Linoleic acid in diets of mice increases total endocannabinoid levels in bowel and liver: modification by dietary glucose

AIM

Linoleic acid (LA) is an essential fatty acid involved in the biosynthesis of arachidonic acid and prostaglandins. LA is known to induce obesity and insulin resistance. In this study, two concentrations of LA with or without added glucose (G) were fed to mice to investigate their effects on endocannabinoid (EC) biology.

MATERIALS AND METHODS

Four groups of C57BL/6 mice were provided with diets containing 1% or 8% LA with or without added G (LAG) for 8 weeks. Body weights, food intake, circulating glucose and insulin levels were measured throughout the study. Following euthanasia, plasma, bowel and hepatic ECs, monoacylglycerol lipase and fatty acid amide hydroxylase protein levels (enzymes responsible for EC degradation) and transcriptional activity of PPARα in liver were quantified. Liver was probed for evidence of insulin receptor activity perturbation.

RESULTS

Increasing dietary LA from 1% to 8% significantly increased circulating, small bowel and hepatic ECs. 1%LAG fed mice had lowest feed efficiency, and only liver levels of both ECs were reduced by addition of G. Addition of G to 1% LA diets resulted in elevated monoacylglycerol lipase and fatty acid amide hydroxylase protein levels (p < 0.001 and p < 0.001, respectively) in liver due to increased transcriptional activity of PPARα (p < 0.05). The reduced EC levels with addition of G also correlated with a measure of enhanced insulin action.

CONCLUSION

In conclusion, body weight of mice is influenced by the source of calorie intake. Furthermore, tissue EC/g are dependent on tissue-specific synthesis and degradation that are modulated by dietary LA and G which also influence food efficiency, and down-stream insulin signalling pathways. The findings could potentially be useful information for weight management efforts in humans.

Subchronic administration of (R,S)-ketamine induces ketamine ring hydroxylation in Wistar rats

Subchronic administration of (R,S)-ketamine, (R,S)-Ket, is used in the treatment of neuropathic pain, in particular Complex Regional Pain Syndrome, but the effect of this protocol on the metabolism of (R,S)-Ket is unknown. In this study, daily administration of a low dose of (R,S)-Ket for 14-days to Wistar rats was conducted to determine the impact of sub-chronic dosing on the pharmacokinetics of (R,S)-Ket and its major metabolites. The data indicate that, relative to a single administration of (R,S)-Ket, subchronic administration resulted in increased clearance of (R,S)-Ket and the N-demethylated metabolite norketamine measured as elimination half-life (t1/2) and decreased plasma concentrations of these compounds. Subchronic administration produced a slight decrease in t1/2 and an increase in plasma concentration of the major metabolite, (2S,6S;2R,6R)-hydroxynorketamine, and produced significant increases in the plasma concentrations of the (2S,6R;2R,6S)-hydroxynorketamine and (2S,4R;2R,4S)-hydroxynorketamine metabolites. The metabolism of (R,S)-Ket predominately occurs via two microsomal enzyme-mediated pathways: (R,S)-Ket⇒(R,S)-norketamine⇒(2S,6S;2R,6R)-hydroxynorketamine and (2S,4R;2R,4S)-hydroxynorketamine and the (R,S)-Ket⇒(2S,6R;2R,6S)-hydroxyketamine⇒(2S,6R;2R,6S)-hydroxynorketamine and (2S,6S;2R,6R)-hydroxynorketamine. The results indicate that the activity of both metabolic pathways are increased by subchronic administration of (R,S)-Ket producing new metabolite patterns and potential differences in clinical effects.

Development and characterization of the α3β4α5 nicotinic receptor cellular membrane affinity chromatography column and its application for on line screening of plant extracts

The α3β4α5 nAChR has been recently shown to be a useful target for smoking cessation pharmacotherapies. Herein, we report on the development and characterization of the α3β4α5 nicotinic receptor column by frontal displacement chromatography. The binding affinity of the nicotine and minor alkaloids found in tobacco smoke condensates were determined for both the α3β4 and α3β4α5 nicotinic receptors. It was demonstrated that while no subtype selectivity was observed for nicotine and nornicotine, anabasine was selective for the α3β4α5 nicotinic receptor. The non-competitive inhibitor binding site was also studied and it was demonstrated while mecamylamine was not selective between subtypes, buproprion showed subtype selectivity for the α3β4 nicotinic receptor. The application of this methodology to complex mixtures was then carried out by screening aqueous-alcoholic solutions of targeted plant extracts, including Lycopodium clavatum L. (Lycopodiaceae) and Trigonella foenum graecum L. (Fabaceae) against both the α3β4 and α3β4α5 nAChRs.

The development of mitochondrial membrane affinity chromatography columns for the study of mitochondrial transmembrane proteins

Mitochondrial membrane fragments from U-87 MG (U87MG) and HEK-293 cells were successfully immobilized onto immobilized artificial membrane (IAM) chromatographic support and surface of activated open tubular (OT) silica capillary, resulting in mitochondrial membrane affinity chromatography (MMAC) columns. Translocator protein (TSPO), located in mitochondrial outer membrane as well as sulfonylurea and mitochondrial permeability transition pore (mPTP) receptors, localized to the inner membrane, were characterized. Frontal displacement experiments with multiple concentrations of dipyridamole (DIPY) and PK-11195 were run on MMAC (U87MG) column, and the binding affinities (Kd) determined were 1.08±0.49 and 0.0086±0.0006μM, respectively, consistent with previously reported values. Furthermore, binding affinities (Ki) for DIPY binding site were determined for TSPO ligands, PK-11195, mesoporphyrin IX, protoporphyrin IX, and rotenone. In addition, the relative ranking of these TSPO ligands based on single displacement studies using DIPY as marker on MMAC (U87MG) was consistent with the obtained Ki values. The immobilization of mitochondrial membrane fragments was also confirmed by confocal microscopy.

The identification of a novel SIRT6 modulator from Trigonella foenum-graecum using ligand fishing with protein coated magnetic beads

SIRT6 is a histone deacetylase that has been proposed as a potential therapeutic target for metabolic disorders and the prevention of age-associated diseases. Thus the identification of compounds that modulate SIRT6 activity could be of great therapeutic importance. We have previously developed an H3K9 deacetylation guided assay with SIRT6 coated magnetic beads (SIRT6-MB). With the developed assay, we identified quercetin, naringenin and vitexin as SIRT6 inhibitors from T. foenum-graecum seed extract using a candidate approach. Currently, the predominant method for the identification of active compounds from a plant extract is carried out through a dereplication process. A novel targeted approach for the direct identification of active compounds from a complex matrix could save time and resources. Herein, we report the application of the SIRT6-MB for 'fishing' experiments utilizing T. foenum-graecum seed extract. In which orientin, and seventeen other compounds were identified as SIRT6 binders. This is the first use of this method for 'fishing' out active ligands from a botanical matrix, and sets the basis for the identification of active compounds from a complex matrix.

Resveratrol and its metabolites bind to PPARs

Resveratrol, a modulator of several signaling proteins, can exert off-target effects involving the peroxisome proliferator-activated receptor (PPAR) transcription factors. However, evidence for the direct interaction between this polyphenol and PPARs is lacking. Here, we addressed the hypothesis that resveratrol and its metabolites control aspects of PPAR transcriptional activity through direct interaction with PPARs. Bioaffinity chromatographic studies with the immobilized ligand-binding domains (LBDs) of PPARγ and PPARα and isothermal titration calorimetry allowed the binding affinities of resveratrol, resveratrol 3-O-glucuronide, resveratrol 4-O-glucuronide, and resveratrol 3-O-sulfate to both PPAR-LBDs to be determined. Interaction of resveratrol, resveratrol 3-O-glucuronide, and resveratrol 4-O-glucuronide with PPARγ-LBD occurred with binding affinities of 1.4, 1.1, and 0.8 μM, respectively, although only resveratrol bound to the PPARα-LBD with a binding affinity of 2.7 μM. Subsequently, X-ray crystallographic studies were carried out to characterize resveratrol binding to the PPARγ-LBD at the molecular level. The electron density map from the crystal structure of the complex between PPARγ-LBD and resveratrol revealed the presence of one molecule of resveratrol bound to the LBD of PPARγ, with the ligand occupying a position close to that of other known PPARγ ligands. Transactivation assays were also performed in HepG2 cells, with the results showing that resveratrol was not a PPAR agonist but instead was able to displace rosiglitazone from PPARγ and Wy-14643 from PPARα with IC50 values of (27.4±1.8) μM and (31.7±2.5) μM, respectively. We propose that resveratrol acts as a PPAR antagonist through its direct interaction with PPARγ and PPARα.

Multiple protein stationary phases: a review

Cellular membrane affinity chromatography stationary phases have been extensively used to characterize immobilized proteins and provide a direct measurement of multiple binding sites, including orthosteric and allosteric sites. This review will address the utilization of immobilized cellular and tissue fragments to characterize multiple transmembrane proteins co-immobilized onto a stationary phase. This approach will be illustrated by demonstrating that multiple transmembrane proteins were immobilized from cell lines and tissue fragments. In addition, the immobilization of individual compartments/organelles within a cell will be discussed and the changes in the proteins binding/kinetics based on their location.

Characterization of a multiple endogenously expressed adenosine triphosphate-binding cassette transporters using nuclear and cellular membrane affinity chromatography columns

Glioblastoma multiforme is an aggressive form of human astrocytoma, with poor prognosis due to multi-drug resistance to a number of anticancer drugs. The observed multi-drug resistance is primarily due to the efflux activity of ATP-Binding Cassette (ABC) efflux transporters such as Pgp, MRP1 and BCRP. The expression of these transporters has been demonstrated in nuclear and cellular membranes of the LN-229 human glioblastoma cell line. Nuclear membrane and cellular membrane fragments from LN-229 cells were immobilized on the IAM stationary phase to create nuclear and cellular membrane affinity chromatography columns, (NMAC(LN-229)) and (CMAC(LN-229)), respectively. Pgp, MRP1 and BCRP transporters co-immobilized on both columns were characterized and compared by establishing the binding affinities for estrone-3-sulfate (3.8 vs. 3.7μM), verapamil (0.6 vs. 0.7μM) and prazosin (0.099 vs. 0.033μM) on each column and no significant differences were observed. Since the marker ligands had overlapping selectivities, the selective characterization of each transporter was carried out by saturation of the binding sites of the non-targeted transporters. The addition of verapamil (Pgp and MRP1 substrate) to the mobile phase allowed the comparative screening of eight compounds at the nuclear and cellular BCRP using etoposide as the marker ligand. AZT increased the retention of etoposide (+15%), a positive allosteric interaction, on the CMAC(LN-229) column and decreased it (-5%) on the NMAC(LN-229), while the opposite effect was produced by rhodamine. The results indicate that there are differences between the cellular and nuclear membrane expressed BCRP and that NMAC and CMAC columns can be used to probe these differences.

Pharmacophore model of the quercetin binding site of the SIRT6 protein

SIRT6 is a histone deacetylase that has been proposed as a potential therapeutic target for metabolic disorders and the prevention of age-associated diseases. We have previously reported on the identification of quercetin and vitexin as SIRT6 inhibitors, and studied structurally related flavonoids including luteolin, kaempferol, apigenin and naringenin. It was determined that the SIRT6 protein remained active after immobilization and that a single frontal displacement could correctly predict the functional activity of the immobilized enzyme. The previous study generated a preliminary pharmacophore for the quercetin binding site on SIRT6, containing 3 hydrogen bond donors and one hydrogen bond acceptor. In this study, we have generated a refined pharmacophore with an additional twelve quercetin analogs. The resulting model had a positive linear behavior between the experimental elution time verses the fit values obtained from the model with a correlation coefficient of 0.8456.

Development and validation of a sensitive LC-MS/MS method for the determination of fenoterol in human plasma and urine samples

Due to the lack of sensitivity in current methods for the determination of fenoterol (Fen), a rapid LC-MS/MS method was developed for the determination of (R,R')-Fen and (R,R';S,S')-Fen in plasma and urine. The method was fully validated and was linear from 50pg/ml to 2000pg/ml for plasma and from 2.500ng/ml to 160ng/ml for urine with a lower limit of quantitation of 52.8pg/ml in plasma. The coefficient of variation was <15% for the high QC standards and <10% for the low QC standards in plasma and was <15% for the high and low QC standards in urine. The relative concentrations of (R,R')-Fen and (S,S')-Fen were determined using a chirobiotic T chiral stationary phase. The method was used to determine the concentration of (R,R')-Fen in plasma and urine samples obtained in an oral cross-over study of (R,R')-Fen and (R,R';S,S')-Fen formulations. The results demonstrated a potential pre-systemic enantioselective interaction in which the (S,S')-Fen reduces the sulfation of the active (R,R')-Fen. The data suggest that a non-racemic mixture of the Fen enantiomers may provide better bioavailability of the active (R,R')-Fen for use in the treatment of cardiovascular disease.

Development and preliminary validation of a plate-based CB1/CB2 receptor functional assay

Cannabinoid (CB) receptors are being targeted therapeutically for the treatment of anxiety, obesity, movement disorders, glaucoma, and pain. More recently, cannabinoid agonists have displayed antiproliferative activity against breast cancer and prostate cancer in animal models. To study cannabinoid receptor ligands, we have developed a novel plate-based assay that measures internalization of CB1/CB2 receptors by determining the change in the intracellular levels of the radiolabeled agonists: [(3)H]Win55-212-2 for CB1 and [(3)H]CP55-940 for CB2. The developed plate-based assay was validated by determining IC50 values for known antagonists: AM251, AM281, AM630, and AM6545. The data obtained were consistent with previously reported values, thereby confirming that the assay can be used to determine the functional binding activities (IC50) of antagonists for the CB1 and CB2 receptors. In addition, we demonstrated that the plate-based assay may be used for screening against complex matrices. Specifically, we demonstrated that the plate-based assay was able to identify which extracts of several species of the genus Zanthoxylum had activity at the CB1/CB2 receptors.

Open tubular columns containing the immobilized ligand binding domain of peroxisome proliferator-activated receptors α and γ for dual agonists characterization by frontal affinity chromatography with mass spectrometry detection

The peroxisome proliferator-activated receptors (PPARs) belong to the nuclear receptor superfamily. In the last years novel PPARs ligands have been identified and these include PPARα/γ dual agonists. To rapidly identify novel PPARs dual ligands, a robust binding assay amenable to high-throughput screening toward PPAR isoforms would be desirable. In this work we describe a parallel assay based on the principles of frontal affinity chromatography coupled to mass spectrometry (FAC-MS) that can be used to characterize dual agonists. For this purpose the ligand binding domain of PPARα receptor was immobilized onto the surface of open tubular capillaries to create new PPAR-alpha-OT columns to be used in parallel with PPAR-gamma-OT columns. The two biochromatographic systems were used in both ranking and Kd experiments toward new ureidofibrate-like dual agonists for subtype selectivity ratio determination. In order to validate the system, the Kd values determined by frontal analysis chromatography were compared to the affinity constants obtained by ITC experiments. The results of this study strongly demonstrate the specific nature of the interaction of the ligands with the two immobilized receptor subtypes.

The synthesis and characterization of a nuclear membrane affinity chromatography column for the study of human breast cancer resistant protein (BCRP) using nuclear membranes obtained from the LN-229 cells

BCRP expression has been reported in glioblastoma cell lines and clinical specimens and has been shown to be expressed both in purified nuclei and in the soluble cytoplasmic fraction. To date, the nuclear BCRP has not been characterized. Our laboratory has previously developed an online chromatographic approach for the study of binding interactions between ligands and protein, cellular membrane affinity chromatography. To this end, we have immobilized the nuclear membrane fragments onto an immobilized artificial membrane stationary phase (IAM), resulting in the nuclear membrane affinity chromatography (NMAC) column. Initial characterization was carried out on the radio flow detector, as well as the LC-MSD, using frontal displacement chromatography techniques. Etoposide, a substrate for BCRP, was initially tested, to determine the functional immobilization of BCRP. Frontal displacement experiments with multiple concentrations of etoposide were run and the binding affinity was determined to be 4.54 μM, which is in close agreement with literature. The BCRP was fully characterized on the NMAC column and this demonstrates that for the first time the nuclear membranes have been successfully immobilized.

The cannabinoid receptor inverse agonist AM251 regulates the expression of the EGF receptor and its ligands via destabilization of oestrogen-related receptor α protein

BACKGROUND AND PURPOSE

AM251 is an inverse agonist of the cannabinoid 1 receptor (CB(1)R) that can exert 'off-target' effects in vitro and in CB(1)R knock-out mice. AM251 is also potent at modulating tumour cell growth, suggesting that growth factor-mediated oncogenic signalling could be regulated by AM251. Since dysregulation of the EGF receptor has been associated with carcinogenesis, we examined AM251 regulation of EGF receptor (EGFR) expression and function.

EXPERIMENTAL APPROACH

The various biological functions of AM251 were measured in CB(1)R-negative human cancer cells. Pharmacological and genetic approaches were used to validate the data.

KEY RESULTS

The mRNA levels for EGFR and its associated ligands, including HB-EGF, were induced several fold in PANC-1 and HCT116 cells in response to AM251. This event was associated with enhanced expression of EGFR on the cell surface with concomitant increase in EGF-induced cellular responses in AM251-treated cells. Exposure to XCT790, a synthetic inverse agonist of the orphan nuclear oestrogen-related receptor α (ERRα), also induced EGFR and HB-EGF expression to the same extent as AM251, whereas pretreatment with the ERRα-selective agonist, biochanin A, blunted AM251 actions. AM251 promoted the degradation of ERRα protein without loss of the corresponding mRNA. Knock-down of ERRα by siRNA-based approach led to constitutive induction of EGFR and HB-EGF levels, and eliminated the biological responses of AM251 and XCT790. Finally, AM251 displaced diethylstilbestrol prebound to the ligand-binding domain of ERRα.

CONCLUSIONS AND IMPLICATIONS

AM251 up-regulates EGFR expression and signalling via a novel non-CB(1)R-mediated pathway involving destabilization of ERRα protein in selected cancer cell lines.

Synthesis and characterization of SIRT6 protein coated magnetic beads: identification of a novel inhibitor of SIRT6 deacetylase from medicinal plant extracts

SIRT6 is a histone deacetylase that has been proposed as a potential therapeutic target for metabolic disorders and the prevention of age-associated diseases. Thus, the identification of compounds that modulate SIRT6 activity could be of great therapeutic importance. The aim of this study was to develop a screening method for the identification of novel modulators of SIRT6 from a natural plant extract. We immobilized SIRT6 onto the surface of magnetic beads, and assessed SIRT6 enzymatic activity on synthetic acetylated histone tails (H3K9Ac) by measuring products of the deacetylation process. The SIRT6 coated magnetic beads were then suspended in fenugreek seed extract (Trigonella foenum-graecum) as a bait to identify active ligands that suppress SIRT6 activity. While the entire extract also inhibited SIRT6 activity in a cell-based assay, the inhibitory effect of two flavonoids from this extract, quercetin and vitexin, was only detected in vitro. This is the first report on the use of protein-coated magnetic beads for the identification of an active ligand from a botanical matrix, and it sets the basis for the de novo identification of SIRT6 modulators from complex biological mixtures.

Development and characterization of immobilized cannabinoid receptor (CB1/CB2) open tubular column for on-line screening

Cannabinoid receptors, CB1 and CB2, are therapeutic targets in the treatment of anxiety, obesity, movement disorders, glaucoma, and pain. We have developed an on-line screening method for CB1 and CB2 ligands, where cellular membrane fragments of a chronic myelogenous leukemia cell line, KU-812, were immobilized onto the surface of an open tubular (OT) capillary to create a CB1/CB2-OT column. The binding activities of the immobilized CB1/CB2 receptors were established using frontal affinity chromatographic techniques. This is the first report that confirms the presence of functional CB1 and CB2 receptors on KU-812 cells. The data from this study confirm that the CB1/CB2-OT column can be used to determine the binding affinities (K(i) values) for a single compound and to screen individual compounds or a mixture of multiple compounds. The CB1/CB2-OT column was also used to screen a botanical matrix, Zanthoxylum clava-herculis, where preliminary results suggest the presence of a high-affinity phytocannabinoid.

Synthesis and characterization of liquid chromatographic columns containing the immobilized ligand binding domain of the estrogen related receptor alpha and estrogen related receptor gamma

The ligand binding domains of the estrogen related receptors, ERRalpha and ERRgamma were covalently immobilized onto the surface of an aminopropyl silica liquid chromatography stationary phase to create the ERRalpha-silica and ERRgamma-silica columns and onto the surface of open tubular capillaries to create the ERRalpha-OT and ERRgamma-OT columns. The ERR-silica and ERR-OT columns were characterized using frontal chromatographic techniques with diethylstibesterol and the binding affinities, K(d) values, to the immobilized receptors were consistent with the values obtained by a radioligand binding assay. The ERRgamma-silica column was also characterized using non-linear chromatographic techniques using a series of tamoxifen derivatives. The relative K(d) values obtained for the derivatives were consistent with the relative ability of the compounds to inhibit the cellular proliferation of the human-derived T98G glioma cell line, expressed as IC(50) values. The results indicate that the columns containing immobilized ERRalpha and ERRgamma can be created and used to characterize the binding of compounds to the immobilized receptors and that the relative retention of compounds on these columns reflects the magnitude of their inhibitory activity.

Determination and modelling of stereoselective interactions of ligands with drug transporters: a key dimension in the understanding of drug disposition

1. Stereochemistry is an important dimension in pharmacology and plays a role in every aspect of the pharmacological fate of chiral xenobiotics. This includes small molecule-drug transporter binding. 2. This paper reviews the reported stereoselectivities of substrate and inhibitor interactions with P-glycoprotein and the organic cation transporter obtained using standard functional and binding studies, as well as data obtained from online cellular membrane affinity chromatography studies. 3. The use of stereochemical data in quantitative structure-activity relationship (QSAR) and pharmacophore modelling is also addressed as is the effect of ignoring the fact that small molecule-drug transporter interactions take place in three-dimensional and asymmetric space.

Characterization of a multiple ligand-gated ion channel cellular membrane affinity chromatography column and identification of endogenously expressed receptors in astrocytoma cell lines

Cellular membranes obtained from the 1321N1 and A172 astrocytoma cell lines were immobilized on a chromatographic phase to create cellular membrane affinity chromatography (CMAC) columns, CMAC(1321N1) and CMAC(A172). The columns were characterized using frontal affinity chromatography with [(3)H]-epibatidine as the marker ligand and epibatidine, nicotine, and methyllycaconitine as the displacers. The results indicated that the columns contained homomeric alpha7 nicotinic acetylcholine receptors (alpha7 nAChR) and heteromeric nicotinic acetylcholine receptors (alpha(x)beta(y) nAChRs), which was confirmed by the addition of subtype-specific inhibitors, alpha-bungarotoxin (alpha7 nAChR) and kappa-bungarotoxin (alpha(x)beta(y) nAChR) to the mobile phase. The presence of two additional ligand-gated ion channels (LGICs), gamma-aminobutyric acid (GABA(A)) and N-methyl-D-aspartic acid (NMDA), was established using frontal affinity chromatography with flunitrazepam and diazepam (GABA(A) receptor) and MK-801 and NMDA (NMDA receptor). The presence of the four LGICs was confirmed using confocal microscopy and flow cytometry. The results indicate that the CMAC(1321N1) and CMAC(A172) columns contain four independently functioning LGICs, that the columns can be used to characterize binding affinities of small molecules to each of the receptors, and that the CMAC approach can be used to probe the expression of endogenous membrane receptors.

Ligand and protein fishing with heat shock protein 90 coated magnetic beads

Heat shock protein 90alpha (Hsp90alpha) is a molecular chaperone that has been targeted for the development of new anticancer therapies. To date, co-immunoprecipitation (IP) has been primarily used to identify novel client proteins. We now report an alternative approach in which Hsp90alpha has been immobilized onto the surface of silica-based magnetic beads. The beads were used to isolate known Hsp90alpha ligands from a mixture containing ligands and nonligands. In addition, they were also used to isolated proteins from a mixture of proteins, as well as a cellular extract. The results indicate that the Hsp90alpha coated magnetic beads can be used to "fish" from complex chemical and biological mixtures for new lead drug candidates and client proteins.

Development and characterization of immobilized human organic anion transporter-based liquid chromatographic stationary phase: hOAT1 and hOAT2

This paper reports the development of liquid chromatographic columns containing immobilized organic anion transporters (hOAT1 and hOAT2). Cellular membrane fragments from MDCK cells expressing hOAT1 and S2 cells expressing hOAT2 were immobilized on the surface of the immobilized artificial membrane (IAM) liquid chromatographic stationary phase. The resulting stationary phases were characterized by frontal affinity chromatography, using the marker ligand [3H]-adefovir for the hOAT1 and [14C]-p-aminohippurate for the hOAT2 in the presence of multiple displacers. The determined binding affinities (Kd) for eight OAT1 ligands and eight OAT2 ligands were correlated with literature values and a statistically significant correlation was obtained for both the hOAT1 and hOAT2 columns: r2=0.688 (p<0.05) and r2=0.9967 (p<0.0001), respectively. The results indicate that the OAT1 and OAT2 have been successfully immobilized with retention of their binding activity. The use of these columns to identify ligands to the respective transporters will be presented.

Automated ligand fishing using human serum albumin-coated magnetic beads

Human serum albumin, HSA, was immobilized onto the surface of silica-based magnetic beads. The beads were used to isolate known HSA ligands from a mixture containing ligands and nonligands. The separation was accomplished manually and was also automated. The results indicate that an automated "ligand-fishing" technique can be developed using magnetic beads containing an immobilized protein.

Pharmacophore modelling of stereoselective binding to the human organic cation transporter (hOCT1)

BACKGROUND AND PURPOSE

The human organic cation transporter-1 (hOCT1) is a polyspecific transporter that plays a role in drug distribution, metabolism and excretion. Previous studies have demonstrated that hOCT1 binding can be stereoselective, but the mechanism for stereochemical recognition has not been described. The purpose of this study was to develop a pharmacophore model to describe stereoselective binding to hOCT1.

EXPERIMENTAL APPROACH

A set of 22 compounds including 8 pairs of enantiomers and five pairs of diastereomers was used to develop a pharmacophore model. The pharmacophore modeling was carried out using Catalyst version 4.11 and HypoGen and was based upon the correlation of the structures and activities (K(i) values) of the compounds used in the study.

KEY RESULTS

The resulting model contained a positive ion, hydrophobic and two hydrogen-bond acceptor interaction sites. The relative enantioselectivity of 8/8 enantiomeric pairs and diastereoselectivity of 5/5 diastereomers was described by mapping to a combination of at least 3 of the 4 functional feature sites of the model.

CONCLUSIONS AND IMPLICATIONS

The pharmacophore model describes stereoselective interactions with hOCT1 at one of the binding sites on the molecule.

Automated ligand fishing using human serum albumin-coated magnetic beads

Human serum albumin, HSA, was immobilized onto the surface of silica-based magnetic beads. The beads were used to isolate known HSA ligands from a mixture containing ligands and nonligands. The separation was accomplished manually and was also automated. The results indicate that an automated "ligand-fishing" technique can be developed using magnetic beads containing an immobilized protein.

Development and characterization of an immobilized human organic cation transporter based liquid chromatographic stationary phase

Membranes from a stably transfected cell line that expresses the human organic cation 1 transporter (hOCT1) have been immobilized on the immobilized artificial membrane (IAM) liquid chromatographic stationary phase to form the hOCT1(+)-IAM stationary phase. Membranes from the parent cell line that does not express the hOCT1 were also immobilized to create the hOCT1(-)-IAM stationary phase. Columns were created using both stationary phases, and frontal displacement chromatography experiments were conducted using [(3)H]-methyl phenyl pyridinium ([(3)H]-MPP(+)) as the marker ligand and MPP(+), verapamil, quinidine, quinine, nicotine, dopamine and vinblastin as the displacers. The K(d) values calculated from the chromatographic studies correlated with previously reported K(i) values (r(2)=0.9987; p<0.001). The data indicate that the hOCT1(+)-IAM column can be used for the on-line determination of binding affinities to the hOCT1 and that these affinities are comparable to those obtained using cellular uptake studies. In addition, the chromatographic method was able to identify a previously undetected high affinity binding site for MPP(+) and to determine that hOCT1 bound (R)-verapamil to a greater extent than (S)-verapamil.

Enantioselective binding to the human organic cation transporter-1 (hOCT1) determined using an immobilized hOCT1 liquid chromatographic stationary phase

A liquid chromatography stationary phase containing immobilized membranes obtained from a cell line that expresses the human organic cation transporter (hOCT1-IAM) has been used to study the binding of the enantiomers of propranolol, atenolol, pseudoephedrine, and alpha-methylbenzylamine to the immobilized hOCT1. Frontal displacement chromatography was used to determine the binding affinities (K(d) values), and the data demonstrate that there was an enantioselective difference in the K(d) values of the enantiomers of propranolol, atenolol, and pseudoephedrine, while alpha-methylbenzylamine did not significantly bind to the transporter. Competitive inhibition studies with the cell line used to create the chromatographic column demonstrated that, for the enantiomers of propranolol, the ratio of the chromatographically determined K(d) values [K(d (+)-(R)-propranolol)/K(d (-)-(S)-propranolol) = 2.98] reflected an enantioselective difference in the functional activity of the two enantiomers [IC(50 (+)-(R)-propranolol)/IC(50 (-)-(S)-propranolol) = 2.75]. The chromatographically determined K(d) values were used to construct an initial pharmacophore which contains a hydrogen bond donating site that appears to be responsible for the observed enantioselectivity.

Novel biotinylated phenylarsonous acids as bifunctional reagents for spatially close thiols: studies on reduced antibodies and the agonist binding site of reduced Torpedo nicotinic receptors

We synthesized three novel organoarsenicals as prototype bifunctional reagents for spatially close thiols, N-(4-arsenosophenyl) hexahydro-2-oxo-(3aS,4S,6aR)-1H-thieno[3, 4-d]imidazole-4-pentamide (1), 2-[4-[(4-arsenosophenyl)amino]-1, 4-dioxobutyl] hydrazide, (3aS,4S,6aR)-hexahydro-2-oxo- 1H-thieno[3, 4-d] imidazole-4-pentanoic acid (2), and [4-[[12-[[5-[(3aS,4S, 6aR)-hexahydro-2-oxo-1H-thieno[3, 4-d]imidazol-4-yl]-1-oxopentyl]amino]-1-oxododecyl]amino]phe nyl]-arso nous acid (3) containing both biotin and arsenic with intervening varying length spacers extending from 2 to 15 A beyond biotin bound to streptavidin. Conceptually, the arsenical group can form a stable, covalent ring structure with appropriately spaced thiols and thereby anchor the reagent to a macromolecule, while biotin allows for the detection of the reagent-macromolecule complex via avidin binding. Because the alpha-subunits of all characterized nicotinic receptors contain an easily reducible disulfide bond between adjacent cysteine residues, the reduced alpha-subunit is an attractive site for labeling. Compounds 1-3 all simultaneously bound streptavidin and dithiols, and all three decreased the number of [125I]alpha-bungarotoxin-binding sites in reduced Torpedo nicotinic receptors (IC50s 10-300 nM). Moreover, arsenylation of the receptors prevented their reoxidation with dithio-bis(nitrobenzoic acid), was reversible with 2,3-dimercaptopropanesulfonic acid, and protected the receptor from irreversible alkylation by bromoacetylcholine. However, in no case did 1-3 allow simultaneous binding to reduced nicotinic receptors and to [125I]streptavidin, although 3 alone allowed simultaneous labeling of a spatially close dithiol located in reduced antibodies.