The quinoid structure is the molecular requirement for recognition of phthaleins by the organic anion carrier at the sinusoidal plasma membrane level in the liver

Chemometrics approach for the prediction of structure-activity relationship for membrane transporter bilitranslocase

Membrane transport proteins are essential for cellular uptake of numerous salts, nutrients and drugs. Bilitranslocase is a transporter, specific for water-soluble organic anions, and is the only known carrier of nucleotides and nucleotide-like compounds. Experimental data of bilitranslocase ligand specificity for 120 compounds were used to construct classification models using counter-propagation artificial neural networks (CP-ANNs) and support vector machines (SVMs). A subset of active compounds with experimentally determined transport rates was used to build predictive QSAR models for estimation of transport rates of unknown compounds. Several modelling methods and techniques were applied, i.e. CP-ANN, genetic algorithm, self-organizing mapping and multiple linear regression method. The best predictions were achieved using CP-ANN coupled with a genetic algorithm, with the external validation parameter QV(2) of 0.96. The applicability domains of the models were defined to determine the chemical space in which reliable predictions can be obtained. The models were applied for the estimation of bilitranslocase transport activity for two sets of pharmaceutically interesting compounds, antioxidants and antiprions. We found that the relative planarity and a high potential for hydrogen bond formation are the common structural features of anticipated substrates of bilitranslocase. These features may serve as guidelines in the design of new pharmaceuticals transported by bilitranslocase.

Assessment of applicability domain for multivariate counter-propagation artificial neural network predictive models by minimum euclidean distance space analysis: a case study

Alongside the validation, the concept of applicability domain (AD) is probably one of the most important aspects which determine the quality as well as reliability of the established quantitative structure-activity relationship (QSAR) models. To date, a variety of approaches for AD estimation have been devised which can be applied to particular type of QSAR models and their practical utilization is extensively elaborated in the literature. The present study introduces a novel, simple, and effective distance-based method for estimation of the AD in case of developed and validated predictive counter-propagation artificial neural network (CP ANN) models through a proficient exploitation of the euclidean distance (ED) metric in the structure-representation vector space. The performance of the method was evaluated and explained in a case study by using a pre-built and validated CP ANN model for prediction of the transport activity of the transmembrane protein bilitranslocase for a diverse set of compounds. The method was tested on two more datasets in order to confirm its performance for evaluation of the applicability domain in CP ANN models. The chemical compounds determined as potential outliers, i.e., outside of the CP ANN model AD, were confirmed in a comparative AD assessment by using the leverage approach. Moreover, the method offers a graphical depiction of the AD for fast and simple determination of the extreme points.

Structural analysis of a peptide fragment of transmembrane transporter protein bilitranslocase

Using a combination of genomic and post-genomic approaches is rapidly altering the number of identified human influx carriers. A transmembrane protein bilitranslocase (TCDB 2.A.65) has long attracted attention because of its function as an organic anion carrier. It has also been identified as a potential membrane transporter for cellular uptake of several drugs and due to its implication in drug uptake, it is extremely important to advance the knowledge about its structure. However, at present, only the primary structure of bilitranslocase is known. In our work, transmembrane subunits of bilitranslocase were predicted by a previously developed chemometrics model and the stability of these polypeptide chains were studied by molecular dynamics (MD) simulation. Furthermore, sodium dodecyl sulfate (SDS) micelles were used as a model of cell membrane and herein we present a high-resolution 3D structure of an 18 amino acid residues long peptide corresponding to the third transmembrane part of bilitranslocase obtained by use of multidimensional NMR spectroscopy. It has been experimentally confirmed that one of the transmembrane segments of bilitranslocase has alpha helical structure with hydrophilic amino acid residues oriented towards one side, thus capable of forming a channel in the membrane.

Electrogenic bromosulfalein transport in isolated membrane vesicles: implementation in both animal and plant preparations for the study of flavonoid transporters

Bromosulfalein is an organic anion dye used in the study of a variety of membrane carriers expressed in animal tissues and involved in transport of drugs and metabolites. The spectrophotometric assay of electrogenic bromosulfalein transport in membrane vesicles, isolated from various mammalian organs or tissues, enables to specifically measure the transport activity of bilitranslocase (TCDB 2.A.65.1.1). The latter is a bilirubin- and flavonoid-specific transporter expressed in rat liver, the organ where its function has been best characterized. The spectrophotometric assay of electrogenic bromosulfalein transport requires minimal volumes of membrane vesicles, is completed within 1 min, and, therefore, is a useful tool to screen the transporter spectrum of potential substrates, by testing them as reversible inhibitors of bromosulfalein transport kinetics. Furthermore, the assay enables to study the progress of time-dependent inactivation of bromosulfalein transport, caused by different protein-specific reagents, including specific anti-sequence antibodies. Inactivation can be retarded by the presence of substrates in a concentration-dependent manner, enabling to derive the dissociation constants of the transporter-substrate complex and thus to gain further insight into the transporter structure-function relationship. This assay, implemented in membrane vesicles isolated from plant organs, has paved the way to the discovery of homologues of bilitranslocase in plants.

Properties of flavonoids influencing the binding to bilitranslocase investigated by neural network modelling

Bilitranslocase is a plasma membrane carrier firstly identified on the sinusoidal (vascular) domain of liver cells and later on also in the gastric epithelium. It transports diverse organic anions, such as bilirubin, some phthaleins and many dietary anthocyanins, suggesting that it could play a role both in the absorption of flavonoids from dietary sources and in their hepatic metabolism. This work was aimed at characterising the interaction of bilitranslocase with flavonols, a flavonoid sub-class. The results obtained show that, contrary to anthocyanins, flavonol glycosides do not interact with the carrier, whereas just some of the corresponding aglycones act as relatively poor ligands to bilitranslocase. These data point to a clear-cut discrimination between anthocyanins and flavonols occurring at the level of the bilitranslocase transport site. A quantitative structure-activity relationship based on counter propagation artificial neural network modelling was undertaken in order to shed light on the nature of flavonoid interaction with bilitranslocase. It was found that binding relies on the ability to establish hydrogen bonds, ruling out the involvement of charge interactions. This requisite might be at the basis of the discrimination between anthocyanins and flavonols by bilitranslocase and could lie behind some aspects of the distinct pharmacokinetic properties of anthocyanins and flavonols in mammals.

Uptake of bilirubin into HepG2 cells assayed by thermal lens spectroscopy. Function of bilitranslocase

Bilitranslocase is a carrier protein localized at the basolateral domain of the hepatocyte plasma membrane. It transports various organic anions, including bromosulfophthalein and anthocyanins. Functional studies in subcellular fractions enriched in plasma membrane revealed a high-affinity binding site for bilirubin, associated with bilitranslocase. The aim of this work was to test whether the liver uptake of bilirubin depends on the activity of bilitranslocase. To this purpose, an assay of bilirubin uptake into HepG2 cell cultures was set up. The transport assay medium contained bilirubin at a concentration of approximately 50 nm in the absence of albumin. To analyse the relative changes in bilirubin concentration in the medium throughout the uptake experiment, a highly sensitive thermal lens spectrometry method was used. The mechanism of bilirubin uptake into HepG2 cells was investigated by using inhibitors such as anti-sequence bilitranslocase antibodies, the protein-modifying reagent phenylmethanesulfonyl fluoride and diverse organic anions, including nicotinic acid, taurocholate and digoxin. To validate the assay further, both bromosulfophthalein and indocyanine green uptake in HepG2 cells was also characterized. The results obtained show that bilitranslocase is a carrier with specificity for both bilirubin and bromosulfophthalein, but not for indocyanine green.

Characterization of electrogenic bromosulfophthalein transport in carnation petal microsomes and its inhibition by antibodies against bilitranslocase

Bilitranslocase is a rat liver plasma membrane carrier, displaying a high-affinity binding site for bilirubin. It is competitively inhibited by grape anthocyanins, including aglycones and their mono- and di-glycosylated derivatives. In plant cells, anthocyanins are synthesized in the cytoplasm and then translocated into the central vacuole, by mechanisms yet to be fully characterized. The aim of this work was to determine whether a homologue of rat liver bilitranslocase is expressed in carnation petals, where it might play a role in the membrane transport of anthocyanins. The bromosulfophthalein-based assay of rat liver bilitranslocase transport activity was implemented in subcellular membrane fractions, leading to the identification of a bromosulfophthalein carrier (K(M) = 5.3 microm), which is competitively inhibited by cyanidine 3-glucoside (Ki = 51.6 microm) and mainly noncompetitively by cyanidin (Ki = 88.3 microm). Two antisequence antibodies against bilitranslocase inhibited this carrier. In analogy to liver bilitranslocase, one antibody identified a bilirubin-binding site (Kd = 1.7 nm) in the carnation carrier. The other antibody identified a high-affinity binding site for cyanidine 3-glucoside (Kd = 1.7 microm) on the carnation carrier only, and a high-affinity bilirubin-binding site (Kd = 0.33 nm) on the liver carrier only. Immunoblots showed a putative homologue of rat liver bilitranslocase in both plasma membrane and tonoplast fractions, isolated from carnation petals. Furthermore, only epidermal cells were immunolabeled in petal sections examined by microscopy. In conclusion, carnation petals express a homologue of rat liver bilitranslocase, with a putative function in the membrane transport of secondary metabolites.

The interaction of anthocyanins with bilitranslocase

Bilitranslocase (TC 2.A.65.1.1) is an organic anion membrane carrier expressed at the sinusoidal domain of the liver plasma membrane and in epithelial cells of the gastric mucosa. Its substrates are sulfobromophthalein, bilirubin, and nicotinic acid. This work reports on the identification of a new class of bilitranslocase substrates, i.e., anthocyanins. Seventeen out thes 20 compounds tested behaved as competitive inhibitors of bilitranslocase transport activity (K(I)=1.4-22 microM). Their structure-activity relationship reveals that mono- and di-glucosyl anthocyanins, the anthocyanin species occurring in food, are better ligands than the corresponding aglycones. Moreover, the first interaction of anthocyanins with the carrier occurs through hydrophilic moieties, such as the 3-glucosyl moiety and the B ring for the monoglucosides, through the 5-glucosyl moiety and the A ring for the diglucosides, and through either the B or the A ring for the aglycones. These findings suggest that bilitranslocase could play a role in the bioavailability of anthocyanins.

The bilirubin-binding motif of bilitranslocase and its relation to conserved motifs in ancient biliproteins

In the primary structure of bilitranslocase, currently under study in our laboratory, an aminoacid motif was identified and found to be conserved in a number of alpha-phycocyanines, ancient biliproteins present in cyanobacteria. To test the possibility that such a motif could be at least part of the binding site for bilirubin, epitope-specific antibodies were raised. The target corresponds to the sequence 65-75 of bilitranslocase and covers the central portion of the motif identified. The antibodies were shown: 1) to inhibit the electrogenic BSP transport by plasmamembrane vesicles; 2) to react with purified bilitranslocase; and 3) to identify only one protein band with electrophoretic mobility identical to bilitranslocase in Western blots of solubilised plasmamembrane vesicles. The presence of either bilirubin or nicotinate during pre-incubation with the antibodies decreases concentration-wise the inhibition kinetics. From these experiments a dissociation constant of 2.2 +/- 0.3 and 11.3 +/- 1.3 nM for bilirubin-bilitranslocase and nicotinate-bilitranslocase complexes were calculated.

Molecular structure influence in the recognition of phthaleins by the electrogenic organic anion carrier at the sinusoidal plasma membrane level in the liver

The hepatocytic uptake of cholephilic organic anions occurs by carrier-mediated mechanisms. Electrogenic and electroneutral transport systems have been described. The aim of this study was to determine the dissociation constant (Kd) of the Electrogenic Carrier System(s) (ECS) for tetrabromosulfophthalein (BSP), dibromosulfophthalein (DBSP), tetrabromophthalein (TBP), tetrabromosulfonephthalein (TBS) and thymol blue (TB). Kd (uM) values for ECS-organic anion complexes were: ECS-BSP = 3.61 +/- 0.18; ECS-DBSP = 11.61 +/- 1.32; ECS-TBP = 0.51 +/- 0.08; ECS-TBS = 1.31 +/- 0.25; ECS-TB = 9.44 +/- 1.80. From these data, it is possible to conclude that molecular characteristics of the organic anions are important factors in determining the dissociation constant for the electrogenic hepatic carrier(s). In this sense, the addition of two sulphonic groups on the phenolic ring; the presence of a sulphonic on the benzenic ring and the absence of two or four bromines on the molecule confers a lower affinity for ECS.

Gender-differential liver plasma membrane affinities in hepatic tetrabromosulfonephthalein (TBS) uptake

The sex difference in the hepatic uptake of tetrabromosulfonephthalein (TBS) was investigated in male and female rats in two different experimental models. In the intact animal, the initial plasma disappearance constant rate, the initial velocity of uptake, and the plasma clearance of TBS were significantly higher in females than in males. In sinusoidal liver plasma membrane vesicles, kinetic parameters of TBS uptake were investigated in both sexes. The Km was lower in females than in males (5.5 +/- 0.4 vs 17 +/- 4 microM, P < 0.05), whereas Vmax showed comparable values (544 +/- 15 vs 581 +/- 60 nmol TBS/min/mg protein, mean +/- SD, NS, females and males, respectively). Collectively, these data indicate that the sex difference in hepatic uptake of TBS is located at the sinusoidal liver plasma membrane and is due to a greater affinity of the electrogenic transport system(s) in females.

Interaction of sulfonylureas with the transport of bile acids into hepatocytes

The sulfonylurea compounds glisoxepide and glibenclamide inhibit the uptake of bile acids into isolated rat hepatocytes. The Ki values for the inhibition of cholate uptake was 9 microM with glibenclamide and 200 microM with glisoxepide. The inhibition of cholate uptake by both sulfonylureas was noncompetitive. Uptake of the conjugated bile acid taurocholate was inhibited by glibenclamide, Ki = 75 microM. Again the inhibition was noncompetitive. Glisoxepide inhibited taurocholate uptake only in the absence of sodium ions. Under sodium-free conditions glisoxepide also strongly inhibited cholate uptake. The inhibition was competitive, Ki = 42 microM. Both bile acids interfered with the hepatocellular uptake of [3H]glisoxepide, with IC50 values of 375 and 467 microM for cholate and taurocholate, respectively. The uptake of [3H]glibenclamide was inhibited by cholate, IC50 = 328 microM, but not by taurocholate. Glisoxepide uptake was further inhibited by blockers of the hepatocellular monocarboxylate transporter, by the loop diuretic bumetanide, by 4,4'-diisothiocyano-2,2'-stilbenedisulfonate (DIDS) and by sulfate. Glibenclamide uptake was weakly inhibited by DIDS and by anthracene-9-carboxylic acid (A-9-C) but not by bumetanide and sulfate. Neither bromosulfophthalein nor the fatty acid oleate inhibited glisoxepide or glibenclamide uptake. These results are consistent with the transport of glisoxepide via the transport system for the unconjugated bile acid cholate. Glibenclamide uptake is mediated by a still unknown hepatocellular transport system.

Hepatic handling of photoirradiated bilirubin. A study in isolated perfused Wistar rat liver

Conjugation has been considered the rate-limiting step for bilirubin hepatic transport, and bypass of this metabolic step could explain why photobilirubins can be rapidly cleared by the liver. In this paper we assessed whether photoirradiation may enhance the bilirubin overall hepatic transport in the isolated perfused Wistar rat liver, a model possessing intact transport and conjugating systems. Bilirubin was administered as a bolus so as to reach a perfusate concentration of approximately 10 microM (bilirubin/albumin molar ratio 1:17). Perfusate light exposure (0.56.10(15) quanta s-1 cm-2) yielded 7-10% of configurational photoisomers, which were further identified as (4Z,15E/4E,15Z)-bilirubin IX alpha. Under such conditions, the perfusate removal rate was increased by 39% over that from dark conditions. Likewise, biliary excretion, estimated as total bilirubin recovery at 60 min, was also increased (+48%). This later improvement was mainly produced at the expense of unconjugated bilirubin, which most likely derived from its configurational photoisomers that, once excreted into bile, readily re-isomerized to the parent compound. In addition, this increment was partially due to a delayed improvement of monoglucuronide pigment excretion. The calculated hepatic pigment content was significantly higher under light conditions. A direct assessment of hepatic content of different bilirubin moieties at 20 min after bilirubin administration confirmed that such an increment was fully accounted for by unconjugated pigment. Our finding that hepatic pigment content rose (despite a higher biliary excretion) when the bilirubin was irradiated suggests a higher net uptake of photoisomers than native pigment. This observation, and the finding that bilirubin photoisomers were usually excreted without undergoing conjugation even if the metabolic system is active, contribute to explain the greater appearance of unconjugated bilirubin in Wistar rat bile under light exposure.

Organization of functional groups of liver bilitranslocase

Bilitranslocase transport activity can be described as consisting of three functional fractions, which depend on two distinct classes of sulfhydryl groups, on the one hand, and on the guanido groups of arginine residues, on the other. Each fraction accounts for approx. 50% transport activity. The pattern of transport activity inhibition resulting from step-wise derivatization of these functional groups indicates that, in general, derivatization of arginine residues prevents that of one class of sulfhydryl groups and vice versa, indicating their close location in the protein. Nevertheless, under appropriate conditions, derivatization of both functional groups can be achieved; however, the inhibitory effect produced is not additive. Hence, these two fractions overlap functionally and are likely to belong to a common functional domain of the protein. On the contrary, the other class of sulfhydryl groups can be derivatized, regardless of the state of the arginine residues.

Abnormal hepatic uptake of low doses of sulfobromophthalein in Gilbert's syndrome: the role of reduced affinity of the plasma membrane carrier of organic anions

The plasma disappearance rate of sulfobromophthalein (VBSP; mumol/kg/min) was measured in 15 Gilbert's syndrome patients and 12 control subjects after intravenous injection of two different doses (0.59 and 5.90 mumol/kg) of the dye. Plasma disappearance rate was significantly reduced in Gilbert's syndrome patients after administration of 0.59 mumol sulfobromophthalein/kg (0.119 +/- 0.016 vs. 0.146 +/- 0.018 mumol/kg/min; mean +/- S.D.; p less than 0.001), whereas no difference was found with the higher dose (0.754 +/- 0.040 vs. 0.767 +/- 0.072 mumol/kg/min). Significant reduction was also found after administration to four Gilbert's syndrome patients and four control subjects of 0.29 and 2.95 mumol sulfobromophthalein (0.060 +/- 0.005 mumol/kg/min vs. 0.077 +/- 0.07 mumol/kg/min and 0.480 +/- 0.012 mumol/kg/min vs. 0.591 +/- 0.015 mumol/kg/min, respectively; p less than 0.01). Competition studies with combined administration of sulfobromophthalein (0.59 mumol/kg) and different doses of rifamycin SV (0.59, 1.47 and 2.95 mumol/kg) showed a significant (p less than 0.001) reduction in plasma disappearance rate in Gilbert's syndrome patients but not in controls. The rifamycin SV dose at which a 50% inhibition in plasma disappearance rate was observed was 0.8 mumol/kg. The apparent affinity (Km) of the hepatic transport was higher in Gilbert's syndrome patients than in control subjects (3.61 +/- 0.37 mumol sulfobromophthalein/kg vs. 2.76 +/- 0.29 mumol sulfobromophthalein/kg, mean +/- S.D.; p less than 0.01), whereas no difference was found in Vmax (0.95 +/- 0.11 mumol sulfobromophthalein/kg vs. 0.93 +/- 0.10 mumol sulfobromophthalein/kg/min, mean +/- S.D.; N.S.).(ABSTRACT TRUNCATED AT 250 WORDS)

Arginine residues are involved in the transport function of bilitranslocase

Specific guanido group reagents inhibit bilitranslocase transport activity in rat liver plasma membrane vesicles. Their reaction is shown to be affected by sulfobromophthalein, Thymol blue and bilirubin, which are translocated by bilitranslocase across the plasma membrane. It is concluded that the transport function of bilitranslocase depends on arginine residues, which are involved in the interaction with the molecules to be translocated.

The sulfhydryl groups responsible for bilitranslocase transport activity respond to the interaction of the carrier with bilirubin and functional analogues

Both inactivation of sulfobromophthalein transport in rat liver plasma membrane vesicles by sulfhydryl group reagents and subsequent reactivation by 2-mercaptoethanol are shown to be modulated by ligands to bilitranslocase. In particular, bilirubin, sulfobromophthalein and Thymol blue behave as negative effectors in the inactivation reaction and as positive effectors in the reactivation reaction. Kinetic data provide further evidence of the existence of two classes of sulfhydryl groups involved in transport activity. The effect brought about by remarkably low concentrations of bilirubin is in line with the physiological function of bilitranslocase as a bilirubin carrier.

The role of sulfhydryl groups in sulfobromophthalein transport in rat liver plasma membrane vesicles

Sulfobromophthalein (BSP) electrogenic transport activity in a plasma membrane vesicle preparation from rat liver is shown to depend on free sulfhydryl groups. These are organized in two classes, one of which does not react with the sulfhydryl group reagent 5,5'-dithiobis(2-nitrobenzoate). The two classes appear to be involved in BSP transport independently. However, reactivity of one class can be shown to be affected by alkylation of the other. Hence, it is concluded that both classes are located on the same carrier system, which previous research has established to be the integral sinusoidal membrane protein bilitranslocase.