Comparative structural analysis of the human and Schistosoma glutathione transferase dimer interface using selective binding of bromosulfophthalein.

The binding channel of Schistosoma glutathione transferase (SGST) has been identified to possess a non-substrate site implicated in enzyme inhibition. This binding channel is formed by the interface of the GST dimer. We produced a comparative characterization of the SGST dimer interface with respect to that of human GST (hGST) analogues using the selective binding of bromosulfophthalein (BSP). First, two SGST and three hGST structures were used as search queries to assemble a data set of 48 empirical GST structures. Sequence alignment to generate a universal residue indexing scheme was then performed, followed by local superposition of the dimer interface. Principal component analysis revealed appreciable variation of the dimer interface, suggesting the potential for selective inhibition of SGST. BSP was found to dock invariably in the dimer interface core pocket, placing it in proximity to the GST catalytic domains, through which it may exert its inhibitory behavior. Binding poses across the GST forms were distinguished with ligand interaction profiling, where SGST complexes showed stabilization of ligand aromatic- and sulfonate moieties, which altogether anchor the ligand and produce a tight association. In comparison, missing aromatic stabilization in the hGST complexes impart large bonding distances, causing mobile poses likely to dissociate. Altogether, this study illustrates the potential for selective inhibition of SGST, rationalizes the selective behavior of the BSP inhibitor, and produces a reliable metric for construction and validation of pharmacophore models of the SGST binding channel.

Liver function from Y to Z: The guidance of William Jakoby.

In the 1960s, my lab was interested in understanding how bilirubin and other organic anions are transferred from the plasma through the liver cell and into the bile. We performed gel filtration of liver supernatants and identified two protein fractions, designated Y and Z, which bound organic anions including bilirubin, and thus we proposed that they were involved in hepatic uptake of organic anions from plasma. Subsequently, the Y and Z proteins responsible for this binding activity were purified, cloned, and sequenced. With Bill Jakoby, we identified Y protein as a member of the glutathione S-transferase (GST) protein family. In separate studies, Z was found to be a member of the fatty acid-binding protein (FABP) family. These proteins have since been shown to have additional surprising roles, but understanding of their full role in physiology and disease has not yet been achieved. In the 1960s, bilirubin metabolism was a "hot" topic. Along with other groups, my lab was studying various forms of inheritable jaundice in an effort to dissect the mechanism of bilirubin's transfer from plasma into the hepatocyte and its role in intracellular metabolism and biliary secretion. These processes were eventually identified and found to be related to the basic mechanisms whereby the liver handles many anionic drugs, metabolites, and hormones. Because the mechanism of hepatic uptake of bilirubin was unknown, A.J. Levi, Z. Gatmaitan, and I took advantage of advances in gel permeation chromatography to study this process. In 1969, we described two hepatic cytoplasmic protein fractions, designated Y and Z, that bound bilirubin and various organic anionic dyes in vivo and in vitro and, based on tissue distribution, abundance, and effects of genetic and pharmacologic models, were proposed to participate in organic anion uptake (Levi et al., 1969) [1]. In the decades since then, the Y and Z proteins have been identified as members of large protein families that were cloned and sequenced. Several surprising functions emerged, whereas others are proposed based on binding properties. Many challenges remain in understanding the full role of these proteins in physiology and disease.

Identification of multiple binding sites for substrate transport in bovine organic anion transporting polypeptide 1a2.

Organic anion transporting polypeptides (OATP) have been extensively recognized as key determinants of absorption, distribution, metabolism, and excretion of various drugs because of their broad substrate specificity, wide tissue distribution, and the involvement of drug-drug interaction. As the first cloned human OATP, OATP1A2 has been found to transport a wide spectrum of endogenous and exogenous compounds. Bovine Oapt1a2 shared high homology with the human transporter and is considered as its functional ortholog. In the present study, we expressed bovine Oatp1a2 in human embryonic kidney 293 cells and found that, unlike human OATP1A2, the transport of estrone-3-sulfate (E-3-S) exhibited biphasic saturation kinetics. The K(m) values are 0.25 ± 0.08 and 46.6 ± 18.5 µM, and V(max) values were 24.5 ±4.4 and 375 ± 142 pmol/mg protein/min for high- and low-affinity sites, respectively, suggesting the presence of multiple binding sites. Further study on other Oatp1a2 substrates showed that the high affinity component for E-3-S is responsible for the interaction with taurocholate, bromsulphthalein, and rifampicin and is sensitive to proton concentration change, whereas the low affinity binding site is only involved in the binding of the antitumor drug methotrexate and had no response to change of pH.

Substrate-dependent inhibition of organic anion transporting polypeptide 1B1: comparative analysis with prototypical probe substrates estradiol-17ß-glucuronide, estrone-3-sulfate, and sulfobromophthalein.

Organic anion transporting polypeptide (OATP) 1B1 plays an important role in the hepatic uptake of many drugs, and the evaluation of OATP1B1-mediated drug-drug interactions (DDIs) is emphasized in the latest DDI (draft) guidance documents from U.S. and E.U. regulatory agencies. It has been suggested that some OATP1B1 inhibitors show a discrepancy in their inhibitory potential, depending on the substrates used in the cell-based assay. In this study, inhibitory effects of 14 compounds on the OATP1B1-mediated uptake of the prototypical substrates [³H]estradiol-17ß-glucuronide (E2G), [³H]estrone-3-sulfate (E1S), and [³H]sulfobromophthalein (BSP) were studied in OATP1B1-transfected cells. Inhibitory potencies of tested compounds varied depending on the substrates. Ritonavir, gemfibrozil, and erythromycin caused remarkable substrate-dependent inhibition with up to 117-, 14-, and 13-fold difference in their IC50 values, respectively. Also, the clinically relevant OATP inhibitors rifampin and cyclosporin A exhibited up to 12- and 6-fold variation in their IC50 values, respectively. Regardless of the inhibitors tested, the most potent OATP1B1 inhibition was observed when [³H]E2G was used as a substrate. Mutual inhibition studies of OATP1B1 indicated that E2G and E1S competitively inhibited each other, whereas BSP noncompetitively inhibited E2G uptake. In addition, BSP inhibited E1S in a competitive manner, but E1S caused an atypical kinetics on BSP uptake. This study showed substrate-dependent inhibition of OATP1B1 and demonstrated that E2G was the most sensitive in vitro OATP1B1 probe substrate among three substrates tested. This will give us an insight into the assessment of clinically relevant OATP1B1-mediated DDI in vitro with minimum potential of false-negative prediction.

Disposition of ceftriaxone in hepatopathic goats following single-intramuscular dosing.

Hepatopathy sometimes may interfere with metabolism and/or elimination of drugs which undergo major hepatic clearance. Twelve healthy goats were equally divided into two groups (I and II) and hepatopathy was induced by carbontetrachloride in the second group (group II). A single dose of ceftriaxone at 50 mg/kg was administered to each group intramuscularly. Disposition of ceftriaxone in plasma of healthy goats showed a typical absorption-reabsorption phase. However, the reabsorption phase was totally absent in hepatopathic goats and the disposition of ceftriaxone showed only absorption and distribution/elimination phase. The drug persisted in plasma for 6 h in hepatopathic animals, whereas the drug can only be detected up to 2 h in healthy animals indicating longer persistence of ceftriaxone in the former group. Ceftizoxime, the active metabolite of ceftriaxone was available in urine of group I animals, whereas only ceftriaxone was detected in the urine of hepatopathic animals suggesting impairment of metabolism of the parent drug in hepatopathy. Therefore, the reabsorption and metabolism of ceftriaxone in goats should be taken into consideration for drug monitoring.

Pharmaceutical excipients influence the function of human uptake transporting proteins.

Although pharmaceutical excipients are supposed to be pharmacologically inactive, solubilizing agents like Cremophor EL have been shown to interact with cytochrome P450 (CYP)-dependent drug metabolism as well as efflux transporters such as P-glycoprotein (ABCB1) and multidrug resistance associated protein 2 (ABCC2). However, knowledge about their influence on the function of uptake transporters important in drug disposition is very limited. In this study we investigated the in vitro influence of polyethylene glycol 400 (PEG), hydroxypropyl-ß-cyclodextrin (HPCD), Solutol HS 15 (SOL), and Cremophor EL (CrEL) on the organic anion transporting polypeptides (OATP) 1A2, OATP2B1, OATP1B1, and OATP1B3 and the Na(+)/taurocholate cotransporting polypeptide (NTCP). In stably transfected human embryonic kidney cells we analyzed the competition of the excipients with the uptake of bromosulfophthalein in OATP1B1, OATP1B3, OATP2B1, and NTCP, estrone-3-sulfate (E(3)S) in OATP1A2, OATP1B1, and OATP2B1, estradiol-17ß-glucuronide in OATP1B3, and taurocholate (TA) in OATP1A2 and NTCP cells. SOL and CrEL were the most potent inhibitors of all transporters with the strongest effect on OATP1A2, OATP1B3, and OATP2B1 (IC(50) < 0.01%). HPCD also strongly inhibited all transport proteins but only for substrates containing a sterane-backbone. Finally, PEG seems to be a selective and potent modulator of OATP1A2 with IC(50) values of 0.05% (TA) and 0.14% (E(3)S). In conclusion, frequently used solubilizing agents were shown to interact substantially with intestinal and hepatic uptake transporters which should be considered in drug development. However, the clinical relevance of these findings needs to be evaluated in further in vivo studies.

Delivery of the bioactive gas hydrogen sulfide during cold preservation of rat liver: effects on hepatic function in an ex vivo model.

The insults sustained by transplanted livers (hepatectomy, hypothermic preservation, and normothermic reperfusion) could compromise hepatic function. Hydrogen sulfide (H2S) is a physiologic gaseous signaling molecule, like nitric oxide (NO) and carbon monoxide (CO). We examined the effect of diallyl disulfide as a H2S donor during hypothermic preservation and reperfusion on intrahepatic resistance (IVR), lactate dehydrogenase (LDH) release, bile production, oxygen consumption, bromosulfophthalein (BSP) depuration and histology in an isolated perfused rat liver model (IPRL), after 48 h of hypothermic storage (4 °C) in University of Wisconsin solution (UW, Viaspan). Livers were retrieved from male Wistar rats. Three experimental groups were analyzed: Control group (CON): IPRL was performed after surgery; UW: IPRL was performed in livers preserved (48 h-4 °C) in UW; and UWS: IPRL was performed in livers preserved (48 h-4 °C) in UW in the presence of 3.4 mM diallyl disulfide. Hypothermic preservation injuries were manifested at reperfusion by a slight increment in IHR and LDH release compared with the control group. Also, bile production for the control group (1.32 µL/min/g of liver) seemed to be diminished after preservation by 73% in UW and 69% in UW H2S group at the end of normothermic reperfusion. Liver samples analyzed by hematoxylin/eosin clearly showed the deleterious effect of cold storage process, partially reversed (dilated sinusoids and vacuolization attenuation) by the addition of a H2S delivery compound to the preservation solution. Hepatic clearance (HC) of BSP was affected by cold storage of livers, but there were no noticeable differences between livers preserved with or without diallyl disulfide. Meanwhile, livers preserved in the presence of H2S donor showed an enhanced capacity for BSP uptake (k(A) CON = 0.29 min⁻¹; k(A) UW = 0.29 min⁻¹ ; k(A) UWS = 0.36 min ⁻¹). In summary, our animal model suggests that hepatic hypothermic preservation for transplantation affects liver function and hepatic depuration of BSP, and implies that the inclusion of an H2S donor during hypothermic preservation could improve standard methods of preparing livers for transplant.

Role of hepatic anion-binding protein in bromsulphthalein conjugation.

Using gel filtration, the binding of both glutathione and Bromsulphthalein (BSP) to a liver-soluble protein was found to be identical. BSP-conjugating activity (glutathione S-aryltransferase) was present only in the fractions corresponding to the two protein-bound markers. Using a highly sensitive assay, with 3,4-dichloronitrobenzene, the pattern of glutathione S-aryltransferase activity was found to coincide with Y protein. This evidence suggests that Y protein, or ligandin, has a dual role in hepatic transport: a specific enzymic function in the conjugation of certain anions with glutathione in addition to a transport function in the intracellular binding of organic anions.

Hepatic uptake of the magnetic resonance imaging contrast agent Gd-EOB-DTPA: role of human organic anion transporters.

Contrast-enhancing magnetic resonance imaging with the liver-specific agent gadolinium-ethoxybenzyl-diethylenetriamine pentaacetic acid (Gd-EOB-DTPA) has been shown to improve the detection rate of focal lesions. There is evidence from preclinical studies that multidrug organic anion transporters are involved in hepatic uptake of Gd-EOB-DTPA. Therefore, we evaluated affinity of the contrast agent to human organic anion-transporting polypeptides (OATP1B1, OATP1B3, OATP2B1) and to the Na(+)/taurocholate cotransporting polypeptide (NTCP) using stable transfected human embryonic kidney (HEK) 293 cells. In competition assays, Gd-EOB-DTPA inhibited the uptake of bromosulfophthalein (BSP) by OATP1B1 (IC(50) = 0.6 mM) and OATP1B3 (IC(50) = 0.4 mM). In comparison, the IC(50) values for rifampicin were 11.9 (OATP1B1), 1.4 (OATP1B3), and 80.5 muM (OATP2B1), respectively. Uptake of BSP by OATP2B1, uptake of taurocholic acid by NTCP, and viability of all HEK cells were not influenced by Gd-EOB-DTPA in concentrations up to 10 mM. In uptake assays using a new liquid chromatography-tandem mass spectrometry method for quantification, Gd-EOB-DTPA was a substrate for OATP1B1 (K(m) = 0.7 mM, V(max) = 10.5 pmol/mg x min), OATP1B3 (K(m) = 4.1 mM, V(max) = 22.7 pmol/mg x min), and NTCP (K(m) = 0.04 mM, V(max) = 1.4 pmol/mg x min). The uptake by OATP2B1 was not different from the vector control. In conclusion, Gd-EOB-DTPA is a substrate of the liver-specific OATP1B1, OATP1B3, and NTCP.

Long-lasting inhibition of the transporter-mediated hepatic uptake of sulfobromophthalein by cyclosporin a in rats.

Cyclosporin A (CsA) is a well known inhibitor of the organic anion-transporting polypeptide (OATP/Oatp) family transporters, causing a large number of transporter-mediated drug-drug interactions in clinical situations. In the present study, we examined the inhibitory effect of CsA on the hepatic uptake of sulfobromophthalein (BSP) in rats, focusing on a long-lasting inhibition. Twenty-one hours after the subcutaneous administration of CsA, the hepatic clearance of BSP was decreased. The liver uptake index study revealed that hepatic uptake of BSP was reduced in CsA-treated rats for at least 3 days. Comparison of uptake studies using isolated hepatocytes prepared from control and CsA-treated rats showed that hepatic uptake in CsA-treated rats was decreased. In primary cultured hepatocytes, after preincubation with CsA, the uptake of [(3)H]BSP was reduced even after removal of CsA from the incubation buffer although a preincubation time dependence was not observed. However, the expression of Oatp1a1 and Oatp1b2, which are involved in the hepatic uptake of BSP, and the amount of intrahepatic glutathione, a driving force of Oatp1a1, did not change in CsA-treated rats. Thus, we can conclude that CsA modulates the transporter function sustainably. It can cause a potent in vivo drug-drug interaction. The modulation of transporters is not caused by reduced expression or driving force of transporters. It may be affected by CsA accumulated in the liver or its metabolites. The inhibitory effect of CsA on the transporter-mediated uptake of BSP cannot be explained by a simple competitive mechanism and a novel mechanism should be considered.

Identification and localization of the bilitranslocase homologue in white grape berries (Vitis vinifera L.) during ripening.

A homologue of the mammalian bilirubin transporter bilitranslocase (BTL) (TCDB 2.A.65.1.1), able to perform an apparent secondary active transport of flavonoids, has previously been found in carnation petals and red grape berries. In the present work, a BTL homologue was also shown in white berries from Vitis vinifera L. cv. Tocai/Friulano, using anti-sequence antibodies specific for rat liver BTL. This transporter, similarly to what found in red grape, was localized in the first layers of the epidermal tissue and in the vascular bundle cells of the mesocarp. In addition, a strong immunochemical reaction was detected in the placental tissue and particularly in peripheral integuments of the seed. The protein was expressed during the last maturation stages in both skin and pulp tissues and exhibited an apparent molecular mass of c. 31 kDa. Furthermore, the transport activity of such a carrier, measured as bromosulphophthalein (BSP) uptake, was detected in berry pulp microsomes, where it was inhibited by specific anti-BTL antibodies. The BTL homologue activity exhibited higher values, for both K(m) and V(max), than those found in the red cultivar. Moreover, two non-pigmented flavonoids, such as quercetin (a flavonol) and eriodictyol (a flavanone), inhibited the uptake of BSP in an uncompetitive manner. Such results strengthen the hypothesis that this BTL homologue acts as a carrier involved also in the membrane transport of colourless flavonoids and demonstrate the presence of such a carrier in different organs and tissues.

Functional activity of the liver during severe compression injury.

Severe compression injury in rats was accompanied by metabolic acidosis, cytolytic syndrome, disturbances in liver excretory function and detoxification, and change in biotransformation processes.

Metabolism and disposition of resveratrol in the isolated perfused rat liver: role of Mrp2 in the biliary excretion of glucuronides.

In this study, the hepatic metabolism and transport system for resveratrol was examined in isolated perfused livers from Wistar and Mrp2-deficient TR(-) rats. Based on extensive metabolism to six glucuronides and sulfates (M1-M6), the hepatic extraction ratio and clearance of resveratrol was very high in Wistar and TR(-) rats (E: 0.998 vs. 0.999; Cl: 34.9 mL/min vs. 36.0 mL/min). However, biliary excretion and efflux of conjugates differs greatly in TR(-) rats. While cumulative biliary excretion of the glucuronides M1, M2, M3, and M5 dropped dramatically to 0-6%, their efflux into perfusate increased by 3.6-, 1.8-, 2.5-, and 1.5-fold. In contrast, biliary secretion of the sulfates M4 and M6 was partially maintained in the Mrp2-deficient rats (61% and 39%) with a concomitant decline of their efflux into perfusate by 33.2% and 78.1%. This indicates that Mrp2 exclusively mediates the biliary excretion of resveratrol glucuronides but only partly that of sulfates. Cumulative secretion of unconjugated resveratrol into bile of TR(-) rats was only reduced by 40%, and into perfusate by 19%, suggesting only a minor role of Mrp2 in resveratrol elimination. In summary, resveratrol was dose-dependently metabolized to several conjugates whereby the canalicular transporter Mrp2 selectively mediated the biliary excretion of glucuronides.

Targeting NLRP3 inflammasome via acetylsalicylic acid: Role in suppressing hepatic dysfunction and insulin resistance induced by atorvastatin in naïve versus alcoholic liver in rats.

BACKGROUND: NLRP3 inflammasome is described in many pathological conditions and is also involved in drug induced liver injury. AIM OF THE WORK: To investigate the role of NLRP3 inflammasome in liver injury induced by chronic alcohol and/or atorvastatin ingestion. MATERIALS AND METHODS: Sixty male Wistar rats were used. They were divided into 5 groups: (I) control naïve (II) Alcoholic: given ethanol 8 g/kg/day, p.o (III) Atorvastatin: given atorvastatin 10 mg/kg/day, p.o. (IV) Alcoholic + atorvastatin (V) Acetylsalicylic acid (ASA): given ASA 10 mg/kg/day, p.o together with alcohol and atorvastatin. Isolated perfused liver, biochemical and histological studies were done. RESULTS: Atorvastatin and alcohol induced liver inflammation with increasing the expression of NLRP3, IL-1ß and caspase-8 immune-reaction. Atorvastatin and alcohol decreased the reduced form of glutathione in hepatic tissues and induced insulin resistance. ASA administration alleviated the hepatotoxic effects of alcohol and atorvastatin to a significant extent. CONCLUSIONS: Acetylsalicylic acid alleviated the hepatotoxic effects of alcohol and atorvastatin through decreasing the production of NLRP3 inflammasome in rats' liver.

Hepatic organic anion uptake in the rat.

The hepatic uptake of bilirubin (BR), indocyanine green (ICG), and sulfobromophthalein (BSP) was studied in 350 anesthetized Sprague-Dawley rats by determining the initial plasma disappearance rate (V) of various doses of unlabeled ICG, or of tracer quantities of [3H]BR or [35S]BSP injected into the jugular vein simultaneously with varying amounts of unlabeled BR or BSP. Similar studies were also performed involving the simultaneous injection of potential inhibitors of hepatic uptake. The results indicate that: (a) hepatic uptake determined by direct tissue measurement could be accurately estimated from the plasma disappearance data; (b) saturation of hepatic uptake with increasing dose was readily demonstrated for each of these three organic anions, and in each instance a plot of V versus dose took the form of a rectangular hyperbola analyzable in terms of Michaelis-Menten kinetics; (c) for BR, the saturable uptake process showed a Vmax more than 100 times the normal net transfer rate from plasma to bile; (d) hepatic uptake of BR, BSP, and ICG showed relatively selective, mutually competitive inhibition; glycoholic acid did not inhibit hepatic uptake of any of these substances; and (e) "counter-transport" could be demonstrated for each of the three test substances. These data are compatible with the existence of a carrier-mediated transport process for hepatic uptake of each of these three organic anions and clarify the relationship of hepatic BR uptake to its overall transport from plasma to bile.

Hepatocellular uptake of sulfobromophthalein and bilirubin is selectively inhibited by an antibody to the liver plasma membrane sulfobromophthalein/bilirubin binding protein.

To clarify sulfobromophthalein (BSP) and bilirubin uptake mechanisms, isolated rat hepatocytes were incubated with [35S]BSP. The initial uptake velocity (V0), determined from the first, linear portion of the cumulative uptake curve, was saturable (Michaelis constant [Km] = 6.2 +/- 0.5 microM; Vmax = 638 +/- 33 pmol X min-1 per 10(5) hepatocytes), maximal at 37 degrees C and pH 7.4, and competitively inhibited by bilirubin, but not by taurocholate, cholate, or oleate. Preloading with unlabeled BSP led to trans-stimulation of V0. Sodium substitution or pretreatment of hepatocytes with ouabain or metabolic inhibitors had no effect on V0; trypsin reduced V0 by 39% (P less than 0.001). A rabbit antiserum to the rat liver plasma membrane (LPM)-BSP/bilirubin binding protein selectively reduced V0 of 5 microM [35S]BSP and [14C]bilirubin by 41 and 42%, respectively (P less than 0.01); uptakes of [3H]oleate, [3H]cholate and [3H]taurocholate were not affected. Hence, the LPM-BSP/bilirubin binding protein plays a role in the carrier-mediated uptake of BSP and bilirubin by hepatocytes.

Identification, purification, and partial characterization of an organic anion binding protein from rat liver cell plasma membrane.

Uptake of bilirubin, sulfobromophthalein (BSP), and other organic anions by the liver is a process with kinetics consistent with carrier mediation. The molecular basis of this transport mechanism is unknown. In the search for the putative organic anion carrier or receptor, the interaction of BSP with rat liver cell plasma membrane (LPM) has been studied. Specific binding of [(35)S]BSP to LPM was determined using a filtration assay. Results revealed high affinity (K(a) = 0.27 muM(-1)), saturable (6.3 nmol/mg protein) binding, which was eliminated after preincubation with trypsin. Although [(35)S]BSP was strongly bound to LPM, the binding was rapidly reversible, preventing direct identification and study of a specific binding site(s). To avoid this problem, a photoaffinity probe was devised, in which [(35)S]BSP is covalently bound to LPM after exposure to ultraviolet light. Subsequent sodium dodecyl sulfate gel electrophoresis and fluorography revealed radioactivity predominantly associated with a single 55,000-mol wt protein. A protein with identical electrophoretic mobility was purified from deoxycholate solubilized LPM after affinity chromatography on glutathione-BSP-agarose gel. This protein migrated as a single band on sodium dodecyl sulfate gel electrophoresis and on urea gel isoelectric focusing. It contained 1-2 residues of sialic acid per 55,000-dalton protein, and was immunologically distinct from rat albumin and ligandin. It bound bilirubin with a K(d) of 20 muM, as determined by tryptophan fluorescence quenching. Although the high affinity of this LPM protein for organic anions suggests that it may function as a hepatocellular organic anion receptor, its role in transport of these compounds is unknown.

Physicochemical and immunohistological studies of a sulfobromophthalein- and bilirubin-binding protein from rat liver plasma membranes.

Affinity chromatography over bilirubinagarose and sulfobromophthalein (BSP)-agarose was used to isolate two proteins, with high affinities for bilirubin and BSP, respectively, from Triton X-100-solubilized rat liver plasma membranes. The protein eluted from either affinity column migrated as a single band of approximately 55,000 D on sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, and either protein cochromatographed with both [14C]bilirubin and [35S]BSP on Sephadex G-75. On gradient gels without reduction or SDS, or on Sephadex G-150, the native BSP-binding protein had an estimated molecular mass of approximately 100,000 D. After incubation with SDS, an additional Sephadex G-150 peak of molecular mass of 56,000 D was observed. Both, the 100,000- and 56,000-D G-150 peaks cochromatographed with [35S]BSP. The native protein had an isoelectric point of 3.5, stained with periodic acid-Schiff but not Sudan black, and contained 4 mol of sialic acid per mol of protein. A rabbit antibody to the BSP-binding protein gave a line of identity with both the BSP- and bilirubin-binding antigens, and inhibited the binding of [14C]bilirubin and [35S]BSP, but not [14C]oleate or [14C]taurocholate, to rat liver plasma membranes. Immunohistochemical studies revealed the presence of the antigen on all surface domains of rat hepatocytes, but not on other cell populations from normal rat liver. It was not found in other organs. These data are compatible with the hypothesis that a specific liver cell plasma membrane protein mediates the hepatocytic sequestration of bilirubin and BSP.

The effect of sodium taurocholate on the hepatic metabolism of sulfobromophthalein sodium (BSP). The role of bile flow.

The influence of bile salts on the hepatic metabolism of sulfobromophthalein sodium (BSP) was studied in the perfused rat liver. During sodium taurocholate infusions, hepatic uptake of BSP from plasma was increased and appeared to be related to an enhanced transit of BSP from liver into bile. BSP-glutathione conjugation was not affected by the bile salt infusions, although bile salts inhibited the enzyme system in vitro. The major effect of bile salts was to increase the BSP transport maximum (Tm). When sodium taurocholate was infused in saline at a rate of 30 mumoles/hr, both bile flow and the BSP Tm increased, and remained at peak levels of 1.5 +/-0.12 mul/min per g liver and 21 +/-3.0 mug/min per g liver, respectively. In contrast, during saline infusion alone both levels were significantly lower (1.06 +/-0.17 mul/min per g liver and 15.8 +/-4.16 mug/min per g liver, respectively), and both fell progressively after the 2nd hr of perfusion. This decline in bile flow and BSP Tm was associated with a decrease in biliary bile salt excretion and was reversed by adding bile salts to the perfusate. Since the biliary concentration of BSP remained within a narrow range in all experiments, the BSP Tm was primarily determined by the rate of bile flow. Dependence of BSP Tm on the rate of bile production was further confirmed by changing the temperature of the perfusate during a constant infusion of taurocholate. BSP Tm paralleled temperature-induced changes in bile flow irrespective of changes in the level of bile salt excretion. Since the biliary concentration of BSP remained within a narrow range in all experiments, the concentrating capacity for BSP in bile may be the major limiting factor in BSP transport. Thus two independent factors appear to determine the BSP Tm: the bile BSP concentration, and the rate of bile production. Because taurocholate enhanced BSP transport only when it increased bile production, its effect on the BSP Tm appears to be attributable to its choleretic properties.

Separate transport systems for biliary secretion of sulfated and unsulfated bile acids in the rat.

Biliary secretion of 3 alpha-sulfated bile acids has been studied in Wistar rats with an autosomal recessive defect in the hepatic transport of bilirubin. Liver function, established by measurement of various enzymes in plasma, by enzyme histochemical methods, and by electron microscopy, appeared to be normal in these rats. Serum levels of unconjugated, monoglucuronidated, and diglucuronidated bilirubin were 0.62, 1.62, and 6.16 mumol/liter, respectively, compared with 0.17, 0.08, and 0.02 mumol/liter in control rats. Biliary bilirubin secretion was strongly reduced in the mutant animals: 0.21 +/- 0.03 vs. 0.39 +/- 0.03 nmol/min per 100 g body wt in control rats. Despite normal biliary bile acid output, bile flow was markedly impaired in the mutant animals, due to a 53% reduction of the bile acid-independent fraction of bile flow. The transport maximum for biliary secretion of dibromosulphthalein (DBSP) was also drastically reduced (-53%). Biliary secretion of intravenously administered trace amounts of the 3 alpha-sulfate esters of 14C-labeled taurocholic acid (-14%), taurochenodeoxycholic acid (-39%), taurolithocholic acid (-73%), and glycolithocholic acid (-91%) was impaired in the jaundiced rats compared with controls, in contrast to the biliary secretion of the unsulfated parent compounds. Hepatic uptake of sulfated glycolithocholic acid was not affected in the jaundiced animals. Preadministration of DBSP (15 mumol/100 g body wt) to normal Wistar rats significantly impaired the biliary secretion of sulfated glycolithocholic acid, but did not affect taurocholic acid secretion. We conclude that separate transport systems in the rat liver exist for biliary secretion of sulfated and unsulfated bile acids; the sulfates probably share secretory pathways with the organic anions bilirubin and DBSP. The described genetic defect in hepatic transport function is associated with a reduced capacity to secrete sulfated bile acids into bile; this becomes more pronounced with a decreasing number of hydroxyl groups on the sulfated bile acid's molecule.