Stimulation of cytochrome P450 reactions by apo-cytochrome b5: evidence against transfer of heme from cytochrome P450 3A4 to apo-cytochrome b5 or heme oxygenase

Proteomics, modeling, and fluorescence assays delineate cytochrome b5 residues involved in binding and stimulation of cytochrome P450 17A1 17,20-lyase

Cytochrome b5 (b5) is known to stimulate some catalytic activities of cytochrome P450 (P450, CYP) enzymes, although mechanisms still need to be defined. The reactions most strongly enhanced by b5 are the 17,20-lyase reactions of P450 17A1 involved in steroid biosynthesis. We had previously used a fluorescently labeled human b5 variant (Alexa 488-T70C-b5) to characterize human P450 17A1-b5 interactions, but subsequent proteomic analyses indicated that lysines in b5 were also modified with Alexa 488 maleimide in addition to Cys-70, due to disulfide dimerization of the T70C mutant. A series of b5 variants were constructed with Cys replacements for the identified lysine residues and labeled with the dye. Fluorescence attenuation and the function of b5 in the steroid lyase reaction depended on the modified position. Apo-b5 (devoid of heme group) studies revealed the lack of involvement of the b5 heme in the fluorescence attenuation. A structural model of b5 with P450 17A1 was predicted using AlphaFold-Multimer algorithms/Rosetta docking, based upon the individual structures, which predicted several new contacts not previously reported, that is, interactions of b5 Glu-48:17A1 Arg-347, b5 Glu-49:17A1 Arg-449, b5 Asp-65:17A1 Arg-126, b5 Asp-65:17A1 Arg-125, and b5 Glu-61:17A1 Lys-91. Fluorescence polarization assays with two modified b5 variants yielded Kd values (for b5-P450 17A1) of 120 to 380 nM, the best estimate of binding affinity. We conclude that both monomeric and dimeric b5 can bind to P450 17A1 and stimulate activity. Results with the mutants indicate that several Lys residues in b5 are sensitive to the interaction with P450 17A1, including Lys-88 and Lys-91.

Species Specificity and Selection of Models for Drug Oxidations Mediated by Polymorphic Human Enzymes

Many drug oxygenations are mainly mediated by polymorphic cytochromes P450 (P450s) and also by flavin-containing monooxygenases (FMOs). More than 50 years of research on P450/FMO-mediated drug oxygenations have clarified their catalytic roles. The natural product coumarin causes hepatotoxicity in rats via the reactive coumarin 3,4-epoxide, a reaction catalyzed by P450 1A2; however, coumarin undergoes rapid 7-hydroxylation by polymorphic P450 2A6 in humans. The primary oxidation product of the teratogen thalidomide in rats is deactivated 5'-hydroxythalidomide plus sulfate and glucuronide conjugates; however, similar 5'-hydroxythalidomide and 5-hydroxythalidomide are formed in rabbits in vivo. Thalidomide causes human P450 3A enzyme induction in liver (and placenta) and is also activated in vitro and in vivo by P450 3A through the primary human metabolite 5-hydroxythalidomide (leading to conjugation with glutathione/nonspecific proteins). Species differences exist in terms of drug metabolism in rodents and humans, and such differences can be very important when determining the contributions of individual enzymes. The approaches used for investigating the roles of human P450 and FMO enzymes in understanding drug oxidations and clinical therapy have not yet reached maturity and still require further development. SIGNIFICANCE STATEMENT: Drug oxidations in animals and humans mediated by P450s and FMOs are important for understanding the pharmacological properties of drugs, such as the species-dependent teratogenicity of the reactive metabolites of thalidomide and the metabolism of food-derived odorous trimethylamine to non-odorous (but proatherogenic) trimethylamine N-oxide. Recognized differences exist in terms of drug metabolism between rodents, non-human primates, and humans, and such differences are important when determining individual liver enzyme contributions with substrates in in vitro and in vivo systems.

Endocrine disrupting chemicals: effects on pituitary, thyroid and adrenal glands

BACKGROUND

In recent years, scientific research has increasingly focused on Endocrine Disrupting Chemicals (EDCs) and demonstrated their relevant role in the functional impairment of endocrine glands. This induced regulatory authorities to ban some of these compounds and to carefully investigate others in order to prevent EDCs-related conditions. As a result, we witnessed a growing awareness and interest on this topic.

AIMS

This paper aims to summarize current evidence regarding the detrimental effects of EDCs on pivotal endocrine glands like pituitary, thyroid and adrenal ones. Particularly, we directed our attention on the known and the hypothesized mechanisms of endocrine dysfunction brought by EDCs. We also gave a glimpse on recent findings from pioneering studies that could in the future shed a light on the pathophysiology of well-known, but poorly understood, endocrine diseases like hormone-producing adenomas.

CONCLUSIONS

Although intriguing, studies on endocrine dysfunctions brought by EDCs are challenging, in particular when investigating long-term effects of EDCs on humans. However, undoubtedly, it represents a new intriguing field of science research.

Cytochrome b 5 Binds Tightly to Several Human Cytochrome P450 Enzymes

Numerous studies have been reported in the past 50-plus years regarding the stimulatory role of cytochrome b ₅ (b ₅) in some, but not all, microsomal cytochrome P450 (P450) reactions with drugs and steroids. A missing element in most of these studies has been a sensitive and accurate measure of binding affinities of b ₅ with P450s. In the course of work with P450 17A1, we developed a fluorescent derivative of a human b ₅ site-directed mutant, Alexa 488-T70C-b ₅, that could be used in binding assays at sub-μM concentrations. Alexa 488-T70C-b ₅ bound to human P450s 1A2, 2B6, 2C8, 2C9, 2E1, 2S1, 4A11, 3A4, and 17A1, with estimated K _d values ranging from 2.5 to 61 nM. Only weak binding was detected with P450 2D6, and no fluorescence attenuation was observed with P450 2A6. All of the P450s that bound b ₅ have some reported activity stimulation except for P450 2S1. The affinity of P450 3A4 for b ₅ was decreased somewhat by the presence of a substrate or inhibitor. The fluorescence of a P450 3A4•Alexa 488-T70C-b ₅ complex was partially restored by titration with NADPH-P450 reductase (POR) (K _d,apparent 89 nM), suggesting the existence of a ternary P450 3A4-b ₅-POR complex, as observed previously with P450 17A1. Gel filtration evidence was also obtained for this ternary complex with P450 3A4. Overall, the results indicated that the affinity of b ₅ for many P450s is very high, and that ternary P450-b ₅-POR complexes are relevant in P450 3A4 reactions as opposed to a shuttle mechanism. SIGNIFICANCE STATEMENT: High-affinity binding of cytochrome b ₅ (b ₅) (K _d < 100 nM) was observed with many drug-metabolizing cytochrome P450 (P450) enzymes. There is some correlation of binding with reported stimulation, with several exceptions. Evidence is provided for a ternary P450 3A4-b ₅-NADPH-P450 reductase complex.

An electron transfer competent structural ensemble of membrane-bound cytochrome P450 1A1 and cytochrome P450 oxidoreductase

Cytochrome P450 (CYP) heme monooxygenases require two electrons for their catalytic cycle. For mammalian microsomal CYPs, key enzymes for xenobiotic metabolism and steroidogenesis and important drug targets and biocatalysts, the electrons are transferred by NADPH-cytochrome P450 oxidoreductase (CPR). No structure of a mammalian CYP-CPR complex has been solved experimentally, hindering understanding of the determinants of electron transfer (ET), which is often rate-limiting for CYP reactions. Here, we investigated the interactions between membrane-bound CYP 1A1, an antitumor drug target, and CPR by a multiresolution computational approach. We find that upon binding to CPR, the CYP 1A1 catalytic domain becomes less embedded in the membrane and reorients, indicating that CPR may affect ligand passage to the CYP active site. Despite the constraints imposed by membrane binding, we identify several arrangements of CPR around CYP 1A1 that are compatible with ET. In the complexes, the interactions of the CPR FMN domain with the proximal side of CYP 1A1 are supplemented by more transient interactions of the CPR NADP domain with the distal side of CYP 1A1. Computed ET rates and pathways agree well with available experimental data and suggest why the CYP-CPR ET rates are low compared to those of soluble bacterial CYPs.

Kinetics of cytochrome P450 3A4 inhibition by heterocyclic drugs defines a general sequential multistep binding process

Cytochrome P450 (P450) 3A4 is the enzyme most involved in the metabolism of drugs and can also oxidize numerous steroids. This enzyme is also involved in one-half of pharmacokinetic drug-drug interactions, but details of the exact mechanisms of P450 3A4 inhibition are still unclear in many cases. Ketoconazole, clotrimazole, ritonavir, indinavir, and itraconazole are strong inhibitors; analysis of the kinetics of reversal of inhibition with the model substrate 7-benzoyl quinoline showed lag phases in several cases, consistent with multiple structures of P450 3A4 inhibitor complexes. Lags in the onset of inhibition were observed when inhibitors were added to P450 3A4 in 7-benzoyl quinoline O-debenzylation reactions, and similar patterns were observed for inhibition of testosterone 6β-hydroxylation by ritonavir and indinavir. Upon mixing with inhibitors, P450 3A4 showed rapid binding as judged by a spectral shift with at least partial high-spin iron character, followed by a slower conversion to a low-spin iron-nitrogen complex. The changes were best described by two intermediate complexes, one being a partial high-spin form and the second another intermediate, with half-lives of seconds. The kinetics could be modeled in a system involving initial loose binding of inhibitor, followed by a slow step leading to a tighter complex on a multisecond time scale. Although some more complex possibilities cannot be dismissed, these results describe a system in which conformationally distinct forms of P450 3A4 bind inhibitors rapidly and two distinct P450-inhibitor complexes exist en route to the final enzyme-inhibitor complex with full inhibitory activity.

Probing protein-protein and protein-substrate interactions in the dynamic membrane-associated ternary complex of cytochromes P450, b5, and reductase

Cytochrome P450 (cytP450) interacts with two redox partners, cytP450 reductase and cytochrome-b5, to metabolize substrates. Using NMR, we reveal changes in the dynamic interplay when all three proteins are incorporated into lipid nanodiscs in the absence and presence of substrates.

Identification of Human Enzymes Oxidizing the Anti-Thyroid-Cancer Drug Vandetanib and Explanation of the High Efficiency of Cytochrome P450 3A4 in its Oxidation

The metabolism of vandetanib, a tyrosine kinase inhibitor used for treatment of symptomatic/progressive medullary thyroid cancer, was studied using human hepatic microsomes, recombinant cytochromes P450 (CYPs) and flavin-containing monooxygenases (FMOs). The role of CYPs and FMOs in the microsomal metabolism of vandetanib to N-desmethylvandetanib and vandetanib-N-oxide was investigated by examining the effects of CYP/FMO inhibitors and by correlating CYP-/FMO-catalytic activities in each microsomal sample with the amounts of N-desmethylvandetanib/vandetanib-N-oxide formed by these samples. CYP3A4/FMO-activities significantly correlated with the formation of N-desmethylvandetanib/ vandetanib-N-oxide. Based on these studies, most of the vandetanib metabolism was attributed to N-desmethylvandetanib/vandetanib-N-oxide to CYP3A4/FMO3. Recombinant CYP3A4 was most efficient to form N-desmethylvandetanib, while FMO1/FMO3 generated N-oxide. Cytochrome b₅ stimulated the CYP3A4-catalyzed formation of N-desmethylvandetanib, which is of great importance because CYP3A4 is not only most efficient in generating N-desmethylvandetanib, but also most significant due to its high expression in human liver. Molecular modeling indicated that binding of more than one molecule of vandetanib into the CYP3A4-active center can be responsible for the high efficiency of CYP3A4 N-demethylating vandetanib. Indeed, the CYP3A4-mediated reaction exhibits kinetics of positive cooperativity and this corresponded to the in silico model, where two vandetanib molecules were found in CYP3A4-active center.

Mechanisms of Cytochrome P450-Catalyzed Oxidations

Enzymes are complex biological catalysts and are critical to life. Most oxidations of chemicals are catalyzed by cytochrome P450 (P450, CYP) enzymes, which generally utilize mixed-function oxidase stoichiometry, utilizing pyridine nucleotides as electron donors: NAD(P)H + O₂ + R → NAD(P)⁺ + RO + H₂O (where R is a carbon substrate and RO is an oxidized product). The catalysis of oxidations is largely understood in the context of the heme iron-oxygen complex generally referred to as Compound I, formally FeO³⁺, whose basis was in peroxidase chemistry. Many X-ray crystal structures of P450s are now available (≥ 822 structures from ≥146 different P450s) and have helped in understanding catalytic specificity. In addition to hydroxylations, P450s catalyze more complex oxidations, including C-C bond formation and cleavage. Enzymes derived from P450s by directed evolution can even catalyze more unusual reactions, e.g. cyclopropanation. Current P450 questions under investigation include the potential role of the intermediate Compound 0 (formally Fe^III-O₂ ^-) in catalysis of some reactions, the roles of high- and low-spin forms of Compound I, the mechanism of desaturation, the roles of open and closed structures of P450s in catalysis, the extent of processivity in multi-step oxidations, and the role of the accessory protein cytochrome b ₅. More global questions include exactly how structure drives function, prediction of catalysis, and roles of multiple protein conformations.

Formation and Cleavage of C-C Bonds by Enzymatic Oxidation-Reduction Reactions

Many oxidation-reduction (redox) enzymes, particularly oxygenases, have roles in reactions that make and break C-C bonds. The list includes cytochrome P450 and other heme-based monooxygenases, heme-based dioxygenases, nonheme iron mono- and dioxygenases, flavoproteins, radical S-adenosylmethionine enzymes, copper enzymes, and peroxidases. Reactions involve steroids, intermediary metabolism, secondary natural products, drugs, and industrial and agricultural chemicals. Many C-C bonds are formed via either (i) coupling of diradicals or (ii) generation of unstable products that rearrange. C-C cleavage reactions involve several themes: (i) rearrangement of unstable oxidized products produced by the enzymes, (ii) oxidation and collapse of radicals or cations via rearrangement, (iii) oxygenation to yield products that are readily hydrolyzed by other enzymes, and (iv) activation of O₂ in systems in which the binding of a substrate facilitates O₂ activation. Many of the enzymes involve metals, but of these, iron is clearly predominant.

Taxadiene-5α-ol is a minor product of CYP725A4 when expressed in Escherichia coli

CYP725A4 is a P450 enzyme from Taxus cuspidata that catalyzes the formation of taxadiene-5α-ol (T5α-ol) from taxadiene in paclitaxel biosynthesis. Past attempts expressing CYP725A4 in heterologous hosts reported the formation of 5(12)-oxa-3(11)-cyclotaxane (OCT) and/or 5(11)-oxa-3(11)-cyclotaxane (iso-OCT) instead of, or in addition to, T5α-ol. Here, we report that T5α-ol is produced as a minor product by Escherichia coli expressing both taxadiene synthase and CYP725A4. The major products were OCT and iso-OCT, while trace amounts of unidentified monooxygenated taxanes were also detected by gas chromatography-mass spectrometry. Since OCT and iso-OCT had not been found in nature, we tested the hypothesis that protein-protein interaction of CYP725A4 with redox partners, such as cytochrome P450 reductase (CPR) and cytochrome b5, may affect the products formed by CYP725A4, possibly favoring the formation of T5α-ol over OCT and iso-OCT. Our results show that coexpression of CYP725A4 with CPR from different organisms did not change the relative ratios of OCT, iso-OCT, and T5α-ol, while cytochrome b5 decreased overall levels of the products formed. Although unsuccessful in finding conditions that promote T5α-ol formation over other products, we used our results to clarify conflicting claims in the literature and discuss other possible approaches to produce paclitaxel via metabolic and enzyme engineering.

[Study specificity of isatin interactions with P450 cytochromes]

Cytochrome P450-dependent monooxygenase systems exist basically in all living organisms, where they perform various important functions. The coordinated functioning of these systems involves many proteins participating in different protein-protein interactions (PPI). Previously, we have found that the endogenous non-peptide bioregulator isatin (indoledione-2,3), synthesized from indole by means of certain cytochromes P450 (e.g. P450 2E1, P450 2C19, P450 2A6) regulates affinity of some PPI. In this work, an attempt has been undertaken to register a direct interaction of isatin with a set of different proteins related to the functioning of cytochrome P450-dependent monooxygenase: five isoforms of cytochromes P450, two isoforms of cytochrome b5, cytochrome P450 reductase, adrenodoxin, adrenodoxin reductase and ferrochelatase. The study has shown that isatin binds specifically only to cytochromes P450 with high affinity (the equilibrium dissociation constant (Kd) is about 10-8 M).

Topography of human cytochrome b5/cytochrome b5 reductase interacting domain and redox alterations upon complex formation

Cytochrome b₅ is the main electron acceptor of cytochrome b₅ reductase. The interacting domain between both human proteins has been unidentified up to date and very little is known about its redox properties modulation upon complex formation. In this article, we characterized the protein/protein interacting interface by solution NMR and molecular docking. In addition, upon complex formation, we measured an increase of cytochrome b₅ reductase flavin autofluorescence that was dependent upon the presence of cytochrome b_5. Data analysis of these results allowed us to calculate a dissociation constant value between proteins of 0.5±0.1μM and a 1:1 stoichiometry for the complex formation. In addition, a 30mV negative shift of cytochrome b₅ reductase redox potential in presence of cytochrome b₅ was also measured. These experiments suggest that the FAD group of cytochrome b₅ reductase increase its solvent exposition upon complex formation promoting an efficient electron transfer between the proteins.

Structural and functional effects of cytochrome b5 interactions with human cytochrome P450 enzymes

The small heme-containing protein cytochrome b₅ can facilitate, inhibit, or have no effect on cytochrome P450 catalysis, often in a P450-dependent and substrate-dependent manner that is not well understood. Herein, solution NMR was used to identify b₅ residues interacting with different human drug-metabolizing P450 enzymes. NMR results revealed that P450 enzymes bound to either b₅ α4-5 (CYP2A6 and CYP2E1) or this region and α2-3 (CYP2D6 and CYP3A4) and suggested variation in the affinity for b₅ Mutations of key b₅ residues suggest not only that different b₅ surfaces are responsible for binding different P450 enzymes, but that these different complexes are relevant to the observed effects on P450 catalysis.

Cytochrome b5 plays a dual role in the reaction cycle of cytochrome P450 3A4 during oxidation of the anticancer drug ellipticine

Abstract

Ellipticine is an anticancer agent that forms covalent DNA adducts after enzymatic activation by cytochrome P450 (CYP) enzymes, mainly by CYP3A4. This process is one of the most important ellipticine DNA-damaging mechanisms for its antitumor action. Here, we investigated the efficiencies of human hepatic microsomes and human recombinant CYP3A4 expressed with its reductase, NADPH:CYP oxidoreductase (POR), NADH:cytochrome b₅ reductase and/or cytochrome b₅ in Supersomes™ to oxidize this drug. We also evaluated the effectiveness of coenzymes of two of the microsomal reductases, NADPH as a coenzyme of POR, and NADH as a coenzyme of NADH:cytochrome b₅ reductase, to mediate ellipticine oxidation in these enzyme systems. Using HPLC analysis we detected up to five ellipticine metabolites, which were formed by human hepatic microsomes and human CYP3A4 in the presence of NADPH or NADH. Among ellipticine metabolites, 9-hydroxy-, 12-hydroxy-, and 13-hydroxyellipticine were formed by hepatic microsomes as the major metabolites, while 7-hydroxyellipticine and the ellipticine N²-oxide were the minor ones. Human CYP3A4 in Supersomes™ generated only three metabolic products, 9-hydroxy-, 12-hydroxy-, and 13-hydroxyellipticine. Using the ³²P-postlabeling method two ellipticine-derived DNA adducts were generated by microsomes and the CYP3A4-Supersome system, both in the presence of NADPH and NADH. These adducts were derived from the reaction of 13-hydroxy- and 12-hydroxyellipticine with deoxyguanosine in DNA. In the presence of NADPH or NADH, cytochrome b₅ stimulated the CYP3A4-mediated oxidation of ellipticine, but the stimulation effect differed for individual ellipticine metabolites. This heme protein also stimulated the formation of both ellipticine-DNA adducts. The results demonstrate that cytochrome b₅ plays a dual role in the CYP3A4-catalyzed oxidation of ellipticine: (1) cytochrome b₅ mediates CYP3A4 catalytic activities by donating the first and second electron to this enzyme in its catalytic cycle, indicating that NADH:cytochrome b₅ reductase can substitute NADPH-dependent POR in this enzymatic reaction and (2) cytochrome b₅ can act as an allosteric modifier of the CYP3A4 oxygenase.

Graphical abstract

Role of cytochrome b5 in the modulation of the enzymatic activities of cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1)

Cytochrome b5 (cyt b5) is a small hemoprotein that plays a significant role in the modulation of activities of an important steroidogenic enzyme, cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1, CYP17A1). Located in the zona fasciculata and zona reticularis of the adrenal cortex and in the gonads, P450 17A1 catalyzes two different reactions in the steroidogenic pathway; the 17α-hydroxylation and 17,20-lyase, in the endoplasmic reticulum of these respective tissues. The activities of P450 17A1 are regulated by cyt b5 that enhances the 17,20-lyase reaction by promoting the coupling of P450 17A1 and cytochrome P450 reductase (CPR), allosterically. Cyt b5 can also act as an electron donor to enhance the 16-ene-synthase activity of human P450 17A1. In this review, we discuss the many roles of cyt b5 and focus on the modulation of CYP17A1 activities by cyt b5 and the mechanisms involved.

Intracellular drug bioavailability: a new predictor of system dependent drug disposition

Intracellular drug exposure is influenced by cell- and tissue-dependent expression of drug-transporting proteins and metabolizing enzymes. Here, we introduce the concept of intracellular bioavailability (F_ic) as the fraction of extracellular drug available to bind intracellular targets, and we assess how F_ic is affected by cellular drug disposition processes. We first investigated the impact of two essential drug transporters separately, one influx transporter (OATP1B1; SLCO1B1) and one efflux transporter (P-gp; ABCB1), in cells overexpressing these proteins. We showed that OATP1B1 increased F_ic of its substrates, while P-gp decreased F_ic. We then investigated the impact of the concerted action of multiple transporters and metabolizing enzymes in freshly-isolated human hepatocytes in culture configurations with different levels of expression and activity of these proteins. We observed that F_ic was up to 35-fold lower in the configuration with high expression of drug-eliminating transporters and enzymes. We conclude that F_ic provides a measurement of the net impact of all cellular drug disposition processes on intracellular bioavailable drug levels. Importantly, no prior knowledge of the involved drug distribution pathways is required, allowing for high-throughput determination of drug access to intracellular targets in highly defined cell systems (e.g., single-transporter transfectants) or in complex ones (including primary human cells).

The effect of isatin on protein-protein interactions between cytochrome b5 and cytochromes P450

Cytochromes P450 (CYP) are involved in numerous biochemical processes including metabolism of xenobiotics, biosynthesis of cholesterol, steroid hormones etc. Since some CYP catalyze indol oxidation to isatin, we have hypothesized that isatin can regulate protein-protein interactions (PPI) between components of the CYP system thus representing a (negative?) feedback mechanism. The aim of this study was to investigate a possible effect of isatin on interaction of human CYP with cytochrome b5 (CYB5A). Using the optical biosensor test system employing surface plasmon resonance (SPR) we have investigated interaction of immobilized CYB5A with various CYP in the absence and in the presence of isatin. The SPR-based experiments have shown that a high concentration of isatin (270 mM) increases Kd values for complexes CYB5A/CYP3А5 and CYB5A/CYP3A4 (twofold and threefold, respectively), but has no influence on complex formation between CYB5A and other CYP (including indol-metabolizing CYP2C19 and CYP2E1). Isatin injection to the optical biosensor chip with the preformed molecular complex CYB5A/CYP3A4 caused a 30%-increase in its dissociation rate. Molecular docking manipulations have shown that isatin can influence interaction of CYP3А5 or CYP3A4 with CYB5A acting at the contact region of CYB5A/CYP.

A Mechanism of O-Demethylation of Aristolochic Acid I by Cytochromes P450 and Their Contributions to This Reaction in Human and Rat Livers: Experimental and Theoretical Approaches

Aristolochic acid I (AAI) is a plant alkaloid causing aristolochic acid nephropathy, Balkan endemic nephropathy and their associated urothelial malignancies. AAI is detoxified by cytochrome P450 (CYP)-mediated O-demethylation to 8-hydroxyaristolochic acid I (aristolochic acid Ia, AAIa). We previously investigated the efficiencies of human and rat CYPs in the presence of two other components of the mixed-functions-oxidase system, NADPH:CYP oxidoreductase and cytochrome b₅, to oxidize AAI. Human and rat CYP1A are the major enzymes oxidizing AAI. Other CYPs such as CYP2C, 3A4, 2D6, 2E1, and 1B1, also form AAIa, but with much lower efficiency than CYP1A. Based on velocities of AAIa formation by examined CYPs and their expression levels in human and rat livers, here we determined the contributions of individual CYPs to AAI oxidation in these organs. Human CYP1A2 followed by CYP2C9, 3A4 and 1A1 were the major enzymes contributing to AAI oxidation in human liver, while CYP2C and 1A were most important in rat liver. We employed flexible in silico docking methods to explain the differences in AAI oxidation in the liver by human CYP1A1, 1A2, 2C9, and 3A4, the enzymes that all O-demethylate AAI, but with different effectiveness. We found that the binding orientations of the methoxy group of AAI in binding centers of the CYP enzymes and the energies of AAI binding to the CYP active sites dictate the efficiency of AAI oxidation. Our results indicate that utilization of experimental and theoretical methods is an appropriate study design to examine the CYP-catalyzed reaction mechanisms of AAI oxidation and contributions of human hepatic CYPs to this metabolism.

Photo-initiated crosslinking extends mapping of the protein-protein interface to membrane-embedded portions of cytochromes P450 2B4 and b₅

Protein-protein interactions play a central role in the regulation of many biochemical processes (e.g. the system participating in enzyme catalysis). Therefore, a deeper understanding of protein-protein interactions may contribute to the elucidation of many biologically important mechanisms. For this purpose, it is necessary to establish the composition and stoichiometry of supramolecular complexes and to identify the crucial portions of the interacting molecules. This study is devoted to structure-functional relationships in the microsomal Mixed Function Oxidase (MFO) complex, which is responsible for biotransformation of many hydrophobic endogenous compounds and xenobiotics. In particular, the cytochrome b5 interaction with MFO terminal oxygenase cytochrome P-450 (P450) was studied. To create photolabile probes suitable for this purpose, we prepared cytochrome b5 which had a photolabile diazirine analog of methionine (pMet) incorporated into the protein sequence, employing recombinant expression in Escherichia coli. In addition to wild-type cytochrome b5, where three methionines (Met) are located at positions 96, 126, and 131, six mutants containing only one Met in the sequence were designed and expressed (see Table 1). In these mutants, a single Met was engineered into the catalytic domain (at positions 23, 41, or 46), into the linker between the protein domains (at position 96), or into the membrane region (at positions 126 or 131). These mutants should confirm or exclude these portions of cytochrome b5 which are involved in the interaction with P450. After UV irradiation, the pMet group(s) in the photolabile cytochrome b5 probe was(were) activated, producing covalent crosslinks with the interacting parts of P450 2B4 in the close vicinity. The covalent complexes were analyzed by the "bottom up" approach with high-accuracy mass spectrometry. The analysis provided an identification of the contacts in the supramolecular complex with low structural resolution. We found that all the above-mentioned cytochrome b5 Met residues can form intermolecular crosslinks and thus participate in the interaction. In addition, our results indicate the existence of at least two P450:cytochrome b5 complexes which differ in the orientation of individual proteins. The results demonstrate the advantages of the photo-initiated crosslinking technique which is able to map the protein-protein interfaces not only in the solvent exposed regions, but also in the membrane-embedded segments (compared to a typical crosslinking approach which generally only identifies crosslinks in solvent exposed regions).

Mechanistic basis of electron transfer to cytochromes p450 by natural redox partners and artificial donor constructs

Cytochromes P450 (P450s) are hemoproteins catalyzing oxidative biotransformation of a vast array of natural and xenobiotic compounds. Reducing equivalents required for dioxygen cleavage and substrate hydroxylation originate from different redox partners including diflavin reductases, flavodoxins, ferredoxins and phthalate dioxygenase reductase (PDR)-type proteins. Accordingly, circumstantial analysis of structural and physicochemical features governing donor-acceptor recognition and electron transfer poses an intriguing challenge. Thus, conformational flexibility reflected by togging between closed and open states of solvent exposed patches on the redox components was shown to be instrumental to steered electron transmission. Here, the membrane-interactive tails of the P450 enzymes and donor proteins were recognized to be crucial to proper orientation toward each other of surface sites on the redox modules steering functional coupling. Also, mobile electron shuttling may come into play. While charge-pairing mechanisms are of primary importance in attraction and complexation of the redox partners, hydrophobic and van der Waals cohesion forces play a minor role in docking events. Due to catalytic plasticity of P450 enzymes, there is considerable promise in biotechnological applications. Here, deeper insight into the mechanistic basis of the redox machinery will permit optimization of redox processes via directed evolution and DNA shuffling. Thus, creation of hybrid systems by fusion of the modified heme domain of P450s with proteinaceous electron carriers helps obviate the tedious reconstitution procedure and induces novel activities. Also, P450-based amperometric biosensors may open new vistas in pharmaceutical and clinical implementation and environmental monitoring.

The anticancer drug ellipticine activated with cytochrome P450 mediates DNA damage determining its pharmacological efficiencies: studies with rats, Hepatic Cytochrome P450 Reductase Null (HRN™) mice and pure enzymes

Ellipticine is a DNA-damaging agent acting as a prodrug whose pharmacological efficiencies and genotoxic side effects are dictated by activation with cytochrome P450 (CYP). Over the last decade we have gained extensive experience in using pure enzymes and various animal models that helped to identify CYPs metabolizing ellipticine. In this review we focus on comparison between the in vitro and in vivo studies and show a necessity of both approaches to obtain valid information on CYP enzymes contributing to ellipticine metabolism. Discrepancies were found between the CYP enzymes activating ellipticine to 13-hydroxy- and 12-hydroxyellipticine generating covalent DNA adducts and those detoxifying this drug to 9-hydroxy- and 7-hydroellipticine in vitro and in vivo. In vivo, formation of ellipticine-DNA adducts is dependent not only on expression levels of CYP3A, catalyzing ellipticine activation in vitro, but also on those of CYP1A that oxidize ellipticine in vitro mainly to the detoxification products. The finding showing that cytochrome b5 alters the ratio of ellipticine metabolites generated by CYP1A1/2 and 3A4 explained this paradox. Whereas the detoxification of ellipticine by CYP1A and 3A is either decreased or not changed by cytochrome b5, activation leading to ellipticine-DNA adducts increased considerably. We show that (I) the pharmacological effects of ellipticine mediated by covalent ellipticine-derived DNA adducts are dictated by expression levels of CYP1A, 3A and cytochrome b5, and its own potency to induce these enzymes in tumor tissues, (II) animal models, where levels of CYPs are either knocked out or induced are appropriate to identify CYPs metabolizing ellipticine in vivo, and (III) extrapolation from in vitro data to the situation in vivo is not always possible, confirming the need for these animal models.

Flexible docking-based molecular dynamics/steered molecular dynamics calculations of protein-protein contacts in a complex of cytochrome P450 1A2 with cytochrome b5

Formation of transient complexes of cytochrome P450 (P450) with another protein of the endoplasmic reticulum membrane, cytochrome b5 (cyt b5), dictates the catalytic activities of several P450s. Therefore, we examined formation and binding modes of the complex of human P450 1A2 with cyt b5. Docking of soluble domains of these proteins was performed using an information-driven flexible docking approach implemented in HADDOCK. Stabilities of the five unique binding modes of the P450 1A2-cyt b5 complex yielded by HADDOCK were evaluated using explicit 10 ns molecular dynamics (MD) simulations in aqueous solution. Further, steered MD was used to compare the stability of the individual P450 1A2-cyt b5 binding modes. The best binding mode was characterized by a T-shaped mutual orientation of the porphyrin rings and a 10.7 Å distance between the two redox centers, thus satisfying the condition for a fast electron transfer. Mutagenesis studies and chemical cross-linking, which, in the absence of crystal structures, were previously used to deduce specific P450-cyt b5 interactions, indicated that the negatively charged convex surface of cyt b5 binds to the positively charged concave surface of P450. Our simulations further elaborate structural details of this interface, including nine ion pairs between R95, R100, R138, R362, K442, K455, and K465 side chains of P450 1A2 and E42, E43, E49, D65, D71, and heme propionates of cyt b5. The universal heme-centric system of internal coordinates was proposed to facilitate consistent classification of the orientation of the two porphyrins in any protein complex.

Role of protein-protein interactions in cytochrome P450-mediated drug metabolism and toxicity

Through their unique oxidative chemistry, cytochrome P450 monooxygenases (CYPs) catalyze the elimination of most drugs and toxins from the human body. Protein–protein interactions play a critical role in this process. Historically, the study of CYP–protein interactions has focused on their electron transfer partners and allosteric mediators, cytochrome P450 reductase and cytochrome b5. However, CYPs can bind other proteins that also affect CYP function. Some examples include the progesterone receptor membrane component 1, damage resistance protein 1, human and bovine serum albumin, and intestinal fatty acid binding protein, in addition to other CYP isoforms. Furthermore, disruption of these interactions can lead to altered paths of metabolism and the production of toxic metabolites. In this review, we summarize the available evidence for CYP protein–protein interactions from the literature and offer a discussion of the potential impact of future studies aimed at characterizing noncanonical protein–protein interactions with CYP enzymes.

Cytochrome b5 and epoxide hydrolase contribute to benzo[a]pyrene-DNA adduct formation catalyzed by cytochrome P450 1A1 under low NADPH:P450 oxidoreductase conditions

In previous studies we had administered benzo[a]pyrene (BaP) to genetically engineered mice (HRN) which do not express NADPH:cytochrome P450 oxidoreductase (POR) in hepatocytes and observed higher DNA adduct levels in livers of these mice than in wild-type mice. To elucidate the reason for this unexpected finding we have used two different settings for in vitro incubations; hepatic microsomes from control and BaP-pretreated HRN mice and reconstituted systems with cytochrome P450 1A1 (CYP1A1), POR, cytochrome b5, and epoxide hydrolase (mEH) in different ratios. In microsomes from BaP-pretreated mice, in which Cyp1a1 was induced, higher levels of BaP metabolites were formed, mainly of BaP-7,8-dihydrodiol. At a low POR:CYP1A1 ratio of 0.05:1 in the reconstituted system, the amounts of BaP diones and BaP-9-ol formed were essentially the same as at an equimolar ratio, but formation of BaP-3-ol was ∼ 1.6-fold higher. Only after addition of mEH were BaP dihydrodiols found. Two BaP-DNA adducts were formed in the presence of mEH, but only one when CYP1A1 and POR were present alone. At a ratio of POR:CYP1A1 of 0.05:1, addition of cytochrome b5 increased CYP1A1-mediated BaP oxidation to most of its metabolites indicating that cytochrome b5 participates in the electron transfer from NADPH to CYP1A1 required for enzyme activity of this CYP. BaP-9-ol was formed even by CYP1A1 reconstituted with cytochrome b5 without POR. Our results suggest that in livers of HRN mice Cyp1a1, cytochrome b5 and mEH can effectively activate BaP to DNA binding species, even in the presence of very low amounts of POR.

Structure and function of heme proteins in non-native states: a mini-review

Heme proteins perform various biological functions ranging from electron transfer, oxygen binding and transport, catalysis, to signaling. Although adopting proper native states is very important for these functions, progresses in representative heme proteins, including cytochrome c (cyt c), cytochrome b5 (cyt b5), myoglobin (Mb), neuroglobin (Ngb), cytochrome P450 (CYP) and heme-based sensor proteins such as CO sensor CooA, showed that various native functions, or new functions evolved, are also closely associated with non-native states. The structure and function relationship of heme proteins in non-native states is thus as important as that in native states for elucidating the precise roles of heme proteins in biological systems.

The involvement of mitochondrial amidoxime reducing components 1 and 2 and mitochondrial cytochrome b5 in N-reductive metabolism in human cells

The mitochondrial amidoxime reducing component mARC is a recently discovered molybdenum enzyme in mammals. mARC is not active as a standalone protein, but together with the electron transport proteins NADH-cytochrome b5 reductase (CYB5R) and cytochrome b5 (CYB5), it catalyzes the reduction of N-hydroxylated compounds such as amidoximes. The mARC-containing enzyme system is therefore considered to be responsible for the activation of amidoxime prodrugs. All hitherto analyzed mammalian genomes code for two mARC genes (also referred to as MOSC1 and MOSC2), which share high sequence similarities. By RNAi experiments in two different human cell lines, we demonstrate for the first time that both mARC proteins are capable of reducing N-hydroxylated substrates in cell metabolism. The extent of involvement is highly dependent on the expression level of the particular mARC protein. Furthermore, the mitochondrial isoform of CYB5 (CYB5B) is clearly identified as an essential component of the mARC-containing N-reductase system in human cells. The participation of the microsomal isoform (CYB5A) in N-reduction could be excluded by siRNA-mediated down-regulation in HEK-293 cells and knock-out in mice. Using heme-free apo-CYB5, the contribution of mitochondrial CYB5 to N-reductive catalysis was proven to strictly depend on heme. Finally, we created recombinant CYB5B variants corresponding to four nonsynonymous single nucleotide polymorphisms (SNPs). Investigated mutations of the heme protein seemed to have no significant impact on N-reductive activity of the reconstituted enzyme system.

Cytochrome b5: novel roles in steroidogenesis

Cytochrome b(5) (cyt-b(5)) is essential for the regulation of steroidogenesis and as such has been implicated in a number of clinical conditions. It is well documented that this small hemoprotein augments the 17,20-lyase activity of cytochrome P450 17α-hydroxylase/17,20-lyase (CYP17A1). Studies have revealed that this augmentation is accomplished by cyt-b(5) enhancing the interaction between cytochrome P450 reductase (POR) and CYP17A1. In this paper we present evidence that cyt-b(5) induces a conformational change in CYP17A1, in addition to facilitating the interaction between CYP17A1 and POR. We also review the recently published finding that cyt-b(5) allosterically augments the activity of 3β-hydroxysteroid dehydrogenase/Δ(5)-Δ(4) isomerase (3βHSD), a non cytochrome P450 enzyme, by increasing the enzymes affinity for its cofactor, NAD(+). The physiological importance of this finding, in terms of understanding adrenal androstenedione production, is examined. Finally, evidence that cyt-b(5) is able to form homomeric complexes in living cells is presented and discussed.

Substrate-modulated cytochrome P450 17A1 and cytochrome b5 interactions revealed by NMR

The membrane heme protein cytochrome b5 (b5) can enhance, inhibit, or have no effect on cytochrome P450 (P450) catalysis, depending on the specific P450, substrate, and reaction conditions, but the structural basis remains unclear. Here the interactions between the soluble domain of microsomal b5 and the catalytic domain of the bifunctional steroidogenic cytochrome P450 17A1 (CYP17A1) were investigated. CYP17A1 performs both steroid hydroxylation, which is unaffected by b5, and an androgen-forming lyase reaction that is facilitated 10-fold by b5. NMR chemical shift mapping of b5 titrations with CYP17A1 indicates that the interaction occurs in an intermediate exchange regime and identifies charged surface residues involved in the protein/protein interface. The role of these residues is confirmed by disruption of the complex upon mutagenesis of either the anionic b5 residues (Glu-48 or Glu-49) or the corresponding cationic CYP17A1 residues (Arg-347, Arg-358, or Arg-449). Cytochrome b5 binding to CYP17A1 is also mutually exclusive with binding of NADPH-cytochrome P450 reductase. To probe the differential effects of b5 on the two CYP17A1-mediated reactions and, thus, communication between the superficial b5 binding site and the buried CYP17A1 active site, CYP17A1/b5 complex formation was characterized with either hydroxylase or lyase substrates bound to CYP17A1. Significantly, the CYP17A1/b5 interaction is stronger when the hydroxylase substrate pregnenolone is present in the CYP17A1 active site than when the lyase substrate 17α-hydroxypregnenolone is in the active site. These findings form the basis for a clearer understanding of this important interaction by directly measuring the reversible binding of the two proteins, providing evidence of communication between the CYP17A1 active site and the superficial proximal b5 binding site.

In vitro metabolism of the 5-hydroxytryptamine1B receptor antagonist elzasonan

The metabolism of elzasonan has been examined in vitro using hepatic microsomes from human and recombinant heterologously expressed P450 enzymes (rCYP). Metabolism occurs primarily via oxidative N-demethylation to form M4 and oxidation reactions to form elzasonan N-oxide (M5) and 5-hydroxyelzasonan metabolite (M3). Additionally, elzasonan was shown to be metabolized to the novel cyclized indole metabolite (M6) which undergoes subsequent oxidation to form the iminium ion metabolite (M3a). The rCYP data was normalized relative to the levels of each CYP form in native human liver microsomes to better assess the contribution of each rCYP in the metabolism of elzasonan. Results demonstrated the involvement of CYP3A4 in the pathways leading to M3a, M3, M5 and M6 and CYP2C8 in the formation of M4. Kinetic constants for the formation of M3 were determined and correlation and inhibition studies suggested that CYP3A4 is primarily responsible for the formation of M3 and CYP2C19 plays a very minor role in its formation. Cytochrome b5 has shown to be an essential component in P450 3A4 catalyzed 5-hydroxyelzasonan formation and provides insights on the disconnect between human liver microsomes data and that of rCYP. Furthermore, rCYP3A4 containing b5 are useful models for predicting the rates for liver microsomes P450-dependent drug oxidations and should be utilized routinely.

Co-expression of human cytochrome b5 increases expression of cytochrome P450 3A4 in Escherichia coli by stabilizing mRNA

CYP3A4 is the most abundant cytochrome P450 in the human liver. The expression level of CYP3A4 when coexpressed with cytochrome b(5) (cyt b(5)) in Escherichia coli was 20-60% higher than that when it was expressed alone over an extended period (48-72 h). This time-dependent elevation in coexpression with cyt b(5) was a result of an increase in CYP3A4 mRNA half-life; no significant change in CYP3A4 degradation was seen in the bacterial protease fraction. These results suggest that the higher CYP3A4 levels observed upon coexpression with cyt b(5) primarily resulted from CYP3A4 mRNA stabilization by cyt b(5).

The action of cytochrome b(5) on CYP2E1 and CYP2C19 activities requires anionic residues D58 and D65

The capacity of cytochrome b(5) (b(5)) to influence cytochrome P450 activities has been extensively studied and physiologically validated. Apo-b(5) enhances the activities of CYP3A4, CYP2A6, CYP2C19, and CYP17A1 but not that of CYP2E1 or CYP2D6, suggesting that the b(5) interaction varies among P450s. We previously showed that b(5) residues E48 and E49 are required to stimulate the 17,20-lyase activity of CYP17A1, but these same residues might not mediate b(5) activation of other P450 reactions, such as CYP2E1-catalyzed oxygenations, which are insensitive to apo-b(5). Using purified P450, b(5), and reductase (POR) in reconstituted assays, the D58G/D65G double mutation, of residues located in a hydrophilic α-helix of b(5), totally abolished the ability to stimulate CYP2E1-catalyzed chlorzoxazone 6-hydroxylation. In sharp contrast, the D58G/D65G double mutation retained the full ability to stimulate the 17,20-lyase activity of CYP17A1. The D58G/D65G double mutation competes poorly with wild-type b(5) for binding to the CYP2E1·POR complex yet accepts electrons from POR at a similar rate. Furthermore, the phospholipid composition markedly influences P450 turnover and b(5) stimulation and specificity, particularly for CYP17A1, in the following order: phosphatidylserine > phosphatidylethanolamine > phosphatidylcholine. The D58G/D65G double mutation also failed to stimulate CYP2C19-catalyzed (S)-mephenytoin 4-hydroxylation, whereas the E48G/E49G double mutation stimulated these activities of CYP2C19 and CYP2E1 equivalent to wild-type b(5). We conclude that b(5) residues D58 and D65 are essential for the stimulation of CYP2E1 and CYP2C19 activities and that the phospholipid composition significantly influences the b(5)-P450 interaction. At least two surfaces of b(5) differentially influence P450 activities, and the critical residues for individual P450 reactions cannot be predicted from sensitivity to apo-b(5) alone.

Cross-linking mass spectrometry and mutagenesis confirm the functional importance of surface interactions between CYP3A4 and holo/apo cytochrome b(5)

Cytochrome b(5) (cyt b(5)) is one of the key components in the microsomal cytochrome P450 monooxygenase system. Consensus has not been reached about the underlying mechanism of cyt b(5) modulation of CYP catalysis. Both cyt b(5) and apo b(5) are reported to stimulate the activity of several P450 isoforms. In this study, the surface interactions of both holo and apo b(5) with CYP3A4 were investigated and compared for the first time. Chemical cross-linking coupled with mass spectrometric analysis was used to identify the potential electrostatic interactions between the protein surfaces. Subsequently, the models of interaction of holo/apo b(5) with CYP3A4 were built using the identified interacting sites as constraints. Both cyt b(5) and apo b(5) were predicted to bind to the same groove on CYP3A4 with close contacts to the B-B' loop of CYP3A4, a substrate recognition site. Mutagenesis studies further confirmed that the interacting sites on CYP3A4 (Lys96, Lys127, and Lys421) are functionally important. Mutation of these residues reduced or abolished cyt b(5) binding affinity. The critical role of Arg446 on CYP3A4 in binding to cyt b(5) and/or cytochrome P450 reductase was also discovered. The results indicated that electrostatic interactions on the interface of the two proteins are functionally important. The results indicate that apo b(5) can dock with CYP3A4 in a manner analogous to that of holo b(5), so electron transfer from cyt b(5) is not required for its effects.

Ellipticine oxidation and DNA adduct formation in human hepatocytes is catalyzed by human cytochromes P450 and enhanced by cytochrome b5

Ellipticine is an antineoplastic agent considered a pro-drug, the pharmacological and genotoxic effects of which are dependent on cytochrome P450 (CYP)- and/or peroxidase-mediated activation to covalent DNA adducts. We investigated whether ellipticine-DNA adducts are formed in human hepatic microsomes and human hepatocytes. We then identified which human CYPs oxidize ellipticine to metabolites forming DNA adducts and the effect of cytochrome b(5) on this oxidation. 13-Hydroxyellipticine, the metabolite forming the major ellipticine-DNA adduct, was generated mainly by CYP3A4 and 1A1, followed by CYP2D6>2C19>1B1>1A2>2E1 and >2C9. Cytochrome b(5) increased formation of this metabolite by human CYPs, predominantly by CYP1A1, 3A4, 1A2 and 2C19. Formation of 12-hydroxyellipticine is generated mainly by CYP2C19, followed by CYP2C9>3A4>2D6>2E1 and >2A6. Other CYPs were less active (CYP2C8 and 2B6) or did not oxidize ellipticine to this metabolite (CYP1A1, 1A2 and 1B1). CYP2D6 was the most efficient enzyme generating ellipticine N(2)-oxide. CYP3A4 and 1A1 in the presence of cytochrome b(5) are mainly responsible for bioactivation of ellipticine to DNA adduct 1 (formed by ellipticine-13-ylium from 13-hydroxyellipticine), while 12-hydroxyellipticine generated during the CYP2C19-mediated ellipticine oxidation is the predominant metabolite forming ellipticine-12-ylium that generates ellipticine-DNA adduct 2. These ellipticine-DNA adducts were also generated by human hepatic microsomes and in primary human hepatocytes exposed to ellipticine. Ellipticine is toxic to these hepatocytes, decreasing their viability; the IC(50) value of ellipticine in these cells was 0.7 μM. In liver CYP3A4 is the predominant ellipticine activating CYP species, which is expected to result in efficient metabolism after oral ingestion of ellipticine in humans.

Experimental approaches to evaluate activities of cytochromes P450 3A

Cytochrome P450 (CYP) is a heme protein oxidizing various xenobiotics, as well as endogenous substrates. Understanding which CYP enzymes are involved in metabolic activation and/or detoxication of different compounds is important in the assessment of an individual's susceptibility to the toxic action of these substances. Therefore, investigation which of several in vitro experimental models are appropriate to mimic metabolism of xenobiotics in organisms is the major challenge for research of many laboratories. The aim of this study was to evaluate the efficiency of different in vitro systems containing individual enzymes of the mixed-function monooxygenase system to oxidize two model substrates of CYP3A enzymes, exogenous and endogenous compounds, α-naphtoflavone (α-NF) and testosterone, respectively. Several different enzymatic systems containing CYP3A enzymes were utilized in the study: (i) human hepatic microsomes rich in CYP3A4, (ii) hepatic microsomes of rabbits treated with a CYP3A6 inducer, rifampicine, (iii) microsomes of Baculovirus transfected insect cells containing recombinant human CYP3A4 and NADPH:CYP reductase with or without cytochrome b(5) (Supersomes™), (iv) membranes isolated from of Escherichia coli, containing recombinant human CYP3A4 and cytochrome b(5), and (v) purified human CYP3A4 or rabbit CYP3A6 reconstituted with NADPH:CYP reductase with or without cytochrome b(5) in liposomes. The most efficient systems oxidizing both compounds were Supersomes™ containing human CYP3A4 and cytochrome b(5). The results presented in this study demonstrate the suitability of the supersomal CYP3A4 systems for studies investigating oxidation of testosterone and α-NF in vitro.

Optimalization of preparation of apo-cytochrome b(5) utilizing apo-myoglobin

Cytochrome b₅ (cyt b₅), a component of endoplasmic reticulum membrane, plays a role in modulation of enzymatic activity of some cytochrome P450 (CYP) enzymes. The effect of apo-cytochrome b₅ on this enzymatic system has not been investigated in details, because preparation of cyt b₅ as a pure protein failed in many laboratories. In order to prepare the native apo-cytochrome b₅ in a large scale we utilized a protein with higher affinity toward the heme; the apo-myoglobin from the equine skeletal muscle. In the first step, we extracted heme moiety from the native myoglobin by butanone extraction. Than the effect of pH on spontaneous heme release from both proteins was investigated: purified rabbit cyt b₅ as well as equine skeletal muscle myoglobin. The prepared apo-myoglobin was incubated with the cyt b₅ and heme transfer was monitored as a shift of absorption maximum from 413 to 409 nm in pH varying between 3–6 (10 mM KH₂PO₄, pH 3–6). Here, we obtained 43 mg of the equine skeletal muscle apo-myoglobin (43% yield). The optimal pH range for heme transfer from cyt b₅ into apo-myoglobin was between 4.2 and 5. Native apo-cytochrome b₅ was successfully prepared using procedure described here.

Microsomal monooxygenase as a multienzyme system: the role of P450-P450 interactions

INTRODUCTION

There is increasing evidence of physical interactions (association) among cytochromes P450 in the membranes of the endoplasmic reticulum. Functional consequences of these interactions are often underestimated.

AREAS COVERED

This article provides a comprehensive overview of available experimental material regarding P450-P450 interactions. Special emphasis is given to the interactions between different P450 species and to the functional consequences of homo- and heterooligomerization.

EXPERT OPINION

Recent advances provide conclusive evidence for a substantial degree of P450 oligomerization in membranes. Interactions between different P450 species resulting in the formation of mixed oligomers with altered activity and substrate specificity have been demonstrated clearly. There are important indications that oligomerization impedes electron flow to a fraction of the P450 population, which renders some P450 species nonfunctional. Functional consequences of P450-P450 interactions make the integrated properties of the microsomal monooxygenase remarkably different from a simple summation of the properties of the individual P450 species. This complexity compromises the predictive power of the current in vitro models of drug metabolism and warrants an urgent need for development of new model systems that consider the interactions of multiple P450 species.

Cytochrome P450 3A activities and their modulation by α-naphthoflavone in vitro are dictated by the efficiencies of model experimental systems

The knowledge on efficiencies of different in vitro systems containing cytochromes P450 (CYP) of a 3A subfamily is crucial to screen potential substrates of these CYPs. We evaluated and compared efficiencies of several in vitro CYP3A enzymatic systems to oxidize the model substrates, α-NF and testosterone, under the standardized experimental conditions. Five CYP3A systems were tested: (i) human hepatic microsomes rich in CYP3A4, (ii) hepatic microsomes of rabbits treated with a CYP3A6 inducer, rifampicine, (iii) microsomes of Baculovirus transfected insect cells containing recombinant human CYP3A4 and NADPH:CYP reductase with or without cytochrome b5 (SupersomesTM), (iv) membranes isolated from Escherichia coli, containing recombinant human CYP3A4, NADPH:CYP reductase and cytochrome b5, and (v) human CYP3A4 or rabbit CYP3A6 reconstituted with NADPH:CYP reductase with or without cytochrome b5 in liposomes. All systems oxidize testosterone to its 6β-hydroxylated metabolite and α-NF to trans-7,8-dihydrodiol and 5,6-epoxide. The most efficient systems oxidizing both compounds were CYP3A4-SupersomesTM containing cytochrome b5, followed by human hepatic microsomes. This finding suggests these systems to be suitable for general evaluating a variety of compounds as potential substrates of CYP3A4. The lowest efficiencies to oxidize α-NF and testosterone were found for CYP3A4 expressed in membranes of E. coli, and for reconstituted CYP3A4 or CYP3A6. Utilizing the tested enzymatic systems, we also explain here the discrepancies, which showed previously the controversial effects of α-NF on CYP3A-mediated reactions. We demonstrate that inhibition or stimulation of the CYP3A-mediated testosterone hydroxylation by α-NF is dictated by efficiencies of individual enzymatic systems to oxidize the CYP3A substrates.

Structural diversity of cytochrome P450 enzyme system

Cytochrome P450 enzyme system consists of P450 and its NAD(P)H-linked reductase or reducing system, and catalyses monooxygenation reactions. The most prevalent type in eukaryotic organisms is 'microsomes type', which consists of membrane-bound P450 and NADPH-P450 reductase. The second type is 'mitochondria type', in which P450 is bound to the inner membrane while the reducing system consisting of an NADPH-linked flavoprotein and a ferredoxin-type iron-sulphur protein is soluble in the matrix space. The third type is 'bacteria type', in which both P450 and the reducing system are soluble in the cytoplasm. In addition to these three types, several forms of P450-reductase fusion proteins have been found in prokaryotic organisms. On the other hand, some P450s catalyse the re-arrangement of the oxygen atoms in the substrate molecules that does not require the supply of reducing equivalents for the reaction. A peculiar P450, P450nor, receives electrons directly from NADH for the reduction of nitric oxide.

Conformational stability of cytochrome b5, enhanced green fluorescent protein, and their fusion protein Hmwb5-EGFP

The conformational stabilities of chimeric protein Hmwb(5)-EGFP and its constituents (cytochrome b(5) and enhanced green fluorescent protein) in guanidine hydrochloride solutions are reported in this paper. Intensity of fluorescence of tryptophan residues, intensity of EGFP fluorescence in the visible region, absorbance of cytochrome b(5) heme and EGFP fluorophore, and fluorescence anisotropy were used to follow the unfolding process. Thermodynamic parameters of protein unfolding were obtained using different approaches. The data were analyzed using a two-stage model and a linear extrapolation method. Unfolding of protein molecules was additionally monitored by measuring Stern-Volmer constants for tryptophan fluorescence quenching by acrylamide, cesium, and iodide. The accessibility of tryptophan residues of both components in the fusion molecule is lower than in the separate molecules. The thermodynamic stability of the protein globules in the fusion protein is much lower than in the individual protein molecules in solution, the difference in free energy of unfolding being more considerable for cytochrome b(5) (29 +/- 4 and 13 +/- 2 kJ/mol) than for EGFP (26 +/- 0.9 and 20 +/- 2.7 kJ/mol). The data indicate that artificial protein fusion can greatly affect total structural stability, and in the case of cytochrome b(5) and EGFP it results in decrease in free energy of transition from native to denatured unfolded form and consequently to decrease in thermodynamic stability of protein globules compared to the separate proteins.

Preparation of a biologically active apo-cytochrome b5 via heterologous expression in Escherichia coli

Cytochrome b(5) (b(5)) has been shown to modulate many cytochrome P450 (CYP)-dependent reactions. In order to elucidate the mechanism of such modulations, it is necessary to evaluate not only the effect of native b(5) on CYP-catalyzed reactions, but also that of the apo-cytochrome b(5) (apo-b(5)). Therefore, the apo-b(5) protein was prepared using a heterologous expression in Escherichia coli. The gene for rabbit b(5) was constructed from synthetic oligonucleotides using polymerase chain reaction (PCR), cloned into pUC19 plasmid and amplified in DH5 alpha cells. The gene sequence was verified by DNA sequencing. The sequence coding b(5) was cleaved from pUC19 by NdeI and XhoI restriction endonucleases and subcloned to the expression vector pET22b. This vector was used to transform E. coli BL-21 (DE3) Gold cells by heat shock. Expression of b(5) was induced with isopropyl beta-D-1-thiogalactopyranoside (IPTG). The b(5) protein, produced predominantly in its apo-form, was purified from isolated membranes of E. coli cells by chromatography on a column of DEAE-Sepharose. Using such procedures, the homogenous preparation of apo-b(5) protein was obtained. Oxidized and reduced forms of the apo-b(5) reconstituted with heme exhibit the same absorbance spectra as native b(5). The prepared recombinant apo-b(5) reconstituted with heme can be reduced by NADPH:CYP reductase. The reconstituted apo-b(5) is also fully biologically active, exhibiting the comparable stimulation effect on the CYP3A4 enzymatic activity towards oxidation of 1-phenylazo-2-hydroxynaphthalene (Sudan I) as native rabbit and human b(5).