Bile-salt-binding polypeptides in plasma membranes of hepatocytes revealed by photoaffinity labelling

Organic anion uptake by hepatocytes

Many of the compounds taken up by the liver are organic anions that circulate tightly bound to protein carriers such as albumin. The fenestrated sinusoidal endothelium of the liver permits these compounds to have access to hepatocytes. Studies to characterize hepatic uptake of organic anions through kinetic analyses, suggested that it was carrier-mediated. Attempts to identify specific transporters by biochemical approaches were largely unsuccessful and were replaced by studies that utilized expression cloning. These studies led to identification of the organic anion transport proteins (oatps), a family of 12 transmembrane domain glycoproteins that have broad and often overlapping substrate specificities. The oatps mediate Na(+)-independent organic anion uptake. Other studies identified a seven transmembrane domain glycoprotein, Na(+)/taurocholate transporting protein (ntcp) as mediating Na(+)-dependent uptake of bile acids as well as other organic anions. Although mutations or deficiencies of specific members of the oatp family have been associated with transport abnormalities, there have been no such reports for ntcp, and its physiologic role remains to be determined, although expression of ntcp in vitro recapitulates the characteristics of Na(+)-dependent bile acid transport that is seen in vivo. Both ntcp and oatps traffic between the cell surface and intracellular vesicular pools. These vesicles move through the cell on microtubules, using the microtubule based motors dynein and kinesins. Factors that regulate this motility are under study and may provide a unique mechanism that can alter the plasma membrane content of these transporters and consequently their accessibility to circulating ligands.

Role of the intestinal bile acid transporters in bile acid and drug disposition

Membrane transporters expressed by the hepatocyte and enterocyte play critical roles in maintaining the enterohepatic circulation of bile acids, an effective recycling and conservation mechanism that largely restricts these potentially cytotoxic detergents to the intestinal and hepatobiliary compartments. In doing so, the hepatic and enterocyte transport systems ensure a continuous supply of bile acids to be used repeatedly during the digestion of multiple meals throughout the day. Absorption of bile acids from the intestinal lumen and export into the portal circulation is mediated by a series of transporters expressed on the enterocyte apical and basolateral membranes. The ileal apical sodium-dependent bile acid cotransporter (abbreviated ASBT; gene symbol, SLC10A2) is responsible for the initial uptake of bile acids across the enterocyte brush border membrane. The bile acids are then efficiently shuttled across the cell and exported across the basolateral membrane by the heteromeric Organic Solute Transporter, OSTα-OSTβ. This chapter briefly reviews the tissue expression, physiology, genetics, pathophysiology, and transport properties of the ASBT and OSTα-OSTβ. In addition, the chapter discusses the relationship between the intestinal bile acid transporters and drug metabolism, including development of ASBT inhibitors as novel hypocholesterolemic or hepatoprotective agents, prodrug targeting of the ASBT to increase oral bioavailability, and involvement of the intestinal bile acid transporters in drug absorption and drug-drug interactions.

Enterohepatic circulation: physiological, pharmacokinetic and clinical implications

Enterohepatic recycling occurs by biliary excretion and intestinal reabsorption of a solute, sometimes with hepatic conjugation and intestinal deconjugation. Cycling is often associated with multiple peaks and a longer apparent half-life in a plasma concentration-time profile. Factors affecting biliary excretion include drug characteristics (chemical structure, polarity and molecular size), transport across sinusoidal plasma membrane and canniculae membranes, biotransformation and possible reabsorption from intrahepatic bile ductules. Intestinal reabsorption to complete the enterohepatic cycle may depend on hydrolysis of a drug conjugate by gut bacteria. Bioavailability is also affected by the extent of intestinal absorption, gut-wall P-glycoprotein efflux and gut-wall metabolism. Recently, there has been a considerable increase in our understanding of the role of transporters, of gene expression of intestinal and hepatic enzymes, and of hepatic zonation. Drugs, disease and genetics may result in induced or inhibited activity of transporters and metabolising enzymes. Reduced expression of one transporter, for example hepatic canalicular multidrug resistance-associated protein (MRP) 2, is often associated with enhanced expression of others, for example the usually quiescent basolateral efflux MRP3, to limit hepatic toxicity. In addition, physiologically relevant pharmacokinetic models, which describe enterohepatic recirculation in terms of its determinants (such as sporadic gall bladder emptying), have been developed. In general, enterohepatic recirculation may prolong the pharmacological effect of certain drugs and drug metabolites. Of particular importance is the potential amplifying effect of enterohepatic variability in defining differences in the bioavailability, apparent volume of distribution and clearance of a given compound. Genetic abnormalities, disease states, orally administered adsorbents and certain coadministered drugs all affect enterohepatic recycling.

Identification of the bile acid-binding site of the ileal lipid-binding protein by photoaffinity labeling, matrix-assisted laser desorption ionization-mass spectrometry, and NMR structure

The ileal lipid-binding protein (ILBP) is the only physiologically relevant bile acid-binding protein in the cytosol of ileocytes. To identify the bile acid-binding site(s) of ILBP, recombinant rabbit ILBP photolabeled with 3-azi- and 7-azi-derivatives of cholyltaurine was analyzed by a combination of enzymatic fragmentation, gel electrophoresis, and matrix-assisted laser desorption ionization (MALDI)-mass spectrometry. The attachment site of the 3-position of cholyltaurine was localized to the amino acid triplet His(100)-Thr(101)-Ser(102) using the photoreactive 3,3-azo-derivative of cholyltaurine. With the corresponding 7,7-azo-derivative, the attachment point of the 7-position could be localized to the C-terminal part (position 112-128) as well as to the N-terminal part suggesting more than one binding site for bile acids. By chemical modification and NMR structure of ILBP, arginine residue 122 was identified as the probable contact point for the negatively charged side chain of cholyltaurine. Consequently, bile acids bind to ILBP with the steroid nucleus deep inside the protein cavity and the negatively charged side chain near the entry portal. The combination of photoaffinity labeling, enzymatic fragmentation, MALDI-mass spectrometry, and NMR structure was successfully used to determine the topology of bile acid binding to ILBP.

Photoaffinity labeling of the aglycon binding site of the recombinant human liver UDP-glucuronosyltransferase UGT1A6 with 7-azido-4-methylcoumarin

7-Azido-4-methylcoumarin (AzMC) is a fluorescent photoactive compound structurally related to 4-methylumbelliferone (4-MU), a marker substrate of the human liver recombinant UDP-glucuronosyltransferase (UGT) 1A6. AzMC was synthesized and utilized to label the substrate binding site of UGT1A6. AzMC exhibits a fluorescence spectrum with maximum excitation and emission wavelengths of 380 and 442 nm, respectively. Upon irradiation, the probe irreversibly inhibited glucuronidation activity measured with para-nitrophenol (pNP) as substrate and interacted with UGT1A6 according to a saturable process indicative of reversible binding before covalent incorporation of the photoaffinity label. This inhibition was both time and concentration dependent and led to the calculation of an inhibition constant, k(2) = 0.113 mM min(-1), and dissociation constant, K(d) = 2.89 mM, for the reaction. Partial photoinactivation of UGT1A6 with AzMC revealed that the probe decreased the apparent V(max) of the pNP glucuronidation reaction, but not the K(m). Moreover, inhibition was partially prevented by 1-naphthol, a surrogate substrate for the enzyme, or by preincubation with an active-site directed inhibitor, 5'-O-[[(2-decanoylamino-3-phenyl-propyloxycarbonyl)amino]-su lfonyl]-2 ',3'-O-isopropylideneuridine. In contrast, UDP-glucuronic acid (UDP-GlcUA) did not have any protective effect against photoinactivation and AzMC did not affect the photoaffinity labeling of UGT1A6 by 5-[beta-(32)P]N(3)UDP-GlcUA, a photoaffinity analog of UDP-GlcUA. Additionally, in the absence of irradiation, AzMC was found to be a competitive inhibitor of 4MU glucuronidation. Collectively, these results strongly indicate that AzMC specifically binds to the UGT1A6 aglycon binding site. Amino acid alignment of phenol-binding proteins revealed a conserved motif, YXXXKXXPXP. It is possible that this motif is involved in phenol binding to UGT1A6 and other phenol-accepting proteins.

Topological photoaffinity labeling of the rabbit ileal Na+/bile-salt-cotransport system

For the investigation of the topology of the rabbit ileal Na+/bile-salt-cotransport system, composed of a 93-kDa integral membrane protein and a peripheral 14-kDa bile-acid-binding protein (ILBP), we have synthesized photolabile dimeric bile-salt-transport inhibitors (photoblockers), G1-X-G2, where two bile acid moieties (G1 and G2) are tethered together via a spacer, X, and where one of the two bile acid moieties carries a photoactivatable group. These photoblockers specifically interact with the ileal Na+/bile-salt-cotransport system as demonstrated by a concentration-dependent inhibition of [3H]cholyltaurine uptake by rabbit ileal brush-border membrane vesicles and by inhibition of photolabeling of the 93-kDa and 14-kDa bile-salt-binding proteins by 7,7-azo and 3,3-azo derivatives of cholyltaurine. Ileal bile-salt uptake was specifically inhibited by the photoblockers, which were not taken up themselves by the small intestine as demonstrated by in vivo ileal perfusion. Dependent on the photoblocker used several polypeptides in the molecular-mass range of 14-130 kDa were labeled. The cytoplasmically attached 14-kDa ILBP was significantly labeled only by inhibitors that are photoactivatable in bile acid moiety G1, suggesting that during binding and translocation of a bile-salt molecule by the ileal bile-salt-transport system the steroid nucleus gets access to the cytoplasmic site of the ileal brush-border membrane first. Photoaffinity labeling in the frozen state with the transportable 3,3-azo and 7,7-azo derivatives of cholyltaurine revealed a time-dependent increase in the extent of labeling of the 14-kDa and 93-kDa proteins, suggesting a labeling of these proteins from the cytoplasmic site of the ileal brush-border membrane. By photoaffinity labeling in the frozen state with the various photoblockers time-dependent changes in the extent of photoaffinity labeling of bile-salt-binding proteins were observed, demonstrating the possibility of topological analysis of the rabbit ileal Na+/bile-salt-cotransport system.

Relationship between structure and intestinal absorption of bile acids with a steroid or side-chain modification

A structure-activity relationship for bile acid (BA) intestinal absorption is known to exist. To better understand the BA structural requirements for optimal BA intestinal absorption, rabbit ileal perfusion studies were performed. Unconjugated BA: Ursodeoxycholic (UDCA) and ursocholic acid (UCA) with methyl (6MUDCA and 6MUCA) or fluoro group (6FUDCA and 6FUCA) in the 6 position and UCA with a methyl group in 23 position (23MUCA) were compared with unconjugated UDCA, UCA, deoxycholic (DCA), chenodeoxycholic (CDCA), hyocholic (HCA), and hyodeoxycholic (HDCA) acid. BA lipophilicity was evaluated by their octanol-water partition coefficient. Conjugated BA: Taurine-conjugated UDCA and UCA with a methyl group in the 23 position (T23MUDCA and T23MUCA) were compared with the corresponding taurine-conjugated natural analogs. An analog of glycine-conjugated UDCA with the C24 amide bond replaced by a -CO-CH2- in the 24 position (24PUDCA) was studied and results were compared with the natural form (GUDCA). Unconjugated BA absorption was dose dependent (i.e., passive) and followed their lipophilicity: DCA > 6MUDCA > CDCA > HDCA > UDCA > HCA > 6FUDCA > 6MUCA > 6FUCA > UCA. Conjugated BA absorption was active, and Vmax was in the following order: TCA > TUDCA > TUCA > T23MUCA > T23MUDCA > 24PUDCA > GUDCA. 24PUDCA transport was also active and higher than GUDCA.

CONCLUSION

Passive transport is dependent on BA lipophilicity. Conjugated BAs are actively transported, and the presence of a 23-C methyl group does not improve transport when compared with the natural analogs. The substitution of the C24 amide bond with a -CO-CH2-still affords interaction of the BA with the intestinal transport carrier.

The rabbit ileal lipid-binding protein. Gene cloning and functional expression of the recombinant protein

A bile-acid-binding protein of Mr 14000 has been previously identified by photoaffinity labeling in rabbit ileal brush border membrane vesicles [Kramer et al. (1993) J. Biol. Chem. 268, 18035-18046]. This peripheral membrane-associated protein was purified and identified as an ileal lipid-binding protein. It was further shown to be identical to the cytosolic 14-kDa bile-acid-binding protein from the same tissue. Starting with sequence information from tryptic fragments, we cloned and sequenced the gene and its transcript. It has four exons (123, 176, 90, 115 bp) and three introns (1372, 2291, 3137 bp) and a similar structure as the genes from other members of the fatty-acid-binding protein family. The deduced protein has 128 amino acid residues and a calculated molecular mass of 14404 Da. It exhibits high similarity to its human (83%), mouse (77%), rat (76%) and porcine (72%) counterparts. Furthermore, the recombinant protein was produced in Escherichia coli and shown to be identical to native protein from ileal tissue. Functionality of the recombinant protein was demonstrated by labeling with various photoaffinity derivatives of bile acids. Ranking of the photolabeling efficiency of these probes towards the recombinant protein was comparable to the respective ranking towards the native protein. Polyclonal antibodies that were raised in hens against the recombinant protein, specifically recognized the ileal lipid-binding protein in the brush border membrane and cytosol from rabbit ileum. In contrast, no labeling was observed with jejunal tissue. Our results suggest a specific role of the membrane-associated ileal lipid-binding protein for the process of ileal bile acid uptake.

Effect of antisense oligonucleotides on the expression of hepatocellular bile acid and organic anion uptake systems in Xenopus laevis oocytes

A Na(+)-dependent bile acid (Na+/taurocholate co-transporting polypeptide; Ntcp) and a Na(+)-independent bromosulphophthalein (BSP)/bile acid uptake system (organic-anion-transporting polypeptide; oatp) have been cloned from rat liver by using functional expression cloning in Xenopus laevis oocytes. To evaluate the extent to which these cloned transporters could account for overall hepatic bile acid and BSP uptake, we used antisense oligonucleotides to inhibit the expression of Ntcp and oatp in Xenopus laevis oocytes injected with total rat liver mRNA. An Ntcp-specific antisense oligonucleotide co-injected with total rat liver mRNA blocked the expression of Na(+)-dependent taurocholate uptake by approx. 95%. In contrast, an oatp-specific antisense oligonucleotide when co-injected with total rat liver mRNA had no effect on the expression of Na(+)-dependent taurocholate uptake, but it blocked Na(+)-independent uptake of taurocholate by approx. 80% and of BSP by 50%. Assuming similar expression of hepatocellular bile acid and organic anion transporters in Xenopus laevis oocytes, these results indicate that Ntcp and oatp respectively represent the major, if not the only, Na(+)-dependent and Na(+)-independent taurocholate uptake systems in rat liver. By contrast, the cloned oatp accounts for only half of BSP transport, suggesting that there must be additional, non-bile acid transporting organic anion uptake systems in rat liver.

Current concepts of hepatic uptake, intracellular transport and biliary secretion of bile acids: physiological basis and pathophysiological changes in cholestatic liver dysfunction

Hepatic sinusoidal uptake of bile acids is mediated by defined carrier proteins against unfavourable concentration and electrical gradients. Putative carrier proteins have been identified using bile acid photoaffinity labels and more recently using immunological probes, such as monoclonal antibodies. At the sinusoidal domain, proteins with molecular weights of 49 and 54 kDa have been shown to be carriers for bile acid transport. The 49 kDa protein has been associated with the Na(+)-dependent uptake of conjugated bile acids, while the 54 kDa carrier has been involved in the Na(+)-independent bile acid uptake process. Within the hepatocyte, cytosolic proteins, such as the glutathione S-transferase (also designated the Y protein), the Y binders and the fatty acid binding proteins, are able to bind bile acids and possibly facilitate their movement to the canalicular domain. At the canalicular domain a 100 kDa carrier protein has been isolated and it has been shown by several laboratories that this particular protein is concerned with canalicular bile acid transport. The system is ATP-dependent and follows Michaelis-Menten kinetics. Interference with bile acid transport has been demonstrated by several chemicals. The mechanisms by which these chemicals inhibit bile acid transport may explain the apparent cholestatic properties observed in patients and experimental animals treated with these agents. Several studies have shown that Na+/K(+)-ATPase activity is markedly decreased in cholestasis induced by ethinyloestradiol, taurolithocholate and chlorpromazine. However, other types of interference have been described and the cholestatic effects may be the result of several mechanisms. Cholestasis is associated with several adaptive changes that may be responsible for the accumulation of bile acids and other cholephilic compounds in the blood of these patients. It may be speculated that the nature of these changes is to protect liver parenchymal cells from an accumulation of bile acids to toxic levels. However, more detailed quantitative experiments are necessary to answer questions with regard to the significance of these changes and the effect of various hepatobiliary disorders in modifying these mechanisms. It is expected that the mechanisms by which bile acid transport is regulated and efforts to understand the molecular basis for these processes will be among the areas of future research.

Species differences in hepatic bile acid uptake: comparative evaluation of taurocholate and tauroursodeoxycholate extraction in rat and rabbit

Dose-response curves for taurocholate and tauroursodeoxycholate were performed in rat and rabbit livers to get more insight into species differences in the hepatic bile acid uptake. The bile acids showed saturation kinetics in both animals, the Vmax in rat being higher than in rabbit and the Km being lower in the rat than in the rabbit for both the bile acids, with no significant difference in the hepatic cells morphometric parameters. Therefore, it is possible that differences in the kinetic parameters are related to the number and, to a lesser extent, to the affinity of the transporters on the sinusoidal plasma membranes.

Radiation-inactivation analysis of the Na+/bile acid co-transport system from rabbit ileum

The functional-unit molecular size of the Na+/bile acid cotransport system and the apparent target size of the bile-acid-binding proteins in brush-border membrane vesicles from rabbit ileum were determined by radiation inactivation with high-energy electrons. The size of the functional transporting unit for Na(+)-dependent taurocholate uptake was determined to 451 +/- 35 kDa, whereas an apparent molecular mass of 434 +/- 39 kDa was measured for the Na(+)-dependent D-glucose transport system. Proteins of 93 kDa and 14 kDa were identified as putative protein components of the ileal Na+/bile acid cotransporter in the rabbit ileum, whereas a protein of 87 kDa may be involved in passive intestinal bile acid uptake. Photoaffinity labelling with 3- and 7-azi-derivatives of taurocholate revealed a target size of 229 +/- 10 kDa for the 93 kDa protein, and 132 +/- 23 kDa for the 14 kDa protein. These findings indicate that the ileal Na+/bile acid co-transport system is in its functional state a protein complex composed of several subunits. The functional molecular sizes for Na(+)-dependent transport activity and the bile-acid-binding proteins suggest that the Na+/bile acid co-transporter from rabbit ileum is a homotetramer (AB)4 composed of four AB subunits, where A represents the integral 93 kDa and B the peripheral 14 kDa brush-border membrane protein.

Binding proteins for cyclosomatostatins and bile acids in basolateral plasma membranes of rat liver

The bile acids cholate and taurocholate on the one hand and the cyclopeptide c(Phe-Thr-Lys-Trp-Phe-D-Pro) (008), an analog of somatostatin with retro sequence, on the other hand, display mutually competitive transport inhibition into isolated rat hepatocytes. This indicates a common transport system for bile acids and cyclosomatostatins in sinusoidal rat liver plasma membranes. In order to identify and isolate common binding and/or transport proteins for bile acids and the cyclopeptides by affinity chromatography, the bile acid derivative 4'-amino-7-benzamidotaurocholate (ABATC) and the cyclosomatostatin-analog 008 were attached to a gel matrix. Two methods were used to prepare integral membrane proteins: (1) alkaline EDTA extraction and (2) Triton X-114 phase separation. Octyl glycoside solubilized, alkaline EDTA-extracted integral basolateral membrane proteins with apparent molecular masses of 52 and 48 kDa bound specifically to the ABATC affinity matrix. Two-phase Triton X-114 separated integral membrane proteins of the same molecular masses bound specifically to the cyclosomatostatin ligand. The 48 kDa ABATC and 008 binding protein was shown to be present in the basolateral plasma membrane fraction and in the microsomal fraction. The isolated 52 kDa ABATC binding protein was localized only in basolateral plasma membranes and could not be found in isolated microsomes.

Purification and partial sequence of proteins involved in the cholic acid transport into rat liver hepatocytes

Two proteins, in previous work labeled by affinity markers derived from taurocholic acid, were purified and partially sequenced. Antibodies were raised against purified proteins, and cross-reactions were carefully checked. The influence of these antibodies upon taurocholic acid import into vesicles from rat liver plasma membranes was measured, and showed a distinct inhibition of transport in the case of the 54 kD protein.

Two different mRNAs from rat liver code for the transport of bumetanide and taurocholate in Xenopus laevis oocytes

The aim of this study was to elucidate whether bumetanide, which is a competitive inhibitor of carrier mediated bile acid uptake in liver cells, is transported by bile acid carriers. The expression of hepatocellular transport proteins for bile acid uptake and the uptake of the loop diuretic bumetanide was therefore studied in Xenopus laevis oocytes by injection of rat liver poly(A)(+)-RNA. Three hours after injection, a 70% increase in [3H]taurocholate uptake versus noninjected oocytes was accompanied by an increase in only 24% in the uptake of [3H]bumetanide. Size fractionation of the poly(A)(+)-RNA yielded 33 mRNA fractions of which fraction 21 accounted for an 800% increase of taurocholate transport with only a slight increase in bumetanide uptake. Bumetanide transport was coded by mRNA-fraction 18, which stimulated uptake by 160-200% with a concomitant small increase in taurocholate uptake. Uptake of cholate was induced by both mRNA fractions with almost 2.5 fold greater expression by the bumetanide fraction. Oocyte transport of taurocholate (expressed by fraction 21) and bumetanide transport (expressed by fraction 18) were characterized in terms of Na+ dependency, inhibition by 4,4'-diisothiocyano-1,2-diphenylethane-2,2'-disulfonic acid (DIDS) and mutual competition. The results indicate that the bumetanide transporter mRNA is clearly different from the mRNA for the taurocholate transport protein. The mRNA fraction 18 was used for the construction of a cDNA library.

Two distinct types of SH-groups are necessary for bumetanide and bile acid uptake into isolated rat hepatocytes

Substances that block SH-groups were studied in respect to their effects on the uptake of the loop diuretic bumetanide and the bile acids cholate and taurocholate into isolated rat hepatocytes. SH-blockers, e.g., p-chloromercuribenzenesulfonate (PCMBS), N-ethylmaleimide (NEM), dithiobis-nitropyridine (DTNP) and dithiobis-2-nitrobenzoic acid (DTNB) reduced bumetanide transport in a concentration-dependent manner. Inhibition of the organic mercurial PCMBS was reversed by the addition of 500 microM dithiothreitol (DTT), indicating an interaction of this substance with free SH-groups. NEM irreversibly blocked SH-groups by covalent binding and was the most effective inhibitor of bumetanide and cholate uptake. In contrast, PCMBS was the most effective inhibitor of taurocholate uptake. Photoaffinity studies with [3H]bumetanide and [3H]7,7-azotaurocholate were performed with isolated rat hepatocytes in the presence of PCMBS and DTNP. Binding of the photolabels was not reduced by SH-group blockers. Newly synthesized sulfhydryl-modifying reagents such as dithio-sulfonate-ethyl-nitrobenzoic acid (DTSNB) and dithio-octyl-nitrobenzoic acid (DTONB), are derivatives of the alkylating agent DTNB. DTSNB is regarded as a selective blocker for SH-groups in a hydrophilic environment, while DTONB is more lipophilic abd interacts with SH-groups in the transmembrane domain of transport proteins. The IC50-values of these blockers for bumetanide uptake (DTSNB 250 microM, DTONB 141 microM) and for cholate uptake (DTSNB 250 microM, DTONB 115 microM) were almost identical. These findings support the concept of a common uptake mechanism for cholate and bumetanide and indicate that two distinct moieties of SH-groups are required for the uptake of both organic anions. One of these is probably located on the outer surface and the other within the membrane of hepatocytes.

Serum bile acids in companion animal medicine

Quantification of total serum bile acids is used as a method for appraising liver function and perfusion in contemporary small animal practice. This article provides a historical perspective and a comprehensive review of bile acid physiology, laboratory methodologies for bile acid quantification, and normal values published for total serum bile acids measured using the spectrophotometric enzymatic method and for serum bile acids measured using validated radioimmunoassay procedures. The variables influencing the enterohepatic circulation of bile acids and, consequently, the fasting and postprandial serum bile acid concentrations are discussed with the intent of clarifying the application of test results to clinical patients. A brief discussion of the therapeutic use of dehydrocholate and ursodeoxycholic acid in clinical patients is provided.

Characterization and chemical modification of the Na(+)-dependent bile-acid transport system in brush-border membrane vesicles from rabbit ileum

The Na(+)-dependent uptake system for bile acids in the ileum from rabbit small intestine was characterized using brush-border membrane vesicles. The uptake of [3H]taurocholate into vesicles prepared from the terminal ileum showed an overshoot uptake in the presence of an inwardly-directed Na(+)-gradient ([Na+]out > [Na+]in), in contrast to vesicles prepared from the jejunum. The Na(+)-dependent [3H]taurocholate uptake was cis-inhibited by natural bile acid derivatives, whereas cholephilic organic compounds, such as phalloidin, bromosulphophthalein, bilirubin, indocyanine green or DIDS - all interfering with hepatic bile-acid uptake - did not show a significant inhibitory effect. Photoaffinity labeling of ileal membrane vesicles with 3,3-azo- and 7,7-azo-derivatives of taurocholate resulted in specific labeling of a membrane polypeptide with apparent molecular mass 90 kDa. Bile-acid derivatives inhibiting [3H]taurocholate uptake by ileal vesicles also inhibited labeling of the 90 kDa polypeptide, whereas compounds with no inhibitory effect on ileal bile-acid transport failed to show a significant effect on the labeling of the 90 kDa polypeptide. The involvement of functional amino-acid side-chains in Na(+)-dependent taurocholate uptake was investigated by chemical modification of ileal brush-border membrane vesicles with a variety of group-specific agents. It was found that (vicinal) thiol groups and amino groups are involved in active ileal bile-acid uptake, whereas carboxyl- and hydroxyl-containing amino acids, as well as tyrosine, histidine or arginine are not essential for Na(+)-dependent bile-acid transport activity. The irreversible inhibition of [3H]taurocholate transport by DTNB or NBD-chloride could be partially reversed by thiols like 2-mercaptoethanol or DTT. Furthermore, increasing concentrations of taurocholate during chemical modification with NBD-chloride were able to protect the ileal bile-acid transporter from inactivation. These findings suggest that a membrane polypeptide of apparent M(r) 90,000 is a component of the active Na(+)-dependent bile-acid reabsorption system in the terminal ileum from rabbit small intestine. Vicinal thiol groups and amino groups of the transport system are involved in Na(+)-dependent transport activity, whereas other functional amino acids are not essential for transport activity.

Separation and purification by two-dimensional gel electrophoresis of a 52-54 kDa bumetanide binding protein from rat liver plasma membranes

By affinity labeling with photolabile [3H]bumetanide, a 52-54 kDa bumetanide binding protein was identified in the sinusoidal plasma membrane fraction from rat liver. The protein is assumed to represent the carrier for hepatic uptake of loop diuretics. By two-dimensional (2D) gel electrophoresis we have purified this protein from hepatocytes, sinusoidal plasma membranes and subfractions of associated and integral plasma membrane proteins. Amongst more than 20 protein spots, a single integral plasma membrane protein was detected. The apparent pI of this molecule is 6.7. Specific labeling of this protein was not found in the fraction of associated plasma membrane proteins. To detect possible binding of radioactive bumetanide to microsomal cytochrome P450s, photolabeling experiments with integral plasma membrane proteins were performed under nitrogen/carbon monoxide atmosphere and in the presence of piperonyl butoxide. Labeling of the 52-54 kDa protein was not affected by these inhibitors of P450 enzymes. Taken together, these results indicate that the bumetanide binding protein is very likely to be a non-microsomal integral plasma membrane protein.

Investigations on the hepatic uptake systems for organic cations with a photoaffinity probe of procainamide ethobromide

Azido procainamide methoiodide (APM), a photolabile derivative of the transport model compound procainamide ethobromide (PAEB), shows a close resemblance to PAEB from a physicochemical point of view. Like PAEB it is effectively taken up by the liver and excreted into bile. Kinetics of the uptake of APM in isolated hepatocytes revealed that in addition to a non-saturable process, two saturable uptake systems are involved (Km1 = 3 microM, Vmax1) = 80 pmol/min/10(6) cells, Km2 = 100 microM, Vmax2 = 130 pmol/min x 10(6) cells). The uptake rate of APM was inhibited markedly in the presence of other organic cations. Organic anions and uncharged compounds generally had no inhibitory effect on the APM uptake. These results support the theory that there is a separate hepatic uptake system for organic cations like APM. Photoaffinity labeling of intact hepatocytes as well as plasma membrane sub-fractions enriched with sinusoidal domains disclosed two major binding polypeptides with apparent M(r) of 48,000 and 72,000. Such labeling patterns were not observed in membranes from hepatoma cells that are deficient in organic solute uptake. Differential photoaffinity labeling with other cationic compounds such as tributylmethyl ammonium and d-tubocurarine reduced the incorporation of APM in these polypeptides. The 48- and 72-kDa proteins might be involved in carrier-mediated transport of type I organic cations at the hepatic uptake level.

Characterization of the transport of a synthetic bile salt, iodinated cholyl-glycyl-tyrosine, in isolated cultured rat hepatocytes

The uptake of tri-hydroxy conjugated bile salts by hepatocytes is principally by a sodium-dependent carrier. We examined the uptake kinetics of the high-specific-activity, hydroxylated, conjugated bile salt 125I-labeled cholyl-glycyl-tyrosine, to determine whether this synthetic bile salt was transported by the sodium-dependent bile salt system. 125I-labeled cholyl-glycyl-tyrosine was synthesized, and its transport kinetics were studied in freshly cultured rat hepatocytes. Uptake into hepatocytes was time and temperature dependent and was decreased by the inhibitors diisothiocyanodisulfonic acid stilbene, probenecid and carbonyl cyanide chlorophenyl hydrazone, demonstrating carrier mediation and energy dependence. At concentrations of iodinated cholyl-glycyl-tyrosine less than 10 mumol/L, uptake was 27% +/- 5% sodium dependent, whereas at concentrations from 10 mumol/L to 40 mumol/L uptake was 52% +/- 4% sodium dependent. The apparent affinity for uptake of 125I-labeled cholyl-glycyl-tyrosine was 8 +/- 2 mumol/L, and the maximal velocity was 50 +/- 20 pmol/micrograms DNA/min. Both taurocholate and indocyanine green inhibited uptake of 125I-labeled cholyl-glycyl-tyrosine. Indocyanine green inhibited the uptake of 125I-labeled cholyl-glycyl-tyrosine (Ki = 10 microns) more effectively than taurocholate (Ki = 20 microns). We conclude that 125I-labeled cholyl-glycyl-tyrosine is not a specific probe for either sodium-dependent bile salt or sodium-independent organic anion carriers, but appears to use both systems in a concentration-dependent manner in cultured rat hepatocytes.

Binding proteins for linear renin-inhibiting peptides in basolateral plasma membranes of rat liver

A linear hydrophobic peptide, (Code no. EMD 55068), a synthetic renin-antagonist, competitively inhibits the uptake of taurocholate and of another linear peptide (EMD 51921) but not of oleic acid, serine or thiamin hydrochloride into isolated rat liver cells. EMD 55068 was attached to a gel matrix at a position that is not involved in the protein ligand interaction. The gel matrix used did not interact nonspecifically with solubilized proteins from rat liver. The quantity of bound ligand was determined to be 3.6 mg/ml of gel matrix. In the fraction of EDTA extracted hydrophilic membrane-associated proteins, no binding proteins were detected. Affinity chromatography of integral plasma membrane proteins resulted in four protein bands with molecular masses of 46, 49, 53 and 56 kDa in SDS-PAGE. In contrast, solubilized plasma membrane proteins from AS-30D ascites hepatoma cells, which are unable to transport bile acids and linear peptides, did not bind specifically to the affinity matrix.

Functional molecular mass of the 14C-azidobenzamidotaurocholic acid binding proteins in hepatocellular bile acid transport systems

The apparent target size of 14C-azidobenzamidotaurocholate binding proteins in basolateral rat liver plasma membranes (blPm) was determined by analysis of the radiation induced decrease of the binding of this photoreactive taurocholate analog to blPm. Radiation causes a dose-dependent mono-exponential reduction of binding of ABATC to the protein subunits with molecular masses of 48-50 and 52-54 kDa in SDS-PAGE. The minimal functional molecular mass of the 48-50 and 52-54 kDa ABATC binding proteins was determined to be 99 +/- 8.2 and 93.2 +/- 7 kDa, respectively.

Subcellular and molecular mechanisms of bile secretion

One of the liver's principal functions is the formation of bile, which is requisite for digestion of fat and elimination of detoxified drugs and metabolites. Bile is a complex fluid made up of water, electrolytes, bile acids, pigments, proteins, lipids, and a multitude of chemical breakdown products. In this review, we have summarized the source of various biliary components, the route by which they end up in bile, including the underlying subcellular and molecular mechanisms, and their contribution to bile formation. One of the reasons why bile formation is so complex is that there are many mechanisms with overlapping substrate specificities, i.e., many biochemically unrelated biliary constituents share common transport mechanisms. Additionally, biliary constituents may reach bile by more than one pathway. Some biliary components are critical for bile formation; others are of minor significance for bile formation but play a major physiological role. The major driving force for bile formation is the uptake and transcellular transport of bile salts by hepatocytes. The energy for bile formation comes from the sodium gradient created by the basolateral Na+/K(+)-ATPase, to which bile salt transport is coupled. The secretory pathway for bile salts involves uptake at the basolateral surface of the hepatocyte, vectorial transcellular movement, and transport across the canalicular membrane into the canalicular lumen. Hydrophilic bile salts are taken up via a sodium-dependent, saturable, carrier-mediated process coupled to the Na+/K(+)-ATPase. This uptake mechanism is also shared by other substrates, such as electroneutral lipids, cyclic oligopeptides, and a wide variety of drugs. Hydrophobic bile acids are taken up by a sodium-independent facilitated carrier-mediated mechanism in common with other organic ions, including sulfated bile acids, sulfobromophthalein, bilirubin, glutathione, and glucuronides, or by nonsaturable passive diffusion. Two major carrier proteins have been identified on the hepatocyte basolateral membrane: a 48-kDa protein that appears to be involved with Na(+)-dependent bile salt uptake, and a 54-kDa protein, thought to be associated with Na(+)-independent bile salt uptake. The intracellular transport of bile salts may involve cytosolic carrier proteins, of which several have been identified. Some evidence suggests a vesicular transport mechanism for bile salts. Since bile acids clearly do not enter the cell by endocytosis, formation of transport vesicles must be a more distal event in the transcellular translocation process. Some bile salts appear to be transported within the same unilamellar vesicles that are involved in the secretion of cholesterol and phospholipid.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolic zonation of the liver: regulation and implications for liver function

Liver parenchyma shows a remarkable heterogeneity of the hepatocytes along the porto-central axis with respect to ultrastructure and enzyme activities resulting in different cellular functions within different zones of the liver lobuli. According to the concept of metabolic zonation, the spatial organization of the various metabolic pathways and functions forms the basis for the efficient adaptation of liver metabolism to the different nutritional requirements of the whole organism in different metabolic states. The present review summarizes current knowledge about this heterogeneity, its development and determination, as well as about its significance for the understanding of all aspects of liver function and pathology, especially of intermediary metabolism, biotransformation of drugs and zonal toxicity of hepatotoxins.

Intrahepatic cholestasis: a review of biochemical-pathological mechanisms

Intrahepatic cholestasis involves impaired excretion of bile via the hepatobiliary system as a consequence of one or more lesions within the liver. In humans, intrahepatic cholestasis most often results as a side-effect of drug therapy and the clinical manifestation of this condition, jaundice, has been estimated to account for hospitalization in 2 to 5% of the cases for the general population and approaches as much as 20% in the elderly. With the aging of the population and the common occurrence of poly-drug therapy in geriatric patients, it is to be expected that jaundice due to drug-induced intrahepatic cholestasis will become even more prevalent, and accordingly the need to understand the basic mechanisms of this disease condition will become more urgent. The list of culprit agents implicated in the induction of intrahepatic cholestasis in humans is continually expanding. These include various steroid hormones, bile acids, drugs and other chemicals. Experimentally, a wide spectrum of agents has been shown to precipitate intrahepatic cholestasis. Over the years, a number of hypotheses on the biochemical and pathological mechanisms of intrahepatic cholestasis has emerged, including the following: impaired sinusoidal membrane function; interference with the distribution and binding of cytoplasmic endogenous carrier proteins; interference with mitochondrial energy supply; defects in the canalicular membrane including altered Na+/K+ -ATP-ase activity; impairment of microfilament and microtubule functions; interference with bile secretion involving bile acid dependent and independent fractions, and altered bile acid metabolism due to "hypoactive hypertrophic smooth endoplasmic reticulum". In partial agreement with the latter hypothesis, our studies indicated that impairment of the endoplasmic reticulum might represent one of the early stages in the development of intrahepatic cholestasis. Various experimental conditions that induce intrahepatic cholestasis to different degrees resulted in an interference of the synthesis of microsomal phospholipids and altered microsomal function. The conditions included the administration of various hepatotoxic compounds or steroids, pregnancy, delayed development of the endoplasmic reticulum in neonates, and dietary methyl donor or choline deficiency. This review reports the biochemical-pathological mechanisms postulated to be involved in the genesis of intrahepatic cholestasis with specific reference to experimental models of drug-induced intrahepatic cholestasis. The important practical implications of cholestasis are also briefly surveyed.

Photoaffinity labeling of plasma membrane proteins involved in the transport of loop diuretics into hepatocytes

To identify proteins involved in the hepatocellular uptake of loop diuretics, [3H]bumetanide was photoactivated by light flash in the presence of either intact isolated rat hepatocytes, rat liver basolateral plasma membranes or integral membrane proteins extracted from the basolateral plasma membranes. Proteins of 52-54, 48, 33, 27, 25 and 23 kDa in sodium dodecyl sulfate (SDS) gel electrophoresis were radiolabeled on intact hepatocytes. On liver basolateral plasma membranes a 50-52 kDa protein was the most intensely labeled protein. After separation into integral and associated membrane proteins by extraction with Triton X-114, radioactive labeling was only found in integral membrane proteins with a molecular weight of 50-52 kDa. Photoactivated bumetanide irreversibly inhibited the hepatocellular uptake of cholate, taurocholate but not of serine. Binding proteins for photoactivated bumetanide were absent on AS 30-D ascites hepatoma cells. Labeling of all proteins was sodium dependent in intact hepatocytes but was sodium independent in plasma membranes. Labeling was prevented by non-labeled bumetanide and by the loop diuretics piretanide and furosemide. Labeling protection was further achieved with organic anions such as bromosulfophthalein, rifampicin, probenecid and by the bile acids taurocholate, deoxycholate and dehydrocholate. The radiolabeled proteins did not belong to the bumetanide-sensitive NaCl/KCl co-transport system which apparently does not occur in intact isolated rat hepatocytes.

Characterization of a novel 14 kDa bile acid-binding protein from rat ileal cytosol

A 14 kDa polypeptide in rat ileal cytosol has been identified as the major intestinal cytosolic bile acid-binding protein (I-BABP) by photoaffinity labeling with the radiolabeled 7,7-azo derivative of taurocholate (7,7-azo-TC). To further characterize I-BABP, the protein was purified by lysylglycocholate Sepharose 4B affinity and DE-52 anion-exchange chromatography. The purified I-BABP contained a single 14 kDa band on SDS-PAGE. The 14 kDa protein showed a 26-fold increase in binding affinity for [3H]7,7-azo-TC compared to cytosolic protein. Immunoblotting of protein fractions separated by affinity chromatography showed that neither liver fatty acid binding protein (L-FABP) nor intestinal fatty acid binding protein (I-FABP) bind to the affinity column and that the 14 kDa protein which bound to the column and was subsequently eluted with detergent did not cross-react with anti-L-FABP or anti-I-FABP. The 14 kDa protein labeled with [3H]7,7-azo-TC was radioimmunoprecipitated from cytosol by rabbit antiserum raised against purified I-BABP. I-BABP was shown to have a blocked N-terminus; however, its mixed internal sequence generated from cyanogen bromide-cleaved protein and amino acid composition indicated that it was related to (although clearly distinct from) both I-FABP and L-FABP. These studies have isolated a 14 kDa bile acid-binding protein from rat ileal cytosol which is immunologically and biochemically distinct from I-FABP and L-FABP.

Membrane transport of conjugated and unconjugated bile acids into hepatocytes is susceptible to SH-blocking reagents

The present study indicates that SH-groups are essential for the uptake of [3H]taurocholate and [14C]cholate into isolated rat hepatocytes. Several sulfhydryl-modifying reagents viz. p-chloromercuribenzenesulfonate (PCMBS), N-ethylmaleimide (NEM), dithio-bis(5-nitropyridine) (DTNP), bromosuccinimide and HgCl2 inhibited uptake of bile acids in a concentration-dependent manner. PCMBS was the most effective inhibitor in the uptake of taurocholate, while NEM is preferentially blocking the cholate uptake. PCMBS inhibited both the sodium- dependent and the sodium-independent bile acid uptake. Two different moieties of SH-groups seemed to be important for bile acid transport. One group was susceptible to DTNP and NEM, whereas PCMBS was able to block another type of SH-groups in addition. Cell viability was altered by SH-blockers, except by PCMBS. Efflux studies with 86Rb+ demonstrated that the transmembrane potential of hepatocytes was less effected by 100 microM PCMBS in contrast to 100 microM HgCl2. Efflux of tetra[3H]phenylphosphonium and of [3H]aflatoxin in PCMBS-treated hepatocytes documented membrane integrity during at least 10 min. PCMBS did not reduce cellular ATP levels significantly (minus 7%) nor did it markedly increase the amount of the Trypan-blue stained hepatocytes (plus 8.5%). The blocking effect of PCMBS was immediate and was completely reversed by the addition of 500 microM dithiothreitol (DTT), indicating a specific interaction with sulfhydryl-groups. This antagonizing effect of DTT depends on the concentration and exposure time of PCMBS. Six other thiols viz. 2-mercaptoethanol, 1,2-dimercaptoethane, 1,4-dimercaptobutane, 1,6-dimercaptohexane, L-cysteine and L-glutathione were less effective. The results suggest that free SH-groups on the outer surface of hepatocytes play an important role in the uptake process for conjugated and unconjugated bile acids.