Quantitative aspects of the interaction of bile acids with human serum albumin

Efficient one-step amide formation using amino porphyrins

Amide bonds are one of the most prevalent phenomena in nature and are utilized frequently in drug and material design. However, forming amide bonds is not always efficient or high yielding, particularly when the amine used to conjugate to a carboxylic acid is a weak nucleophile. This limitation precludes many useful amino compounds from participating in conjugation reactions to form amides. A particularly valuable amino compound, which is also a very weak nucleophile, is the amino porphyrin, valued for its role as a photosensitizer, fluorescent agent, catalyst, or, upon metalation, even a very efficient contrast agent for magnetic resonance imaging (MRI). In this work, we propose fast and high-yield coupling of an unreactive amine - the amino porphyrin - to carboxylic acid via isothiocyanate conjugation. Reactions can be achieved in one step at room temperature in one hour, achieving quantitative conversion and near perfect selectivity. Both metalated and unmetalated porphyrin, as well as fluorescein isothiocyanate (FITC), demonstrated efficient conjugation. To illustrate the value of the proposed method, we created a new blood-pool MRI contrast agent that reversibly binds to serum albumin. This new blood-pool agent, known as MITC-Deox (MRI isothiocyanate that links with deoxycholic acid), substantially reduced T1 relaxation times in blood vessels in mice, remained stable for 1 hour, cleared from blood by 24 hours, and was eliminated from the body after 4 days. The proposed method for efficient amide formation is a superior alternative to existing coupling methods, opening a door to novel synthesis of MRI contrast agents and beyond.

Transfer of ANS-Like Drugs from Micellar Drug Delivery Systems to Albumin Is Highly Favorable and Protected from Competition with Surfactant by "Reserved" Binding Sites

Micellar drug delivery systems (MDDS) for the intravenous administration of poorly soluble drugs have great advantages over alternative formulations in terms of the safety of their excipients, storage stability, and straightforward production. A classic example is mixed micelles of glycocholate (GC) and lecithin, both endogenous substances in human blood. What limits the use of MDDS is the complexity of the transitions after injection. In particular, as the MDDS disintegrate partially or completely after injection, the drug has to be transferred safely to endogenous carriers in the blood, such as human serum albumin (HSA). If this transfer is compromised, the drug might precipitate─a process that needs to be excluded under all circumstances. The key question of this paper is whether the high local concentration of GC at the moment and site of MDDS dissolution might transiently saturate HSA binding sites and, hence, endanger quick drug transfer. To address this question, we have used a new approach, which is time-resolved fluorescence spectroscopy of the single tryptophan in HSA, Trp-214, to characterize the competitive binding of GC and the drug substitute anilinonaphthalenesulfonate (ANS) to HSA. Time-resolved fluorescence of Trp-214 showed important advantages over established methods for tackling this problem. ANS has been the standard "model drug" to study albumin binding for decades, given its structural similarity to the class of naphthalene-containing acidic drugs and the fact that it is displaced from HSA by numerous drugs (which presumably bind to the same sites). Our complex global fit uses the critical approximation that the average lifetimes behave similarly to a single lifetime, but the resulting errors are found to be moderate and the results provide a convincing explanation of the, at first glance, counterintuitive behavior. Accordingly, and largely in line with the literature, we observed two types of sites binding ANS at HSA: 3 type A, rather peripheral, and 2 type B, likely more central sites. The latter quench Trp-214 by Förster Resonance Energy Transfer (FRET) with a rate constant of ≈0.4 ns-1 per ANS. Adding millimolar concentrations of GC displaces ANS from the A sites but not from B sites. At incomplete ANS saturation, this causes a GC-induced translocation of ANS from A to the more FRET-active B sites. This leads to the apparent paradox that the partial displacement of ANS from HSA increases its quenching effect on Trp-214. The most important conclusion is that (ANS-like) drugs cannot be displaced from the type-B sites, and consequently, drug transfer to these sites is not impaired by competitive binding of GC in the vicinity of a dissolving micelle. The second conclusion is that for unbound GC above the CMC (9 mM), ANS equilibrates between HSA and GC micelles but with a strong preference for free sites on HSA. That means that even persisting micelles would lose their cargo readily once exposed to HSA. For all MDDS sharing this property, targeted drug delivery approaches involving them as the nanocarrier would be pointless.

Circulating neutrophil anti-pathogen dysfunction in cirrhosis

Neutrophils are the largest population of leucocytes and are among the first cells of the innate immune system to fight against intruding pathogens. In patients with cirrhosis, neutrophils exhibit altered functionality, including changes in phagocytic ability, bacterial killing, chemotaxis, degranulation, reactive oxygen species production and NET (neutrophil extracellular trap) formation. This results in their inability to mount an adequate antibacterial response and protect the individual from infection. Prognosis and survival in patients with cirrhosis are greatly influenced by the development of infectious complications. Multidrug-resistant bacterial infections in patients with cirrhosis are currently a growing problem worldwide; therefore, alternative methods for the prevention and treatment of bacterial infections in cirrhosis are urgently needed. The prevention and treatment of neutrophil dysfunction could be a potential way to protect patients from bacterial infections. However, the reasons for changes in neutrophil function in cirrhosis are still not completely understood, which limits the development of efficient therapeutic strategies. Both cellular and serum factors have been proposed to contribute to the functional impairment of neutrophils. Herein, we review the current knowledge on features and proposed causes of neutrophil dysfunction in cirrhosis, with a focus on current knowledge gaps and limitations, as well as opportunities for future investigations in this field.

The Pathological Effects of Circulating Hydrophobic Bile Acids in Alzheimer's Disease

Recent clinical studies have revealed that the serum levels of toxic hydrophobic bile acids (deoxy cholic acid, lithocholic acid [LCA], and glycoursodeoxycholic acid) are significantly higher in patients with Alzheimer's disease (AD) and amnestic mild cognitive impairment (aMCI) when compared to control subjects. The elevated serum bile acids may be the result of hepatic peroxisomal dysfunction. Circulating hydrophobic bile acids are able to disrupt the blood-brain barrier and promote the formation of amyloid-β plaques through enhancing the oxidation of docosahexaenoic acid. Hydrophobic bile acid may find their ways into the neurons via the apical sodium-dependent bile acid transporter. It has been shown that hydrophobic bile acids impose their pathological effects by activating farnesoid X receptor and suppressing bile acid synthesis in the brain, blocking NMDA receptors, lowering brain oxysterol levels, and interfering with 17β-estradiol actions such as LCA by binding to E2 receptors (molecular modelling data exclusive to this paper). Hydrophobic bile acids may interfere with the sonic hedgehog signaling through alteration of cell membrane rafts and reducing brain 24(S)-hydroxycholesterol. This article will 1) analyze the pathological roles of circulating hydrophobic bile acids in the brain, 2) propose therapeutic approaches, and 3) conclude that consideration be given to reducing/monitoring toxic bile acid levels in patients with AD or aMCI, prior/in combination with other treatments.

Extending the Pseudo-Phase Model of Detergent-Lipid Dispersions by a Detergent-Binding Protein

Mixed micellar drug delivery systems for poorly soluble active pharmaceutical ingredients (APIs) are easy to produce and long-term stable, because they represent equilibrium structures. However, their fate after intravenous injection is still largely unknown. Once injected into the bloodstream, they can potentially convert to vesicles or disappear altogether, with both API and excipients being picked up by blood components. Our study aimed at reducing the gap between the good, quantitative understanding of aqueous glycocholate (GC)-lecithin dispersions alone and the highly complex situation in the blood. To this end, we extended the pseudophase model previously established for lipid-detergent dispersions to include the detergent-binding protein albumin as another component. The model predicted a quaternary phase diagram with planar phase boundaries defined by key parameters of the ternary subsystems, which were then determined by isothermal titration calorimetry. They include the aqueous GC concentration upon bilayer-micelle coexistence, 5.2 mM, the GC-to-lipid mole ratios in coexisting bilayers (R_e^sat = 0.2) and micelles (R_e^sol = 0.7), as well as the capacity of the albumin to bind 0.1 GC molecules with a dissociation constant of K_D = 0.1 mM and 6 GC molecules with K_D = 0.7 mM. Subsequent measurements in the quaternary system showed phase boundaries in good agreement with the model predictions. In addition, the critical micelle concentration of GC shows a minimal value (midpoint of transition) of 9.1 mM at the temperature of 24 °C where the demicellization enthalpy is zero. The demicellization process is accompanied by a heat capacity change of 29 cal/mol K. The model improves the understanding of the mixed micellar drug delivery systems. The success of the approach encourages including even more blood components, like lipoproteins, to a quantitative treatment.

Population pharmacokinetic model to generate mechanistic insights in bile acid homeostasis and drug-induced cholestasis

Bile acids (BA) fulfill a wide range of physiological functions, but are also involved in pathologies, such as cholestasis. Cholestasis is characterized by an intrahepatic accumulation of BAs and subsequent spillage to the systemic circulation. The aim of the present study was to develop physiologically based kinetic (PBK) models that would provide a tool to predict dose-dependent BA accumulation in humans upon treatment with a Bile Salt Export Pump (BSEP) inhibitor. We developed a PBK model describing the BA homeostasis using glycochenodeoxycholic acid as an exemplary BA. Population wide distributions of BSEP abundances were incorporated in the PBK model using Markov Chain Monte Carlo simulations, and alternatively the total amount of BAs was scaled empirically to describe interindividual differences in plasma BA levels. Next, the effects of the BSEP inhibitor bosentan on the BA levels were simulated. The PBK model developed adequately predicted the in vivo BA dynamics. Both the Markov Chain Monte Carlo simulations based on a distribution of BSEP abundances and empirical scaling of the total BA pool readily described the variations within and between data in human volunteers. Bosentan treatment disproportionally increased the maximum BA concentration in individuals with a large total BA pool or low BSEP abundance. Especially individuals having a large total BA pool size and a low BSEP abundance were predicted to be at risk for rapid saturation of BSEP and subsequent intrahepatic BA accumulation. This model provides a first estimate of personalized safe therapeutic external dose levels of compounds with BSEP-inhibitory properties.

New Insights into Bile Acids Related Signaling Pathways in the Onset of Colorectal Cancer

Colorectal cancer (CRC) ranks as the second among the causes of tumor death worldwide, with an estimation of 1.9 million new cases in 2020 and more than 900,000 deaths. This rate might increase by 60% over the next 10 years. These data are unacceptable considering that CRC could be successfully treated if diagnosed in the early stages. A high-fat diet promotes the hepatic synthesis of bile acids (BAs) increasing their delivery to the colonic lumen and numerous scientific reports correlate BAs, especially secondary BAs, with CRC incidence. We reviewed the physicochemical and biological characteristics of BAs, focusing on the major pathways involved in CRC risk and progression. We specifically pointed out the role of BAs as signaling molecules and the tangled relationships among their nuclear and membrane receptors with the big bang of molecular and cellular events that trigger CRC occurrence.

Bile Acids, Liver Cirrhosis, and Extrahepatic Vascular Dysfunction

The bile acid pool with its individual bile acids (BA) is modulated in the enterohepatic circulation by the liver as the primary site of synthesis, the motility of the gallbladder and of the intestinal tract, as well as by bacterial enzymes in the intestine. The nuclear receptor farnesoid X receptor (FXR) and Gpbar1 (TGR5) are important set screws in this process. Bile acids have a vasodilatory effect, at least according to in vitro studies. The present review examines the question of the extent to which the increase in bile acids in plasma could be responsible for the hyperdynamic circulatory disturbance of liver cirrhosis and whether modulation of the bile acid pool, for example, via administration of ursodeoxycholic acid (UDCA) or via modulation of the dysbiosis present in liver cirrhosis could influence the hemodynamic disorder of liver cirrhosis. According to our analysis, the evidence for this is limited. Long-term studies on this question are lacking.

A microbial metabolite remodels the gut-liver axis following bariatric surgery

Bariatric surgery is the most effective treatment for type 2 diabetes and is associated with changes in gut metabolites. Previous work uncovered a gut-restricted TGR5 agonist with anti-diabetic properties-cholic acid-7-sulfate (CA7S)-that is elevated following sleeve gastrectomy (SG). Here, we elucidate a microbiome-dependent pathway by which SG increases CA7S production. We show that a microbial metabolite, lithocholic acid (LCA), is increased in murine portal veins post-SG and by activating the vitamin D receptor, induces hepatic mSult2A1/hSULT2A expression to drive CA7S production. An SG-induced shift in the microbiome increases gut expression of the bile acid transporters Asbt and Ostα, which in turn facilitate selective transport of LCA across the gut epithelium. Cecal microbiota transplant from SG animals is sufficient to recreate the pathway in germ-free (GF) animals. Activation of this gut-liver pathway leads to CA7S synthesis and GLP-1 secretion, causally connecting a microbial metabolite with the improvement of diabetic phenotypes.

Untargeted Defining Protein-Metabolites Interaction Based on Label-Free Kinetic Size Exclusion Chromatography-Mass Spectrometry

The specific interactions between protein and metabolites (PMIs) are closely related to many cellular processes and play a vital role in signal transduction and regulating material and energy metabolism. However, most of the available analytical strategies for PMIs involve chemical modification of metabolites or immobilization of protein, which has restricted current PMIs study mainly to lipid-protein and hydrophobic metabolites. In this work, a label-free online kinetic size exclusion chromatography-mass spectrometry (KSEC-MS) method combined with untargeted metabolomics was developed to define PMIs in a complex system. The metabolite mixture and target protein were injected into the SEC column sequentially without preincubation, and the separation results of KSEC were monitored by global metabolite profiling with mass spectrometry. The potential ligands in the metabolite mixture can be discovered if their migration patterns were affected by the target protein and the variation was positively correlated with the concentration of target protein. To verify this approach, carbonic anhydrase was first selected as a test protein, and acetazolamide as its known inhibitor was successfully defined. Furthermore, human serum albumin (HSA) as the common transport carrier of metabolites was selected as a target protein to demonstrate the usefulness of this approach. Multiple endogenous ligands of HSA were simultaneously defined from the extracted metabolites of human serum; most of them are polar metabolites rather than nonpolar lipids. This approach can provide a novel way for mapping and identifying unknown PMIs in a complex system, especially for polar metabolites-protein interactions.

Further evidence for a continuous flux of bile acids into the brain: trapping of bile acids in subdural hematomas

Bile acids are known to pass the blood-brain barrier and are present at low concentrations in the brain. In a previous work, it was shown that subdural hematomas are enriched with bile acids and that the levels in such hematomas are higher than in the peripheral circulation. The mechanism behind this enrichment was never elucidated. Bile acids have a high affinity to albumin, and subdural hematomas contain almost as high albumin levels as the peripheral circulation. A subdural hematoma is encapsulated by fibrin which may allow passage of small molecules like bile acids. We hypothesized that bile acids originating from the circulation may be 'trapped' in the albumin in subdural hematomas. In the present work, we measured the conjugated and unconjugated primary bile acids cholic acid and chenodeoxycholic acid in subdural hematomas and in peripheral circulation of 24 patients. In most patients, the levels of both conjugated and free bile acids were higher in the hematomas than in the circulation, but the enrichment of unconjugated bile acids was markedly higher than that of conjugated bile acids. In patients with a known time interval between the primary bleeding and the operation, there was a correlation between this time period and the accumulation of bile acids. This relation was most obvious for unconjugated bile acids. The results are consistent with a continuous flux of bile acids, in particular unconjugated bile acids, across the blood-brain barrier. We discuss the possible physiological importance of bile acid accumulation in subdural hematomas.

An overview of albumin and alpha-1-acid glycoprotein main characteristics: highlighting the roles of amino acids in binding kinetics and molecular interactions

Although Albumin (ALB) and alpha-1-acid glycoprotein (AGP) have distinctive structural and functional characteristics, they both play a key role in binding a large variety of endogenous and exogenous ligands. An extensive binding to these plasma proteins could have a potential impact on drugs disposition (e.g. bioavailability, distribution and clearance), on their innocuity and their efficacy. This review summarizes the common knowledge about the structural and molecular characteristics of both ALB and AGP in humans, and about the most involved amino acids in their high-affinity binding pockets. However, the variability in residues found in binding pockets, for the same species, allows each plasma protein to interact differently with the ligands. The protein-ligand interaction influences differently the disposition of drugs that bind to either of these plasma proteins. The content of this review is useful for the design of new drug entities with high-binding characteristics, in qualitative and quantitative modelling (e.g. in vitro-in vivo extrapolations, 3D molecular docking, interspecies extrapolations), and for other interdisciplinary research.

A Physiology-Based Model of Human Bile Acid Metabolism for Predicting Bile Acid Tissue Levels After Drug Administration in Healthy Subjects and BRIC Type 2 Patients

Drug-induced liver injury (DILI) is a matter of concern in the course of drug development and patient safety, often leading to discontinuation of drug-development programs or early withdrawal of drugs from market. Hepatocellular toxicity or impairment of bile acid (BA) metabolism, known as cholestasis, are the two clinical forms of DILI. Whole-body physiology-based modelling allows a mechanistic investigation of the physiological processes leading to cholestasis in man. Objectives of the present study were: (1) the development of a physiology-based model of the human BA metabolism, (2) population-based model validation and characterisation, and (3) the prediction and quantification of altered BA levels in special genotype subgroups and after drug administration. The developed physiology-based bile acid (PBBA) model describes the systemic BA circulation in humans and includes mechanistically relevant active and passive processes such as the hepatic synthesis, gallbladder emptying, transition through the gastrointestinal tract, reabsorption into the liver, distribution within the whole body, and excretion via urine and faeces. The kinetics of active processes were determined for the exemplary BA glycochenodeoxycholic acid (GCDCA) based on blood plasma concentration-time profiles. The robustness of our PBBA model was verified with population simulations of healthy individuals. In addition to plasma levels, the possibility to estimate BA concentrations in relevant tissues like the intracellular space of the liver enhance the mechanistic understanding of cholestasis. We analysed BA levels in various tissues of Benign Recurrent Intrahepatic Cholestasis type 2 (BRIC2) patients and our simulations suggest a higher susceptibility of BRIC2 patients toward cholestatic DILI due to BA accumulation in the liver. The effect of drugs on systemic BA levels were simulated for cyclosporine A (CsA). Our results confirmed the higher risk of DILI after CsA administration in healthy and BRIC2 patients. The presented PBBA model enhances our mechanistic understanding underlying cholestasis and drug-induced alterations of BA levels in blood and organs. The developed PBBA model might be applied in the future to anticipate potential risk of cholestasis in patients.

Development of liposome as a novel adsorbent for artificial liver support system in liver failure

Artificial liver support systems (ALSS), represented by albumin dialysis, are designed to replace the liver detoxification function and to serve as supportive therapy until liver transplantation or liver regeneration. We introduce liposome, which is majorly formed by soybean lecithin as the adsorbent nanomaterial in dialysate for the removal of protein-bound and liver failure-related solutes. The binding rate was detected by ultrafiltration column. In vitro and in vivo dialysis was performed in a recirculation system. Unconjugated bilirubin (52.83-99.87%) and bile salts (50.54-94.75%) were bound by liposomes (5-80 g/L) in a dose-response relationship. The in vitro haemodialysis model showed that the concentration of unconjugated bilirubin (45.64 ± 0.90 μmol/L vs. 54.47 ± 3.48 μmol/L, p < 0.05) and bile salts (153.75 ± 7.72 μmol/L vs. 180.72 ± 7.95 μmol/L, p < 0.05) were significantly decreased in the liposome dialysis group than in the phosphate buffer saline group. The in vivo haemodialysis model showed that 40 g/L liposome-containing dialysate led to a significant higher reduction ratio in total bilirubin (6.56 ± 5.72% vs. -1.86 ± 5.99%, p < 0.05) and more total bile acids (7.63 ± 5.27 μmol vs. 2.13 ± 2.32 μmol, p < 0.05) extracted in the dialysate in comparison with the conventional dialysate. In conclusion, the liposome-added dialysate proved to impose good extraction effects on the unconjugated bilirubin and bile salts. These findings indicate that conventional dialysate supported by this nanomaterial can markedly improve the removal of protein-bound and liver failure-related solutes, thus suggesting a novel and promising liver dialysis system.

Synthesis, physicochemical properties, and biological activity of bile acids 3-glucuronides: Novel insights into bile acid signalling and detoxification

Glucuronidation is considered an important detoxification pathway of bile acids especially in cholestatic conditions. Glucuronides are less toxic than the parent free forms and are more easily excreted in urine. However, the pathophysiological significance of bile acid glucuronidation is still controversial and debated among the scientific community. Progress in this field has been strongly limited by the lack of appropriate methods for the preparation of pure glucuronides in the amount needed for biological and pharmacological studies. In this work, we have developed a new synthesis of bile acid C3-glucuronides enabling the convenient preparation of gram-scale quantities. The synthesized compounds have been characterized in terms of physicochemical properties and abilities to modulate key nuclear receptors including the farnesoid X receptor (FXR). In particular, we found that C3-glucuronides of chenodeoxycholic acid and lithocholic acid, respectively the most abundant and potentially cytotoxic species formed in patients affected by cholestasis, behave as FXR agonists and positively regulate the gene expression of transporter proteins, the function of which is critical in human conditions related to imbalances of bile acid homeostasis.

Removal of bile acids by extracorporeal therapies: an in vitro study

INTRODUCTION

Bile acids (BAs) accumulating in the circulation in patients with liver failure are considered to be responsible for pruritus, which strongly impairs quality of life of the affected patients. The aim of this study was the in vitro characterization of different BAs regarding their removability with high-flux dialysis as well as with different adsorbents, and the evaluation of their binding to plasma proteins.

METHODS

Dialysis experiments were conducted in pediatric circuits with human plasma. For the adsorption studies, batch tests using 10% adsorbent in spiked human plasma were carried out. The binding of BAs to plasma compounds was determined by centrifugation of spiked plasma through spin columns. Sieving coefficients were determined using an albumin filter and a high-flux dialyzer.

RESULTS

With high-flux dialysis, only hydrophilic BAs such as glycocholic and taurocholic acid could be removed efficiently, while all tested BAs were removed by adsorption.

CONCLUSIONS

In conclusion, the hydrophilicity of BAs plays a major role in their removability using extracorporeal approaches. Adsorption-based systems offer particular advantages for the removal of hydrophobic BAs.

Unconjugated bile acids in rat brain: Analytical method based on LC/ESI-MS/MS with chemical derivatization and estimation of their origin by comparison to serum levels

Although some studies have revealed the implication of bile acids (BAs) and neurological diseases, the levels and origin of the BAs in the brain are not fully understood. In this study, we first developed and validated a sensitive and specific method for the determination of three unconjugated BAs [cholic acid (CA), chenodeoxycholic acid (CDCA) and deoxycholic acid (DCA)] in the rat brain by liquid chromatography/electrospray ionization-tandem mass spectrometry combined with chemical derivatization. The measured brain concentrations (mean±standard deviation, n=10) of normal rats were 58.7±48.8, 14.2±11.7 and 13.2±8.7ng/g tissue for CA, CDCA and DCA, respectively. For their origin, we developed the hypothesis that they might be mostly derived from the periphery. To test this hypothesis, the brain BA levels were compared with the serum levels. The brain levels had high correlations with the serum levels, and were always lower than the serum levels for the three unconjugated BAs. Furthermore, the higher brain-to-serum concentration ratios were found for the BAs with higher logD values (higher lipophilicity). Moreover, the brains of the rats intraperitoneally administered with deuterium-labeled CA and CDCA were also analyzed; the deuterium-labeled BAs were detected in the brain of the rats administered with these compounds. Based on all the results, we concluded that the BAs found in the brain are mostly derived from the periphery and the major mechanism for the transportation of the unconjugated BAs to the brain is by passive diffusion.

Age-Related Changes of Plasma Bile Acid Concentrations in Healthy Adults--Results from the Cross-Sectional KarMeN Study

Bile acids (BA) play an important role in lipid metabolism. They facilitate intestinal lipid absorption, and BA synthesis is the main catabolic pathway for cholesterol. The objective of this study was to investigate associations of age, sex, diet (fat intake) and parameters of lipid metabolism (triglycerides, LDL, HDL, body fat content) with fasting plasma BA concentration of healthy individuals. Fasting plasma samples from a cross-sectional study were used to determine the concentrations of 14 BA using an LC-MS stable isotope dilution assay. Triglycerides, LDL and HDL were analyzed by standard clinical chemistry methods and body fat content was measured with a DXA instrument. The dietary fat intake of the 24 h period prior to the sampling was assessed on the basis of a 24 h recall. Subsequent statistical data processing was done by means of a median regression model. Results revealed large inter-individual variations. Overall, higher median plasma concentrations of BA were observed in men than in women. Quantile regression showed significant interactions of selected BA with age and sex, affecting primarily chenodeoxycholic acid and its conjugates. No associations were found for LDL and the amount of fat intake (based on the percentage of energy intake from dietary fat as well as total fat intake). Additional associations regarding body fat content, HDL and triglycerides were found for some secondary BA plasma concentrations. We conclude that age and sex are associated with the fasting plasma concentrations. Those associations are significant and need to be considered in studies investigating the role of BA in the human metabolism.

Differential sensing for the regio- and stereoselective identification and quantitation of glycerides

Glycerides are of interest to the areas of food science and medicine because they are the main component of fat. From a chemical sensing perspective, glycerides are challenging analytes because they are structurally similar to one another and lack diversity in terms of functional groups. Furthermore, because animal and plant fat consists of a number of stereo- and regioisomeric acylglycerols, their components remain challenging analytes for chromatographic and mass spectrometric determination, particularly the quantitation of species in mixtures. In this study, we demonstrated the use of an array of cross-reactive serum albumins and fluorescent indicators with chemometric analysis to differentiate a panel of mono-, di-, and triglycerides. Due to the difficulties in identifying the regio- and stereochemistry of the unsaturated glycerides, a sample pretreatment consisting of olefin cross-metathesis with an allyl fluorescein species was used before array analysis. Using this simple assay, we successfully discriminated 20 glycerides via principal component analysis and linear discriminant analysis (PCA and LDA, respectively), including stereo- and regioisomeric pairs. The resulting chemometric patterns were used as a training space for which the structural characteristics of unknown glycerides were identified. In addition, by using our array to perform a standard addition analysis on a mixture of triglycerides and using a method introduced herein, we demonstrated the ability to quantitate glyceride components in a mixture.

Effects of Pringle maneuver and partial hepatectomy on the pharmacokinetics and blood-brain barrier permeability of sodium fluorescein in rats

Liver diseases are known to affect the function of remote organs. The aim of the present study was to investigate the effects of Pringle maneuver, which results in hepatic ischemia-reperfusion (IR) injury, and partial hepatectomy (Hx) on the pharmacokinetics and brain distribution of sodium fluorescein (FL), which is a widely used marker of blood-brain barrier (BBB) permeability. Rats were subjected to Pringle maneuver (total hepatic ischemia) for 20 min with (HxIR) or without (IR) 70% hepatectomy. Sham-operated animals underwent laparotomy only. After 15 min or 8h of reperfusion, a single 25-mg/kg dose of FL was injected intravenously and serial (0-30 min) blood and bile and terminal brain samples were collected. Total and free (ultrafiltration) plasma, total brain homogenate, and bile concentrations of FL and/or its glucuronidated metabolite (FL-Glu) were determined by HPLC. Both IR and HxIR caused significant reductions in the biliary excretions of FL and FL-Glu, resulting in significant increases in the plasma AUC of the marker. Additionally, the free fraction of FL in plasma was significantly increased by HxIR. Although the brain concentrations of FL were increased by almost twofold in both IR and HxIR animals, the brain concentrations corrected by the free FL AUC (and not the total AUC) were similar in both groups at either time points. It is concluded that Pringle maneuver and/or partial hepatectomy substantially alters the hepatobiliary disposition, plasma AUC, plasma free fraction, and brain accumulation of FL without altering the BBB permeability to the marker.

Bile acid-induced necrosis in primary human hepatocytes and in patients with obstructive cholestasis

Accumulation of bile acids is a major mediator of cholestatic liver injury. Recent studies indicate bile acid composition between humans and rodents is dramatically different, as humans have a higher percent of glycine conjugated bile acids and increased chenodeoxycholate content, which increases the hydrophobicity index of bile acids. This increase may lead to direct toxicity that kills hepatocytes, and promotes inflammation. To address this issue, this study assessed how pathophysiological concentrations of bile acids measured in cholestatic patients affected primary human hepatocytes. Individual bile acid levels were determined in serum and bile by UPLC/QTOFMS in patients with extrahepatic cholestasis with, or without, concurrent increases in serum transaminases. Bile acid levels increased in serum of patients with liver injury, while biliary levels decreased, implicating infarction of the biliary tracts. To assess bile acid-induced toxicity in man, primary human hepatocytes were treated with relevant concentrations, derived from patient data, of the model bile acid glycochenodeoxycholic acid (GCDC). Treatment with GCDC resulted in necrosis with no increase in apoptotic parameters. This was recapitulated by treatment with biliary bile acid concentrations, but not serum concentrations. Marked elevations in serum full-length cytokeratin-18, high mobility group box 1 protein (HMGB1), and acetylated HMGB1 confirmed inflammatory necrosis in injured patients; only modest elevations in caspase-cleaved cytokeratin-18 were observed. These data suggest human hepatocytes are more resistant to human-relevant bile acids than rodent hepatocytes, and die through necrosis when exposed to bile acids. These mechanisms of cholestasis in humans are fundamentally different to mechanisms observed in rodent models.

Treatment of bile acid amidation defects with glycocholic acid

Bile acid amidation defects were predicted to present with fat/fat soluble vitamin malabsorption with minimal cholestasis. We identified and treated five patients (one male, four females) from four families with defective bile acid amidation due to a genetically confirmed deficiency in bile acid CoA:amino acid N-acyl transferase (BAAT) with the conjugated bile acid, glycocholic acid (GCA). Fast atom bombardment-mass spectrometry analysis of urine and bile at baseline revealed predominantly unconjugated cholic acid and absence of the usual glycine and taurine conjugated primary bile acids. Treatment with 15 mg/kg GCA resulted in total duodenal bile acid concentrations of 23.3 ± 19.1 mmol/L (mean ± SD) and 63.5 ± 4.0% of the bile acids were secreted in bile in the conjugated form, of which GCA represented 59.6 ± 9.3% of the total biliary bile acids. Unconjugated cholic acid continued to be present in high concentrations in bile because of partial intestinal deconjugation of orally administered GCA. Serum total bile acid concentrations did not significantly differ between pretreatment and posttreatment samples and serum contained predominantly unconjugated cholic acid. These findings confirmed efficient intestinal absorption, hepatic extraction, and biliary secretion of the administered GCA. Oral tolerance tests for vitamin D2 (1,000 IU vitamin D2/kg) and tocopherol (100 IU/kg tocopherol acetate) demonstrated improvement in fat-soluble vitamin absorption after GCA treatment. Growth improved in 3/3 growth-delayed prepubertal patients.

CONCLUSION

Oral glycocholic acid therapy is safe and effective in improving growth and fat-soluble vitamin absorption in children and adolescents with inborn errors of bile acid metabolism due to amidation defects.

Influence of human serum albumin on the bile acid-mediated inhibition of liver microsomal type 1 11β-hydroxysteroid dehydrogenase

The influence of human serum albumin (HSA) on the bile acid-mediated inhibition of liver microsomal type 1 11β-hydroxysteroid dehydrogenase (11β-HSD1) was studied in vitro. A rat liver microsomal fraction was prepared, and the 11β-HSD1 enzyme activity in the presence of various concentrations of bile acids and HSA was determined using hydrocortisone as the substrate. The products of the reaction were extracted and analyzed using high-performance liquid chromatography. The magnitude of the inhibition decreased with the addition of HSA in a dose-dependent manner. Four percent human albumin decreased the inhibitory effects of 100 μM chenodeoxycholic acid and lithocholic acid from 89.9 ± 5.6 to 54.5 ± 6.1% and from 83.8 ± 4.8 to 20.8 ± 4.2%, respectively. In contrast, ursodeoxycholic acid and deoxycholic acid showed no inhibitory effect on the enzyme activity in the presence of 4% human serum albumin, and the addition of 1% γ-globulin to the assay mixture in the presence of bile acids did not affect the enzyme activity. Our in vitro study showed that the addition of HSA ameliorated the inhibition of 11β-HSD1 and that the magnitude of the change is dependent on the species of bile acid, presumably based on the numbers of hydroxyl groups. These results suggest that HSA seems to protect the bile acid-mediated inhibition of 11β-HSD1 in the healthy subject. On the other hand, in the patients with obstructive biliary diseases, not only elevated serum bile acid but also the accompanying hypoalbuminemia is important to evaluate the pathophysiology of the bile acid-mediated inhibition of 11β-HSD1 of the disease.

Effects of short-term portacaval anastomosis on the peripheral and brain disposition of the blood-brain barrier permeability marker sodium fluorescein in rats

Contradictory results have been reported with regard to the effects of various models of hepatic encephalopathy on the blood-brain barrier (BBB) permeability, which may be due partly to the use of brain concentrations of BBB markers without attention to their peripheral pharmacokinetics. The purpose of the current study was to investigate the effects of short-term portacaval anastomosis (PCA), a type B model of hepatic encephalopathy, on the peripheral pharmacokinetics and brain distribution of sodium fluorescein (FL), which is a small molecule marker of BBB passive permeability. A single 25mg/kg dose of FL was administered intravenously to 10-day PCA and sham-operated rats, and serial blood and bile (0-30min) and terminal (30min) brain samples were collected, and the concentrations of FL and its glucuronidated metabolite (FL-Glu) were measured by HPLC. Additionally, the free fractions of FL (fu) in all the plasma samples were determined, and the effects of bile salts on fu were investigated in vitro. Passive permeability of BBB to FL was estimated by brain uptake clearance (Kin) based on both the brain concentrations of FL and plasma concentrations of free (unbound) FL. PCA caused a 26% increase in the fu of FL in plasma, which was due to competition of bile acids with FL for binding to plasma proteins. Additionally, PCA reduced the biliary excretion of FL-Glu by 55%. However, free Kin values (µl/min/g brain) for the sham (0.265±0.034) and PCA (0.228±0.038) rats were not significantly different. It is concluded that whereas 10-day PCA alters the peripheral pharmacokinetics of FL, it does not significantly affect the BBB permeability to the marker.

A new point-of-care portable immunosensor for non-invasive assessment of oro-ileal transit time by oral fluid tauroursodeoxycholate measurement after its oral load

A non-invasive test for oro-ileal transit time (OITT) evaluation was developed, based on the measurement of tauroursodeoxycholic acid (TUDCA) oral fluid concentration profile after its oral administration. Exploiting the fact that TUDCA is actively absorbed only in the ileum, OITT is measured as the time corresponding to TUDCA maximum oral fluid concentration (tmax). To measure oral fluid TUDCA concentration in a point-of-care setting, an ultrasensitive portable immunosensor was developed, based on a competitive chemiluminescent enzyme immunoassay (CL-EIA), using immobilized anti-TUDCA antibody and an ursodeoxycholic acid (UDCA)-peroxidase conjugate as tracer, detected by enhanced chemiluminescence employing a portable charge-coupled device (CCD)-based device. The test was validated in 24 healthy subjects before and after treatment with Loperamide, a drug that increases OITT. The developed CL-EIA was accurate and precise, with a LLOQ of 50 pmol L(-1). The measured OITT for healthy subjects (291 ± 50 min) was fairly well correlated with OITT values obtained by measuring TUDCA in serum (r=0.89). An increased OITT was observed in all the studied subjects after Loperamide treatment. The CL immunosensor can be employed directly in gastroenterology and paediatric units and it can thus represent a new non-invasive simple test for OITT evaluation in a point-of-care setting, with improved diagnostic utility.

Macroporous Composite Cryogels with Embedded Polystyrene Divinylbenzene Microparticles for the Adsorption of Toxic Metabolites from Blood

Composite monolithic adsorbents were prepared by the incorporation of neutral polystyrene divinylbenzene (PS-DVB) microparticles into macroporous polymer structures produced by cryogelation of agarose or poly(vinyl alcohol). The composite materials exhibited excellent flow-through properties. Scanning electron microscopy of the composite cryogels revealed that the microparticles were covered by thin films of poly(vinyl alcohol) or agarose and thus were withheld in the monolith structure. Plain PS-DVB microparticles showed efficient adsorption of albumin-bound toxins related to liver failure (bilirubin and cholic acid) and of cytokines (tumor necrosis factor-alpha and interleukin-6). The rates of adsorption and the amount of adsorbed factors were lower for the embedded microparticles as compared to the parent PS-DVB microparticles, indicating the importance of the accessibility of the adsorbent pores. Still, the macroporous composite materials showed efficient adsorption of albumin-bound toxins related to liver failure as well as efficient binding of cytokines, combined with good blood compatibility. Thus, the incorporation of microparticles into macroporous polymer structures may provide an option for the development of adsorption modules for extracorporeal blood purification.

Ultrasound-triggered BSA/SPION hybrid nanoclusters for liver-specific magnetic resonance imaging

Nanoclusters of superparamagnetic iron oxide nanoparticles (SPION) are developed for liver-specific magnetic resonance imaging (MRI) by a unique synthesis route. The process is efficient, environmentally benign, and straight forward within five minutes. The clustering effect is triggered in the presence of bovine serum albumin (BSA) aqueous phase under ultrasonication condition. The hydrophobic SPION are densely self-assembled into BSA/SPION hybrid nanoclusters with a uniform size of ~86 nm. The as-prepared BSA/SPION hybrid nanoclusters are found to be biocompatible and stable. They exhibit high transverse relaxivity and longitudinal relaxivity in water (r(2) and r(1) values are 600.8 and 4.3 s(-1) per mM of Fe(3+), respectively). In vivo T(2)-weighted MRI shows excellent enhancement in liver with an imaging time-window up to 48 h. In vivo biodistribution study indicates a gradual excretion of the nanoclusters via hepatobiliary (HB) processing. No toxicity is observed in the in vivo and ex vivo experiments. The BSA/SPION hybrid nanoclusters present great potential in MRI as the liver-specific contrast agents (CAs).

Spectrophotometric and thermodynamic studies of the interactions of 4-carboxyl-2,6-dinitrophenylazohydroxynaphthalenes with bovine serum albumin

The intermolecular interaction occurring between bovine serum albumin and 4-carboxyl-2,6-dinitrophenylazohydroxynaphthalene in aqueous media has been studied spectrophotometrically at different temperatures including body temperature. Evidence for the formation of new molecular complexes was established by hypsochromic and hypochromic shifts of the spectra of dye-BSA complexes compared to the spectra of unbound dyes. One congener (AZ-02) gave a minor peak characteristic of charge-transfer complexation. The binding constants of the four monoazo dyes were investigated and found to vary according to the dye structure and temperature of investigation. AZ-01 and -04 combined with BSA at approximately 1:1 mol ratio while the other two congeners with additional proton donors gave greater than this mole ratio. Thermodynamic considerations established that the dyes utilized the various forms of binding modes; hydrogen bonding, hydrophobic bonding, van der Waals and AZ-03 was particularly involved in electrostatic interactions giving positive entropy change for a small enthalpy change. The observed properties were correlated with the genotoxicity potentials of the monoazo dyes. The results obtained should prove beneficial in designing other molecules in this category of chemical class.

Bile Acid Inhibition of N-type Calcium Channel Currents from Sympathetic Ganglion Neurons

Under some pathological conditions as bile flow obstruction or liver diseases with the enterohepatic circulation being disrupted, regurgitation of bile acids into the systemic circulation occurs and the plasma level of bile acids increases. Bile acids in circulation may affect the nervous system. We examined this possibility by studying the effects of bile acids on gating of neuronal (N)-type Ca²⁺ channel that is essential for neurotransmitter release at synapses of the peripheral and central nervous system. N-type Ca²⁺ channel currents were recorded from bullfrog sympathetic neuron under a cell-attached mode using 100 mM Ba²⁺ as a charge carrier. Cholic acid (CA, 10^-6 M) that is relatively hydrophilic thus less cytotoxic was included in the pipette solution. CA suppressed the open probability of N-type Ca²⁺ channel, which appeared to be due to an increase in null (no activity) sweeps. For example, the proportion of null sweep in the presence of CA was ~40% at +40 mV as compared with ~8% in the control recorded without CA. Other single channel properties including slope conductance, single channel current amplitude, open and shut times were not significantly affected by CA being present. The results suggest that CA could modulate N-type Ca²⁺ channel gating at a concentration as low as 10^-6 M. Bile acids have been shown to activate nonselective cation conductance and depolarize the cell membrane. Under pathological conditions with increased circulating bile acids, CA suppression of N-type Ca²⁺ channel function may be beneficial against overexcitation of the synapses.

The bile steroid chenodeoxycholate is a potent antagonist at NMDA and GABA(A) receptors

The bile steroids (BS) cholic acid and chenodeoxycholic acid are produced in hepatocytes and in the brain. Nothing is known about neuronal actions of BS. Deficiency in a 27-hydroxylase enzyme coincides with reduced production of chenodeoxycholic acid (CDCA) and a relative increase in cholic acid in an inherited lipid storage disease, cerebrotendinous xanthomatosis, characterized by neurological dysfunctions, which can be treated by dietary CDCA. We have examined the modulation of hypothalamic network activity by nine common BS. Cholate and CDCA significantly reduced the firing of hypothalamic neurons and synchronized network activity with CDCA being nearly 10 times more potent. The synthetic BS dehydrocholate synchronized the activity without affecting the firing rate. Gabazine, a GABA(A) receptor antagonist, occluded synchronization by BS. Whole-cell patch clamp recordings revealed a block of NMDA- and GABA(A)-receptors by BS. Potencies of nine common BS differed between NMDA and GABA(A) receptors, however in both cases they correlated with BS affinities for albumin but not with their lipophilicity, supporting a direct action at ligand gated ion channels. GABAergic synaptic currents displayed a faster decay under BS. Our data provide new insight into extrahepatic functions of BS revealing their neuroactive potential.

Activation-dependent adsorption of cytokines and toxins related to liver failure to carbon beads

In the course of severe pathological conditions, such as acute liver failure and sepsis, toxic metabolites and mediators of inflammation are released into the patient's circulation. One option for the supportive treatment of these conditions is plasmapheresis, in which plasma, after being separated from the cellular components of the blood, is cleansed by adsorption of harmful molecules on polymers or activated carbon. In this work, the adsorption characteristics of activated carbon beads with levels of activation ranging from 0 to 86% were assessed for both hydrophobic compounds accumulating in liver failure (bilirubin, cholic acid, phenol and tryptophan) and cytokines (tumor necrosis factor α and interleukin-6). Progressive activation resulted in significant gradual reduction of both bulk density and mean particle size, in an increase in the specific surface area, and to changes in pore size distribution with progressive broadening of micropores. These structural changes went hand in hand with enhanced adsorption of small adsorbates, such as IL-6 and cholic acid and, to a lesser extent, also of large molecules, such as TNF-α.

Liver and bone

Osteoporosis is a frequent complication in patients with chronic liver disease, especially in end-stages and in cases with chronic cholestasis, hemochromatosis and alcohol abuse. The problem is more critical in transplant patients when bone loss is accelerated during the period immediately after transplantation, leading to a greater incidence of fractures. Advanced age, low body mass index and severity of the liver disease are the main risk factors for bone disease in patients with cholestasis. Mechanisms underlying osteoporosis in chronic liver disease are complex and poorly understood, but osteoporosis mainly results from low bone formation, related to the effects of retained substances of cholestasis, such as bilirubin and bile acids, or to the effects of alcohol on osteoblastic cells. Increased bone resorption has also been described in cholestatic women with advanced disease. Although there is no specific treatment, bisphosphonates associated with supplements of calcium and vitamin D are effective for increasing bone mass in patients with chronic cholestasis and after liver transplantation. The outcome in reducing the incidence of fractures has not been adequately demonstrated essentially because of the low number of patients included in the therapeutic trials. Randomized studies assessing bisphosphonates in larger series of patients, the development of new drugs for osteoporosis and the improvement in the management of liver transplant recipients may change the future.

Ligand binding strategies of human serum albumin: how can the cargo be utilized?

Human serum albumin (HSA), being the most abundant carrier protein in blood and a modern day clinical tool for drug delivery, attracts high attention among biologists. Hence, its unfolding/refolding strategies and exogenous/endogenous ligand binding preference are of immense use in therapeutics and clinical biochemistry. Among its fellow proteins albumin is known to carry almost every small molecule. Thus, it is a potential contender for being a molecular cargo/or nanovehicle for clinical, biophysical and industrial purposes. Nonetheless, its structure and function are largely regulated by various chemical and physical factors to accommodate HSA to its functional purpose. This multifunctional protein also possesses enzymatic properties which may be used to convert prodrugs to active therapeutics. This review aims to highlight current overview on the binding strategies of protein to various ligands that may be expected to lead to significant clinical applications.

Lithocholic acid downregulates vitamin D effects in human osteoblasts

BACKGROUND

Osteoporosis is a common complication in chronic cholestasis. It has been proposed that retained substances such as bile acids may produce a damaging effect on bone cells. This study analyses the effects of lithocholic acid (LCA) on cell survival and vitamin D metabolism in human osteoblasts (hOB).

MATERIALS AND METHODS

Human osteoblasts cultures were performed with or without foetal bovine serum (FBS) or human albumin (HA) at different LCA concentrations and times with or without vitamin D.

RESULTS

Lithocholic acid at concentrations higher than 10(-5 )M decreased cell survival. This effect was partially prevented by the presence of FBS or HA. Vitamin D stimulated CYP24A, BGLAP and TNFSF11 expression in hOB and these effects were modified by nontoxic LCA concentrations. LCA significantly decreased vitamin D stimulation of CYP24A, BGLAP and TNFSF11 gene expression at 72%, 79% and 56% (respectively). LCA alone has an agonistic effect, as has vitamin D, thus partially increasing CYP24A and BGLAP expression, but with no changes on TNFRSF11B expression. Equivalent effects of the LCA were observed by performing gene reporter assays using MG-63 cells transfected with constructs containing CYP24A1 promoter regions.

CONCLUSIONS

Lithocholic acid decreases the stimulatory effect of vitamin D on CYP24A, BGLAP and TNFSF11 expression in hOB. This effect is produced through vitamin D response elements (VDREs), located in the promoter regions of these genes, suggesting that LCA acts as a mild analogous of vitamin D, interacting with the vitamin D receptor. These results may explain the potential deleterious effects of retained bile acids on hOB.