (under single turnover conditions) generated a progress curve showing changes in the concentrations of the substrate, intermediates, and product. A minimal kinetic model containing the individual rate constants for the steps in P450 19A1 catalysis was developed to globally fit the time course of the overall reaction, the dissociation constants, the two-step ligand binding, the distributive character, the iron-reduction rates, and the steady-state conversion of the 19-hydroxy androstenedione and 19-aldehyde androstenedione intermediates to estrone.

X-ray structure of human aromatase reveals an androgen-specific active site

Aromatase is a unique cytochrome P450 that catalyzes the removal of the 19-methyl group and aromatization of the A-ring of androgens for the synthesis of estrogens. All human estrogens are synthesized via this enzymatic aromatization pathway. Aromatase inhibitors thus constitute a frontline therapy for estrogen-dependent breast cancer. Despite decades of intense investigation, this enzyme of the endoplasmic reticulum membrane has eluded all structure determination efforts. We have determined the crystal structure of the highly active aromatase purified from human placenta, in complex with its natural substrate androstenedione. The structure shows the binding mode of androstenedione in the catalytically active oxidized high-spin ferric state of the enzyme. Hydrogen bond-forming interactions and tight packing hydrophobic side chains that complement the puckering of the steroid backbone provide the molecular basis for the exclusive androgenic specificity of aromatase. Locations of catalytic residues and water molecules shed new light on the mechanism of the aromatization step. The structure also suggests a membrane integration model indicative of the passage of steroids through the lipid bilayer.

History of aromatase: saga of an important biological mediator and therapeutic target

Aromatase is the enzyme that catalyzes the conversion of androgens to estrogens. Initial studies of its enzymatic activity and function took place in an environment focused on estrogen as a component of the birth control pill. At an early stage, investigators recognized that inhibition of this enzyme could have major practical applications for treatment of hormone-dependent breast cancer, alterations of ovarian and endometrial function, and treatment of benign disorders such as gynecomastia. Two general approaches ultimately led to the development of potent and selective aromatase inhibitors. One targeted the enzyme using analogs of natural steroidal substrates to work out the relationships between structure and function. The other approach initially sought to block adrenal function as a treatment for breast cancer but led to the serendipitous finding that a nonsteroidal P450 steroidogenesis inhibitor, aminoglutethimide, served as a potent but nonselective aromatase inhibitor. Proof of the therapeutic concept of aromatase inhibition involved a variety of studies with aminoglutethimide and the selective steroidal inhibitor, formestane. The requirement for even more potent and selective inhibitors led to intensive molecular studies to identify the structure of aromatase, to development of high-sensitivity estrogen assays, and to "mega" clinical trials of the third-generation aromatase inhibitors, letrozole, anastrozole, and exemestane, which are now in clinical use in breast cancer. During these studies, unexpected findings led investigators to appreciate the important role of estrogens in males as well as in females and in multiple organs, particularly the bone and brain. These studies identified the important regulatory properties of aromatase acting in an autocrine, paracrine, intracrine, neurocrine, and juxtacrine fashion and the organ-specific enhancers and promoters controlling its transcription. The saga of these studies of aromatase and the ultimate utilization of inhibitors as highly effective treatments of breast cancer and for use in reproductive disorders serves as the basis for this first Endocrine Reviews history manuscript.

Structural basis for androgen specificity and oestrogen synthesis in human aromatase

Aromatase cytochrome P450 is the only enzyme in vertebrates known to catalyse the biosynthesis of all oestrogens from androgens. Aromatase inhibitors therefore constitute a frontline therapy for oestrogen-dependent breast cancer. In a three-step process, each step requiring 1 mol of O(2), 1 mol of NADPH, and coupling with its redox partner cytochrome P450 reductase, aromatase converts androstenedione, testosterone and 16alpha-hydroxytestosterone to oestrone, 17beta-oestradiol and 17beta,16alpha-oestriol, respectively. The first two steps are C19-methyl hydroxylation steps, and the third involves the aromatization of the steroid A-ring, unique to aromatase. Whereas most P450s are not highly substrate selective, it is the hallmark androgenic specificity that sets aromatase apart. The structure of this enzyme of the endoplasmic reticulum membrane has remained unknown for decades, hindering elucidation of the biochemical mechanism. Here we present the crystal structure of human placental aromatase, the only natural mammalian, full-length P450 and P450 in hormone biosynthetic pathways to be crystallized so far. Unlike the active sites of many microsomal P450s that metabolize drugs and xenobiotics, aromatase has an androgen-specific cleft that binds the androstenedione molecule snugly. Hydrophobic and polar residues exquisitely complement the steroid backbone. The locations of catalytically important residues shed light on the reaction mechanism. The relative juxtaposition of the hydrophobic amino-terminal region and the opening to the catalytic cleft shows why membrane anchoring is necessary for the lipophilic substrates to gain access to the active site. The molecular basis for the enzyme's androgenic specificity and unique catalytic mechanism can be used for developing next-generation aromatase inhibitors.

NADPH- and iron-dependent lipid peroxidation inhibit aromatase activity in human placental microsomes

During pregnancy placenta is the most significant source of lipid hydroperoxides and other reactive oxygen species (ROS). The increased production of lipid peroxides and other ROS is often linked to pre-eclampsia. It is already proved that placental endoplasmic reticulum may be an important place of lipid peroxides and superoxide radical production. In the present study we revealed that NADPH- and iron-dependent lipid peroxidation in human placental microsomes (HPM) inhibit placental aromatase--a key enzyme of estrogen biosynthesis in human placenta. We showed that significant inhibition of this enzyme is caused by small lipid peroxidation (TBARS (thiobarbituric acid-reactive substances)<4nmol/mg microsomal protein (m.p.)). More intensive lipid peroxidation (TBARS>9nmol/mg microsomal protein) diminishes aromatase activity to value being less than 5% of initial value. NADPH- and iron-dependent lipid peroxidation also causes disappearance of cytochrome P450 parallel to observed aromatase activity inhibition. EDTA, alpha-tocopherol, MgCl(2) and superoxide dismutase (SOD) prevent aromatase activity inhibition and cytochrome P450(AROM) degradation. Mannitol and catalase have not effect on TBARS synthesis, aromatase activity and cytochrome P450 degradation. In view of the above we postulate that the inhibition of aromatase activity observed is mainly a consequence of cytochrome P450(AROM) degradation induced by lipid radicals. The role of hydroxyl radical in cytochrome P450 degradation is negligible in our experimental conditions. The results presented here also suggest that the inhibition of aromatase activity can also take place in placenta at in vivo conditions.

Molecular basis for the aromatization reaction and exemestane-mediated irreversible inhibition of human aromatase

Aromatase converts androgens to aromatic estrogens. Aromatase inhibitors have been used as first-line drugs in the treatment of hormone-dependent breast cancer. Structural basis of the aromatization reaction and drug recognition by aromatase has remained elusive because of its unknown three-dimensional structure. In this study, recombinant human aromatase was expressed and purified from Escherichia coli. Using this purified and active preparation, the three-dimensional folding of aromatase was revealed by proteomic analysis. Combined with site-directed mutagenesis, several critical residues involved in enzyme catalysis and suicide inhibition by exemestane were evaluated. Based on our results, a new clamping mechanism of substrate/exemestane binding to the active site is proposed. These structure-function studies of aromatase would provide useful information to design more effective aromatase inhibitors for the prevention and the treatment of hormone-dependent breast cancer.

Structure-function studies of aromatase and its inhibitors: a progress report

The utilization of computer modeling, site-directed mutagenesis, inhibition kinetic analysis and reaction metabolite analysis allows us to better understand the structure-function relationship between aromatase and its inhibitors. Our results have helped in determining how steroidal and nonsteriodal aromatase inhibitors bind to the active site of the enzyme. This information has also aided in the understanding of the reaction mechanism of aromatase. Furthermore, our structure-function studies of aromatase have generated important information for predicting how environmental chemicals interact with the enzyme. During the last 2 years, a new aromatase computer model based on the X-ray structure of rabbit cytochrome P450 2C5 has been generated and used to evaluate the results obtained from new aromatase mutants produced in this laboratory. In addition, we have succeeded in the expression and purification of functionally active aromatase using an Escherichia coli expression method. The catalytic properties of this recombinant aromatase are similar to those properties exhibited by the human placental aromatase preparation and the mammalian cell-expressed enzyme. The E. coli expressed aromatase will be very useful for further structure-function studies of aromatase. Our laboratory has also evaluated the growth-inhibiting activity of aromatase inhibitors in estrogen receptor-positive breast cancer using three-dimensional cell cultures of aromatase-over expressing MCF-7 and T-47D cell lines (i.e. MCF-7aro and T-47Daro). Our results demonstrate that these three-dimensional cultures are valuable approaches to assess the growth-inhibiting activity of aromatase inhibitors. Finally, we have identified several phytochemicals to be potent inhibitors of aromatase. To demonstrate the impact of the phytochemicals on estrogen formation in vivo, we showed that the intake of anti-aromatase chemicals from red wine was capable of suppressing MCF-7aro-mediated tumor formation in nude mice and aromatase-induced hyperplasia in a transgenic mouse model in which aromatase is over-expressed in the mammary tissue.

Expression of human aromatase (CYP19) in Escherichia coli by N-terminal replacement and induction of cold stress response

CYP19 (P450arom) catalyzes the aromatization reaction of C19 steroids leading to estrogens. While readily expressed in insect cells, the human P450arom has been a difficult P450 to express in Escherichia coli at useful levels. In the present study, we replaced the N-terminal sequence in human CYP19 with the corresponding sequences of other microsomal P450s (CYP2C11 and CYP17) that are efficiently expressed in E. coli. Although the N-terminal replacement alone was not sufficient for the expression, human P450arom was successfully expressed up to the level of 240nmol/l culture by the combination of the N-terminal replacement and the induction of cold stress response by 1 microg/ml chloramphenicol. Membrane fractions containing the expressed P450arom catalyzed aromatization of androstenedione with a specific activity of 4.9 nmol/min/nmol P450. Our results are important to provide large quantities of human P450arom as an active form for structure-function studies.

Expression and purification of a recombinant form of human aromatase from Escherichia coli

Aromatase converts androgen to estrogen, a hormone that plays an important role in the development of breast cancer. Aromatase inhibitors have been shown to be a useful endocrine regimen for estrogen-dependent breast cancer. Structure-function studies of aromatase can generate critical structural information for designing highly potent and specific inhibitors. However, aromatase structure-function studies have been hampered by a lack of purified protein. In this report, we describe the construction and expression of a recombinant derivative of human aromatase in Escherichia coli using the pET vector system, and the purification of the enzyme by means of nickel-agarose affinity chromatography. We examined the expression of the full-length, Del-38, C-6xHis-tagged Del-38, and NC-6xHis-tagged Del-38 forms of aromatase. The recombinant aromatase without the first 38 amino acids from the amino-terminus (i.e. Del-38) was found to have a higher activity than the full-length enzyme. Moreover, the addition of two separate hexameric histidine tags at both the amino and the carboxyl-termini (i.e. NC-6xHis-tagged Del-38) increased the binding affinity of the recombinant enzyme to the nickel-agarose. The expressed aromatase (i.e. NC-6xHis-tagged Del-38 aromatase) was eluted from the nickel-agarose with 80 mM EDTA. The total aromatase activity of the 80 mM EDTA-eluted fractions was significantly higher than the detergent-solubilized protein extract, indicating a renaturation process during the nickel-agarose affinity chromatography. Purified aromatase exhibited a single band when analyzed by SDS-PAGE, and activity up to 5.8 nmol/mg/min was obtained using the tritiated water release assay. The K(m) value for androstenedione was determined to be 62+/-24 nM by enzyme kinetic analysis. The recombinant aromatase preparation was also characterized by reduced CO-difference spectral analysis, reaction product extraction assay, and inhibition studies using two aromatase inhibitors (letrozole and anastrozole). The results indicate that the recombinant aromatase from E. coli has catalytic properties identical to those of the enzyme expressed in human tissue and will be very useful for further structure-function studies of aromatase.

Cloning of two mRNA variants of brain aromatase cytochrome P450 in rainbow trout (Oncorhynchus mykiss Walbaum)

This work describes the molecular cloning of the cDNA encoding the rainbow trout (Oncorhynchus mykiss Walbaum) brain cytochrome P450arom by means of reverse transcriptase and polymerase chain reaction (RT-PCR) and 5'- and 3'-rapid amplification of cDNA ends (RACE) analyses. The results obtained demonstrate that, as in other teleost fishes, the trout genome contains, besides the gene previously identified in the ovary, a second CYP19 gene (CYP19B) expressed at high level in the brain. Moreover, two P450aromB mRNAs, forms I and II, were found to be transcribed in trout. Form I (1816 sequenced nt) contains an open reading frame (ORF) of 1464b, a 5'-untranslated terminal region (UTR) of 124b and at least 228b in the 3'-UTR (incomplete, as the polyadenylation signal was not determined). Form II (1930 sequenced nt) contains an ORF of 1362b, a 5'-UTR of 340b and the same 3'-UTR as form I. Form II lacks the first 34 amino acids of form I, corresponding to the membrane-anchoring segment, whereas the sequence of the remaining coding region is almost the same in the two forms, resulting in proteins of 454 and 488 amino acids, respectively. Whether the two transcripts derive from the same gene by alternative splicing or are encoded by different CYP19B genes remains to be clarified. On Northern blot analyses with brain and ovary specific ORF probes and poly(A)(+)-enriched RNAs from trout ovary and brain, a transcript of about 2.6kb was identified in the ovary, as expected, whereas the full-length mRNA of brain P450arom is about 3.8kb. The brain form is expressed in the brain and gonads, whereas expression in peripheral tissues is limited mostly to the gills. The two trout CYP19 genes are not equivalent in tissue-specific expression, indicating the possibility of distinct promoters and regulatory mechanisms.

Structural determinants of aromatase cytochrome p450 inhibition in substrate recognition site-1

The porcine gonadal form of aromatase cytochrome P450 (P450arom) exhibits higher sensitivity to inhibition by the imidazole, etomidate, than the placental isozyme. The residue(s) responsible for this functional difference was mapped using chimeragenesis and point mutation analysis of the placental isozyme, and the kinetic analysis was conducted on native and mutant enzymes after overexpression in insect cells. The etomidate sensitivity of the placental isozyme was markedly increased by substitution of the predicted substrate recognition site-1 (SRS-1) and essentially reproduced that of the gonadal isozyme by substitution of SRS-1 and the predicted B helix. A single isoleucine (I) to methionine (M) substitution at position 133 of the placental isozyme (I(133)M) was proven to be the critical residue within SRS-1. Residue 133 is located in the B'-C loop and has been shown to be equally important in other steroid-metabolizing P450s. Single point mutations (including residues 110, 114, 120, 128, 137, and combinations thereof among others) and mutation of the entire B and C helixes were without marked effect on etomidate inhibitory sensitivity. The same mutation (I(133)M) introduced into human P450arom also markedly increased etomidate sensitivity. Mutation of Ile(133) to either alanine (I(133)A) or tyrosine (I(133)Y) decreased apparent enzyme activity, but the I(133)A mutant was sensitive to etomidate inhibition, suggesting that it is Ile(133) that decreases etomidate binding rather than Met(133) increasing enzyme sensitivity. Androstenedione turnover and affinity were similar for the I(133)M mutant and the native placental isozyme. These data suggest that Ile(133) is a contact residue in SRS-1 of P450arom, emphasize the functional conservation that exists in SRS-1 of a number of steroid-hydroxylating P450 enzymes, and suggest that substrate and inhibitor binding are dependent on different contact points to varying degrees.

Functional characterization of 102-amino acid-deleted form of human aromatase (delta102-aromatase)

A truncate form of human aromatase cDNA that corresponds to the recently identified rat cortical type aromatase mRNA variant (Yamada-Mouri et al., J. Steroid Biochem. Molec. Biol., 60: 325-329, 1997) has been generated, and the amino-terminus deleted form of the enzyme has been expressed in CHO cells. The resulting product lacking 102 residues from the N-terminus of aromatase (i.e. 102-aromatase) showed an extremely low enzyme activity using an 'In-cell' assay. A strong aromatase activity, however, was observed for the delta102-aromatase using an in vitro method on the solublized preparations. The in vitro activity was dependent on both incubation time and NADPH concentration as well as inclusion of NADPH-cytochrome P450 reductase in the assay mixture. The average turnover rate of aromatization of the reconstituted delta102-aromatase was 6.8 min(-1). The results of the immunosuppression assay suggested that delta102-aromatase still holds the epitope interactive to MAb3-2C2, a monoclonal antibody raised agaist human placental aromatase P450. Furthermore, the IC50 values of MAb3-2C2 were determined to be 24 and 23 microg/ml for the whole homogenate and the 105,000 x g precipitate fractions prepared from the truncated aromatase expressing cells, respectively, whereas an IC50 of 1.3 microg/ml was shown for the full-length human aromatase. These results indicate that the delta102-aromatase P450 can be expressed and is catalytically competent as the full-length enzyme, but the epitope structure for the monoclonal antibody MAb3-2C2 is altered from that of the native enzyme. In addition, the intracellular distribution of delta102-aromatase may be different from that of the wild-type enzyme, explaining why very low activity was measured using an 'In-cell' assay.

Expression and characterization of human glutamate-cysteine ligase

Glutamate-cysteine ligase (GLCL) catalyzes the rate-limiting step in glutathione biosynthesis. GLCL comprises regulatory (GLCLR) and catalytic (GLCLC) subunits. To understand better the structure-function relationship of GLCL subunits and holoenzyme, human GLCLR and GLCLC genes were inserted into the baculovirus genome. Recombinant hGLCLR andhGLCLC were produced in cells infected with recombinant baculoviruses, and homogeneous hGLCL subunits and holoenzyme were purified from cell lysates with a Ni-NTA resin. Purified recombinant hGLCL holoenzyme was catalytically more active than hGLCLC with L-glutamate, L-alpha-aminobutyrate, and ATP as substrates. The selectivity of purified hGLCL holoenzyme for L-glutamate, L-alpha-aminobutyrate, or L-cysteine was significantly higher than for hGLCLC. Glutathione was a noncompetitive inhibitor for both hGLCL holoenzyme and hGLCLC. hGLCLC was more sensitive to inhibition by glutathione than hGLCL holoenzyme. Deletion of the first 25 amino acid residues at the amino terminus of GLCLC dramatically decreased GLCL activity, indicating that the amino terminus of GLCLC is required for full catalytic activity. Expressed and purified hGLCL provides a useful tool to investigate glutathione biosynthesis in vitro.

Baculovirus vectors for expression in insect cells

Recombinant baculoviruses now represent a mature technology in which vector development, particularly for the control of expression level, has reached a plateau. However, other aspects of expression, such as the production of multiple proteins, improved product purification or maximizing protein processing, remain areas for novel vector and host cell development. This year has seen these topics come to the fore in descriptions of new expression systems.

Detection of aromatase cytochrome P450, 17 alpha-hydroxylase cytochrome P450 and NADPH:P450 reductase on the surface of cells in which they are expressed

Using the membrane impermeant probe NHS-LC-biotin, we show in this report that a fraction of aromatase P450 (P450arom), the enzyme that catalyzes estrogen biosynthesis, is present at the surface of cells in which it is expressed, either endogenously or as a consequence of transfection. The same findings were obtained for a truncated form of P450arom lacking the putative membrane-spanning region, thus suggesting the presence of other membrane-spanning region(s) within its structure. P450arom is not unique in this regard as we find that a fraction of 17 alpha-hydroxylase P450 as well as NADPH:P450 reductase also are present at the cell surface. It is therefore possible that a number of microsomal P450s are expressed at the cell surface in this fashion.