Organization of NADPH-cytochrome P450 reductase and CYP1A2 in the endoplasmic reticulum--microdomain localization affects monooxygenase function

Influence of cholesterol on kinetic parameters for human aromatase (P450 19A1) in phospholipid nanodiscs

Cholesterol, a significant constituent of the endoplasmic reticulum membrane, exerts a substantial effect on the membrane's biophysical and mechanical properties. Cholesterol, however, is often neglected in model systems used to study membrane-bound proteins. For example, the influence of cholesterol on the enzymatic functions of type 2 cytochromes P450, which require a phospholipid bilayer and the redox partner P450-oxidoreductase (POR) for activity, are rarely investigated. Human aromatase (P450 19A1) catalyzes three sequential oxygenations of 19‑carbon steroids to estrogens and is widely expressed across various tissues, which are characterized by varying cholesterol compositions. Our study examined the impact of cholesterol on the functionality of the P450 19A1 complex with POR. Nanodiscs containing P450 19A1 with 20% cholesterol/80% phospholipid had similar rates and affinity of androstenedione binding as phospholipid-only P450 19A1 nanodiscs, and rates of product formation were indistinguishable among these conditions. In contrast, the rate of the first electron transfer from POR to P450 19A1 was 3-fold faster in cholesterol-containing nanodiscs than in phospholipid-only nanodiscs. These results suggest that cholesterol influences some aspects of POR interaction with P450 19A1 and might serve as an additional regulatory mechanism in this catalytic system.

The Influence of Lipid Microdomain Heterogeneity on Protein-Protein Interactions: Proteomic Analysis of Co-Immunoprecipitated Binding Partners of P450 1A2 and P450 3A in Rat Liver Microsomes

Liver cytochrome P450s (CYPs) of the endoplasmic reticulum (ER) are involved in the metabolism of exogenous and endogenous chemicals. The ER is not uniform, but possesses ordered lipid microdomains containing higher levels of saturated fatty acids, sphingomyelin, and cholesterol and disordered regions containing higher levels of polyunsaturated fatty acid chains. The various forms of drug-metabolizing P450s partition to either the ordered or disordered lipid microdomains with different degrees of specificity. P450s readily form complexes with ER-resident proteins, including other forms of P450. This study aims to ascertain whether lipid microdomain localization influences protein-P450 interactions in rat liver microsomes. Thus, liver microsomes were co-immunoprecipitated with CYP1A2-specific and CYP3A-specific antibodies, and the co-immunoprecipitating proteins were identified by liquid chromatography mass spectrometry proteomic analysis. These two P450s preferentially partition to ordered and disordered microdomains, respectively. More than 100 proteins were co-immunoprecipitated with each P450. Segregation of proteins into different microdomains did not preclude their interaction. However, CYP3A interacted broadly with proteins from ordered microdomains, whereas CYP1A2 reacted with a limited subset of these proteins. This is consistent with the concept of lipid raft heterogeneity and may indicate that CYP1A2 is targeted to a specific type of lipid raft. Although many of the interacting proteins for both P450s were other-drug metabolizing enzymes, other interactions were also evident. The consistent CYP3A binding partners were predominantly involved in phase I/II drug metabolism; however, CYP1A2 interacted not only with xenobiotic metabolizing enzymes, but also with enzymes involved in diverse cellular responses such as ER stress and protein folding. SIGNIFICANCE STATEMENT: This work describes the protein interactomes in rat liver microsomes of two important cytochromes P450s (CYP1A2 and CYP3A) in drug metabolism and describes the relationship of the interacting proteins to lipid microdomain distribution.

Heterologous Expression of Recombinant Human Cytochrome P450 (CYP) in Escherichia coli: N-Terminal Modification, Expression, Isolation, Purification, and Reconstitution

Cytochrome P450 (CYP) enzymes play important roles in metabolising endogenous and xenobiotic substances. Characterisations of human CYP proteins have been advanced with the rapid development of molecular technology that allows heterologous expression of human CYPs. Among several hosts, bacteria systems such as Escherichia coli (E. coli) have been widely used thanks to their ease of use, high level of protein yields, and affordable maintenance costs. However, the levels of expression in E. coli reported in the literature sometimes differ significantly. This paper aims to review several contributing factors, including N-terminal modifications, co-expression with a chaperon, selections of vectors and E. coli strains, bacteria culture and protein expression conditions, bacteria membrane preparations, CYP protein solubilizations, CYP protein purifications, and reconstitution of CYP catalytic systems. The common factors that would most likely lead to high expression of CYPs were identified and summarised. Nevertheless, each factor may still require careful evaluation for individual CYP isoforms to achieve a maximal expression level and catalytic activity. Recombinant E. coli systems have been evidenced as a useful tool in obtaining the ideal level of human CYP proteins, which ultimately allows for subsequent characterisations of structures and functions.

A peptide-derived strategy for specifically targeting the mitochondria and ER of cancer cells: a new approach in fighting cancer

An effective anti-cancer therapy should exclusively target cancer cells and trigger in them a broad spectrum of cell death pathways that will prevent avoidance. Here, we present a new approach in cancer therapy that specifically targets the mitochondria and ER of cancer cells. We developed a peptide derived from the flexible and transmembrane domains of the human protein NAF-1/CISD2. This peptide (NAF-144-67) specifically permeates through the plasma membranes of human epithelial breast cancer cells, abolishes their mitochondria and ER, and triggers cell death with characteristics of apoptosis, ferroptosis and necroptosis. In vivo analysis revealed that the peptide significantly decreases tumor growth in mice carrying xenograft human tumors. Computational simulations of cancer vs. normal cell membranes reveal that the specificity of the peptide to cancer cells is due to its selective recognition of their membrane composition. NAF-144-67 represents a promising anti-cancer lead compound that acts via a unique mechanism.

Reactive Oxygen Species (ROS) Regulates Different Types of Cell Death by Acting as a Rheostat

Reactive oxygen species (ROS) are essential for cellular signaling and response to stress. The level of ROS and the type of ROS determine the ability of cells to undergo cell death. Furthermore, dysregulation of the antioxidant pathways is associated with many diseases. It has become apparent that cell death can occur through different mechanisms leading to the classifications of different types of cell death such as apoptosis, ferroptosis, and necroptosis. ROS play essential roles in all forms of cell death, but it is only now coming into focus that ROS control and determine the type of cell death that occurs in any given cell. Indeed, ROS may act as a rheostat allowing different cell death mechanisms to be engaged and crosstalk with different cell death types. In this review, we will describe the ROS regulatory pathways and how they control different types of cell death under normal and disease states. We will also propose how ROS could provide a mechanism of crosstalk between cell death mechanisms and act as a rheostat determining the type of cell death.

Hepatic endoplasmic reticulum calcium fluxes: effect of free fatty acids and KATP channel involvement

As a common sequel to obesity, plasma and intracellular free fatty acid (FFA) concentrations are elevated and, as a consequence, manifold disturbances in metabolism may ensue. Biochemical processes in the cytosol and organelles, such as mitochondria and endoplasmic reticulum (ER), can be disturbed. In the ER, the maintenance of a high calcium gradient is indispensable for viability. In sarcoplasmic reticulum, selective FFA can induce ER stress by disrupting luminal calcium homeostasis; however, there are limited studies in hepatic microsomes. Our studies found that FFA has a noxious effect on rat hepatic microsomal calcium flux, and the extent of which depended on the number of double bonds and charge. Furthermore, insofar as the FFA had no effect on microsomal calcium efflux, their inhibitory action primarily involves calcium influx. Finally, other cationic channels have been found in hepatic ER, and evidence is presented of their interaction with the Ca²⁺ ATPase pump.

Advanced Understanding of the Electron Transfer Pathway of Cytochrome P450s

Cytochrome P450s are heme-thiolate enzymes that participate in carbon source assimilation, natural compound biosynthesis and xenobiotic metabolism in all kingdoms of life. P450s can catalyze various reactions by using a wide range of organic compounds, thus exhibiting great potential in biotechnological applications. The catalytic reactions of P450s are driven by electron equivalents that are sourced from pyridine nucleotides and delivered by cognate or matching redox partners (RPs). The electron transfer (ET) route from RPs to P450s involves one or more redox center-containing domains. As the rate of ET is one of the main determinants of P450 efficacy, an in-depth understanding of the P450 ET pathway should increase our knowledge of these important enzymes and benefit their further applications. Here, the various P450 RP systems along with current understanding of their ET routes will be reviewed. Notably, state-of-the-art structural studies of the two main types of self-sufficient P450 will also be summarized.

A Guide to Tracking Single Membrane Proteins and Their Interactions in Supported Lipid Bilayers

The purpose of this chapter is to serve as a guide for those who wish to carry out experiments tracking single proteins in planar supported biomimetic membranes. This chapter describes, in detail, the construction of a simple single molecule microscope, which includes: (1) a parts list, (2) temperature control, (3) an alignment procedure, (4) a calibration procedure, and (5) a procedure for measuring the mechanical stability of the instrument. It also gives procedures for making planar supported bilayers on hydrophilically treated borosilicate and quartz. These include (1) POPC bilayers, (2) POPC/PEG-PE cushioned bilayers, (3) POPC/PEG-PE cushioned bilayers on BSA passivated substrates, and (4) a cushioned biomimetic membrane of the endoplasmic reticulum (ER). A procedure for the detergent mediated incorporation of the transmembrane protein 5HT_3A (a serotonin receptor) is also described and can be used as a starting point for other large non-self-inserting transmembrane proteins. A procedure for the detergent-free incorporation of cytochrome P450 reductase (CPR) and cytochrome P450 enzymes (P450) into an ER biomimetic is also described. The final experimental section of this chapter details different procedures for data analysis including (1) quantitative analysis of mean squared displacements from individually tracked proteins, (2) gamma distribution analysis of diffusion coefficients from a small ensemble of individually tracked proteins, (3) average mean squared displacement analysis, (4) Gaussian analysis of step-size distributions, (5) Arrhenius analysis of temperature dependent data, (6) the determination of equilibrium constants from a step-size distribution, and (7) a perspective associated with the interpretation of single particle tracking data.

Lipid composition and macromolecular crowding effects on CYP2J2-mediated drug metabolism in nanodiscs

Lipid composition and macromolecular crowding are key external effectors of protein activity and stability whose role varies between different proteins. Therefore, it is imperative to study their effects on individual protein function. CYP2J2 is a membrane-bound cytochrome P450 in the heart involved in the metabolism of fatty acids and xenobiotics. In order to facilitate this metabolism, cytochrome P450 reductase (CPR), transfers electrons to CYP2J2 from NADPH. Herein, we use nanodiscs to show that lipid composition of the membrane bilayer affects substrate metabolism of the CYP2J2-CPR nanodisc (ND) system. Differential effects on both NADPH oxidation and substrate metabolism by CYP2J2-CPR are dependent on the lipid composition. For instance, sphingomyelin containing nanodiscs produced more secondary substrate metabolites than discs of other lipid compositions, implying a possible conformational change leading to processive metabolism. Furthermore, we demonstrate that macromolecular crowding plays a role in the lipid-solubilized CYP2J2-CPR system by increasing the K_m and decreasing the V_max , and effect that is size-dependent. Crowding also affects the CYP2J2-CPR-ND system by decreasing both the K_m and V_max for Dextran-based macromolecular crowding agents, implying an increase in substrate affinity but a lack of metabolism. Finally, protein denaturation studies show that crowding agents destabilize CYP2J2, while the multidomain protein CPR is stabilized. Overall, these studies are the first report on the role of the surrounding lipid environment and macromolecular crowding in modulating enzymatic function of CYP2J2-CPR membrane protein system.

Lipid-exchange in nanodiscs discloses membrane boundaries of cytochrome-P450 reductase

Lipids are critical for the function of membrane proteins. NADPH-cytochrome-P450-reductase, the sole electron transferase for microsomal oxygenases, possesses a conformational dynamics entwined with its topology. Here, we use peptide-nanodiscs to unveil cytochrome-P450-reductase's lipid boundaries, demonstrating a protein-driven enrichment of ethanolamine lipids (by 25%) which ameliorates by 3-fold CPR's electron-transfer ability.

Cytochrome-P450-Induced Ordering of Microsomal Membranes Modulates Affinity for Drugs

Although membrane environment is known to boost drug metabolism by mammalian cytochrome P450s, the factors that stabilize the structural folding and enhance protein function are unclear. In this study, we use peptide-based lipid nanodiscs to "trap" the lipid boundaries of microsomal cytochrome P450 2B4. We report the first evidence that CYP2B4 is able to induce the formation of raft domains in a biomimetic compound of the endoplasmic reticulum. NMR experiments were used to identify and quantitatively determine the lipids present in nanodiscs. A combination of biophysical experiments and molecular dynamics simulations revealed a sphingomyelin binding region in CYP2B4. The protein-induced lipid raft formation increased the thermal stability of P450 and dramatically altered ligand binding kinetics of the hydrophilic ligand BHT. These results unveil membrane/protein dynamics that contribute to the delicate mechanism of redox catalysis in lipid membrane.

Review of biomarkers to assess the effects of switching from cigarettes to modified risk tobacco products

Context: One approach to reducing the harm caused by cigarette smoking, at both individual and population level, is to develop, assess and commercialize modified risk alternatives that adult smokers can switch to. Studies to demonstrate the exposure and risk reduction potential of such products generally involve the measuring of biomarkers, of both exposure and effect, sampled in various biological matrices.Objective: In this review, we detail the pros and cons for using several biomarkers as indicators of effects of changing from conventional cigarettes to modified risk products.Materials and methods: English language publications between 2008 and 2017 were retrieved from PubMed using the same search criteria for each of the 25 assessed biomarkers. Nine exclusion criteria were applied to exclude non-relevant publications.Results: A total of 8876 articles were retrieved (of which 7476 were excluded according to the exclusion criteria). The literature indicates that not all assessed biomarkers return to baseline levels following smoking cessation during the study periods but that nine had potential for use in medium to long-term studies.Discussion and conclusion: In clinical studies, it is important to choose biomarkers that show the biological effect of cessation within the duration of the study.

Characterization of Interactions Among CYP1A2, CYP2B4, and NADPH-cytochrome P450 Reductase: Identification of Specific Protein Complexes

Cytochromes P450s (P450s) catalyze oxygenation reactions via interactions with their redox partners. However, other proteins, particularly other P450s, also have been shown to form complexes that modulate P450 function. Previous studies showed that CYP1A2 and CYP2B4 form a complex when reconstituted into phospholipid vesicles; however, details of the interactions among the P450s and NADPH-cytochrome P450 reductase (POR) have not been fully characterized. The goal of this study was to examine P450 complex formation in living cells, using bioluminescence resonance energy transfer (BRET). Various pairs of P450 and POR constructs were tagged with either green fluorescent protein or Renilla luciferase, and transfected into human embryonic kidney 293T cells. Complexes were demonstrated by measuring energy transfer between the tags, and disruption of the complex was verified by cotransfection with unlabeled P450-system proteins. CYP1A2 and CYP2B4 formed a stable complex that could not be disrupted by cotransfection of untagged POR. Interactions of both P450s with POR were detected, with untagged CYP1A2 disrupting the POR-CYP2B4 interaction. In contrast, untagged CYP2B4 did not affect the POR-CYP1A2 interaction. These data are consistent with POR preferentially binding to the CYP1A2 moiety of CYP1A2-CYP2B4. BRET-detectable homomeric CYP1A2-CYP1A2 also was detected, and was disrupted by cotransfection of either POR or CYP2B4. Both CYP1A2 and CYP2B4 activities were affected by their coexpression in a manner consistent with formation of the high-affinity POR-CYP1A2-CYP2B4 complex. These findings demonstrate that CYP1A2 and CYP2B4 form a heteromeric POR-CYP1A2-CYP2B4 complex in living cells that has altered catalytic activities relative to the homomeric enzymes.

Dissociation Constants of Cytochrome P450 2C9/Cytochrome P450 Reductase Complexes in a Lipid Bilayer Membrane Depend on NADPH: A Single-Protein Tracking Study

Cytochrome P450-reductase (CPR) is a versatile NADPH-dependent electron donor located in the cytoplasmic side of the endoplasmic reticulum. It is an electron transferase that is able to deliver electrons to a variety of membrane-bound oxidative partners, including the drug-metabolizing enzymes of the cytochrome P450s (P450). CPR is also stoichiometrically limited compared to its oxidative counterparts, and hypotheses have arisen about possible models that can overcome the stoichiometric imbalance, including quaternary organization of P450 and diffusion-limited models. Described here are results from a single-protein tracking study of fluorescently labeled CPR and cytochrome P450 2C9 (CYP2C9) molecules in which stochastic analysis was used to determine the dissociation constants of CPR/CYP2C9 complexes in a lipid bilayer membrane for the first time. Single-protein trajectories demonstrate the transient nature of these CPR-CYP2C9 interactions, and the measured K_d values are highly dependent on the redox state of CPR. It is shown that CPR^ox/CYP2C9 complexes have a much higher dissociation constant than CPR^2-/CYP2C9 or CPR^4-/CYP2C9 complexes, and a model is presented to account for these results. An Arrhenius analysis of diffusion constants was also carried out, demonstrating that the reduced forms of CPR and CYP2C9 interact differently with the biomimetic ER and may, in addition to protein conformational changes, contribute to the observed NADPH-dependent shift in K_d. Finally, it is also shown that the CPR^ox/CYP2C9 affinity depends on the nature of the ligand, being higher when a substrate is bound, compared to an inhibitor.

Liver microsomal lipid enhances the activity and redox coupling of colocalized cytochrome P450 reductase-cytochrome P450 3A4 in nanodiscs

The haem-containing mono-oxygenase cytochrome P450 3A4 (CYP3A4) and its redox partner NADPH-dependent cytochrome P450 oxidoreductase (CPR) are among the most important enzymes in human liver for metabolizing drugs and xenobiotic compounds. They are membrane-bound in the endoplasmic reticulum (ER). How ER colocalization and the complex ER phospholipid composition influence enzyme activity are not well understood. CPR and CYP3A4 were incorporated into phospholipid bilayer nanodiscs, both singly, and together in a 1 : 1 ratio, to investigate the significance of membrane insertion and the influence of varying membrane composition on steady-state reaction kinetics. Reaction kinetics were analysed using a fluorimetric assay with 7-benzyloxyquinoline as substrate for CYP3A4. Full activity of the mono-oxygenase system, with electron transfer from NADPH via CPR, could only be reconstituted when CPR and CYP3A4 were colocalized within the same nanodiscs. No activity was observed when CPR and CYP3A4 were each incorporated separately into nanodiscs then mixed together, or when soluble forms of CPR were mixed with preassembled CYP3A4-nanodiscs. Membrane integration and colocalization are therefore essential for electron transfer. Liver microsomal lipid had an enhancing effect compared with phosphatidylcholine on the activity of CPR alone in nanodiscs, and a greater enhancing effect on the activity of CPR-CYP3A4 nanodisc complexes, which was not matched by a phospholipid mixture designed to mimic the ER composition. Furthermore, liver lipid enhanced redox coupling within the system. Thus, natural ER lipids possess properties or include components important for enhanced catalysis by CPR-CYP3A4 nanodisc complexes. Our findings demonstrate the importance of using natural lipid preparations for the detailed analysis of membrane protein activity.

Activating nuclear xenobiotic receptors and triggering ER stress and hepatic cytochromes P450 systems in quails (Coturnix C. coturnix) during atrazine exposure

Atrazine (ATR) is one of the most widely detected contaminant in the ecosystem. Nuclear xenobiotic receptors are activated by herbicides and induce the transcription of CYP450 isoforms involved in xenobiotic metabolism and transport. However, little is known about hepatic nuclear xenobiotic receptors in birds are responsible for ATR-induced hepatotoxicity via regulating the cytochrome P450 enzyme systems (CYP450s). The objective of this study was to investigate the mechanism of ATR hepatotoxicity in quails. For this purpose, male quails were dosed by oral gavage from sexual immaturity to maturity with 0, 50, 250, and 500 mg/kg/day ATR for 45 days. The results showed that ATR exposure caused the hepatotoxicity damage and endoplasmic reticulum (ER) degeneration. It suggested that ER is a target organelle of ATR toxicity in hepatocytes. ATR exposure disrupted the hepatic CYP450s homeostasis. This study also demonstrated that ATR triggered the CYP450 isoforms transcription via activating the hepatic CAR/PXR pathway. The present study provides new insights regarding the mechanism of the ATR-induced hepatotoxicity through activating nuclear xenobiotic receptors and triggering ER stress and hepatic CYP450s in quails.

Fenofibrate inhibits tumour intravasation by several independent mechanisms in a 3-dimensional co-culture model

Lymph node metastasis of breast cancer is a clinical marker of poor prognosis. Yet, there exist no therapies targeting mechanisms of intravasation into lymphatics. Herein we report on an effect of the antidyslipidemic drug fenofibrate with vasoprotective activity, which attenuates breast cancer intravasation in vitro, and describe the potential mechanisms. To measure intravasation in a 3-dimensional co-culture model MDA-MB231 and MCF-7 breast cancer spheroids were placed on immortalised lymphendothelial cell (LEC) monolayers. This provokes the formation of circular chemorepellent induced defects (CCIDs) in the LEC barrier resembling entry ports for the intravasating tumour. Furthermore, the expression of adhesion molecules ICAM-1, CD31 and FAK was investigated in LECs by western blotting as well as cell-cell adhesion and NF-κB activity by respective assays. In MDA-MB231 cells the activity of CYP1A1 was measured by EROD assay. Fenofibrate inhibited CCID formation in the MDA-MB231/LEC- and MCF-7/LEC models and the activity of NF-κB, which in turn downregulated ICAM-1 in LECs and the adhesion of cancer cells to LECs. Furthermore, CD31 and the activity of FAK were inhibited. In MDA-MB231 cells, fenofibrate attenuated CYP1A1 activity. Combinations with other FDA-approved drugs, which reportedly inhibit different ion channels, attenuated CCID formation additively or synergistically. In summary, fenofibrate inhibited NF-κB and ICAM-1, and inactivated FAK, thereby attenuating tumour intravasation in vitro. A combination with other FDA-approved drugs further improved this effect. Our new concept may lead to a novel therapy for cancer patients.

Single-Protein Tracking Reveals That NADPH Mediates the Insertion of Cytochrome P450 Reductase into a Biomimetic of the Endoplasmic Reticulum

Cytochrome P450 reductase (CPR) is the redox partner for most human cytochrome P450 enzymes. It is also believed that CPR is an integral membrane protein exclusively. Herein, we report that, contrary to this belief, CPR can exist as a peripheral membrane protein in the absence of NADPH and will transition to an integral membrane protein in the presence of stoichiometric amounts of NADPH or greater. All experiments were performed in a solid-supported cushioned lipid bilayer that closely matched the chemical composition of the human endoplasmic reticulum and served as an ER biomimetic. The phase characteristics and fluidity of the ER biomimetic was characterized with fluorescence micrographs and temperature-dependent fluorescence recovery after photobleaching. The interactions of CPR with the ER biomimetic were directly observed by tracking single CPR molecules using time-lapse single-molecule fluorescence imaging and subsequent analysis of tracks. These studies revealed dramatic changes in diffusion coefficient and the degree of partitioning of CPR as a function of NADPH concentration.

Assembly of Dynamic P450-Mediated Metabolons-Order Versus Chaos

Purpose of Review

We provide an overview of the current knowledge on cytochrome P450-mediated metabolism organized as metabolons and factors that facilitate their stabilization. Essential parameters will be discussed including those that are commonly disregarded using the dhurrin metabolon from Sorghum bicolor as a case study.

Recent Findings

Sessile plants control their metabolism to prioritize their resources between growth and development, or defense. This requires fine-tuned complex dynamic regulation of the metabolic networks involved. Within the recent years, numerous studies point to the formation of dynamic metabolons playing a major role in controlling the metabolic fluxes within such networks.

Summary

We propose that P450s and their partners interact and associate dynamically with POR, which acts as a charging station possibly in concert with Cytb5. Solvent environment, lipid composition, and non-catalytic proteins guide metabolon formation and thereby activity, which have important implications for synthetic biology approaches aiming to produce high-value specialized metabolites in heterologous hosts.

Membrane Fluidity Modulates Thermal Stability and Ligand Binding of Cytochrome P4503A4 in Lipid Nanodiscs

Cytochrome P4503A4 (CYP3A4) is a peripheral membrane protein that plays a major role in enzymatic detoxification of many drugs and toxins. CYP3A4 has an integral membrane N-terminal helix and a localized patch comprised of the G' and F' helix regions that are embedded in the membrane, but the effects of membrane composition on CYP3A4 function are unknown. Here, circular dichroism and differential scanning calorimetry were used to compare the stability of CYP3A4 in lipid bilayer nanodiscs with varying ratios of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine to 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). These lipids differ in the acyl-chain length and their degree of unsaturation. The thermal denaturation of CYP3A4 in nanodiscs occurs in a temperature range distinct from that of the nanodisc denaturation so it can be monitored calorimetrically. Melting temperatures (T_m), heat capacities (ΔC_p), and calorimetric enthalpies (ΔH_cal) for denaturation of CYP3A4 each increased with an increasing fraction of DMPC, with a maximum at 50% DMPC, before decreasing at 75% DMPC. Addition of the inhibitor ketoconazole results in increased thermal stability, and larger ΔC_p and ΔH_cal values, with different sensitivities to lipid composition. Effects of lipid composition on ligand binding dynamics were also studied. Equilibrium binding affinities of both ketoconazole (KTZ) and testosterone (TST) were minimally affected by lipid composition. However, stopped-flow analyses indicate that the rates of KTZ binding reach a maximum in membranes containing 50% DMPC, whereas the rate of TST binding decreases continuously with an increasing DMPC concentration. These results indicate that CYP3A4 is highly sensitive to the acyl-chain composition of the lipids and fluidity of the membrane in which it is embedded.

Cytochrome P450 Organization and Function Are Modulated by Endoplasmic Reticulum Phospholipid Heterogeneity

Cytochrome P450s (P450s) comprise a superfamily of proteins that catalyze numerous monooxygenase reactions in animals, plants, and bacteria. In eukaryotic organisms, these proteins not only carry out reactions necessary for the metabolism of endogenous compounds, but they are also important in the oxidation of exogenous drugs and other foreign compounds. Eukaryotic P450 system proteins generally reside in membranes, primarily the endoplasmic reticulum or the mitochondrial membrane. These membranes provide a scaffold for the P450 system proteins that facilitate interactions with their redox partners as well as other P450s. This review focuses on the ability of specific lipid components to influence P450 activities, as well as the role of the membrane in P450 function. These studies have shown that P450s and NADPH-cytochrome P450 reductase appear to selectively associate with specific phospholipids and that these lipid-protein interactions influence P450 activities. Finally, because of the heterogeneous nature of the endoplasmic reticulum as well as other biologic membranes, the phospholipids are not arranged randomly but associate to generate lipid microdomains. Together, these characteristics can affect P450 function by 1) altering the conformation of the proteins, 2) influencing the P450 interactions with their redox partners, and 3) affecting the localization of the proteins into specific membrane microdomains.

The Role of Protein-Protein and Protein-Membrane Interactions on P450 Function

This symposium summary, sponsored by the ASPET, was held at Experimental Biology 2015 on March 29, 2015, in Boston, Massachusetts. The symposium focused on: 1) the interactions of cytochrome P450s (P450s) with their redox partners; and 2) the role of the lipid membrane in their orientation and stabilization. Two presentations discussed the interactions of P450s with NADPH-P450 reductase (CPR) and cytochrome b5. First, solution nuclear magnetic resonance was used to compare the protein interactions that facilitated either the hydroxylase or lyase activities of CYP17A1. The lyase interaction was stimulated by the presence of b5 and 17α-hydroxypregnenolone, whereas the hydroxylase reaction was predominant in the absence of b5. The role of b5 was also shown in vivo by selective hepatic knockout of b5 from mice expressing CYP3A4 and CYP2D6; the lack of b5 caused a decrease in the clearance of several substrates. The role of the membrane on P450 orientation was examined using computational methods, showing that the proximal region of the P450 molecule faced the aqueous phase. The distal region, containing the substrate-access channel, was associated with the membrane. The interaction of NADPH-P450 reductase (CPR) with the membrane was also described, showing the ability of CPR to "helicopter" above the membrane. Finally, the endoplasmic reticulum (ER) was shown to be heterogeneous, having ordered membrane regions containing cholesterol and more disordered regions. Interestingly, two closely related P450s, CYP1A1 and CYP1A2, resided in different regions of the ER. The structural characteristics of their localization were examined. These studies emphasize the importance of P450 protein organization to their function.

The Localization of Cytochrome P450s CYP1A1 and CYP1A2 into Different Lipid Microdomains Is Governed by Their N-terminal and Internal Protein Regions

In cellular membranes, different lipid species are heterogeneously distributed forming domains with different characteristics. Ordered domains are tightly packed with cholesterol, sphingomyelin, and saturated fatty acids, whereas disordered domains contain high levels of unsaturated fatty acids. Our laboratory has shown that membrane heterogeneity affects the organization of cytochrome P450s and their cognate redox partner, the cytochrome P450 reductase (CPR). Despite the high degree of sequence similarity, CYP1A1 was found to localize to disordered regions, whereas CYP1A2 resided in ordered domains. We hypothesized that regions of amino acid sequence variability may contain signal motifs that direct CYP1A proteins into ordered or disordered domains. Thus, chimeric constructs of CYP1A1 and CYP1A2 were created, and their localization was tested in HEK293T cells. CYP1A2, containing the N-terminal regions from CYP1A1, no longer localized in ordered domains, whereas the N terminus of CYP1A2 partially directed CYP1A1 into ordered regions. In addition, intact CYP1A2 containing a 206-302-residue peptide segment of CYP1A1 had less affinity to bind to ordered microdomains. After expression, the catalytic activity of CYP1A2 was higher than that of the CYP1A1-CYP1A2 chimera containing the N-terminal end of CYP1A1 with subsaturating CPR concentrations, but it was approximately equal with excess CPR suggesting that the localization of the CYP1A enzyme in ordered domains favored its interaction with CPR. These data demonstrate that both the N-terminal end and an internal region of CYP1A2 play roles in targeting CYP1A2 to ordered domains, and domain localization may influence P450 function under conditions that resemble those found in vivo.

Cytochrome P450 system proteins reside in different regions of the endoplasmic reticulum

Cytochrome P450 (P450) function is dependent on the ability of these enzymes to successfully interact with their redox partners, NADPH-cytochrome P450 reductase (CPR) and cytochrome b5, in the endoplasmic reticulum (ER). Because the ER is heterogeneous in lipid composition, membrane microdomains with different characteristics are formed. Ordered microdomains are more tightly packed, and enriched in saturated fatty acids, sphingomyelin and cholesterol, whereas disordered regions contain higher levels of unsaturated fatty acids. The goal of the present study was to determine whether the P450 system proteins localize to different regions of the ER. The localization of CYP1A2, CYP2B4 and CYP2E1 within the ER was determined by partial membrane solubilization with Brij 98, centrifugation on a discontinuous sucrose gradient and immune blotting of the gradient fractions to identify ordered and disordered microdomains. CYP1A2 resided almost entirely in the ordered regions of the ER with CPR also localized predominantly to this region. CYP2B4 was equally distributed between the ordered and disordered domains. In contrast, CYP2E1 localized to the disordered membrane regions. Removal of cholesterol (an important constituent of ordered domains) led to the relocation of CYP1A2, CYP2B4 and CPR to the disordered regions. Interestingly, CYP1A1 and CYP1A2 localized to different membrane microdomains, despite their high degree of sequence similarity. These data demonstrate that P450 system enzymes are organized in specific membrane regions, and their localization can be affected by depletion of membrane cholesterol. The differential localization of different P450 in specific membrane regions may provide a novel mechanism for modulating P450 function.

Classification of oxidative stress based on its intensity

In living organisms production of reactive oxygen species (ROS) is counterbalanced by their elimination and/or prevention of formation which in concert can typically maintain a steady-state (stationary) ROS level. However, this balance may be disturbed and lead to elevated ROS levels called oxidative stress. To our best knowledge, there is no broadly acceptable system of classification of oxidative stress based on its intensity due to which proposed here system may be helpful for interpretation of experimental data. Oxidative stress field is the hot topic in biology and, to date, many details related to ROS-induced damage to cellular components, ROS-based signaling, cellular responses and adaptation have been disclosed. However, it is common situation when researchers experience substantial difficulties in the correct interpretation of oxidative stress development especially when there is a need to characterize its intensity. Careful selection of specific biomarkers (ROS-modified targets) and some system may be helpful here. A classification of oxidative stress based on its intensity is proposed here. According to this classification there are four zones of function in the relationship between "Dose/concentration of inducer" and the measured "Endpoint": I - basal oxidative stress (BOS); II - low intensity oxidative stress (LOS); III - intermediate intensity oxidative stress (IOS); IV - high intensity oxidative stress (HOS). The proposed classification will be helpful to describe experimental data where oxidative stress is induced and systematize it based on its intensity, but further studies will be in need to clear discriminate between stress of different intensity.

Relationship between CYP1A2 localization and lipid microdomain formation as a function of lipid composition

Cytochrome P450 (P450) function requires the interaction of P450 and NADPH-cytochrome P450 reductase (CPR) in membranes, and is frequently studied using reconstituted systems composed solely of phosphatidylcholine. There is increasing evidence that other endoplasmic reticulum (ER) lipids can affect P450 structure, activity, and interactions with CPR. Some of these lipid effects have been attributed to the formation of organized liquid-ordered (l(o)) domains. The goal of this study was to determine if l(o) domains were formed in P450 reconstituted systems mimicking the ER membrane. CYP1A2, when incorporated in "ER-like" lipid vesicles, displayed detergent insolubility after treatment with Brij 98 and centrifugation in a sucrose gradient. Lipid probes were employed to identify domain formation in both ER-like vesicles and model membranes known to form l(o) domains. Changes in fluorescence resonance energy transfer (FRET) using an established donor/acceptor FRET pair in both ER-like and model l(o)-forming systems demonstrated the coexistence of l(o)- and liquid-disordered domains as a function of cholesterol and sphingomyelin content. Similarly, 6-dodecanoyl-2-dimethylaminonaphthalene (laurdan), a probe that reports on membrane organization, showed that cholesterol and sphingomyelin increased membrane order. Finally, brominated-phosphatidylcholine allowed for monitoring of the location of both CPR and CYP1A2 within the l(o) regions of ER-like systems. Taken together, the results demonstrate that ER-like vesicles generate microdomains, and both CYP1A2 and CPR predominantly localize into l(o) membrane regions. Probe fluorescent responses suggest that lipid microdomains form in these vesicles whether or not enzymes are included in the reconstituted systems. Thus, it does not appear that the proteins are critical for stabilizing l(o) domains.

Interactions between CYP2E1 and CYP2B4: effects on affinity for NADPH-cytochrome P450 reductase and substrate metabolism

Studies in microsomal and reconstituted systems have shown that the presence of one cytochrome P450 isoform can significantly influence the catalytic activity of another isoform. In this study, we assessed whether CYP2E1 could influence the catalytic activity of CYP2B4 under steady-state turnover conditions. The results show that CYP2E1 inhibits CYP2B4-mediated metabolism of benzphetamine (BNZ) with a K(i) of 0.04 µM. However, CYP2B4 is not an inhibitor of CYP2E1-mediated p-nitrophenol hydroxylation. When these inhibition studies were performed with the artificial oxidant tert-butyl hydroperoxide, CYP2E1 did not significantly inhibit CYP2B4 activity. Determinations of the apparent K(M) and k(cat) of CYP2B4 for CPR in the presence of increasing concentrations of CYP2E1 revealed a mixed inhibition of CYP2B4 by CYP2E1. At low concentrations of CYP2E1, the apparent K(M) of CYP2B4 for CPR increased up to 23-fold with virtually no change in the k(cat) for the reaction, however, at higher concentrations of CYP2E1, the apparent K(M) of CYP2B4 for CPR decreased to levels similar to those observed in the absence of CYP2E1 and the k(cat) also decreased by 11-fold. Additionally, CYP2E1 increased the apparent K(M) of CYP2B4 for BNZ by 8-fold and the apparent K(M) did not decrease to its original value when saturating concentrations of CPR were used. While the individual apparent K(M) values of CYP2B4 and CYP2E1 for CPR are similar, the apparent K(M) of CYP2E1 for CPR in the presence of CYP2B4 decreased significantly, thus suggesting that CYP2B4 enhances the affinity of CYP2E1 for CPR and this may allow CYP2E1 to out-compete CYP2B4 for CPR.

A novel type of allosteric regulation: functional cooperativity in monomeric proteins

Cooperative functional properties and allosteric regulation in cytochromes P450 play an important role in xenobiotic metabolism and define one of the main mechanisms of drug-drug interactions. Recent experimental results suggest that ability to bind simultaneously two or more small organic molecules can be the essential feature of cytochrome P450 fold, and often results in rich and complex pattern of allosteric behavior. Manifestations of non-Michaelis kinetics include homotropic and heterotropic activation and inhibition effects depending on the stoichiometric ratios of substrate and effector, changes in the regio- and stereospecificity of catalytic transformations, and often give rise to the clinically important drug-drug interactions. In addition, functional response of P450 systems is modulated by the presence of specific and non-specific effector molecules, metal ions, membrane incorporation, formation of homo- and hetero-oligomers, and interactions with the protein redox partners. In this article we briefly overview the main factors contributing to the allosteric effects in cytochromes P450 with the main focus on the sources of cooperative behavior in xenobiotic metabolizing monomeric heme enzymes with their conformational flexibility and extremely broad substrate specificity. The novel mechanism of functional cooperativity in P450 enzymes does not require substantial binding cooperativity, rather it implies the presence of one or more binding sites with higher affinity than the single catalytically active site in the vicinity of the heme iron.