Photoaffinity labeling of retinoic acid-binding proteins

Identification of Apolipoprotein A-I as a Retinoic Acid-binding Protein in the Eye

All-trans-retinoic acid may be an important molecular signal in the postnatal control of eye size. The goal of this study was to identify retinoic acid-binding proteins secreted by the choroid and sclera during visually guided ocular growth. Following photoaffinity labeling with all-trans-[11,12-(3)H]retinoic acid, the most abundant labeled protein detected in the conditioned medium of choroid or sclera had an apparent Mr of 27,000 Da. Following purification and mass spectrometry, the Mr 27,000 band was identified as apolipoprotein A-I. Affinity capture of the radioactive Mr 27,000 band by anti-chick apolipoprotein A-I antibodies confirmed its identity as apolipoprotein A-I. Photoaffinity labeling and fluorescence quenching experiments demonstrated that binding of retinoic acid to apolipoprotein A-I is 1) concentration-dependent, 2) selective for all-trans-retinoic acid, and 3) requires the presence of apolipoprotein A-I-associated lipids for retinoid binding. Expression of apolipoprotein A-I mRNA and protein synthesis were markedly up-regulated in choroids of chick eyes during the recovery from induced myopia, and apolipoprotein A-I mRNA was significantly increased in choroids following retinoic acid treatment. Together, these data suggest that apolipoprotein A-I may participate in a regulatory feedback mechanism with retinoic acid to control the action of retinoic acid on ocular targets during postnatal ocular growth.

Active Pin1 is a key target of all-trans retinoic acid in acute promyelocytic leukemia and breast cancer

A common key regulator of oncogenic signaling pathways in multiple tumor types is the unique isomerase Pin1. However, available Pin1 inhibitors lack the required specificity and potency for inhibiting Pin1 function in vivo. By using mechanism-based screening, here we find that all-trans retinoic acid (ATRA)--a therapy for acute promyelocytic leukemia (APL) that is considered the first example of targeted therapy in cancer, but whose drug target remains elusive--inhibits and degrades active Pin1 selectively in cancer cells by directly binding to the substrate phosphate- and proline-binding pockets in the Pin1 active site. ATRA-induced Pin1 ablation degrades the protein encoded by the fusion oncogene PML-RARA and treats APL in APL cell and animal models as well as in human patients. ATRA-induced Pin1 ablation also potently inhibits triple-negative breast cancer cell growth in human cells and in animal models by acting on many Pin1 substrate oncogenes and tumor suppressors. Thus, ATRA simultaneously blocks multiple Pin1-regulated cancer-driving pathways, an attractive property for treating aggressive and drug-resistant tumors.

Photoaffinity labeling of plasma proteins

Photoaffinity labeling is a powerful technique for identifying a target protein. A high degree of labeling specificity can be achieved with this method in comparison to chemical labeling. Human serum albumin (HSA) and α₁-acid glycoprotein (AGP) are two plasma proteins that bind a variety of endogenous and exogenous substances. The ligand binding mechanism of these two proteins is complex. Fatty acids, which are known to be transported in plasma by HSA, cause conformational changes and participate in allosteric ligand binding to HSA. HSA undergoes an N-B transition, a conformational change at alkaline pH, that has been reported to result in increased ligand binding. Attempts have been made to investigate the impact of fatty acids and the N-B transition on ligand binding in HSA using ketoprofen and flunitrazepam as photolabeling agents. Meanwhile, plasma AGP is a mixture of genetic variants of the protein. The photolabeling of AGP with flunitrazepam has been utilized to shed light on the topology of the protein ligand binding site. Furthermore, a review of photoaffinity labeling performed on other major plasma proteins will also be discussed. Using a photoreactive natural ligand as a photolabeling agent to identify target protein in the plasma would reduce non-specific labeling.

Spatiotemporal manipulation of retinoic acid activity in zebrafish hindbrain development via photo-isomerization

All-trans retinoic acid (RA) is a key player in many developmental pathways. Most methods used to study its effects in development involve continuous all-trans RA activation by incubation in a solution of all-trans RA or by implanting all-trans RA-soaked beads at desired locations in the embryo. Here we show that the UV-driven photo-isomerization of 13-cis RA to the trans-isomer (and vice versa) can be used to non-invasively and quantitatively control the concentration of all-trans RA in a developing embryo in time and space. This facilitates the global or local perturbation of developmental pathways with a pulse of all-trans RA of known concentration or its inactivation by UV illumination. In zebrafish embryos in which endogenous synthesis of all-trans RA is impaired, incubation for as little as 5 minutes in 1 nM all-trans RA (a pulse) or 5 nM 13-cis RA followed by 1-minute UV illumination is sufficient to rescue the development of the hindbrain if performed no later than bud stage. However, if subsequent to this all-trans RA pulse the embryo is illuminated (no later than bud stage) for 1 minute with UV light (to isomerize, i.e. deactivate, all-trans RA), the rescue of hindbrain development is impaired. This suggests that all-trans RA is sequestered in embryos that have been transiently exposed to it. Using 13-cis RA isomerization with UV light, we further show that local illumination at bud stage of the head region (but not the tail) is sufficient to rescue hindbrain formation in embryos whose all-trans RA synthetic pathway has been impaired.

Influence of all-trans-retinoic acid on oxoglutarate carrier via retinoylation reaction

All-trans-retinoic acid (atRA), an activated metabolite of vitamin A, is incorporated covalently into proteins both invivo and invitro. AtRA reduced the transport activity of the oxoglutarate carrier (OGC) isolated from testes mitochondria to 58% of control via retinoylation reaction. Labeling of testes mitochondrial proteins with (3)HatRA demonstrated the binding of atRA to a 31.5 KDa protein. This protein was identified as OGC due to the competition for the labeling reaction with 2-oxoglutarate, the specific OGC substrate. The role of retinoylated proteins is currently being explored and here we have the first evidence that retinoic acids bind directly to OGC and inhibit its activity in rat testes mitochondria via retinoylation reaction. This study indicates the evidence of a specific interaction between atRA and OGC and establishes a novel mechanism for atRA action, which could influence the physiological biosynthesis of testosterone in situations such as retinoic acid treatment.

Rhodopsin formation in Drosophila is dependent on the PINTA retinoid-binding protein

Retinoids participate in many essential processes including the initial event in photoreception. 11-cis-retinal binds to opsin and undergoes a light-driven isomerization to all-trans-retinal. In mammals, the all-trans-retinal is converted to vitamin A (all-trans-retinol) and is transported to the retinal pigment epithelium (RPE), where along with dietary vitamin A, it is converted into 11-cis-retinal. Although this cycle has been studied extensively in mammals, many questions remain, including the specific roles of retinoid-binding proteins. Here, we establish the Drosophila visual system as a genetic model for characterizing retinoid-binding proteins. In a genetic screen for mutations that affect the biosynthesis of rhodopsin, we identified a novel CRAL-TRIO domain protein, prolonged depolarization afterpotential is not apparent (PINTA), which binds to all-trans-retinol. We demonstrate that PINTA functions subsequent to the production of vitamin A and is expressed and required in the retinal pigment cells. These results represent the first genetic evidence for a role for the retinal pigment cells in the visual response. Moreover, our data implicate Drosophila retinal pigment cells as functioning in the conversion of dietary all-trans-retinol to 11-cis-retinal and suggest that these cells are the closest invertebrate equivalent to the RPE.

The discovery and mechanism of action of novel tumor-selective and apoptosis-inducing 3,5-diaryl-1,2,4-oxadiazole series using a chemical genetics approach

A novel series of 3,5-diaryl-oxadiazoles was identified as apoptosis-inducing agents through our cell and chemical genetics-based screening assay for compounds that induce apoptosis using a chemical genetics approach. Several analogues from this series including MX-74420 and MX-126374 were further characterized. MX-126374, a lead compound from this series, was shown to induce apoptosis and inhibit cell growth selectively in tumor cells. To elucidate the mechanism(s) by which this class of compounds alters the signal transduction pathway that ultimately leads to apoptosis, expression profiling using the Affymetrix Gene Chip array technology was done along with other molecular and biochemical analyses. Interestingly, we have identified several key genes (cyclin D1, transforming growth factor-beta1, p21, and insulin-like growth factor-BP3) that are altered in the presence of this compound, leading to characterization of the pathway for activation of apoptosis. MX-126374 also showed significant inhibition of tumor growth as a single agent and in combination with paclitaxel in murine tumor models. Using photoaffinity labeling, tail-interacting protein 47, an insulin-like growth factor-II receptor binding protein, was identified as the molecular target. Further studies indicated that down-regulation of tail-interacting protein 47 in cancer cells by small interfering RNA shows a similar pathway profile as compound treatment. These data suggest that 3,5-diaryl-oxadiazoles may be a new class of anticancer drugs that are tumor-selective and further support the discovery of novel drugs and drug targets using chemical genetic approaches.

Structure of UDP‐Glucuronosyltransferases in Membranes

This chapter presents the most recent experimental approaches to the investigation of UDP-glucuronosyltransferase (UGTs) in membranes. The first topic described is the subcellular localization of UGTs with special emphasis on the association of these proteins with the endoplasmic reticulum (ER). Experimental methods include subfractionation of tissue for microsome preparation, evaluation of the purity of the membrane fraction obtained, and measurement of UGT activity in the presence of detergents. Next, the recently demonstrated formation of UGT homo- and heterodimer formation and its functional relevance is discussed and the appropriate methods used to characterize such interactions are given (radiation inactivation, size exclusion chromatography, immunopurification, cross-linking, two-hybrid system). The structural determinants of UGTs in relation to membrane association, residency, and enzymatic activity are the next topic, supplemented by a description of the appropriate methods, including the design and expression of chimeric proteins, membrane insertion, and subcellular localization by immunofluorescence. Also presented is new information on the structure and function of UGTs obtained by molecular modeling, bioinformatics (sequence alignment), and comparison with selected crystallized glycosyltransferases. Finally, we discuss the important, and still not fully developed, issue of UGT active site architecture and organization within the ER. This is addressed from two perspectives: (1) chemical modification of UGT active sites by amino acid-specific probes and (2) photoaffinity labeling of UGTs. The detailed synthesis of a photoaffinity probe for an aglycon-binding site is provided and the use of this probe and direct photoaffinity labeling with retinoids is discussed. The application of proteomics techniques, including proteolytic digestion and protein sequencing by liquid chromatography/tandem mass spectrometry and matrix-assisted laser desorption ionization/time of flight, to the identification of crucial amino acids of the active sites, and subsequent site-directed mutagenesis of identified amino acids, is discussed in detail.

Identification of CRALBP Ligand Interactions by Photoaffinity Labeling, Hydrogen/Deuterium Exchange, and Structural Modeling

Cellular retinaldehyde-binding protein (CRALBP) functions in the retinal pigment epithelium (RPE) as an acceptor of 11-cis-retinol in the isomerization step of the rod visual cycle and as a substrate carrier for 11-cis-retinol dehydrogenase. Toward a better understanding of CRALBP function, the ligand binding cavity in human recombinant CRALBP (rCRALBP) was characterized by photoaffinity labeling with 3-diazo-4-keto-11-cis-retinal and by high resolution mass spectrometric topological analyses. Eight photoaffinity-modified residues were identified in rCRALBP by liquid chromatography tandem mass spectrometry, including Tyr(179), Phe(197), Cys(198), Met(208), Lys(221), Met(222), Val(223), and Met(225). Multiple different adduct masses were found on the photolabeled residues, and the molecular identity of each modification remains unknown. Supporting the specificity of photo-labeling, 50% of the modified residues have been associate with retinoid interactions by independent analyses. In addition, topological analysis of apo- and holo-rCRALBP by hydrogen/deuterium exchange and mass spectrometry demonstrated residues 198-255 incorporate significantly less deuterium when the retinoid binding pocket is occupied with 11-cis-retinal. This hydrophobic region encompasses all but one of the photo-labeled residues. A structural model of CRALBP ligand binding domain was constructed based on the crystal structures of three homologues in the CRAL-TRIO family of lipid-binding proteins. In the model, all of the photolabeled residues line the ligand binding cavity except Met(208), which appears to reside in a flexible loop at the entrance/exit of the ligand cavity. Overall, the results expand to 12 the number of residues proposed to interact with ligand and provide further insight into CRALBP ligand and protein interactions.

All-trans-retinoic acid binds to and inhibits adenine nucleotide translocase and induces mitochondrial permeability transition

We investigated the effects of retinoic acids on mitochondrial permeability transition (MPT) measured as changes in rhodamine 123 fluorescence from both isolated heart mitochondria and HeLa cells. We report that all-trans-retinoic acid (atRA), 9-cis-retinoic acid, and 13-cis-retinoic acid induce a drop in mitochondrial membrane potential in isolated mitochondria. The atRA effect was done through the induction of MPT because it was dependent on Ca(2+), in a synergic mechanism, and inhibited by cyclosporin A (CsA). Furthermore, atRA also opened MPT in vivo, because treatment of HeLa cells with atRA results in a CsA-sensitive drop of mitochondrial membrane potential. We demonstrated for the first time that retinoic acids inhibit adenine nucleotide translocase (ANT) activity in heart and liver mitochondria. Kinetic studies revealed atRA as an uncompetitive inhibitor of ANT. Photoaffinity labeling of mitochondrial proteins with [3H]atRA demonstrated the binding of a 31-kDa protein to atRA. This protein was identified as ANT because the presence of carboxyatractyloside, a specific ANT inhibitor, prevented labeling. The specific photolabeling of ANT was also prevented in a concentration-dependent manner by nonlabeled atRA, whereas palmitic acid was ineffective. This study indicates that specific interaction between atRA and ANT takes place regulating MPT opening and adenylate transport. These observations establish a novel mechanism for atRA action, which could control both energetic and apoptotic mitochondrial processes in situations such as retinoic acid treatment.

Photoaffinity labeling of the uncoupling protein UCP1 with retinoic acid: ubiquinone favors binding

Retinoic acid is a potent activator of the uncoupling protein-1 (UCP1) both at the gene and mitochondrial level. Irradiation with ultraviolet light can be used to directly photolabel proteins with retinoic acid. The procedure has been applied to investigate its interaction with UCP1 isolated from brown adipose tissue mitochondria. All-trans-retinoic acid binds to UCP1 with high affinity and the labeling is only partially protected by guanosine diphosphate. Ubiquinone (UQ) has been described to be an obligatory cofactor for uncoupling protein function and we demonstrate that it greatly increases the affinity of UCP1 for retinoic acid. Data support the notion of a direct interaction between UQ and retinoic acid.

Binding of retinoic acid by the inhibitory serpin protein C inhibitor

The serpin superfamily includes inhibitors of serine proteases and noninhibitory members with other functions (e.g. the hormone precursor angiotensinogen and the hormone carriers corticosteroid-binding globulin and thyroxine-binding globulin). It is not known whether inhibitory serpins have additional, noninhibitory functions. We studied binding of (3)H-labeled hydrophobic hormones (estradiol, progesterone, testosterone, cortisol, aldosterone, and all-trans-retinoic acid) to the inhibitory serpins antithrombin III, heparin cofactor II, plasminogen activator inhibitor-1, and protein C inhibitor (PCI). All-trans-[(3)H]retinoic acid bound in a specific dose-dependent and time-dependent way to PCI (apparent K(d) = 2.43 microm, 0.8 binding sites per molecule of PCI). We did not observe binding of other hormones to serpins. Intact and protease-cleaved PCI bound retinoic acid equally well, and retinoic acid did not influence inhibition of tissue kallikrein by PCI. Gel filtration confirmed binding of retinoic acid to PCI in purified systems and suggested that PCI may also function as a retinoic acid-binding protein in seminal plasma. Therefore, our present data, together with the fact that PCI is abundantly expressed in tissues requiring retinoic acid for differentiation processes (e.g. the male reproductive tract, epithelia in various organs), suggest an additional biological role for PCI as a retinoic acid-binding and/or delivering serpin.

Photoaffinity labeling of human IRBP with all-trans-retinoic acid

Interphotoreceptor retinoid-binding protein (IRBP), found only in photosensitive tissues, is a large approximately 135-kDa glycoprotein that contains a fourfold repeat structure. IRBP may function as a buffer and prevent retinoid toxicity and retinoid degeneration. Here we asked (i) whether each repeat of IRBP possesses the capability of photo-crosslinking all-trans-retinoic acid (RA), (ii) within Repeat 1 whether a single retinoic acid-binding domain exists, and (iii) whether protease and CNBr digestion of Repeat 1 bound RA indicate the exact location of the binding site. 3H-RA cross-linked to all four repeats, consistent with the current model of multiple binding sites in IRBP. Acetone precipitation was effective in removing unbound 3H-RA. LysC and tryptic digestion of the RA-Repeat 1 detected 18- and 5-kDa bands, respectively. CNBr digestion showed two bands about 9 and 11 kDa in size. Our data suggests a single binding site near positions 151-160 in the center of Repeat 1.

Application of photoaffinity labeling with [(3)H] all trans- and 9-cis-retinoic acids for characterization of cellular retinoic acid--binding proteins I and II

Cellular retinoic acid-binding proteins (CRABPs) are carrier proteins thought to play a crucial role in the transport and metabolism of all-trans-retinoic acid (atRA) and its derivatives within the cell. This report describes a novel photoaffinity-based binding assay involving competition between potential ligands of CRABP and [(3)H]atRA or [(3)H]-9-cis-RA for binding to the atRA-binding sites of CRABP I and II. Photoaffinity labeling of purified CRABPs with [(3)H]atRA was light- and concentration-dependent, saturable, and protected by several retinoids in a concentration-dependent manner, indicating that binding occurred in the CRABP atRA-binding site. Structure-function relationship studies demonstrated that oxidative changes to the atRA beta-ionone ring did not affect ligand potency. However, derivatives lacking a terminal carboxyl group and some cis isomers did not bind to CRABPs. These studies also identified two novel ligands for CRABPs: 5,6-epoxy-RA and retinoyl-beta-D-glucuronide (RAG). The labeling of both CRABPs with 9-cis-RA occurred with much lower affinity. Experimental evidence excluded nonspecific binding of RAG to CRABPs and UDP-glucuronosyltransferases, the enzymes responsible for RAG synthesis. These results established that RAG is an effective ligand of CRABPs. Therefore, photoaffinity labeling with [(3)H]atRA can be used to identify new ligands for CRABP and retinoid nuclear receptors and also provide information concerning the identity of amino acid(s) localized in the atRA-binding site of these proteins.

Direct interaction of all-trans-retinoic acid with protein kinase C (PKC). Implications for PKC signaling and cancer therapy

Protein kinase C (PKC) regulates fundamental cellular functions including proliferation, differentiation, tumorigenesis, and apoptosis. All-trans-retinoic acid (atRA) modulates PKC activity, but the mechanism of this regulation is unknown. Amino acid alignments and crystal structure analysis of retinoic acid (RA)-binding proteins revealed a putative atRA-binding motif in PKC, suggesting existence of an atRA binding site on the PKC molecule. This was supported by photolabeling studies showing concentration- and UV-dependent photoincorporation of [(3)H]atRA into PKCalpha, which was effectively protected by 4-OH-atRA, 9-cis-RA, and atRA glucuronide, but not by retinol. Photoaffinity labeling demonstrated strong competition between atRA and phosphatidylserine (PS) for binding to PKCalpha, a slight competition with phorbol-12-myristate-13-acetate, and none with diacylglycerol, fatty acids, or Ca(2+). At pharmacological concentrations (10 micrometer), atRA decreased PKCalpha activity through the competition with PS but not phorbol-12-myristate-13-acetate, diacylglycerol, or Ca(2+). These results let us hypothesize that in vivo, pharmacological concentrations of atRA may hamper binding of PS to PKCalpha and prevent PKCalpha activation. Thus, this study provides the first evidence for direct binding of atRA to PKC isozymes and suggests the existence of a general mechanism for regulation of PKC activity during exposure to retinoids, as in retinoid-based cancer therapy.

Choroidal retinoic acid synthesis: a possible mediator between refractive error and compensatory eye growth

Research over the past two decades has shown that the growth of young eyes is guided by vision. If near- or far-sightedness is artificially imposed by spectacle lenses, eyes of primates and chicks compensate by changing their rate of elongation, thereby growing back to the pre-lens optical condition. Little is known about what chemical signals might mediate between visual effects on the retina and alterations of eye growth. We present five findings that point to choroidal retinoic acid possibly being such a mediator. First, the chick choroid can convert retinol into all-trans-retinoic acid at the rate of 11 +/- 3 pmoles mg protein(-1) hr(-1), compared to 1.3 +/- 0.3 for retina/RPE and no conversion for sclera. Second, those visual conditions that cause increased rates of ocular elongation (diffusers or negative lens wear) produce a sharp decrease in all-trans-retinoic acid synthesis to levels barely detectable with our assay. In contrast, visual conditions which result in decreased rates of ocular elongation (recovery from diffusers or positive lens wear) produce a four- to five-fold increase in the formation of all-trans-retinoic acid. Third, the choroidal retinoic acid is found bound to a 28-32 kD protein. Fourth, a large fraction of the choroidal retinoic acid synthesized in culture is found in a nucleus-enriched fraction of sclera. Finally, application of retinoic acid to cultured sclera at physiological concentrations produced an inhibition of proteoglycan production (as assessed by measuring sulfate incorporation) with a EC50 of 8 x 10(-7) M. These results show that the synthesis of choroidal retinoic acid is modulated by those visual manipulations that influence ocular elongation and that this retinoic acid may reach the sclera in concentrations adequate to modulate scleral proteoglycan formation.

Structural and functional studies of UDP-glucuronosyltransferases

UDP-Glucuronosyltransferases (UGTs) are glycoproteins localized in the endoplasmic reticulum (ER) which catalyze the conjugation of a broad variety of lipophilic aglycon substrates with glucuronic acid using UDP-glucuronic acid (UDP-GIcUA) as the sugar donor. Glucuronidation is a major factor in the elimination of lipophilic compounds from the body. In this review, current information on the substrate specificities of UGT1A and 2B family isoforms is discussed. Recent findings with regard to UGT structure and topology are presented, including a dynamic topological model of UGTs in the ER. Evidence from experiments on UGT interactions with inhibitors directed at specific amino acids, photoaffinity labeling, and analysis of amino acid alignments suggest that UDP-GIcUA interacts with residues in both the N- and C-terminal domains, whereas aglycon binding sites are localized in the N-terminal domain. The amino acids identified so far as crucial for substrate binding and catalysis are arginine, lysine, histidine, proline, and residues containing carboxylic acid. Site-directed mutagenesis experiments are critical for unambiguous identification of the active-site architecture.

Mannose-6-phosphate/insulin-like growth factor-II receptor is a receptor for retinoic acid

Retinoic acid (RA) exerts diverse biological effects in the control of cell growth in embryogenesis and oncogenesis. These effects of RA are thought to be mediated by the nuclear retinoid receptors. Mannose-6-phosphate (M6P)/insulin-like growth factor-II (IGF-II) receptor is a multifunctional membrane glycoprotein that is known to bind both M6P and IGF-II and function primarily in the binding and trafficking of lysosomal enzymes, the activation of transforming growth factor-beta, and the degradation of IGF-II. M6P/IGF-II receptor has recently been implicated in fetal development and carcinogenesis. Despite the functional similarities between RA and the M6P/IGF-II receptor, no direct biochemical link has been established. Here, we show that the M6P/IGF-II receptor also binds RA with high affinity at a site that is distinct from those for M6P and IGF-II, as identified by a photoaffinity labeling technique. We also show that the binding of RA to the M6P/IGF-II receptor enhances the primary functions of this receptor. The biological consequence of the interaction appears to be the suppression of cell proliferation and/or induction of apoptosis. These findings suggest that the M6P/IGF-II receptor mediates a RA response pathway that is important in cell growth regulation. This discovery of the interaction of RA with the M6P/IGF-II receptor may have important implications for our understanding of the roles of RA and the M6P/IGF-II receptor in development, carcinogenesis, and lysosomal enzyme-related diseases.

Application of photoaffinity labeling with [11,12-3H]all-trans-retinoic acid to characterization of rat liver microsomal UDP-glucuronosyltransferase(s) with activity toward retinoic acid

[3H]All-trans-retinoic acid has been shown to be an effective photoaffinity label for microsomal UDP-glucuronosyltransferases. Labeling of rat liver microsomal proteins with [3H]all-trans-retinoic acid and [32P]5-azido-UDP-glucuronic acid has shown that at least one protein in the 50-56 kDa mass range encompassing the UDP-glucuronosyltransferases photoincorporated both probes. The fraction of solubilized microsomal protein eluted from a UDP-hexanolamine affinity column with 50 microM UDP-glucuronic acid contained two protein bands, both of which photoincorporated [3H] all-trans-retinoic acid and were detected on Western blot by anti-UDP-glucuronosyltransferase antibodies. Enzymatic glucuronidation activity toward atRA in the same fraction was enriched five-fold over that of native or solubilized microsomes.