Membrane properties induced by anionic phospholipids and phosphatidylethanolamine are critical for the membrane binding and catalytic activity of human cytochrome P450 3A4

Small angle scattering reveals the orientation of cytochrome P450 19A1 in lipoprotein nanodiscs

Human aromatase (CYP19A1), the cytochrome P450 enzyme responsible for conversion of androgens to estrogens, was incorporated into lipoprotein nanodiscs (NDs) and interrogated by small angle X-ray and neutron scattering (SAXS/SANS). CYP19A1 was associated with the surface and centered at the edge of the long axis of the ND membrane. In the absence of the N-terminal anchor, the amphipathic A'- and G'-helices were predominately buried in the lipid head groups, with the possibly that their hydrophobic side chains protrude into the hydrophobic, aliphatic tails. The prediction is like that for CYP3A4 based on SAXS employing a similar modeling approach. The orientation of CYP19A1 in a ND is consistent with our previous predictions based on molecular dynamics simulations and lends additional credibility to the notion that CYP19A1 captures substrates from the membrane.

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.

Silybin regulates P450s activity by attenuating endoplasmic reticulum stress in mouse nonalcoholic fatty liver disease

Cytochrome P450s are important phase I metabolic enzymes located on endoplasmic reticulum (ER) involved in the metabolism of endogenous and exogenous substances. Our previous study showed that a hepatoprotective agent silybin restored CYP3A expression in mouse nonalcoholic fatty liver disease (NAFLD). In this study we investigated how silybin regulated P450s activity during NAFLD. C57BL/6 mice were fed a high-fat-diet (HFD) for 8 weeks to induce NAFLD, and were administered silybin (50, 100 mg ·kg^-1 ·d^-1, i.g.) in the last 4 weeks. We showed that HFD intake induced hepatic steatosis and ER stress, leading to significant inhibition on the activity of five primary P450s including CYP1A2, CYP2B6, CYP2C19, CYP2D6, and CYP3A in liver microsomes. These changes were dose-dependently reversed by silybin administration. The beneficial effects of silybin were also observed in TG-stimulated HepG2 cells in vitro. To clarify the underlying mechanism, we examined the components involved in the P450 catalytic system, membrane phospholipids and ER membrane fluidity, and found that cytochrome b5 (cyt b5) was significantly downregulated during ER stress, and ER membrane fluidity was also reduced evidenced by DPH polarization and lower polyunsaturated phospholipids levels. The increased ratios of NADP⁺/NADPH and PC/PE implied Ca²⁺ release and disruption of cellular Ca²⁺ homeostasis resulted from mitochondria dysfunction and cytochrome c (cyt c) release. The interaction between cyt c and cyt b5 under ER stress was an important reason for P450s activity inhibition. The effect of silybin throughout the whole course suggested that it regulated P450s activity through its anti-ER stress effect in NAFLD. Our results suggest that ER stress may be crucial for the inhibition of P450s activity in mouse NAFLD and silybin regulates P450s activity by attenuating ER stress.

Effect of Lactobacillus rhamnosus hsryfm 1301 Fermented Milk on Lipid Metabolism Disorders in High-Fat-Diet Rats

To further explore and improve the mechanism of probiotics to alleviate the disorder of lipid metabolism, transcriptomic and metabolomic with bioinformatic analysis were combined. In the present study, we successfully established a rat model of lipid metabolism disorder using a high-fat diet. Intervention with Lactobacillus rhamnosus hsryfm 1301 fermented milk resulted in a significant reduction in body weight, serum free fatty acid and blood lipid levels (p < 0.05), which predicted that the lipid metabolism disorder was alleviated in rats. Metabolomics and transcriptomics identified a total of 33 significantly different metabolites and 183 significantly different genes screened in the intervention group compared to the model group. Comparative analysis of Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway annotations identified a total of 61 pathways in which differential metabolites and genes were jointly involved, with linoleic acid metabolism, glycine, serine and threonine metabolism and glutamatergic synapse in both transcriptome and metabolome being found to be significantly altered (p < 0.05). Lactobacillus rhamnosus hsryfm 1301 fermented milk was able to directly regulate lipid metabolism disorders by regulating the metabolic pathways of linoleic acid metabolism, glycerophospholipid metabolism, fatty acid biosynthesis, alpha-linolenic acid metabolism, fatty acid degradation, glycerolipid metabolism and arachidonic acid metabolism. In addition, we found that Lactobacillus rhamnosus hsryfm 1301 fermented milk indirectly regulates lipid metabolism through regulating amino acid metabolism, the nervous system, the endocrine system and other pathways. Lactobacillus rhamnosus hsryfm 1301 fermented milk could alleviate the disorders of lipid metabolism caused by high-fat diet through multi-target synergy.

Functional Interaction between Cytochrome P450 and UDP-Glucuronosyltransferase on the Endoplasmic Reticulum Membrane: One of Post-translational Factors Which Possibly Contributes to Their Inter-Individual Differences

Cytochrome P450 (P450) and uridine 5'-diphosphate (UDP)-glucuronosyltransferase (UGT) catalyze oxidation and glucuronidation in drug metabolism, respectively. It is believed that P450 and UGT work separately because they perform distinct reactions and exhibit opposite membrane topologies on the endoplasmic reticulum (ER). However, given that some chemicals are sequentially metabolized by P450 and UGT, it is reasonable to consider that the enzymes may interact and work cooperatively. Previous research by our team detected protein-protein interactions between P450 and UGT by analyzing solubilized rat liver microsomes with P450-immobilized affinity column chromatography. Although P450 and UGT have been known to form homo- and hetero-oligomers, this is the first report indicating a P450-UGT association. Based on our previous study, we focused on the P450-UGT interaction and reported lines of evidence that the P450-UGT association is a functional protein-protein interaction that can alter the enzymatic capabilities, including enhancement or suppression of the activities of P450 and UGT, helping UGT to acquire novel regioselectivity, and inhibiting substrate binding to P450. Biochemical and molecular bioscientific approaches suggested that P450 and UGT interact with each other at their internal hydrophobic domains in the ER membrane. Furthermore, several in vivo studies have reported the presence of a functional P450-UGT association under physiological conditions. The P450-UGT interaction is expected to function as a novel post-translational factor for inter-individual differences in the drug-metabolizing enzymes.

Variability in Human In Vitro Enzyme Kinetics

There are many factors which are known to cause variability in human in vitro enzyme kinetic data. Factors such as the source of enzyme and how it was prepared, the genetics and background of the donor, how the in vitro studies are designed, and how the data are analyzed contribute to variability in the resulting kinetic parameters. It is important to consider not only the factors which cause variability within an experiment, such as selection of a probe substrate, but also those that cause variability when comparing kinetic data across studies and laboratories. For example, the artificial nature of the microsomal lipid membrane and microenvironment in some recombinantly expressed enzymes, relative to those found in native tissue microsomes, has been shown to influence enzyme activity and thus can be a source of variability when comparing across the two different systems. All of these factors, and several others, are discussed in detail in the chapter below. In addition, approaches which can be used to visualize the uncertainty arising from the use of enzyme kinetic data within the context of predicting human pharmacokinetics are discussed.

CYP2J2 Molecular Recognition: A New Axis for Therapeutic Design

Cytochrome P450 (CYP) epoxygenases are a special subset of heme-containing CYP enzymes capable of performing the epoxidation of polyunsaturated fatty acids (PUFA) and the metabolism of xenobiotics. This dual functionality positions epoxygenases along a metabolic crossroad. Therefore, structure-function studies are critical for understanding their role in bioactive oxy-lipid synthesis, drug-PUFA interactions, and for designing therapeutics that directly target the epoxygenases. To better exploit CYP epoxygenases as therapeutic targets, there is a need for improved understanding of epoxygenase structure-function. Of the characterized epoxygenases, human CYP2J2 stands out as a potential target because of its role in cardiovascular physiology. In this review, the early research on the discovery and activity of epoxygenases is contextualized to more recent advances in CYP epoxygenase enzymology with respect to PUFA and drug metabolism. Additionally, this review employs CYP2J2 epoxygenase as a model system to highlight both the seminal works and recent advances in epoxygenase enzymology. Herein we cover CYP2J2's interactions with PUFAs and xenobiotics, its tissue-specific physiological roles in diseased states, and its structural features that enable epoxygenase function. Additionally, the enumeration of research on CYP2J2 identifies the future needs for the molecular characterization of CYP2J2 to enable a new axis of therapeutic design.

Nanodiscs as a New Tool to Examine Lipid-Protein Interactions

The interactions between lipids and proteins are one of the most fundamental processes in living organisms, responsible for critical cellular events ranging from replication, cell division, signaling, and movement. Enabling the central coupling responsible for maintaining the functionality of the breadth of proteins, receptors, and enzymes that find their natural home in biological membranes, the fundamental mechanisms of recognition of protein for lipid, and vice versa, have been a focal point of biochemical and biophysical investigations for many decades. Complexes of lipids and proteins, such as the various lipoprotein factions, play central roles in the trafficking of important proteins, small molecules and metabolites and are often implicated in disease states. Recently an engineered lipoprotein particle, termed the nanodisc, a modified form of the human high density lipoprotein fraction, has served as a membrane mimetic for the investigation of membrane proteins and studies of lipid-protein interactions. In this review, we summarize the current knowledge regarding this self-assembling lipid-protein complex and provide examples for its utility in the investigation of a large number of biological systems.

Association between the membrane transporter proteins and type 2 diabetes mellitus

Introduction: The prevalence rate of diabetes is increasing day by day and the current scenario of the available agents for its treatment has given rise to stimulation in the search for new therapeutic targets and agents. Therefore the present review will examine the role of membrane composition in the pathophysiology of Type 2 Diabetes and the possible therapeutic approaches for this.Areas covered: Glucose transporter proteins (GLUTs) are integral membrane proteins which are responsible for facilitated glucose transport over the plasma membrane into cells. Thus, this chapter is an attempt to interpret the co-relation between membrane transporter proteins and lipid molecules of cell membrane and their implications in type 2 diabetes mellitus. The relationship between the composition controlled flexibility of the membrane in the insertion of GLUTs into cell membrane as well as its fusion with the membrane is the focus of this chapter.Expert opinion: There is increasing data on the central role of phospholipid composition toward T2DM. Plasma membrane lipid composition plays a key role in maintaining the machinery for insulin-independent GLUT insertion into the membrane as well as insulin-dependent GLUT4 containing vesicles. As a therapeutic option, the designing of new chemical entities should be aimed to decrease saturated fatty acids of lipid bilayer phospholipids to target type 2 diabetes mellitus.

UDP-Glucuronosyltransferase (UGT)-mediated attenuations of cytochrome P450 3A4 activity: UGT isoform-dependent mechanism of suppression

BACKGROUND AND PURPOSE

Cytochrome P450 (CYP, P450) 3A4 is involved in the metabolism of 50% of drugs and its catalytic activity in vivo is not explained only by hepatic expression levels. We previously demonstrated that UDP-glucuronosyltransferase (UGT) 2B7 suppressed CYP3A4 activity through an interaction. In the present study, we target UGT1A9 as another candidate modulator of CYP3A4.

EXPERIMENTAL APPROACH

We prepared co-expressed enzymes using the baculovirus-insect cell expression system and compared CYP3A4 activity in the presence and absence of UGT1A9. Wistar rats were treated with dexamethasone and liver microsomes were used to elucidate the role of CYP3A-UGT1A interactions.

KEY RESULTS

UGT1A9 and UGT2B7 interacted with and suppressed CYP3A4. Kinetic analyses showed that both of the UGTs significantly reduced V_max of CYP3A4 activity. In addition, C-terminal truncated mutants of UGT1A9 and UGT2B7 still retained the suppressive capacity. Dexamethasone treatment induced hepatic CYP3As and UGT1As at different magnitudes. Turnover of CYP3A was enhanced about twofold by this treatment.

CONCLUSION AND IMPLICATIONS

The changes of kinetic parameters suggested that UGT1A9 suppressed CYP3A4 activity with almost the same mechanism as UGT2B7. The luminal domain of UGTs contains the suppressive interaction site(s), whereas the C-terminal domain may contribute to modulating suppression in a UGT isoform-specific manner. CYP3A-UGT1A interaction seemed to be disturbed by dexamethasone treatment and the suppression was partially cancelled. CYP3A4-UGT interactions would help to better understand the causes of inter/intra-individual differences in CYP3A4 activity.

Characterization of kinetics of human cytochrome P450s involved in bioactivation of flucloxacillin: inhibition of CYP3A-catalysed hydroxylation by sulfaphenazole

Background and Purpose

The aim of this study was to characterize the human cytochrome P450s (CYPs) involved in oxidative bioactivation of flucloxacillin to 5‐hydroxymethyl flucloxacillin, a metabolite with high cytotoxicity towards biliary epithelial cells.

Experimental Approach

The CYPs involved in hydroxylation of flucloxacillin were characterized using recombinant human CYPs, pooled liver microsomes in the presence of CYP‐specific inhibitors and by correlation analysis using a panel of liver microsomes from 16 donors.

Key Results

Recombinant CYPs showing the highest specific activity were CYP3A4, CYP3A7 and to lower extent CYP2C9 and CTP2C8. Michaelis–Menten enzyme kinetics were determined for pooled human liver microsomes, recombinant CYP3A4, CYP3A7 and CYP2C9. Surprisingly, sulfaphenazole appeared to be a potent inhibitor of 5′‐hydroxylation of flucloxacillin by both recombinant CYP3A4 and CYP3A7.

Conclusions and Implications

The combined results show that the 5′‐hydroxylation of flucloxacillin is primarily catalysed by CYP3A4, CYP3A7 and CYP2C9. The large variability of the hepatic expression of these enzymes could affect the formation of 5′‐hydroxymethyl flucloxacillin, which may determine the differences in susceptibility to flucloxacillin‐induced liver injury. Additionally, the strong inhibition in CYP3A‐catalysed flucloxacillin metabolism by sulfaphenazole suggests that unanticipated drug–drug interactions could occur with coadministered drugs.

Catalytic modulation of human cytochromes P450 17A1 and P450 11B2 by phospholipid

Unlike most of the drug-metabolizing cytochrome P450s, microsomal P450 17A1 and mitochondrial P450 11B2 catalyze sequential multi-step reactions in steroid biosynthesis. The membrane phospholipid composition might be one parameter that modulates the efficiency and processivity of specific pathways. Here we systematically examined the effects of physiologically relevant phospholipids on the catalysis of purified P450 17A1, P450 11B2, and P450 11B1 in reconstituted assay systems. Both dioleoylphosphatidylcholine (DOPC, 18:1) and dilauroylphosphatidylcholine (DLPC, 12:0) were found to be very efficient in reconstituting 17-hydroxylase and 1720-lyase reactions of P450 17A1. Phosphatidylethanolamine (PE) specifically enhanced 1720-lyase activity up to 2.4-fold in the presence of phosphatidylcholine. On the other hand, P450 11B2-catalyzed production of aldosterone from 11-deoxycorticosterone was very low and from 18-hydroxycorticosterone nil, implying low processivity. DOPC or cardiolipin, which is exclusively located in the inner mitochondrial membrane, maximized aldosterone yield. In sharp contrast, reconstitution of homologous P450 11B1 with DOPC significantly decreased corticosterone formation without affecting the synthesis of 18-hydroxycorticosterone. The intrinsic fluorescence of P450 17A1 and 11B2 increased in the presence of DOPC, DLPC and PE. Acrylamide quenching studies showed that PE decreased solvent accessibility for tryptophan in P450 17A1, as did 20:4 PC or 18:2 PC for P450 11B2. A moderately positive correlation between the proportion of high-spin substrate-bound species and catalytic activity was only observed in the presence of phosphatidylcholines with low-temperature phase transition. These results demonstrate the potential for phospholipids to regulate the activity of steroidogenic P450 activities and thereby steroid hormone biosynthetic pathways.

Recent advances in nanodisc technology for membrane protein studies (2012-2017)

Historically, the main barrier to membrane protein investigations has been the tendency of membrane proteins to aggregate (due to their hydrophobic nature), in aqueous solution as well as on surfaces. The introduction of biomembrane mimetics has since stimulated momentum in the field. One such mimetic, the nanodisc (ND) system, has proved to be an exceptional system for solubilizing membrane proteins. Herein, we critically evaluate the advantages and imperfections of employing nanodiscs in biophysical and biochemical studies. Specifically, we examine the techniques that have been modified to study membrane proteins in nanodiscs. Techniques discussed here include fluorescence microscopy, solution-state/solid-state nuclear magnetic resonance, electron microscopy, small-angle X-ray scattering, and several mass spectroscopy methods. Newer techniques such as SPR, charge-sensitive optical detection, and scintillation proximity assays are also reviewed. Lastly, we cover how nanodiscs are advancing nanotechnology through nanoplasmonic biosensing, lipoprotein-nanoplatelets, and sortase-mediated labeling of nanodiscs.

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.

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.

The catalytic function of cytochrome P450 is entwined with its membrane-bound nature

Cytochrome P450, a family of monooxygenase enzymes, is organized as a catalytic metabolon, which requires enzymatic partners as well as environmental factors that tune its complex dynamic. P450 and its reducing counterparts—cytochrome P450-reductase and cytochrome b ₅—are membrane-bound proteins located in the cytosolic side of the endoplasmic reticulum. They are believed to dynamically associate to form functional complexes. Increasing experimental evidence signifies the role(s) played by both protein-protein and protein-lipid interactions in P450 catalytic function and efficiency. However, the biophysical challenges posed by their membrane-bound nature have severely limited high-resolution understanding of the molecular interfaces of these interactions. In this article, we provide an overview of the current knowledge on cytochrome P450, highlighting the environmental factors that are entwined with its metabolic function. Recent advances in structural biophysics are also discussed, setting up the bases for a new paradigm in the study of this important class of membrane-bound enzymes.

Mammalian Glucose Transporter Activity Is Dependent upon Anionic and Conical Phospholipids

The regulated movement of glucose across mammalian cell membranes is mediated by facilitative glucose transporters (GLUTs) embedded in lipid bilayers. Despite the known importance of phospholipids in regulating protein structure and activity, the lipid-induced effects on the GLUTs remain poorly understood. We systematically examined the effects of physiologically relevant phospholipids on glucose transport in liposomes containing purified GLUT4 and GLUT3. The anionic phospholipids, phosphatidic acid, phosphatidylserine, phosphatidylglycerol, and phosphatidylinositol, were found to be essential for transporter function by activating it and stabilizing its structure. Conical lipids, phosphatidylethanolamine and diacylglycerol, enhanced transporter activity up to 3-fold in the presence of anionic phospholipids but did not stabilize protein structure. Kinetic analyses revealed that both lipids increase the k_cat of transport without changing the K_m values. These results allowed us to elucidate the activation of GLUT by plasma membrane phospholipids and to extend the field of membrane protein-lipid interactions to the family of structurally and functionally related human solute carriers.

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.

Suppression of Cytochrome P450 3A4 Function by UDP-Glucuronosyltransferase 2B7 through a Protein-Protein Interaction: Cooperative Roles of the Cytosolic Carboxyl-Terminal Domain and the Luminal Anchoring Region

There is a large discrepancy between the interindividual difference in the hepatic expression level of cytochrome P450 3A4 (CYP3A4) and that of drug clearance mediated by this enzyme. However, the reason for this discrepancy remains largely unknown. Because CYP3A4 interacts with UDP-glucuronosyltransferase 2B7 (UGT2B7) to alter its function, the reverse regulation is expected to modulate CYP3A4-catalyzed activity. To address this issue, we investigated whether protein-protein interaction between CYP3A4 and UGT2B7 modulates CYP3A4 function. For this purpose, we coexpressed CYP3A4, NADPH-cytochrome P450 reductase, and UGT2B7 using a baculovirus-insect cell system. The activity of CYP3A4 was significantly suppressed by coexpressing UGT2B7, and this suppressive effect was lost when UGT2B7 was replaced with calnexin (CNX). These results strongly suggest that UGT2B7 negatively regulates CYP3A4 activity through a protein-protein interaction. To identify the UGT2B7 domain associated with CYP3A4 suppression we generated 12 mutants including chimeras with CNX. Mutations introduced into the UGT2B7 carboxyl-terminal transmembrane helix caused a loss of the suppressive effect on CYP3A4. Thus, this hydrophobic region is necessary for the suppression of CYP3A4 activity. Replacement of the hydrophilic end of UGT2B7 with that of CNX produced a similar suppressive effect as the native enzyme. The data using chimeric protein demonstrated that the internal membrane-anchoring region of UGT2B7 is also needed for the association with CYP3A4. These data suggest that 1) UGT2B7 suppresses CYP3A4 function, and 2) both hydrophobic domains located near the C terminus and within UGT2B7 are needed for interaction with CYP3A4.

Incorporation of charged residues in the CYP2J2 F-G loop disrupts CYP2J2-lipid bilayer interactions

CYP2J2 epoxygenase is an extrahepatic, membrane bound cytochrome P450 (CYP) that is primarily found in the heart and mediates endogenous fatty acid metabolism. CYP2J2 interacts with membranes through an N-terminal anchor and various non-contiguous hydrophobic residues. The molecular details of the motifs that mediate membrane interactions are complex and not fully understood. To gain better insights of these complex protein-lipid interactions, we employed molecular dynamics (MD) simulations using a highly mobile membrane mimetic (HMMM) model that enabled multiple independent spontaneous membrane binding events to be captured. Simulations revealed that CYP2J2 engages with the membrane at the F-G loop through hydrophobic residues Trp-235, Ille-236, and Phe-239. To explore the role of these residues, three F-G loop mutants were modeled from the truncated CYP2J2 construct (Δ34) which included Δ34-I236D, Δ34-F239H and Δ34-I236D/F239H. Using the HMMM coordinates of CYP2J2, the simulations were extended to a full POPC membrane which showed a significant decrease in the depth of insertion for each of the F-G loop mutants. The CYP2J2 F-G loop mutants were expressed in E. coli and were shown to be localized to the cytosolic fraction at a greater percentage relative to construct Δ34. Notably, the functional data demonstrated that the double mutant, Δ34-I236D/F239H, maintained native-like enzymatic activity. The membrane insertion characteristics were examined by monitoring CYP2J2 Trp-quenching fluorescence spectroscopy upon binding nanodiscs containing pyrene phospholipids. Relative to the Δ34 construct, the F-G loop mutants exhibited lower Trp quenching and membrane insertion. Taken together, the results suggest that the mutants exhibit a different membrane topology in agreement with the MD simulations and provide important evidence towards the involvement of key residues in the F-G loop of CYP2J2.

Anion-Dependent Stimulation of CYP3A4 Monooxygenase

We co-crystallized human cytochrome P450 3A4 (CYP3A4) with progesterone (PRG) under two different conditions, but the resulting complexes contained only one PRG molecule bound to the previously identified peripheral site. A novel feature in one of our structures is a citrate ion, originating from the crystallization solution. The citrate-binding site is located in an area where the N-terminus splits from the protein core and, thus, is suitable for the interaction with the anionic phospholipids of the microsomal membrane. We investigated how citrate affects the function of a soluble CYP3A4 monooxygenase system consisting of equimolar amounts of CYP3A4 and cytochrome P450 reductase (CPR). Citrate was found to affect the properties of both redox partners and stimulated their catalytic activities in a concentration-dependent manner via a complex mechanism. CYP3A4-substrate binding, reduction of CPR with NADPH, and interflavin and interprotein electron transfer were identified as citrate-sensitive steps. Comparative analysis of various negatively charged organic compounds indicated that, in addition to alterations caused by changes in ionic strength, anions modulate the properties of CYP3A4 and CPR through specific anion-protein interactions. Our results help to better understand previous observations and provide new mechanistic insights into CYP3A4 function.

Vinegar-baked Radix Bupleuri modulates the cell membrane constituents and inhibits the P-gp activity in rat hepatocytes

Background

Vinegar-baked Radix Bupleuri (VBRB) enhances the effects of other drugs on the liver by increasing drug distribution to the liver, but the mechanism of action remains unclear. The present study was designed to determine the effects of VBRB on the membrane permeability, constituents, and P-glycoprotein (P-gp) activity of hepatocyte BRL cells, in order to interpret the liver targeting enhancing effects of VBRB.

Methods

The membrane permeability and P-gp expression were analyzed by flow cytometry. The membrane constituents were determined by an automatic biochemistry analyzer and thin-layer chromatography.

Results

The results showed that, compared with the control, VBRB enhanced the membrane permeability by 41-67% (P < 0.05), which occurred in the absence of any cytotoxicity. VBRB had marginal effects on the cholesterol content, but significantly affected the total protein contents and the lipid constituents of the cell membrane in a dose- and time-dependent manner. VBRB inhibited P-gp expression in the cell membrane by 59-86% (P < 0.01).

Conclusion

VBRB affects the constituents of BRL cells and increases its permeability, which may help explain its liver-targeting effects.

Affinity purification of recombinant human cytochrome P450s 3A4 and 1A2 using mixed micelle systems

Recombinant cytochrome P450 (CYP or P450) enzymes are useful for drug metabolism research and thereby many expression and purification systems have been developed. Here, we provide a method for the purification of human P450s 3A4 and 1A2 expressed in Escherichia coli using mixed micelles containing anionic phospholipids. This method does not require any protein-tagging system for protein isolation and has a further advantage that the purification is concomitantly conducted with reconstitution of the enzymes into a phospholipid environment, which is crucial for the catalytic activity assay of P450 enzyme. This method may also be applied to high-throughput catalytic assays of the enzymes because the purification procedures can be undertaken in a 96-well plate.

Variability in human in vitro enzyme kinetics

There are many factors which are known to cause variability in human in vitro enzyme kinetic data. Factors such as the source of enzyme and how it was prepared, the genetics and background of the donor, how the in vitro studies are designed, and how the data are analyzed contribute to variability in the resulting kinetic parameters. It is important to consider not only the factors which cause variability within an experiment, such as selection of a probe substrate, but also those that cause variability when comparing kinetic data across studies and laboratories. For example, the artificial nature of the microsomal lipid membrane and microenvironment in some recombinantly expressed enzymes, relative to those found in native tissue microsomes, has been shown to influence enzyme activity and thus can be a source of variability when comparing across the two different systems. All of these factors, and several others, are discussed in detail in the chapter below.

Two surfaces of cytochrome b5 with major and minor contributions to CYP3A4-catalyzed steroid and nifedipine oxygenation chemistries

Conserved human cytochrome b5 (b5) residues D58 and D65 are critical for interactions with CYP2E1 and CYP2C19, whereas E48 and E49 are essential for stimulating the 17,20-lyase activity of CYP17A1. Here, we show that b5 mutations E48G, E49G, D58G, and D65G have reduced capacity to stimulate CYP3A4-catalyzed progesterone and testosterone 6β-hydroxylation or nifedipine oxidation. The b5 double mutation D58G/D65G fails to stimulate these reactions, similar to CYP2E1 and CYP2C19, whereas mutation E48G/E49G retains 23-42% of wild-type stimulation. Neither mutation impairs the activity stimulation of wild-type b5, nor does mutation D58G/D65G impair the partial stimulation of mutations E48G or E48G/E49G. For assays reconstituted with a single phospholipid, phosphatidyl serine afforded the highest testosterone 6β-hydroxylase activity with wild-type b5 but the poorest activity with b5 mutation E48G/E49G, and the activity stimulation of mutation E48G/E49G was lost at [NaCl]>50mM. Cross-linking of CYP3A4 and b5 decreased in the order wild-type>E48G/E49G>D58G/D65G and varied with phospholipid. We conclude that two b5 acidic surfaces, primarily the domain including residues D58-D65, participate in the stimulation of CYP3A4 activities. Our data suggest that a minor population of CYP3A4 molecules remains sensitive to b5 mutation E48G/E49G, consistent with phospholipid-dependent conformational heterogeneity of CYP3A4.

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.

Membrane phospholipid augments cytochrome P4501a enzymatic activity by modulating structural conformation during detoxification of xenobiotics

Cytochrome P450 is a superfamily of membrane-bound hemoprotein that gets involved with the degradation of xenobiotics and internal metabolites. Accumulated body of evidence indicates that phospholipids play a crucial role in determining the enzymatic activity of cytochrome P450 in the microenvironment by modulating its structure during detoxification; however, the structure-function relationship of cytochrome P4501A, a family of enzymes responsible for degrading lipophilic aromatic hydrocarbons, is still not well defined. Inducibility of cytochrome P4501A in cultured catfish hepatocytes in response to carbofuran, a widely used pesticide around the world, was studied earlier in our laboratory. In this present investigation, we observed that treating catfish with carbofuran augmented total phospholipid in the liver. We examined the role of phospholipid on the of cytochrome P4501A-marker enzyme which is known as ethoxyresorufin-O-deethylase (EROD) in the context of structure and function. We purified the carbofuran-induced cytochrome P4501A protein from catfish liver. Subsequently, we examined the enzymatic activity of purified P4501A protein in the presence of phospholipid, and studied how the structure of purified protein was influenced in the phospholipid environment. Membrane phospholipid appeared to accelerate the enzymatic activity of EROD by changing its structural conformation and thus controlling the detoxification of xenobiotics. Our study revealed the missing link of how the cytochrome P450 restores its enzymatic activity by changing its structural conformation in the phospholipid microenvironment.

Increase of human CYP1B1 activities by acidic phospholipids and kinetic deuterium isotope effects on CYP1B1 substrate oxidation

The effect of phospholipids on the kinetic parameters of three substrates, 7-ethoxy-4 -(trifluoromethyl)coumarin (7-EFC), 7-ethoxycoumarin (7-EC) and 17β-estradiol (E(2)), of human CYP1B1 was studied. In general, anionic phospholipids, phosphatidic acid and cardiolipin increased catalytic efficiency by increasing k(cat) values or decreasing K(m) values. The advantages of using the 7-EFC as a substrate over 7-EC and E(2) include high k(cat), low K(m) and high catalytic efficiency. Spectral binding titrations indicated that the binding affinity of 7-EFC to CYP1B1 in the presence or absence of phospholipids is higher than that of 7-EC or E(2). Furthermore, phosphatidylcholine increased the binding affinity of the substrates to the CYP1B1. High non-competitive intermolecular kinetic deuterium isotope effects (values 5.4-12) were observed for O-deethylation of 7-EFC and 7-EC with deuterium substitution at the ethoxy group, indicating that the C-H bond-breaking step makes a major contribution to the rate of these CYP1B1-catalyzed reactions. However, the intermolecular kinetic deuterium isotope effect is ~2 for the E(2) 4-hydroxylation reaction, indicating that the C-H bond-breaking step contributes only partially to the rate of this CYP1B1-catalyzed reaction. These results indicate that the reaction mechanism of CYP1B1-catalyzed reactions is distinct for each substrate.

Modeling chemical interaction profiles: II. Molecular docking, spectral data-activity relationship, and structure-activity relationship models for potent and weak inhibitors of cytochrome P450 CYP3A4 isozyme

Polypharmacy increasingly has become a topic of public health concern, particularly as the U.S. population ages. Drug labels often contain insufficient information to enable the clinician to safely use multiple drugs. Because many of the drugs are bio-transformed by cytochrome P450 (CYP) enzymes, inhibition of CYP activity has long been associated with potentially adverse health effects. In an attempt to reduce the uncertainty pertaining to CYP-mediated drug-drug/chemical interactions, an interagency collaborative group developed a consensus approach to prioritizing information concerning CYP inhibition. The consensus involved computational molecular docking, spectral data-activity relationship (SDAR), and structure-activity relationship (SAR) models that addressed the clinical potency of CYP inhibition. The models were built upon chemicals that were categorized as either potent or weak inhibitors of the CYP3A4 isozyme. The categorization was carried out using information from clinical trials because currently available in vitro high-throughput screening data were not fully representative of the in vivo potency of inhibition. During categorization it was found that compounds, which break the Lipinski rule of five by molecular weight, were about twice more likely to be inhibitors of CYP3A4 compared to those, which obey the rule. Similarly, among inhibitors that break the rule, potent inhibitors were 2–3 times more frequent. The molecular docking classification relied on logistic regression, by which the docking scores from different docking algorithms, CYP3A4 three-dimensional structures, and binding sites on them were combined in a unified probabilistic model. The SDAR models employed a multiple linear regression approach applied to binned 1D ¹³C-NMR and 1D ¹⁵N-NMR spectral descriptors. Structure-based and physical-chemical descriptors were used as the basis for developing SAR models by the decision forest method. Thirty-three potent inhibitors and 88 weak inhibitors of CYP3A4 were used to train the models. Using these models, a synthetic majority rules consensus classifier was implemented, while the confidence of estimation was assigned following the percent agreement strategy. The classifier was applied to a testing set of 120 inhibitors not included in the development of the models. Five compounds of the test set, including known strong inhibitors dalfopristin and tioconazole, were classified as probable potent inhibitors of CYP3A4. Other known strong inhibitors, such as lopinavir, oltipraz, quercetin, raloxifene, and troglitazone, were among 18 compounds classified as plausible potent inhibitors of CYP3A4. The consensus estimation of inhibition potency is expected to aid in the nomination of pharmaceuticals, dietary supplements, environmental pollutants, and occupational and other chemicals for in-depth evaluation of the CYP3A4 inhibitory activity. It may serve also as an estimate of chemical interactions via CYP3A4 metabolic pharmacokinetic pathways occurring through polypharmacy and nutritional and environmental exposures to chemical mixtures.

Kinetics of electron transfer between NADPH-cytochrome P450 reductase and cytochrome P450 3A4

We have incorporated CYP3A4 (cytochrome P450 3A4) and CPR (NADPH-cytochrome P450 reductase) into liposomes with a high lipid/protein ratio by an improved method. In the purified proteoliposomes, CYP3A4 binds testosterone with Kd (app)=36±6 μM and Hill coefficient=1.5±0.3, and 75±4% of the CYP3A4 can be reduced by NADPH in the presence of testosterone. Transfer of the first electron from CPR to CYP3A4 was measured by stopped-flow, trapping the reduced CYP3A4 as its Fe(II)-CO complex and measuring the characteristic absorbance change. Rapid electron transfer is observed in the presence of testosterone, with the fast phase, representing 90% of the total absorbance change, having a rate of 14±2 s(-1). Measurements of the first electron transfer were performed at various molar ratios of CPR/CYP3A4 in proteoliposomes; the rate was unaffected, consistent with a model in which first electron transfer takes place within a relatively stable CPR-CYP3A4 complex. Steady-state rates of NADPH oxidation and of 6β-hydroxytestosterone formation were also measured as a function of the molar ratio of CPR/CYP3A4 in the proteoliposomes. These rates increased with increasing CPR/CYP3A4 ratio, showing a hyperbolic dependency indicating a Kd (app) of ~0.4 μM. This suggests that the CPR-CYP3A4 complex can dissociate and reform between the first and second electron transfers.

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

Cytochrome P450 is part of an electron transport chain found in the endoplasmic reticulum (ER), with its catalytic function requiring interactions with NADPH-cytochrome P450 reductase (CPR). The goals of this study were to examine how the P450 system proteins are organized in the membrane and to determine whether they are distributed in detergent-resistant lipid microdomains (DRM). Isolated liver microsomes from untreated rabbits were treated with 1% Brij 98, and DRMs were isolated via sucrose gradient centrifugation. Lipid analysis showed that DRM fractions were enriched in cholesterol and sphingomyelin, similar to that found with plasma membrane DRMs. Approximately 73% of CYP1A2 and 68% of CPR resided in DRM fractions, compared with only 33% of total ER proteins. These DRMs were found to be cholesterol-dependent: CPR and CYP1A2 migrated to the more dense regions of the sucrose gradient after cholesterol depletion. CYP1A2 function was studied in three purified lipid vesicles consisting of 1) phosphatidylcholine (V-PC), 2) lipids with a composition similar to ER lipids (V-ER), and 3) lipids with a composition similar to the DRM fractions (V-DRM). Each system showed similar substrate binding characteristics. However, when the association between CPR and CYP1A2 was measured, V-ER and V-DRM liposomes produced lower apparent K(m) values compared with V-PC without any significant change in V(max). These findings suggest that CYP1A2 and CPR reside in ER-DRMs and that the unique lipid components of these domains enhance CYP1A2 substrate metabolism through greater efficiency in CPR-CYP1A2 binding.

Lipid-protein correlations in nanoscale phospholipid bilayers determined by solid-state nuclear magnetic resonance

Nanodiscs are examples of discoidal nanoscale lipid-protein particles that have been extremely useful for the biochemical and biophysical characterization of membrane proteins. They are discoidal lipid bilayer fragments encircled and stabilized by two amphipathic helical proteins named membrane scaffolding protein (MSP), ~10 nm in size. Nanodiscs are homogeneous, easily prepared with reproducible success, amenable to preparations with a variety of lipids, and stable over a range of temperatures. Here we present solid-state nuclear magnetic resonance (SSNMR) studies on lyophilized, rehydrated POPC Nanodiscs prepared with uniformly (13)C-, (15)N-labeled MSP1D1 (Δ1-11 truncated MSP). Under these conditions, by SSNMR we directly determine the gel-to-liquid crystal lipid phase transition to be at 3 ± 2 °C. Above this phase transition, the lipid (1)H signals have slow transverse relaxation, enabling filtering experiments as previously demonstrated for lipid vesicles. We incorporate this approach into two- and three-dimensional heteronuclear SSNMR experiments to examine the MSP1D1 residues interfacing with the lipid bilayer. These (1)H-(13)C and (1)H-(13)C-(13)C correlation spectra are used to identify and quantify the number of lipid-correlated and solvent-exposed residues by amino acid type, which furthermore is compared with molecular dynamics studies of MSP1D1 in Nanodiscs. This study demonstrates the utility of SSNMR experiments with Nanodiscs for examining lipid-protein interfaces and has important applications for future structural studies of membrane proteins in physiologically relevant formulations.

Modulation of UDP-glucuronosyltransferase activity by protein-protein association

Drug oxidation and conjugation mediated by cytochrome P450 (P450) and UDP-glucuronosyltransferase (UGT) have long been considered to take place separately. However, our recent studies have suggested that CYP3A4 specifically associates with UGT2B7 and alters the regioselectivity of morphine glucuronidation. This observation strongly supports the view that there is functional cooperation between P450 and UGT to facilitate multistep drug metabolism. In recent years, accumulating evidence has suggested an interaction between UGT isoforms or between P450 and UGTs and a change in UGT function by protein-protein association. In this review, we summarize these interactions and discuss their relevance to UGT function.

Modulation of the cytochrome P450 reductase redox potential by the phospholipid bilayer

Cytochrome P450 reductase (CPR) is a tethered membrane protein which transfers electrons from NADPH to microsomal P450s. We show that the lipid bilayer has a role in defining the redox potential of the CPR flavin domains. In order to quantitate the electrochemical behavior of this central redox protein, full-length CPR was incorporated into soluble nanometer scale discoidal membrane bilayers (nanodiscs), and potentials were measured using spectropotentiometry. The redox potentials of both FMN and FAD were found to shift to more positive values when in a membrane bilayer as compared to a solubilized version of the reductase. The potentials of the semiquinone/hydroquinone couple of both FMN and FAD are altered to a larger extent than the oxidized/semiquinone couple which is understood by a simple electrostatic model. When anionic lipids were used to change the membrane composition of the CPR-nanodisc, the redox potential of both flavins became more negative, favoring electron transfer from CPR to cytochrome P450.

Modular synthesis of biologically active phosphatidic acid probes using click chemistry

Phosphatidic acid (PA) is an important signaling lipid that plays roles in a range of biological processes including both physiological and pathophysiological events. PA is one of a number of signaling lipids that can act as site-specific ligands for protein receptors in binding events that enforce membrane association and generally regulate both receptor function and subcellular localization. However, elucidation of the full scope of PA activities has proven problematic, primarily due to the lack of a consensus sequence among PA-binding receptors. Thus, experimental approaches, such as those employing lipid probes, are necessary for characterizing interactions at the molecular level. Herein, we describe an efficient modular approach to the synthesis of a range of PA probes that employs a late stage introduction of reporter groups. This strategy was exploited in the synthesis of PA probes bearing fluorescent and photoaffinity tags as well as a bifunctional probe containing both a photoaffinity moiety and an azide as a secondary handle for purification purposes. To discern the ability of these PA analogs to mimic the natural lipid in protein-binding properties, each compound was incorporated into vesicles for binding studies using a known PA receptor, the C2 domain of PKCalpha. In these studies, each compound exhibited binding properties that were comparable to those of synthetic PA, indicating their viability as probes for effectively studying the activities of PA in cellular processes.

Interaction of cytochrome P450 3A4 and UDP-glucuronosyltransferase 2B7: evidence for protein-protein association and possible involvement of CYP3A4 J-helix in the interaction

We have reported that the protein-protein interaction between UDP-glucuronosyltransferase (UGT) 2B7 and cytochrome P450 3A4 (CYP3A4) alters UGT2B7 function. However, the domain(s) involved in the interaction are largely unknown. To address this issue, we examined in more detail the CYP3A4-UGT2B7 association by means of immunoprecipitation, overlay assay, and cross-linking involving 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide. Purified CYP3A4 or glutathione transferase (GST)-tagged CYP3A4 was cross-linked to UGT2B7 in solubilized baculosomes. The formation of the cross-linked complex was detected by immunoblotting using both antibodies against CYP3A4 and UGTs. Although the GST-tagged CYP3A4 containing the region ranging from Tyr25 to Ala503 was cross-linked to UGT2B7, the same did not occur when another construct containing Met145 to His267 was used. This observation was consistent with the result of the overlay assay indicating that CYP3A4 lacking the N-terminal hydrophobic segment retains the ability to associate with UGT2B7, whereas the Met145-to-His267 region loses this capacity. Although the Met145-to-His267 peptide was recognized by one anti-CYP3A4 antibody that has the ability to coimmunoprecipitate UGT2B7, it was not recognized by another antibody incapable of coimmunoprecipitating UGT2B7. The epitope of the latter antibody was mapped to the Leu331-to-Lys342 region, which is located on the J-helix of CYP3A4. Taken together, the results obtained suggest that 1) CYP3A4 and UGT2B7 are a pair of enzymes in proximity to each other and 2) either the Leu331-to-Lys342 domain or the surrounding region plays a role in the interaction with UGT2B7, whereas the hydrophobic Met145-to-His267 region does not contribute to this interaction.

An NH2-terminal truncated cytochrome P450 CYP3A4 showing catalytic activity is present in the cytoplasm of human liver cells

Cytochrome P450 3A4 (CYP3A4), is the dominant human liver hemoprotein enzyme localized in the endoplasmic reticulum (ER), and is responsible for the metabolism of more than 50% of clinically relevant drugs. While we were studying CYP3A4 expression and activity in human liver, we found that anti-CYP3A4 antibody cross-reacted with a lower band in liver cytoplasmic fraction. We assessed the activities of CYP3A4 and its truncated form in the microsomal and cytoplasmic fraction, respectively. In the cytoplasmic fraction, truncated CYP3A4 showed catalytic activity when reconstituted with NADPH-cytochrome P-450 reductase and cytochrome b5. In order to determine which site was deleted in the truncated form in vitro, we transfected cells with N-terminal tagged or C-terminal tagged human CYP3A4 cDNA. The truncated CYP3A4 is the N-terminal deleted form and was present in the soluble cytoplasmic fraction. Our result shows, for the first time, that N-terminal truncated, catalytically active CYP3A4 is present principally in the cytoplasm of human liver cells.

Phospholipase D in human platelets: presence of isoenzymes and participation of autocrine stimulation during thrombin activation

Phospholipase D (PLD), which hydrolyzes phosphatidylcholine to phosphatidic acid (PA) and choline, is present in human platelets. Thrombin and other agonists have been shown to activate PLD but the precise mechanisms of activation and PLDs role in platelet activation remains unclear. We measured thrombin-stimulated PLD activity in platelets as formation of phosphatidylethanol. Since no specific PLD inhibitors exist, we investigated possible roles for PLD in platelets by correlating PLD activity with platelet responses such as thrombin-mediated secretion and F-actin formation (part of platelet shape change). Extracellular Ca2+ potentiated thrombin-stimulated PLD, but did not stimulate PLD in the absence of thrombin. Thrombin-induced PLD activity was enhanced by secreted ADP and binding of fibrinogen to its receptors. In contrast to others, we also found a basal PLD activity. Comparison of time courses and dose responses of platelets with PLD showed many points of correlation between PLD activation and lysosomal secretion and F-actin formation. The finding of different PLD activities suggested that different PLD isoenzymes exist in platelets as reported for other cells. Here we present evidence for the presence of both PLD1 and PLD2 in platelets by use of specific antibodies with immunoblotting and immunohistochemistry. Both isoforms were randomly localized in resting platelets, but became rapidly translocated to the proximity of the plasma membrane upon thrombin stimulation, thus indicating a role for PLD in platelet activation.

Magic-angle spinning solid-state NMR spectroscopy of nanodisc-embedded human CYP3A4

Cytochrome P450 (CYP) 3A4 contributes to the metabolism of approximately 50% of commercial drugs by oxidizing a large number of structurally diverse substrates. Like other endoplasmic reticulum-localized P450s, CYP3A4 contains a membrane-anchoring N-terminal helix and a significant number of hydrophobic domains, important for the interaction between CYP3A4 and the membrane. Although the membrane affects specificity of CYP3A4 ligand binding, the structural details of the interaction have not been revealed so far because X-ray crystallography studies are available only for the soluble domain of CYP3A4. Here we report sample preparation and initial magic-angle spinning (MAS) solid-state NMR (SSNMR) of CYP3A4 (Delta3-12) embedded in a nanoscale membrane bilayer, or Nanodisc. The growth protocol yields approximately 2.5 mg of the enzymatically active, uniformly 13C,15N-enriched CYP3A4 from 1 L of growth medium. Polyethylene glycol 3350-precipitated CYP3A4 in Nanodiscs yields spectra of high resolution and sensitivity, consistent with a folded, homogeneous protein. CYP3A4 in Nanodiscs remains enzymatically active throughout the precipitation protocol as monitored by bromocriptine binding. The 13C line widths measured from 13C-13C 2D chemical shift correlation spectra are approximately 0.5 ppm. The secondary structure distribution within several amino acid types determined from 13C chemical shifts is consistent with the ligand-free X-ray structures. These results demonstrate that MAS SSNMR can be performed on Nanodisc-embedded membrane proteins in a folded, active state. The combination of SSNMR and Nanodisc methodologies opens up new possibilities for obtaining structural information on CYP3A4 and other integral membrane proteins with full retention of functionality.

Lateral segregation of anionic phospholipids in model membranes induced by cytochrome P450 2B1: bi-directional coupling between CYP2B1 and anionic phospholipid

The lateral segregation of anionic phospholipids phosphatidic acid (PA), phosphatidylinositol (PI), and phosphatidylserine (PS) was detected after addition of cytochrome P450 2B1 (CYP2B1). The tendency of lipid clustering was highly dependent on the type of anionic phospholipids examined. PA was the most highly clustered while PI and PS clustered to a lesser degree. Moreover, liposomes containing anionic phospholipids form anionic phospholipid-rich microdomains in the presence of CYP2B1. Anionic phospholipids (mostly notably PA) also increased the ability of CYP2B1 to bind to lipid monolayers. In addition to the ability of CYP2B1 to modulate the physical properties of the membrane, the membrane itself can have reciprocal effects on the activity and conformation of CYP2B1. The catalytic activity of CYP2B1 increased as a function of anionic phospholipid concentration and in the presence of 10 mol% PA, the activity increased by 85%. These results suggest a bi-directional coupling between the CYP2B1 and anionic phospholipids.