Construction of plasmids and expression in Escherichia coli of enzymatically active fusion proteins containing the heme-domain of a P450 linked to NADPH-P450 reductase

NADPH P450 oxidoreductase: structure, function, and pathology of diseases

Cytochrome P450 oxidoreductase (POR) is an enzyme that is essential for multiple metabolic processes, chiefly among them are reactions catalyzed by cytochrome P450 proteins for metabolism of steroid hormones, drugs and xenobiotics. Mutations in POR cause a complex set of disorders that often resemble defects in steroid metabolizing enzymes 17α-hydroxylase, 21-hydroxylase and aromatase. Since our initial reports of POR mutations in 2004, more than 200 different mutations and polymorphisms in POR gene have been identified. Several missense variations in POR have been tested for their effect on activities of multiple steroid and drug metabolizing P450 proteins. Mutations in POR may have variable effects on different P450 partner proteins depending on the location of the mutation. The POR mutations that disrupt the binding of co-factors have negative impact on all partner proteins, while mutations causing subtle structural changes may lead to altered interaction with specific partner proteins and the overall effect may be different for each partner. This review summarizes the recent discoveries related to mutations and polymorphisms in POR and discusses these mutations in the context of historical developments in the discovery and characterization of POR as an electron transfer protein. The review is focused on the structural, enzymatic and clinical implications of the mutations linked to newly identified disorders in humans, now categorized as POR deficiency.

Downstream reactions and engineering in the microbially reconstituted pathway for Taxol

Taxol (a trademarked product of Bristol-Myers Squibb) is a complex isoprenoid natural product which has displayed potent anticancer activity. Originally isolated from the Pacific yew tree (Taxus brevifolia), Taxol has been mass-produced through processes reliant on plant-derived biosynthesis. Recently, there have been alternative efforts to reconstitute the biosynthetic process through technically convenient microbial hosts, which offer unmatched growth kinetics and engineering potential. Such an approach is made challenging by the need to successfully introduce the significantly foreign enzymatic steps responsible for eventual biosynthesis. Doing so, however, offers the potential to engineer more efficient and economical production processes and the opportunity to design and produce tailored analog compounds with enhanced properties. This mini review will specifically focus on heterologous biosynthesis as it applies to Taxol with an emphasis on the challenges associated with introducing and reconstituting the downstream reaction steps needed for final bioactivity.

Genetic variation in human P450 oxidoreductase

Catalysis by all 50 Type II (microsomal) P450 enzymes, including steroidogenic P450c17, P450c21, and P450aro and hepatic drug-metabolizing enzymes requires electron donation from P450 oxidoreductase (POR). POR knockout mice are embryonic lethal, but human POR mutations cause a complex disorder of steroidogenesis. Disorders of hepatic drug metabolism in human POR deficiency have not yet been described. To understand the potential contribution of POR to pharmacogenetics, we sequenced the POR gene in 842 normal persons from 4 ethnic groups. We detected 140 single nucleotide sequence variants of which 43 were in >1% of alleles, including 15 missense mutants; this brings the total of known POR missense mutants to 35. A503V was found on 28% of alleles, varying from 19% in African Americans to 37% in Chinese Americans. We expressed all 35 missense mutants in E. coli and assayed their activities to: oxidize NADPH, reduce cytochrome c, support the 17alpha-hydroxylase and 17,20 lyase activities of bacterially expressed human P450c17, and support the metabolism of fluorogenic EOMCC by bacterially expressed human CYP1A2 and CYP2C19. These data show that there are great differences in the activities of some POR mutants depending on the electron recipient assayed; for example, Q153R causes severely impaired steroid biosynthesis in human patients and in vitro, but is a gain-of-function mutant with CYP1A2 and 2C19. A503V reduces both activities of P450c17 in half, but had no effect on CYP1A2 or 2C19. POR variants are a previously unappreciated source of genetic variation in patterns of steroid synthesis and drug metabolism.

Cytochromes P450—A Family of Proteins and Scientists–Understanding their Relationships

The unifying thread of this review involves NADPH-cytochrome P450 reductase (CYPOR), the microsomal enzyme responsible for transferring electrons to cytochromes P450, as well as several other monooxygenase systems, a lifelong interest of the corresponding author. The intersection of her research with that of Dr. David Kupfer, their resulting collaboration, and the beginning of a long-standing study of fatty acid- and eicosanoid-metabolizing cytochromes P450 (CYP4A gene subfamily), including the role of cytochrome b5, will be reported. The culmination of this interest now involves purification and characterization of the human mutants of CYPOR that have been implicated in pathologies, such as Antley-Bixler syndrome.

Clinical, structural and functional implications of mutations and polymorphisms in human NADPH P450 oxidoreductase

Cytochrome P450 proteins are involved in metabolism of drugs and xenobiotics. In the endoplasmic reticulum a single nicotinamide adenine dinucleotide phosphate (NADPH) P450 oxidoreductase (POR) supplies electrons to all microsomal P450s for catalytic activity. POR is a flavoprotein that contains both flavin mononucleotide and flavin adenine dinucleotide as cofactors and uses NADPH as the source of electrons. We have recently reported a number of POR mutations in the patients with disordered steroidogenesis. In the first report we had described missense mutations (A287P, R457H, V492E, C569Y, and V608F) identified in four patients with defects in steroid production. Each POR variant was produced as recombinant N-27 form of the enzyme in bacteria and as full-length form in yeast. Membranes from bacteria or yeast expressing normal or variant POR were purified and their activities were characterized in cytochrome c and CYP17A1 assays. Later we have published a larger study that described a whole range of POR mutations and characterized the mutants/polymorphisms A115V, T142A, M263V, Y459H, A503V, G539R, L565P, R616X, V631I, and F646del from the sequencing of patient DNA. We also studied POR variants Y181D, P228L, R316W, G413S, and G504R that were available in public databases or published literature. Three-dimensional structure of rat POR is known and we have used this structure to deduce the structure-function correlation of POR mutations in human. The missense mutations found in patients with disordered steroidogenesis are generally in the co-factor binding and functionally important domains of POR and the apparent polymorphisms are found in regions with lesser structural importance. A variation in POR can alter the activity of all microsomal P450s, and therefore, can affect the metabolism of drugs and xenobiotics even when the P450s involved are otherwise normal. It is important to study the genetic and biochemical basis of POR variants in human population to gain information about possible differences in P450 mediated reactions among the individuals carrying a variant or polymorphic form of POR that could impact their metabolism.

Biochemical analysis of mutations in P450 oxidoreductase

All microsomal P450s require POR (cytochrome P450 reductase) for catalytic activity. Most of the clinically used drugs are metabolized by a small number of P450s and polymorphisms in the cytochrome P450s are known to cause changes in drug metabolism. We have recently found a number of POR missense mutations in the patients with disordered steroidogenesis. Our initial report described five missense mutations (A284P, R454H, V489E, C566Y and V605F) identified in four patients. We built bacterial expression vectors for each POR variant, purified the membranes expressing normal or variant POR and characterized their activities with cytochrome c and P450c17 assays. We have recently completed an extensive study of the range of POR mutations and characterized the mutants/polymorphisms A112V, T139A, M260V, Y456H, A500V, G536R, L562P, R613X, V628I and F643del from sequencing of patient DNA. We also studied POR variants Y179D, P225L, R313W, G410S and G501R that were available in databases or the published literature. We analysed the mutations with a three-dimensional model of human POR that was based on an essentially similar rat POR with known crystal structure. The missense mutations found in patients with disordered steroidogenesis mapped to functionally important domains of POR and the apparent polymorphisms mapped to less crucial regions. Since a variation in POR can alter the activity of all microsomal P450s, it can also affect the drug metabolism even with a normal P450. Understanding the genetic and biochemical basis of POR-mediated drug metabolism will provide valuable information about possible differences in P450-mediated reactions among the individuals carrying a variant or polymorphic form of POR.

P450 oxidoreductase deficiency: a new form of congenital adrenal hyperplasia

PURPOSE OF REVIEW

P450 oxidoreductase deficiency--a newly described form of congenital adrenal hyperplasia--typically presents a steroid profile suggesting combined deficiencies of steroid 21-hydroxylase and 17alpha-hydroxylase/17,20-lyase activities. These and other enzymes require electron donation from P450 oxidoreductase. The clinical spectrum of P450 oxidoreductase deficiency ranges from severely affected children with ambiguous genitalia, adrenal insufficiency and the Antley-Bixler skeletal malformation syndrome to mildly affected individuals with polycystic ovary syndrome. We review current knowledge of P450 oxidoreductase deficiency and its broader implications.

RECENT FINDINGS

Since the first report in 2004, at least 21 P450 oxidoreductase mutations have been reported in over 40 patients. The often subtle manifestations of P450 oxidoreductase deficiency suggest it may be relatively common. P450 oxidoreductase deficiency, with or without Antley-Bixler syndrome, is autosomal recessive, whereas Antley-Bixler syndrome without disordered steroidogenesis is caused by autosomal dominant fibroblast growth factor receptor 2 mutations. In-vitro assays of P450 oxidoreductase missense mutations based on P450 oxidoreductase-supported P450c17 activities provide excellent genotype/phenotype correlations. The causal connection between P450 oxidoreductase deficiency and disordered bone formation remains unclear.

SUMMARY

P450 oxidoreductase mutations cause combined partial deficiency of 17alpha-hydroxylase and 21-hydroxylase. Individuals with an Antley-Bixler syndrome-like phenotype presenting with sexual ambiguity or other abnormalities in steroidogenesis should be analyzed for P450 oxidoreductase deficiency.

P450 oxidoreductase deficiency: a new disorder of steroidogenesis

Microsomal P450 enzymes, which metabolize drugs and catalyze steroid biosynthesis require electron donation from NADPH via P450 oxidoreductase (POR). POR knockout mice are embryonically lethal, but we found recessive human POR missense mutations causing disordered steroidogenesis and Antley-Bixler syndrome (ABS), a skeletal malformation syndrome featuring craniosynostosis. Dominant mutations in exons 8 and 10 of fibroblast growth factor receptor 2 (FGFR2) cause phenotypically related craniosynostosis syndromes and were reported in patients with ABS and normal steroidogenesis. Sequencing POR and FGFR2 exons in 32 patients with ABS and/or hormonal findings suggesting POR deficiency showed complete genetic segregation of POR and FGFR2 mutations. Fifteen patients carried POR mutations on both alleles, four carried POR mutations on 1 allele, nine carried FGFR2/3 mutations on one allele and no mutation was found in three patients. The 34 affected POR alleles included 10 with A287P, 7 with R457H, 9 other missense mutations and 7 frameshifts. These 11 missense mutations and 10 others identified by database mining were expressed in E. coli, purified to apparent homogeneity, and their catalytic capacities were measured in four assays: reduction of cytochrome c, oxidation of NADPH, and support of the 17alpha-hydroxylase and 17,20 lyase activities of human P450c17. As assessed by Vmax/Km, 17,20 lyase activity provided the best correlation with clinical findings. Modeling human POR on the X-ray crystal structure of rat POR shows that these mutant activities correlate well with their locations in the structure. POR deficiency is a new disease, distinct from the craniosynostosis syndromes caused by FGFR mutations.

A virus-directed enzyme prodrug therapy (VDEPT) strategy for lung cancer using a CYP2B6/NADPH-cytochrome P450 reductase fusion protein

Virus-directed enzyme prodrug therapy (VDEPT) is an emerging strategy against cancer. Our approach is a P450-based VDEPT that consists of using cyclophosphamide (CPA) as a prodrug and a Cytochrome P450 2B6/NADPH cytochrome P450 reductase fusion protein (CYP2B6/RED) as a prodrug-activating enzyme. Due to the heterogenous expression of proteins in tumor cells, basal reductase activity may not be sufficient to supply CYP2B6 with electrons, the fusion protein should enable the expression of both proteins at high levels in tumor cells. CYP/RED fusion proteins have never been previously expressed in mammalian cells, to enable expression the fusion protein was cloned into an adenoviral vector and subsequently several pulmonary tumor cell lines were infected. The CYP2B6/RED fusion protein was detected by Western blot, its mRNA by Northern blot, and its heme incorporation into an active form by spectral analysis. Infection with the fusion gene increased RED activity in microsomes by a factor of 3 compared to the control. After infection and treatment with CPA, in cell lines with low endogenous RED, the fusion protein mediated significantly higher CPA-induced cytotoxicity compared to cells expressing solely CYP2B6. In conclusion, the fusion protein is functional for VDEPT by providing one protein for higher levels of CPA metabolism.

Minireview: regulation of steroidogenesis by electron transfer

Cytochrome P450 enzymes catalyze the degradation of drugs and xenobiotics, but also catalyze a wide variety of biosynthetic processes, including most steps in steroidogenesis. The catalytic rate of a P450 enzyme is determined in large part by the rate of electron transfer from its redox partners. Type I P450 enzymes, found in mitochondria, receive electrons from reduced nicotinamide adenine dinucleotide (NADPH) via the intermediacy of two proteins-ferredoxin reductase (a flavoprotein) and ferredoxin (an iron/sulfur protein). Type I P450 enzymes include the cholesterol side-chain cleavage enzyme (P450scc), the two isozymes of 11-hydroxylase (P450c11beta and P450c11AS), and several vitamin D-metabolizing enzymes. Disorders of these enzymes, but not of the two redox partners, have been described. Type II P450 enzymes, found in the endoplasmic reticulum, receive electrons from NADPH via P450 oxidoreductase (POR), which contains two flavin moieties. Steroidogenic Type II P450 enzymes include 17alpha-hydroxylase/17,20 lyase (P450c17), 21-hydroxylase (P450c21), and aromatase (P450aro). All P450 enzymes catalyze multiple reactions, but P450c17 appears to be unique in that the ratio of its activities is regulated at a posttranslational level. Three factors can increase the degree of 17,20 lyase activity relative to the 17alpha-hydroxylase activity by increasing electron flow from POR: a high molar ratio of POR to P450c17, serine phosphorylation of P450c17, and the presence of cytochrome b(5), acting as an allosteric factor to promote the interaction of POR with P450c17. POR is required for the activity of all 50 human Type II P450 enzymes, and ablation of the Por gene in mice causes embryonic lethality. Nevertheless, mutation of the human POR gene is compatible with life, causing multiple steroidogenic defects and a skeletal dysplasia called Antley-Bixler syndrome.

Diversity and function of mutations in p450 oxidoreductase in patients with Antley-Bixler syndrome and disordered steroidogenesis

P450 oxidoreductase (POR) is the obligatory flavoprotein intermediate that transfers electrons from reduced nicotinamide adenine dinucleotide phosphate (NADPH) to all microsomal cytochrome P450 enzymes. Although mouse Por gene ablation causes embryonic lethality, POR missense mutations cause disordered steroidogenesis, ambiguous genitalia, and Antley-Bixler syndrome (ABS), which has also been attributed to fibroblast growth factor receptor 2 (FGFR2) mutations. We sequenced the POR gene and FGFR2 exons 8 and 10 in 32 individuals with ABS and/or hormonal findings that suggested POR deficiency. POR and FGFR2 mutations segregated completely. Fifteen patients carried POR mutations on both alleles, 4 carried mutations on only one allele, 10 carried FGFR2 or FGFR3 mutations, and 3 patients carried no mutations. The 34 affected POR alleles included 10 with A287P (all from whites) and 7 with R457H (four Japanese, one African, two whites); 17 of the 34 alleles carried 16 "private" mutations, including 9 missense and 7 frameshift mutations. These 11 missense mutations, plus 10 others found in databases or reported elsewhere, were recreated by site-directed mutagenesis and were assessed by four assays: reduction of cytochrome c, oxidation of NADPH, support of 17alpha-hydroxylase activity, and support of 17,20 lyase using human P450c17. Assays that were based on cytochrome c, which is not a physiologic substrate for POR, correlated poorly with clinical phenotype, but assays that were based on POR's support of catalysis by P450c17--the enzyme most closely associated with the hormonal phenotype--provided an excellent genotype/phenotype correlation. Our large survey of patients with ABS shows that individuals with an ABS-like phenotype and normal steroidogenesis have FGFR mutations, whereas those with ambiguous genitalia and disordered steroidogenesis should be recognized as having a distinct new disease: POR deficiency.

Electrochemical reduction of cytochrome P450 as an approach to the construction of biosensors and bioreactors

In the present review an attempt was made to present an up-to-date amount of the data on electrochemical reduction of the hemoprotein cytochrome P450. The concept and potentialities of enzyme electrodes--transducers--as the main element for construction of electrochemical biosensors were discussed. Different types of electrodes for bioelectrochemistry were analysed. New nanotechnological approaches to cytochrome P450 immobilisation were reported. It was shown that nanobiotechnology in electrochemistry has potential application in manufacturing biosensors and bioreactors for clinical medicine and pharmacology.

Optimizing bacterial expression of catalytically active human cytochromes P450: comparison of CYP2C8 and CYP2C9

1. Methods for the co-expression in Escherichia coli of human cytochrome P450 (CYP) 2C8 and CYP2C9 with NADPH-cytochrome P450 reductase (OxR) to produce a catalytically active system were compared. 2. Approaches assessed were expression of a CYP:OxR fusion construct, bicistronic plasmids, simultaneous transformation with CYP and OxR plasmids, and separate expression of CYP and OxR with reconstitution of activity by mixing the bacterial membranes. Two N-terminal modifications (Delta3-20 and 17alpha-leader) of the individual P450s were additionally investigated. 3. Each approach gave efficient expression of CYP2C8 and CYP2C9, but the bicistronic constructs under the expression conditions used gave low OxR expression and low catalytic activity. CYP expression was higher with the Delta3-20 construct for CYP2C9 and with the 17alpha-presequence construct for CYP2C8. 4. Using torsemide as substrate, all methods gave catalytically active systems with K(m) values similar to human liver microsomes. Mixing bacterial membranes containing separately expressed CYP and OxR reconstituted a catalytically active system with the Delta3-20 construct for CYP2C9 but not for CYP2C8, and with neither of the 17alpha- presequence constructs. OxR co-expressed with CYP in the same membrane interacted with CYP to reconstitute activity more effectively than addition of exogenous OxR membranes. 5. Expression construct and OxR co-expression strategy should be individualized for CYP isoforms.

Functional Characterization of Single Nucleotide Polymorphisms with Amino Acid Substitution in CYP1A2, CYP2A6, and CYP2B6 Found in the Japanese Population

As a part of the studies conducted by the Pharma SNPs Consortium (PSC), the enzyme activities of CYP1A2, CYP2A6 and CYP2B6 variants with altered amino acids as a result of single nucleotide polymorphisms (SNPs) found among the Japanese population were analyzed under a unified protocol using the same lots of reagents by the laboratories participating in the PSC. Mutations in CYP1A2, CYP2A6 and CYP2B6 were introduced by site-directed mutagenesis and the wild type and mutated CYP molecules were expressed in Escherichia coli. The expressed cytochrome P450s were purified and the enzyme activities were measured in reconstitution systems. CYP1A2 and CYP1A2Gln478His did not show any differences in 7-ethoxyresorufin O-deethylase activity. CYP2A6 and CYP2A6Glu419Asp metabolized coumarin to form 7-hydroxycoumarin in a similar manner, whereas CYP2A6Ile471Thr showed low activity compared to the wild-type CYP2A6. CYP2B6, CYP2B6Pro167Ala and CYP2B6Arg487Cys showed the same activity for 7-ethoxy-4-triflouromethyl-coumarin O-deethylation. However, CYP2B6Gln172His was roughly twice as active as CYP2B6 and the other CYP2B6 variants for 7-ethoxy-4-triflouromethylcoumarin O-deethylation activity. Although higher inter- and intra-laboratory variations were observed for the calculated Km and V(max) values because the studies were conducted in several different laboratories, the degree of variations was reduced by the increased number of analyses and the adoption of a simple analysis system.

Engineering herbicide metabolism in tobacco and Arabidopsis with CYP76B1, a cytochrome P450 enzyme from Jerusalem artichoke

The Jerusalem artichoke (Helianthus tuberosus) xenobiotic inducible cytochrome P450, CYP76B1, catalyzes rapid oxidative dealkylation of various phenylurea herbicides to yield nonphytotoxic metabolites. We have found that increased herbicide metabolism and tolerance can be achieved by ectopic constitutive expression of CYP76B1 in tobacco (Nicotiana tabacum) and Arabidopsis. Transformation with CYP76B1 conferred on tobacco and Arabidopsis a 20-fold increase in tolerance to linuron, a compound detoxified by a single dealkylation, and a 10-fold increase in tolerance to isoproturon or chlortoluron, which need successive catalytic steps for detoxification. Two constructs for expression of translational fusions of CYP76B1 with P450 reductase were prepared to test if they would yield even greater herbicide tolerance. Plants expressing these constructs had lower herbicide tolerance than CYP76B1 alone, which is apparently a consequence of reduced stability of the fusion proteins. In all cases, increased herbicide tolerance results from more extensive metabolism, as demonstrated with exogenously fed phenylurea. Beside increased herbicide tolerance, expression of CYP76B1 has no other visible phenotype in the transgenic plants. Our data indicate that CYP76B1 can function as a selectable marker for plant transformation, allowing efficient selection in vitro and in soil-grown plants. Plants expressing CYP76B1 may also be a potential tool for phytoremediation of contaminated sites.

Engineering the CYP101 system for in vivo oxidation of unnatural substrates

The protein engineering of CYP enzymes for structure-activity studies and the oxidation of unnatural substrates for biotechnological applications will be greatly facilitated by the availability of functional, whole-cell systems for substrate oxidation. We report the construction of a tricistronic plasmid that expresses the CYP101 monooxygenase from Pseudomonas putida, and its physiological electron transfer co-factor proteins putidaredoxin reductase and putidaredoxin in Escherichia coli, giving a functional in vivo catalytic system. Wild-type CYP101 expressed in this system efficiently transforms camphor to 5-exo-hydroxycamphor without further oxidation to 5-oxo-camphor until >95% of camphor has been consumed. CYP101 mutants with increased activity for the oxidation of diphenylmethane (the Y96F-I395G mutant), styrene and ethylbenzene (the Y96F-V247L mutant) have been engineered. In particular, the Y96F-V247L mutant shows coupling efficiency of approximately 60% for styrene and ethylbenzene oxidation, with substrate oxidation rates of approximately 100/min. Escherichia coli cells transformed with tricistronic plasmids expressing these mutants readily gave 100-mg quantities of 4-hydroxydiphenylmethane and 1-phenylethanol in 24-72 h. This new in vivo system can be used for preparative scale reactions for product characterization, and will greatly facilitate directed evolution of the CYP101 enzyme for enhanced activity and selectivity of substrate oxidation.

An enzymatically active chimeric protein containing the hydrophilic form of NADPH-cytochrome P450 reductase fused to the membrane-binding domain of cytochrome b5

The microsomal flavoprotein NADPH-cytochrome P450 reductase (CPR) contains an N-terminal hydrophobic membrane-binding domain required for reconstitution of hydroxylation activities with cytochrome P450s. In contrast, cytochrome b5 (b5) contains a C-terminal hydrophobic membrane-binding domain required for interaction with P450s. We have constructed, expressed and purified a chimeric flavoprotein (hdb5-CPR) where the C-terminal 45 amino acid residues of b5 have replaced the N-terminal 56 amino acid domain of CPR. This hybrid flavoprotein retains the catalytic properties of the native CPR and is able to reconstitute fatty acid and steroid hydroxylation activities with CYP4A1 and CYP17A. However hdb5-CPR is much less effective than CPR for reconstituting activity with CYP3A4. We conclude that differences on the surface of the P450s reflect unique and specific information essential for the recognition needed to establish reactions of intermolecular electron transfer from the flavoprotein CPR.

Expression, purification, and physical properties of recombinant flavocytochrome fusion proteins containing rat cytochrome b(5) linked to NADPH-cytochrome P450 reductase by different membrane-binding segments

Reconstitution of the enzymatic activities using purified microsomal cytochrome P450s (P450) requires the presence of a membrane-binding segment in the mammalian flavoprotein, NADPH--cytochrome P450 reductase (CPR), and the hemeprotein, cytochrome b(5) (b(5)). The mechanism(s) by which the membrane-binding segments of these proteins exert such a critical role in influencing the reconstitution of the NADPH-supported activity of a P450 remains undefined. In the present work we describe the construction, expression, and purification of four different types of recombinant flavocytochromes containing rat b(5) and rat CPR linked by various membrane-binding segments. The physical properties of these artificial fusion proteins have been studied to determine their ability to serve as electron transfer agents. These studies are a prelude to the subsequent study (accompanying paper) evaluating the functional roles of the hydrophobic (membrane-binding) sequences of b(5) and CPR in the reconstitution of P450 activities. The present study shows that the purified recombinant fusion proteins can serve as active electron transport carriers from NADPH to cytochrome c as well as b(5) by intramolecular as well as intermolecular reactions. It is shown here that the electron transport properties of these purified fusion proteins are influenced by high concentrations of KCl, suggesting a role for charged amino acids in protein-protein interactions. The present study illustrates the application of artificial recombinant flavocytochromes as useful proteins for the study of intramolecular electron transport reactions for comparison with intermolecular interactions.

Protein engineering of cytochromes P-450

The cytochromes P-450 are an immensely important superfamily of heme-containing enzymes. They catalyze the monooxygenation of an enormous range of substrates. In bacteria, cytochromes P-450 are known to catalyze the hydroxylation of environmentally significant substrates such as camphor, phenolic compounds and many herbicides. In eukaryotes, these enzymes perform key roles in the synthesis and interconversion of steroids, while in mammals hepatic cytochromes P-450 are vital for the detoxification of many drugs. As such, the cytochromes P-450 are of considerable interest in medicine and biotechnology and are obvious targets for protein engineering. The purpose of this article is to illustrate the ways in which protein engineering has been used to investigate and modify the properties of cytochromes P-450. Illustrative examples include: the manipulation of substrate selectivity and regiospecificity, the alteration of membrane binding properties, and probing the route of electron transfer.

Cytochrome P450-based cancer gene therapy: recent advances and future prospects

Cytochrome P450-based cancer gene therapy is a novel prodrug activation strategy for cancer treatment that has substantial potential for improving the safety and efficacy of cancer chemotherapeutics. The primary goal of this strategy is to selectively increase tumor cell exposure to cytotoxic drug metabolites generated locally by a prodrug-activating P450 enzyme. This strategy has been exemplified for the alkylating agents cyclophosphamide and ifosfamide, which are bioactivated by select P450 enzymes whose expression is generally high in liver and deficient in tumor cells. Transduction of tumors with a prodrug-activating P450 gene, followed by prodrug treatment, greatly increases intratumoral formation of activated drug metabolites. This leads to more efficient killing of the transduced tumor cells without a significant increase in host toxicity. P450 gene therapy is accompanied by substantial bystander cytotoxicity which greatly enhances the therapeutic effect by extending it to nearby tumor cells not transduced with the therapeutic P450 gene. Although endogenous P450 reductase is not expected to be a limiting factor in prodrug activation in tumor cells that express moderate levels of an exogenous P450 gene, P450 reductase transduction has recently been found to substantially enhance intratumoral prodrug activation and its associated therapeutic effects. Using this gene combination, an overall 50- to 100-fold increase in tumor cell kill in vivo over that provided by hepatic drug activation alone has been observed. Striking improvements in therapeutic effects can thus be achieved using an established anticancer drug in an intratumoral prodrug activation strategy based on the combination of a cytochrome P450 gene with the gene encoding NADPH-P450 reductase. This strategy is readily extendable to several other widely used P450-activated cancer chemotherapeutic prodrugs, as well as to prodrugs that undergo P450 reductase-dependent bioreductive activation and which may exhibit synergy when combined with P450-activated prodrugs in a P450/P450 reductase-based cancer gene therapeutic regimen.