Dynamic interactions of rabbit liver cytochromes P450IA2 and P450IIB4 with cytochrome b5 and NADPH-cytochrome P450 reductase in proteoliposomes

Human steroid biosynthesis, metabolism and excretion are differentially reflected by serum and urine steroid metabolomes: A comprehensive review

Advances in technology have allowed for the sensitive, specific, and simultaneous quantitative profiling of steroid precursors, bioactive steroids and inactive metabolites, facilitating comprehensive characterization of the serum and urine steroid metabolomes. The quantification of steroid panels is therefore gaining favor over quantification of single marker metabolites in the clinical and research laboratories. However, although the biochemical pathways for the biosynthesis and metabolism of steroid hormones are now well defined, a gulf still exists between this knowledge and its application to the measured steroid profiles. In this review, we present an overview of steroid hormone biosynthesis and metabolism by the liver and peripheral tissues, specifically highlighting the pathways linking and differentiating the serum and urine steroid metabolomes. A brief overview of the methodology used in steroid profiling is also provided.

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.

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.

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.

Cytochrome b5 modulates multiple reactions in steroidogenesis by diverse mechanisms

Cytochrome b5 (cyt-b5) is a relatively small haemoprotein which plays an important role in the regulation of mammalian steroidogenesis. This unique protein has the ability to modulate the activity of key steroidogenic enzymes via a number of diverse reaction mechanisms. Cyt-b5 can augment the 17,20-lyase activity of CYP17A1 by promoting the interaction of CYP17A1 and POR; enhance the 16-ene-synthase activity of CYP17A1 by acting as an electron donor; and enhance the activity of 3βHSD by increasing the affinity of 3βHSD for its cofactor NAD(+). We review the modulation of CYP17A1 and 3βHSD activity by cyt-b5 and discuss the reaction mechanisms associated with each activity. The physiological importance of cyt-b5 in regulating mammalian steroidogenesis is presented and the impact of inactivating cyt-b5 mutations are reviewed. This article is part of a Special Issue entitled 'Steroid/Sterol signaling'.

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

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

Pivotal role of P450-P450 interactions in CYP3A4 allostery: the case of α-naphthoflavone

We investigated the relationship between oligomerization of CYP3A4 (cytochrome P450 3A4) and its response to ANF (α-naphthoflavone), a prototypical heterotropic activator. The addition of ANF resulted in over a 2-fold increase in the rate of CYP3A4-dependent debenzylation of 7-BFC [7-benzyloxy-4-(trifluoromethyl)coumarin] in HLM (human liver microsomes), but failed to produce activation in BD Supersomes or Baculosomes containing recombinant CYP3A4 and NADPH-CPR (cytochrome P450 reductase). However, incorporation of purified CYP3A4 into Supersomes containing only recombinant CPR reproduced the behaviour observed with HLM. The activation in this system was dependent on the surface density of the enzyme. Although no activation was detectable at an L/P (lipid/P450) ratio ≥750, it reached 225% at an L/P ratio of 140. To explore the relationship between this effect and CYP3A4 oligomerization, we probed P450-P450 interactions with a new technique that employs LRET (luminescence resonance energy transfer). The amplitude of LRET in mixed oligomers of the haem protein labelled with donor and acceptor fluorophores exhibited a sigmoidal dependence on the surface density of CYP3A4 in Supersomes™. The addition of ANF eliminated this sigmoidal character and increased the degree of oligomerization at low enzyme concentrations. Therefore the mechanisms of CYP3A4 allostery with ANF involve effector-dependent modulation of P450-P450 interactions.

Cytochrome b5: novel roles in steroidogenesis

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

Formation of P450 · P450 complexes and their effect on P450 function

Cytochromes P450 (P450) are membrane-bound enzymes that catalyze the monooxygenation of a diverse array of xenobiotic and endogenous compounds. The P450s responsible for foreign compound metabolism generally are localized in the endoplasmic reticulum of the liver, lung and small intestine. P450 enzymes do not act alone but require an interaction with other electron transfer proteins such as NADPH-cytochrome P450 reductase (CPR) and cytochrome b(5). Because P450s are localized in the endoplasmic reticulum with these and other ER-resident proteins, there is a potential for protein-protein interactions to influence P450 function. There has been increasing evidence that P450 enzymes form complexes in the ER, with compelling support that formation of P450 · P450 complexes can significantly influence their function. Our goal is to review the research supporting the formation of P450 · P450 complexes, their specificity, and how drug metabolism may be affected. This review describes the potential mechanisms by which P450s may interact, and provides evidence to support each of the possible mechanisms. Additionally, evidence for the formation of both heteromeric and homomeric P450 complexes are reviewed. Finally, direct physical evidence for P450 complex formation in solution and in membranes is summarized, and questions directing the future research of functional P450 interactions are discussed with respect to their potential impact on drug metabolism.

Identification of cytochrome P450 2C2 protein complexes in mouse liver

Interactions of microsomal cytochromes P450 (CYPs) with other proteins in the microsomal membrane are important for their function. In addition to their redox partners, CYPs have been reported to interact with other proteins not directly involved in their enzymatic function. In this study, proteins were identified that interact with CYP2C2 in vivo in mouse liver. Flag-tagged CYP2C2 was expressed exogenously in mouse liver and was affinity purified, along with associated proteins which were identified by MS and confirmed by Western blotting. Over 20 proteins reproducibly copurified with CYP2C2. The heterogeneous sedimentation velocity of CYP2C2 and associated proteins by centrifugation in sucrose gradients and sequential immunoprecipitation analysis were consistent with multiple CYP2C2 complexes of differing composition. The abundance of CYPs and other drug metabolizing enzymes and NAD/NADP requiring enzymes associated with CYP2C2 suggest that complexes of these proteins may improve enzymatic efficiency or facilitate sequential metabolic steps. Chaperones, which may be important for maintaining CYP function, and reticulons, endoplasmic reticulum proteins that shape the morphology of the endoplasmic reticulum and are potential endoplasmic reticulum retention proteins for CYPs, were also associated with CYP2C2.

Experimental approaches to evaluate activities of cytochromes P450 3A

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

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

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

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

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

Allosteric P450 mechanisms: multiple binding sites, multiple conformers or both?

According to the initial hypothesis on the mechanisms of cooperativity in drug-metabolizing cytochromes P450, a loose fit of a single substrate molecule in the P450 active site results in a requirement for the binding of multiple ligand molecules for efficient catalysis. Although simultaneous occupancy of the active site by multiple ligands is now well established, there is increasing evidence that the mechanistic basis of cooperativity also involves an important ligand-induced conformational transition. Moreover, recent studies demonstrate that the conformational heterogeneity of the enzyme is stabilized by ligand-dependent interactions of several P450 molecules. Application of the concept of an oligomeric allosteric enzyme to microsomal cytochromes P450 in combination with a general paradigm of multiple ligand occupancy of the active site provides an excellent explanation for complex manifestations of the atypical kinetic behavior of the enzyme.

Heteromeric complex formation between CYP2E1 and CYP1A2: evidence for the involvement of electrostatic interactions

Mixed reconstituted systems containing CYP2B4, CYP1A2, and NADPH-cytochrome P450 reductase were previously shown to exhibit a dramatic inhibition of 7-pentoxyresorufin O-dealkylation (PROD) when compared to simple reconstituted systems containing reductase and a single P450 enzyme, results consistent with the formation of CYP1A2-CYP2B4 complexes where the reductase binds with high affinity to the CYP1A2 moiety of the complex. In this report, we provide evidence for an interaction between CYP1A2 and CYP2E1. Synergism of 7-ethoxyresorufin O-deethylation (EROD) and PROD was observed when these P450s were combined in mixed reconstituted systems at subsaturating reductase concentrations. Higher ionic strength attenuated the synergistic stimulation of both PROD and EROD in mixed reconstituted systems, consistent with disruption of heteromeric CYP2E1-CYP1A2 complexes. The effect of ionic strength was further examined as a function of reductase concentration. At lower ionic strength, there was a significant synergistic stimulation of EROD. This synergistic stimulation diminished with increasing reductase concentration, resulting in an additive response as reductase became saturating. Interestingly, at high ionic strength, the synergism of EROD in the mixed reconstituted system was not observed. In contrast, mixed reconstituted systems containing CYP2E1 and CYP2B4 did not provide evidence for the formation of these heteromeric P450-P450 complexes. The synergistic stimulation observed with the reductase-CYP1A2-CYP2E1 mixed reconstituted system is consistent with the formation of a CYP1A2-CYP2E1 complex. Taken together with the lack of a kinetically detectable interaction between CYP2B4 and CYP2E1, and the previously reported CYP1A2-CYP2B4 interaction, these results suggest that CYP1A2 may facilitate the formation of complexes with other P450 enzymes.

Molecular mechanism of phase I and phase II drug-metabolizing enzymes: implications for detoxification

Enzymes that catalyze the biotransformation of drugs and xenobiotics are generally referred to as drug-metabolizing enzymes (DMEs). DMEs can be classified into two main groups: oxidative or conjugative. The NADPH-cytochrome P450 reductase (P450R)/cytochrome P450 (P450) electron transfer systems are oxidative enzymes that mediate phase I reactions, whereas the UDP-glucuronosyltransferases (UGTs) are conjugative enzymes that mediate phase II enzymes. Both enzyme systems are localized to the endoplasmic reticulum (ER) where a number of drugs are sequentially metabolized. DMEs, including P450s and UGTs, generally have a highly plastic active site that can accommodate a wide variety of substrates. The P450 and UGT genes constitute a supergene family, in which UGT proteins are encoded by distinct genes and a complex gene. Both the P450 and UGT genes have evolved to diversify their functions. This chapter reviews advances in understanding the structure and function of the P450R/P450 and UGT enzyme systems. In particular, the coordinate biotransformation of xenobiotics by phase I and II enzymes in the ER membrane is examined.

Role of cytochrome B5 in modulating peroxide-supported cyp3a4 activity: evidence for a conformational transition and cytochrome P450 heterogeneity

The role of cytochrome b(5) (b(5)) in the alpha-naphthoflavone (alpha-NF)-mediated inhibition of H(2)O(2)-supported 7-benzyloxyquinoline (7-BQ) debenzylation by heterologously expressed and purified cytochrome P450 3A4 (CYP3A4) was studied. Although alpha-NF showed negligible effect in an NADPH-dependent reconstituted system, inhibition of 7-BQ oxidation was observed in the H(2)O(2) system. Analysis of the effect of various constituents of a standard reconstituted system on H(2)O(2)-supported activity showed that b(5) alone resulted in a 2.5-fold increase in the k(cat) value and reversed the inhibitory effect of alpha-NF. In addition, titration with b(5) suggested that only 65% of the CYP3A4 participated in the interaction with b(5), consistent with cytochrome P450 (P450) heterogeneity. Study of the influence of b(5) on the kinetics of H(2)O(2)-dependent destruction of the P450 heme moiety suggested two distinct conformers of CYP3A4 with different sensitivity to heme loss. In the absence of b(5), 66% of the wild-type enzyme was bleached in the fast phase, whereas the addition of b(5) decreased the fraction of the fast phase to 16%. Finally, to locate amino acid residues that might influence b(5) action, several active site mutants were tested. Substitution of Ser-119, Ile-301, Ala-305, Ile-369, or Ala-370 with the larger Phe or Trp decreased or even abolished the activation by b(5). Ser-119 is in the B'-C loop, a predicted b(5)-P450 interaction site, and Ile-301 and Ala-305 are closest to the heme. In conclusion, the interaction of b(5) with P450 apparently leads to a conformational transition, which results in redistribution of the CYP3A4 pool.

Effects of ionic strength on the functional interactions between CYP2B4 and CYP1A2

The presence of one P450 can influence the catalytic characteristics of a second enzyme through the formation of heteromeric P450 complexes. Such a complex has been reported for mixed reconstituted systems containing NADPH-cytochrome P450 reductase, CYP2B4, and CYP1A2, where a dramatic inhibition of 7-pentoxyresorufin-O-dealkylation (PROD) was observed when compared to simple reconstituted systems containing reductase and a single P450 enzyme. The goal of the present study was to characterize this interaction by examining the potential of the CYP1A2-CYP2B4 complex to be formed by charge-pair interactions. With ionic interactions being sensitive to the surrounding ionic environment, monooxygenase activities were measured in both simple systems and mixed reconstituted systems as a function of ionic strength. PROD was found to be decreased at high ionic strength in both simple and mixed reconstituted systems, due to disruption of reductase-P450 complexes. Additionally, the inhibition of PROD in mixed reconstituted systems was relieved at high ionic strength, consistent with disruption of the CYP2B4-CYP1A2 complex. When ionic strength was measured as a function of CYP1A2 concentration, a shift to the right in the inflection point of the biphasic curve occurred at high ionic strength, consistent with a loss in CYP1A2 affinity for CYP2B4. When this analysis was applied to the same systems using a different substrate, 7-EFC, evidence for a high-affinity complex was not observed, demonstrating that the characteristics of the CYP1A2-CYP2B4 complex are influenced by the substrates present. These results support the role for a substrate specific electrostatic interaction between these P450 enzymes.

Structure of Human Microsomal Cytochrome P450 2C8

A 2.7-Angstrom molecular structure of human microsomal cytochrome P450 2C8 (CYP2C8) was determined by x-ray crystallography. The membrane protein was modified for crystallization by replacement of the hydrophobic N-terminal transmembrane domain with a short hydrophilic sequence before residue 28. The structure of the native sequence is complete from residue 28 to the beginning of a C-terminal histidine tag used for purification. CYP2C8 is one of the principal hepatic drug-metabolizing enzymes that oxidizes therapeutic drugs such as taxol and cerivastatin and endobiotics such as retinoic acid and arachidonic acid. Consistent with the relatively large size of its preferred substrates, the active site volume is twice that observed for the structure of CYP2C5. The extended active site cavity is bounded by the beta1 sheet and helix F' that have not previously been implicated in substrate recognition by mammalian P450s. CYP2C8 crystallized as a symmetric dimer formed by the interaction of helices F, F', G', and G. Two molecules of palmitic acid are bound in the dimer interface. The dimer is observed in solution, and mass spectrometry confirmed the association of palmitic acid with the enzyme. This novel finding identifies a peripheral binding site in P450s that may contribute to drug-drug interactions in P450 metabolism.

Fluorescence resonance energy transfer analysis of cytochromes P450 2C2 and 2E1 molecular interactions in living cells

The molecular organization of microsomal cytochromes P450 (P450s) and formation of complexes with P450 reductase have been studied previously with isolated proteins and in reconstituted systems. Although these studies demonstrated that some P450s oligomerize in vitro, neither oligomerization nor interactions of P450 with P450 reductase have been studied in living cells. Here we have used fluorescence resonance energy transfer (FRET) to study P450 oligomerization and binding to P450 reductase in live transfected cells. Cytochrome P450 2C2, but not P450 2E1, forms homo-oligomeric structures, and this self-association is mediated by the signal-anchor sequence. Because P450 2C2, in contrast to P450 2E1, is directly retained in the endoplasmic reticulum (ER), these results could suggest that oligomerization may prevent transport from the ER. However, P450 2C1 signal-anchor sequence chimera defective in ER retention also formed oligomers, and chimera containing the cytoplasmic domain of P450 2C2, which is directly retained in the ER, did not exhibit self-oligomerization, which indicates that oligomerization is not correlated with direct retention. By using FRET, we have also detected binding of P450 2C2 and P450 2E1 to P450 reductase. In contrast to self-oligomerization, the catalytic domain can mediate an interaction of P450 2C2 with P450 reductase. These results suggest that microsomal P450s may differ in their quaternary structure but that these differences do not detectably affect interaction with the reductase or transport from the ER.

Organization of multiple cytochrome P450s with NADPH-cytochrome P450 reductase in membranes

Microsomal P450-mediated monooxygenase activity supported by NADPH requires an interaction between flavoprotein NADPH-cytochrome P450 reductase and cytochrome P450. These proteins have been identified as the simplest system (with the inclusion of a phospholipid (PL) component) that possesses monooxygenase function; however, little is known about the organization of these proteins in the microsomal membrane. Although reductase and P450 are known to form a 1:1 functional complex, there exists a 10- to 20-fold excess of P450 over the reductase. This raises several questions including "How are the enzymes of the P450 system organized in the microsomal membrane?" and "Can one P450 enzyme affect the functional characteristics of another P450?" This review summarizes evidence supporting the potential for enzymes involved in the P450 system to interact, focusing on the interactions between reductase and P450 and interactions between multiple P450 enzymes. Studies on the aggregation characteristics of P450 as well as on rotational diffusion are detailed, with a special emphasis on the potential for P450 enzymes to produce oligomeric complexes and to suggest the environment in which P450 exists in the endoplasmic reticulum. Finally, more recent studies describing the potential for multiple P450s to exist as complexes and their effect on P450 function are presented, including studies using reconstituted systems as well as systems where two P450s are coexpressed in the presence of reductase. An understanding of the interactions among reductase and multiple P450s is important for predicting conditions where the drug disposition may be altered by the direct effects of P450-P450 complex formation. Furthermore, the potential for one P450 enzyme to affect the behavior of another P450 may be extremely important for drug screening and development, requiring metabolic screening of a drug with reconstituted systems containing multiple P450s rather than simpler systems containing only a single form.

Effect of microsome-liposome fusion on the rotational mobility of cytochrome P450IIB4 in rabbit liver microsomes

Membrane fusion of microsomes with soybean phospholipid vesicles was performed at pH 6.5 to investigate the effect of lipid-enrichment in the membrane on the rotational mobility of cytochrome P450. Rotational diffusion of cytochrome P450 in the microsomal membrane of phenobarbital-induced rabbit liver was measured by detecting the decay of absorption anisotropy after photolysis of the heme CO complex by a vertically polarized laser flash. The fusion procedures yielded three separate fractions upon sucrose density gradient centrifugation with lipid-to-protein ratio in weight (L/P) as follows: 1.5 in the bottom fraction, 2.2 in the middle fraction, and 3.9 in the top fraction. In each fraction, co-existence of mobile and immobile cytochrome P450 was observed. The percentage of rotationally mobile P450 (with the mean rotational relaxation time of phi=505-828 micros) in each of the different bands was found to be 59% in the bottom fraction, 61% in the middle fraction, and 68% in the top fraction. This increase in mobile population of P450 due to lipid-enrichment indicates that aggregated proteins in microsomal membranes dissociate with increasing L/P which is inversely proportional to the protein concentration in the membrane. With freeze-fracture electron microscopy, it was shown that the average distance increased between intramembrane particles by lipid-enrichment. Thus, the significant immobile population (32%) of P450 in microsomal membranes can be explained by nonspecific protein aggregation which is a consequence of the low L/P of 0.8. The decrease in the mobile population in the bottom fraction compared with intact microsomes was shown to be due to the pH 6.5 incubation used for fusion.

Real-time fluorescence analysis on molecular mechanisms for regulation of cytochrome P450scc activity upon steroidogenic stimulation in adrenocortical cells

Real-time fluorescence analysis revealed that the activity of cytochrome P450scc was related to Ca2+ signals arising from extracellular NADPH, ACTH and ATP stimulation in adrenocortical fasciculata cells. The side-chain cleavage reaction by cytochrome P450scc was measured with 3beta-hydroxy-22,23-bisnor-5-cholenyl ether (cholesterol-resorufin) by observing the distinct increase in fluorescence upon conversion of cholesterol-resorufin to resorufin and pregnenolone. Adrenocorticotropic hormone (ACTH) induced a relatively small stimulation of the P450scc activity. A significant production of resorufin was revealed after stimulation of cell cultures with 100 pM, 1 nM of ACTH for 3 h. On the other hand, extracellular NADPH was found to rapidly and greatly stimulate the resorufin production in intact cells immediately after the addition of 50-500 microM NADPH. The extracellular NADPH stimulation was prevented by the addition of thapsigargin and EGTA which abolished Ca2+ oscillations induced by NADPH. Suramin, a specific antagonist of the P2y type ATP receptor, also completely abolished the NADPH-induced cholesterol-resorufin conversion. These results imply that extracellular NADPH (membrane impermeable) produced Ca2+ oscillations through its binding to ATP receptor thereby stimulating the activity of P450scc. The application of 45-500 microM extracellular ATP to cells did not, however, significantly increase the resorufin production. These three stimulators produced very different types of Ca2+ signals. ACTH induced mainly a series of Ca2+ spikes superimposed on a long-lasting basal Ca2+ elevation. The Ca2+ signals induced by NADPH showed predominantly a series of Ca2+ spikes without elevation of the basal Ca2+ concentration. Only long-lasting Ca2+ elevation was induced by extracellular ATP. The stimulation of cytochrome P450scc may thus be correlated with the different patterns of Ca2+ signals.