Isolation and characterization of the putative canalicular bile salt transport system of rat liver

Use ofin vitrotransporter assays to understand hepatic and renal disposition of new drug candidates

Hepatic and renal transporters contribute to the uptake, secretion and reabsorption of endogenous compounds, xenobiotics and their metabolites and have been implicated in drug-drug interactions and toxicities. Characterising the renal and hepatic disposition of drug candidates early in development would lead to more rational drug design, as chemotypes with 'ideal' pharmacokinetic characteristics could be identified and further refined. Because transporters are often organ specific, 'custom' transporter panels need to be identified for each major organ and chemotype to be evaluated, and appropriate studies planned. This review outlines the major renal and hepatic transporters and some of the in vitro transporter reagents, assays and processes that can be used to evaluate the renal and hepatic disposition of new chemical entities during drug discovery and development.

Functional expression of the canalicular bile salt export pump of human liver

BACKGROUND & AIMS

Hepatic bile salt secretion is an essential function of vertebrate liver. Rat and mouse bile salt export pump (Bsep) are adenosine triphosphate (ATP)-dependent bile salt transporters. Mutations in human BSEP were identified as the cause of progressive familial intrahepatic cholestasis type 2. BSEP protein is highly identical with its rat and mouse orthologs and has not yet been functionally characterized; the effect of BSEP mutations on its function has also not been studied. Therefore, the aim of this study was to functionally characterize human BSEP.

METHODS

Complementary DNA for BSEP was isolated from human liver and expressed with the baculovirus system in Sf9 cells. ATP-dependent bile salt transport assays were performed with Sf9 cell vesicles expressing BSEP and a rapid filtration assay.

RESULTS

Cloning of human BSEP required the inactivation of a bacterial cryptic promoter motif within its coding region. BSEP expressed in Sf9 cells transports different bile salts in an ATP-dependent manner with Michaelis constant values as follows: taurocholate, 7.9 +/- 2.1 micromol/L; glycocholate, 11.1 +/- 3.3 micromol/L; taurochenodeoxycholate, 4.8 +/- 1.7 micromol/L; tauroursodeoxycholate, 11.9 +/- 1.8 micromol/L. The rank order of the intrinsic clearance of bile salts was taurochenodeoxycholate > taurocholate > tauroursodeoxycholate > glycocholate.

CONCLUSIONS

This study characterizes human BSEP as an ATP-dependent bile salt export pump with transport properties similar to its rat and mouse orthologs. Expression of BSEP in Sf9 cells will enable functional characterization of the consequences of mutations in the human BSEP gene.

Benign recurrent intrahepatic cholestasis

Benign recurrent intrahepatic cholestasis is a rare autosomal recessive disorder characterized by repeated episodes of intense pruritus and jaundice. Each attack lasts from several weeks to months before resolving spontaneously. Patients are completely asymptomatic for months to years between symptomatic periods. The disorder does not lead to progressive liver disease. Although attacks seem to be associated with a viral prodrome, an inciting viral agent or toxin has not been defined. Genetic studies have mapped the defect of this disorder to the long arm of chromosome 18 and a gene that codes for a P-type ATPase, which appears to be involved in aminophospholipid transport. Therapy during symptomatic periods is supportive and aimed at relief of severe pruritus until the episode resolves spontaneously.

Enterohepatic circulation of scymnol sulfate in an elasmobranch, the little skate (Raja erinacea)

The sulfated bile alcohol scymnol sulfate (ScyS), 3 alpha,7 alpha,12 alpha,24 xi, 26,27-hexahydroxy-5 beta-cholestane-26(27)-sulfate, is the major bile salt in bile of an elasmobranch, the little skate. To investigate hepatic transport of bile alcohols in skate liver, [3H]ScyS and a potential precursor, 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestane (chtriol), were used as model compounds. Their transport into isolated hepatocytes was partially saturable, temperature sensitive, and Na+ independent. The uptake of ScyS was inhibited by cholyltaurine, and uptake of cholyltaurine was inhibited by ScyS in a competitive manner. In contrast, uptake of chtriol was not inhibited by cholyltaurine, suggesting separate transport systems. ScyS and chtriol showed a choleretic effect in isolated perfused livers. When ScyS was added to the perfusate of isolated perfused livers, > 25% was found in bile within 7 h. When chtriol was added to the perfusate, 10% of the dose was secreted into the bile mainly in the form of polar metabolites, whereas only nonmetabolized chtriol remained in the livers. The slow bile flow of 40-50 microliters/h and the high recovery in the liver suggest that metabolism may be the rate-limiting step in the hepatic elimination of chtriol. The major metabolites secreted into bile were identified by mass spectrometry and chromatography as scymnol and ScyS. To study the enterohepatic circulation, [3H]ScyS or [3H]chtriol was administered into the duodenum of free-swimming skates, and bile was collected through exteriorized indwelling cannulas over a 4-day period. More than 90% of the radioactivity was recovered from bile, indicating that there was a highly effective absorption in the intestinal epithelium, as well as specific transport mechanisms for hepatic uptake and biliary secretion of these compounds. This is the first direct demonstration of an enterohepatic circulation for a bile alcohol sulfate in fish liver.

Immunochemical identification of mouse hepatic protein adducts derived from the nonsteroidal anti-inflammatory drugs diclofenac, sulindac, and ibuprofen

Reactive metabolite-modified hepatic protein adducts have been proposed to play important roles in the mechanism(s) of hepatotoxicity of nonsteroidal anti-inflammatory drugs (NSAIDs). In the present study, immunochemical techniques have been used to compare the patterns of drug-protein adducts expressed in livers of mice given single doses of one or other of three different NSAIDs. These were diclofenac and sulindac, which are widely used but potentially hepatotoxic drugs, and ibuprofen, which is considered to be nonhepatotoxic. Specific polyclonal antisera were produced by immunization of rabbits with conjugates prepared by coupling each of the NSAIDs to the carrier protein keyhole limpet hemocyanin. Immunoblotting studies revealed dose-dependent formation of major 110 kDa polypeptide adducts in livers from mice sacrificed 6 h after administration of single doses of either diclofenac (0-300 mg/kg) or sulindac (0-100 mg/kg). Lower levels of several other adducts, of 140 and 200 kDa, were also expressed in livers from these animals. In contrast, livers from mice treated with ibuprofen (0-200 mg/kg) predominantly expressed a 60 kDa adduct and only relatively low levels of a 110 kDa adduct. The various adducts were shown by differential centrifugation to be concentrated in the nuclear fraction of liver homogenates. Those derived from diclofenac and sulindac were further localized, by Percoll density gradient centrifugation, to a subfraction which contained a high activity of the bile canalicular marker enzyme alkaline phosphatase. This suggests that they are concentrated in the bile canalicular domain of hepatocytes. The different patterns of adduct formation raise the possibility that formation of certain NSAID protein adducts, particularly 110 kDa adducts, has toxicological significance.

Differential ontogenic regulation of basolateral and canalicular bile acid transport proteins in rat liver

The hepatic transport systems mediating bile acid uptake and excretion undergo independent, stage-specific expression during development in the rat. In this study, the mechanisms underlying ontogenic regulation of both the Na(+)-dependent basolateral bile acid transporter and canalicular bile acid transporter/ecto-ATPase were examined. Steady state mRNA levels for the basolateral transporter were less than 20% of adult values prior to birth, increased to 35% on the first postnatal day, and reached adult levels by 1 week of age. This was paralleled by transcription rates, which were low prior to birth, reached 47% by day 1, and were maximal by 1 week of age. Steady state mRNA levels for ecto-ATPase were 12% of adult values prior to birth and showed a 2-fold increase by the first day of life. Thereafter, there was a gradual increase in mRNA for this transporter, with adult levels being reached at 4 weeks of age. Transcription rates paralleled this increment, although adult levels were reached earlier. Surprisingly, for both transporters, the full complement of protein was present well before adult levels of mRNA were reached. The basolateral protein was expressed at 82% of adult levels on the first day of life but was of lower apparent molecular mass (39 kDa), a difference that persisted until 4 weeks of age. N-Glycanase digestion suggested that this difference could be fully accounted for by N-linked glycosylation. The ecto-ATPase protein was present at 33% of adult levels prior to birth, 77% by 1 day, and 84% of adult levels by 1 week of age. Unlike the basolateral transporter, the apparent molecular weight of this protein did not change during development. In summary, the ontogeny of bile acid transporters on the plasma membrane of the hepatocyte is complex and appears to be regulated at transcriptional, translational, and post-translational levels.

Hepatic uptake and intestinal absorption of bile acids in the rabbit

The existence of transporters for bile acids (BA) in liver and intestine has been well documented, but information is still needed as to their respective transport capacity. In the present investigation, we compared the hepatic and intestinal transport rates for BA, using perfused livers and intestines. The livers and intestines were separately perfused and dose-response curves (0.25-10 mM) for tauroursodeoxycholate, taurocholate and taurodeoxycholate were obtained. The intestinal and mesenteric concentration and bile acid pattern were also evaluated in six non-fasting rabbits. Taurocholic, tauroursodeoxycholic and taurodeoxycholic acid ileal absorption showed saturation kinetics in the intestine as in the liver; the maximal uptake velocity for each bile acid in the liver was tenfold higher than the respective maximal transport velocity in the intestine; the Km values obtained in the liver were of the same order of magnitude, i.e. in the millimolar range. Taurocholic, tauroursodeoxycholic and taurodeoxycholic acid transport differences in the liver paralleled those in the intestine. Although the intestine was not homogeneously filled, the bile acid concentration in the ileal content fell into the range of the Km for the three studied bile acids, while the portal blood total bile acid concentration was inferior to the observed Kms of liver uptake. Therefore, both the hepatic and intestinal systems do not operate at their maximal transport rates at the prevailing concentrations in portal blood and luminal content, and the hepatic transport occurs at its highest efficiency (below the Km values) in physiological conditions.

Evidence for defective primary bile acid secretion in children with progressive familial intrahepatic cholestasis (Byler disease)

To clarify the relationship of progressive familial intrahepatic cholestasis (Byler disease) to bile acid metabolism, we analysed, by high performance liquid chromatography, the bile acid composition of serum and bile in seven children with Byler disease and in eight control children with other cholestatic diseases. In serum, total bile acid concentration was increased in patients with Byler disease (0.30 +/- 0.05 mmol/l) and in control patients (0.21 +/- 0.08 mmol/l). Cholate (C) and chenodeoxycholate (CDC) comprised the major proportion of total bile acids in patients with Byler disease as in control patients. Hyocholate (HC) was only detected in patients with Byler disease and lithocholate was only present in control children. In bile, total bile acid concentration was very low in patients with Byler disease (1.1 +/- 1.4 mmol/l) compared to control patients (88.9 +/- 83.2 mmol/l). C and CDC were the major bile acids in control patients, whereas C and HC comprised the major proportion of bile acids in patients with Byler disease. These results suggest the existence of a defect of primary bile acid secretion in Byler disease characterized by the presence of high concentration of bile acids in serum and absence or very low concentration of bile acids in bile.

Secretin prevents taurocholate-induced intrahepatic cholestasis in the rat

Secretin is known to stimulate the flow of bicarbonate-rich bile from the bile-duct epithelium, but has no effect on hepatocytes. To investigate the effects of secretin on bile production during intrahepatic cholestasis, we infused secretin into rats with taurocholate-induced cholestasis. Secretin was given at 0.25 and 0.50 units.min-1.kg-1 to Wistar rats that simultaneously received a continuous infusion of taurocholic acid at above its maximum hepatic transport capacity to produce cholestasis. When taurocholic acid was infused at doses of 1.4 and 1.6 mumol.min-1.100 g b.w.-1, bile volume decreased in control rats. In contrast, the simultaneous infusion of secretin significantly increased bile flow and the biliary excretion of bile acids and bicarbonate. The serum taurocholic acid level at the end of the experiment was significantly lower in the secretin-treated groups than in the control group. These findings indicate that secretin prevents taurocholate-induced cholestasis and may enhance the biliary excretion of bile acids.

Characterisation of the ATP-dependent taurocholate-carrier protein (gp110) of the hepatocyte canalicular membrane

The canalicular domain-specific glycoprotein gp110, which recently has been shown to function as an ATP-dependent taurocholate transporter, has been purified 1800-fold from rat liver plasma membranes. gp110 has been characterised as an integral plasma membrane protein with M(r) of 100,000-115,000 and pI of 2.5-3.5 and possesses a highly glycosylated and negatively charged extra-cellular domain. The broad range of M(r) and pI values results from the existence of numerous glycoforms composed of sialylated N-glycans. After deglycosylation, the polypeptide has M(r) 48,000 and pI 5.0. In primary cultures of rat hepatocytes, gp110 is synthesised with M(r) 110,000, while in the presence of tunicamycin the non-glycosylated form has M(r) 48,000. In the presence of 1-deoxymannojirimycin, two forms of M(r) 83,000 and M(r) 91,000 were found, which were converted by endo-beta-N-acetylglucosaminidase H into a single 52,000-M(r) band, indicating the existence of two basic glycoforms at the oligomannosyl stage of biosynthesis. gp110 was phosphorylated at serine residues in primary cultures of hepatocytes. The sequences of ten internal peptides of gp110 were identical to the sequence of the high-M(r) form of ecto-ATPase, but ecto-ATPase activity from plasma-membrane extracts was not depleted by anti-(gp110) serum. In contrast, Fab fragments of these antibodies inhibit the aggregation of freshly isolated hepatocytes.

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)

Effect of antiserum to a 99 kDa polypeptide on the uptake of taurocholic acid by rat ileal brush border membrane vesicles

A 99 kDa polypeptide in rat ileal brush border membrane (BBM), regarded as a component of the active bile acid transport system on account of photoaffinity labeling, has been purified by affinity chromatography and preparative gel electrophoresis and utilized as an immunogen for raising polyclonal antibody. Immune serum, but not preimmune serum, specifically recognized a single band of 99 kDa protein on immunoblots of ileal and renal BBM. In contrast, no reactivity was observed with proteins in jejunal BBM. This polyclonal antibody, compared with preimmune serum and anticytosolic bile acid binding protein (14 kDa) serum, significantly inhibited the Na+ dependent uptake of [3H] taurocholate by BBM vesicles (p less than 0.01). [14C] D-glucose uptake by BBM vesicles was not influenced by the immune serum (p less than 0.01). Thus, these studies provide further support for the specific role of a 99 kDa protein in ileal BBM bile acid transport.

Characterization of [3H]estradiol-17 beta-(beta-D-glucuronide) binding sites in basolateral and canalicular liver plasma membranes

The specific binding of [3H]estradiol-17 beta-(beta-D-glucuronide) ([3H]E217G) was examined in isolated basolateral (bLPM) and canalicular (cLPM) liver plasma membranes. Two distinct binding sites were identified in each membrane fraction by competition and saturation experiments. Binding parameters obtained from competition studies were: Kd1 = 26 nM, Bmax1 = 0.26 pmol/mg protein; Kd2 = 2.6 microM, Bmax2 = 27 pmol/mg protein for bLPM; and Kd1 = 81 nM, Bmax1 = 0.61 pmol/mg protein; Kd2 = 6.7 microM, Bmax2 = 79 pmol/mg protein for cLPM. Binding parameters obtained from saturation experiments were not significantly different. There was no Na+ requirement for binding. Kinetic dissociation experiments showed that binding was reversible and revealed two components. The dissociation rate constants did not vary with the method of dilution of radioligand, i.e. by "infinite" volume, or excess unlabeled ligand, thus ruling out the possibility of cooperativity. The ability of a series of compounds to inhibit the binding of [3H]E217G was also examined. In bLPM, taurocholate (TC), estrone sulfate (E1SO4) and bromosulfophthalein (BSP) were able to compete with both binding sites, whereas estriol-17 beta-(beta-D-glucuronide) (E317G), estriol-16 alpha-(beta-D-glucuronide) (E316G), testosterone glucuronide (TG), estradiol-3-(beta-D-glucuronide) (E23G), estriol-3-(beta-D-glucuronide) (E(3)3G), cholate and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) were able to inhibit binding to only the low-affinity site. In cLPM, only the cholestatic steroid D-ring glucuronides (E(3)17G, E(3)16G and TG) and TC were able to compete with both sites, whereas the non-cholestatic steroid A-ring glucuronides (E(2)3G and E(3)3G), BSP and DIDS competed for only the low-affinity site. Based on the observed substrate specificities, the low-affinity sites in bLPM and cLPM are postulated to represent multispecific organic anion carriers. The high-affinity site in cLPM may play a role in mediating steroid D-ring glucuronide-induced cholestasis.

Modern approaches to bile acid transport proteins

New techniques for studying solute uptake across epithelial membranes in the gut are providing a clearer picture of bile acid handling. In the small intestine, liver, and other sites, membrane proteins that mediate active bile acid transport are key players. Better understanding of these transporters may lead to elucidation of disorders of bile handling and better treatment of such disorders.

Identification of the bile canalicular cell surface molecule GP110 as the ectopeptidase dipeptidyl peptidase IV: an analysis by tissue distribution, purification and N-terminal amino acid sequence

This paper describes the tissue distribution, purification and N-terminal amino acid sequence of the bile canalicular cell surface molecule dipeptidyl peptidase IV. Immunoperoxidase staining of cryostat sections of rat liver with a monoclonal antibody, Medical Research Council OX-61, indicated specific binding to hepatocyte bile canalicular domains and brush borders of bile ducts. Additional staining was seen in other epithelial brush borders (small intestine, kidney, colon, pancreatic duct); acinar structures in salivary glands; endothelial structures and T cell areas in thymus, spleen and lymph node. The tissue distribution suggested that monoclonal antibody OX-61 binds to the ectoenzyme dipeptidyl peptidase IV. This was confirmed by depletion of dipeptidyl peptidase IV activity from tissue homogenates by monoclonal antibody OX-61 coupled to Sepharose. The molecule recognized by OX-61 was then purified from liver and kidney by monoclonal antibody affinity chromatography. The molecule had a molecular weight of 110 kD under reducing conditions. The purified molecule was subsequently analyzed for amino acid composition and N-terminal amino acid sequence. Thirty-one N-terminal amino acids were sequenced and indicated identity with part of the predicted N-terminus of the previously cloned bile canalicular molecule GP110. On review, other similarities between dipeptidyl peptidase IV and GP110 were detected: molecular weight, deglycosylated form and metabolic half-life. Finally, the recent cloning of dipeptidyl peptidase IV permitted a comparison between the molecule recognized by monoclonal antibody OX-61, GP110 and dipeptidyl peptidase IV. It is concluded that these three molecules are almost certainly identical.

Alteration in the regulation of plasma membrane glycoproteins of the hepatocyte during ontogeny

The expression of four integral membrane glycoproteins was examined in detail utilizing monospecific antibodies during liver development. These included asialoglycoprotein receptor, a hepatocyte glycoprotein residing in the sinusoidal domain, and three bile canalicular glycoproteins, leucine aminopeptidase, dipeptidyl peptidase IV, and a Mr 110,000 glycoprotein denoted GP 110. It was observed that asialoglycoprotein receptor, GP 110, and dipeptidyl peptidase IV were present in low amounts in fetal liver and reached adult levels between 1 to 3 weeks. In contrast, leucine aminopeptidase was present in nearly adult amounts in 18-day-old fetal livers. These observations were qualitatively confirmed by indirect immunofluorescent staining of frozen thin liver sections obtained from fetal and adult rats. Further, in fetal livers it was found that leucine aminopeptidase was not localized to typical bile canalicular areas. Immunoprecipitation studies performed in the presence of proteolytic inhibitors using detergent-solubilized extracts of metabolically labeled liver minces revealed that GP 110 was present in low amounts as Mr 110,000 and Mr 105,000 polypeptides in 17-day fetal livers but by 21 days of gestation the larger polypeptide was the major synthesis product. Conversely, the apparent molecular weights of leucine aminopeptidase and dipeptidyl peptidase IV were not altered during development. Experiments determining relative rates of synthesis using excess amounts of antibodies showed that the concentrations of the three bile canalicular glycoproteins in liver during ontogeny reflect their rates of synthesis. These results underscore that plasma membrane constituents of the hepatocyte undergo dramatic changes in expression and localization as the liver changes its physiological role at birth.

Carrier-mediated transport in the hepatic distribution and elimination of drugs, with special reference to the category of organic cations

Carrier-mediated transport of drugs occurs in various tissues in the body and may largely affect the rate of distribution and elimination. Saturable translocation mechanisms allowing competitive interactions have been identified in the kidneys (tubular secretion), mucosal cells in the gut (intestinal absorption and secretion), choroid plexus (removal of drug from the cerebrospinal fluid), and liver (hepatobiliary excretion). Drugs with quaternary and tertiary amine groups represent the large category of organic cations that can be transported via such mechanisms. The hepatic and to a lesser extent the intestinal cation carrier systems preferentially recognize relatively large molecular weight amphipathic compounds. In the case of multivalent cationic drugs, efficient transport only occurs if large hydrophobic ring structures provide a sufficient lipophilicity-hydrophilicity balance within the drug molecule. At least two separate carrier systems for hepatic uptake of organic cations have been identified through kinetic and photoaffinity labeling studies. In addition absorptive endocytosis may play a role that along with proton-antiport systems and membrane potential driven transport may lead to intracellular sequestration in lysosomes and mitochondria. Concentration gradients of inorganic ions may represent the driving forces for hepatic uptake and biliary excretion of drugs. Recent studies that aim to the identification of potential membrane carrier proteins indicate multiple carriers for organic anions, cations, and uncharged compounds with molecular weights around 50,000 Da. They may represent a family of closely related proteins exhibiting overlapping substrate specificity or, alternatively, an aspecific transport system that mediates translocation of various forms of drugs coupled with inorganic ions. Consequently, extensive pharmacokinetic interactions can be anticipated at the level of uptake and secretion of drugs regardless of their charge.

Hepatocyte plasma membrane ECTO-ATPase (pp120/HA4) is a substrate for tyrosine kinase activity of the insulin receptor

pp120/HA4, a membrane protein found in hepatocyte plasma membranes and a substrate for the insulin receptor tyrosine kinase, was purified to homogeneity, subjected to partial proteolysis, and peptides were sequenced by Edman degradation. Six amino acid sequences were obtained, and they matched the deduced amino acid sequences of six regions of a hepatocyte membrane protein called ecto-ATPase.

Hepatic transport mechanisms for bivalent organic cations. Subcellular distribution and hepato-biliary concentration gradients of some steroidal muscle relaxants

In order to characterize the hepato-biliary transport of bivalent cations in more detail, the subcellular distribution of three steroidal muscle relaxants, that differ physicochemically and kinetically, was studied by differential centrifugation of liver homogenates. Binding of the muscle relaxants to macromolecular compounds was measured in Krebs-albumin solution, in cytosolic fraction of liver homogenate and in bile, to estimate the unbound concentrations in the particular fluids. Cytosol/plasma concentration ratios increased in the order pancuronium less than Org 6368 less than vecuronium, but for all of the compounds did not exceed the value that would be attained by passive equilibration according to the membrane potential. The subcellular distribution patterns of the three substances indicated that the mitochondrial fraction is a major storage compartment in the liver. Yet Org 6368 was bound to the particulate fraction of liver homogenate to a larger extent than pancuronium and vecuronium. The high bile/cytosol concentration ratios indicate that for all of these cations an active transport system is involved in the biliary excretion process. For Org 6368 and vecuronium the bile/cytosol concentration ratios are in the same range (about 30) and substantially higher than for pancuronium (about 6). This suggests that for Org 6368 and vecuronium the transport across the canalicular membrane is more efficient than for pancuronium. The combined data indicate that the extensive binding of Org 6368 to particles within the cell is a major factor in the relative efficient hepatic uptake and the modest biliary excretion of this agent. The limited hepato-biliary transport of pancuronium appears to be due to a relatively small net transport, both at the sinusoidal land at the canalicular membrane.

Direct photoaffinity labeling of leukotriene binding sites

Due to their conjugated double bonds the leukotrienes themselves are photolabile compounds and may therefore be used directly for photoaffinity labeling of leukotriene binding sites. Cryofixation eliminates unspecific labeling taking place in solution by photoisomers and photodegradation products of leukotrienes. After fixation of receptor ligand interactions by shock-freezing of the samples, irradiation-induced highly reactive excited states and/or intermediates can form covalent bonds with the respective binding site in the frozen state. After cryofixation of a solution of albumin incubated with [3H8]leukotriene E4, irradiation at 300 nm resulted in time-dependent incorporation of radioactivity into the protein. Photoaffinity labeling of rat as well as of human blood serum with [3H8]leukotriene E4 after cryofixation revealed that only one polypeptide with an Mr of 67,000 was labeled. This polypeptide was identified as albumin. Photoaffinity labeling of rat liver membrane subfractions enriched with sinusoidal membranes resulted in the labeling of a polypeptide with an apparent Mr of 48,000, whereas no polypeptide was predominantly labeled in the subfraction enriched with canalicular membranes. Photoaffinity labeling of isolated hepatocytes disclosed different leukotriene E4 binding polypeptides. In the particulate fraction of hepatocytes a polypeptide with an apparent Mr of 48,000 was labeled predominantly, whereas in the soluble fraction several polypeptides were labeled to a similar extent. One of these, with an apparent Mr of 25,000, was identified as subunit 1 of glutathione transferases by immunoprecipitation. The method of direct photoaffinity labeling in the frozen state after cryofixation using leukotrienes as photoactivatable compounds, as exemplified by leukotriene E4, may be most useful for the identification and characterization of various leukotriene binding sites, including receptors, leukotriene-metabolizing enzymes, and transport systems.

Hepatocellular bile acid transport and ursodeoxycholic acid hypercholeresis

This review focuses on mechanisms of bile acid transport across the basolateral and canalicular hepatocyte plasma membranes and on ursodeoxycholic acid (UDCA) hypercholeresis and biotransformation. Conjugated trihydroxy bile acids enter hepatocytes via a sodium-coupled mechanism localized to the basolateral membrane, which is saturable, concentrative, inhibited by other bile acids as well as by furosemide and bumetanide, and exhibits developmental changes in rats and probably also in humans. The stoichiometry of sodium-coupled bile acid uptake has been controversial. Hydrophobic, unconjugated dihydroxy and monohydroxy bile acids, including UDCA, enter hepatocytes more rapidly than does taurocholate, and their uptake is largely nonsaturable and sodium independent. A hydroxyl-exchange mechanism that mediates the uptake of cholic acid has also been reported, but its existence is controversial. Current evidence suggests that a 49-kDa protein mediates Na+-dependent taurocholate uptake and that a 54-kDa protein is involved in Na+-independent bile acid uptake. Studies with canalicular membrane vesicles have demonstrated saturable, sodium-independent taurocholate transport, which is sensitive to electrical potential, exhibits trans-stimulation, and appears to be mediated by a 100-kDa canalicular membrane glycoprotein. Studies in mutant rats with conjugated hyperbilirubinemia suggest the presence of a separate canalicular transport mechanism utilized by sulfated bile acids and organic anions such as bilirubin and sulfobromophthalein. UDCA produces in some species a dramatic hypercholeresis that is greater than expected based on the osmotic effect of the secreted bile acid. The hypercholeresis appears attributable to stimulation of biliary bicarbonate output and is decreased or abolished in the perfused rat liver by amiloride or perfusate Na+ substitution. These same maneuvers dramatically alter UDCA biotransformation (unconjugated UDCA disappears from bile, and UDCA glucuronide becomes a major metabolite) and lower hepatocyte intracellular pH. These and other findings indicate that UDCA hypercholeresis is tightly linked to biliary excretion of the unconjugated species and suggest that UDCA biotransformation may be influenced by intracellular pH.

Localization of a putative cell adhesion molecule (gp110) in Wistar and Fischer rat tissues

A plasma membrane glycoprotein (gp110) involved in cellular adhesion was studied in Wistar and Fischer rats. For quantitative analysis of the gp110 molecule a sandwich-ELISA was used. High quantities of gp110 were found especially in the liver, small intestine, submandibular gland and lung. The distribution and localization of the gp110 were investigated by immunohistochemistry utilizing soluble complexes of alkaline phosphatase and monoclonal anti-alkaline phosphatase antibodies. Immunoreactivity was present in plasma membranes of vascular endothelial cells of some organs. Furthermore, immunostaining also occurred in plasma membranes of lymphocytes, exocrine gland cells, excretory duct cells, hepatocytes, epithelial cells of the small intestine, kidney and vesicular gland and in the cytoplasm of renal connecting and collecting duct cells. The localization of gp110 in the luminal domain of the plasma membrane at many sites suggests that this glycoprotein is also involved in processes distinct from cell adhesion.