Effect of protein-bound uraemic toxins on the thermodynamic characteristics of human albumin

Uraemic solutes as therapeutic targets in CKD-associated cardiovascular disease

Chronic kidney disease (CKD) is characterized by the retention of a myriad of solutes termed uraemic (or uremic) toxins, which inflict damage to several organs, including the cardiovascular system. Uraemic toxins can induce hallmarks of cardiovascular disease (CVD), such as atherothrombosis, heart failure, dysrhythmias, vessel calcification and dysregulated angiogenesis. CVD is an important driver of mortality in patients with CKD; however, reliance on conventional approaches to managing CVD risk is insufficient in these patients, underscoring a need to target risk factors that are specific to CKD. Mounting evidence suggests that targeting uraemic toxins and/or pathways induced by uraemic toxins, including tryptophan metabolites and trimethylamine N-oxide (TMAO), can lower the risk of CVD in patients with CKD. Although tangible therapies resulting from our growing knowledge of uraemic toxicity are yet to materialize, a number of pharmacological and non-pharmacological approaches have the potential to abrogate the effects of uraemic toxins, for example, by decreasing the production of uraemic toxins, by modifying metabolic pathways induced by uraemic toxins such as those controlled by aryl hydrocarbon receptor signalling and by augmenting the clearance of uraemic toxins.

The effect of two formulations of carbon enterosorbents on oxidative stress indexes and molecular conformation of serum albumin in experimental animals exposed to CCl4

The liver failure means inability to perform its normal synthetic, biotransformation and excretory functions. The disturbance of metabolic processes leads to the development of "metabolic endogenous intoxication" resulting in oxidative stress. Oxidative stress initiates the processes of oxidation of amino acid residues of blood plasma proteins causing the changes in their structure and functions. The effect of administration of highly activated porous carbonic enterosorbents on oxidative stress manifestations and molecular conformation of serum albumin in blood of experimental animals with acute liver failure induced by carbon tetrachloride (CCl4) needs to be investigated. Two forms of activated carbonic enterosorbents such as AC1 (primary beads with the range of diameters of 125-250 μm) and AC2 (secondary granules prepared from micronized AC1 having the mean particle size of ~1 μm) derived from phenol-formaldehyde resin were used in rat model with CCl4 intoxication. The total level of reactive oxygen species (ROS) in blood plasma, the activity of catalase (CAT) in blood hemolysates; the content of reduced glutathione (GSH) in liver homogenates, and the level of oxidative modification of proteins (OMP) such as aldehyde-dinitrophenylhydrazone (A-DNPH) and ketone-dinitrophenylhydrazone (K-DNPH) derivatives in blood plasma and liver homogenates were determined. In addition, the level of pro/antioxidant ratio in blood hemolysates and the content of lipid peroxidation product - malondialdehyde (MDA), in blood plasma and liver were determined. Melting thermograms of blood plasma proteins (BPP) and molecular conformation changes of serum albumin were analyzed by biophysical methods (differential scanning microcalorimetry and spectrofluorimetry). The extent of CCl4-induced oxidative damage in blood and liver of experimental animals was shown to be less expressed for AC1 in comparison with AC2 enterosorbent. However, AC2 used in the form of secondary granules positively influenced some biophysical properties of albumin molecule (temperature of melting, shape of melting endotherm and intrinsic fluorescence) after rats exposure to CCl4. In general, administration of both AC1 and AC2 led to the reduction of oxidative stress manifestations and partial restoration of native molecular conformation of serum albumin. These observations are promising in terms of achieving recovery of detoxification potential of organism after severe liver injury.

Improving the clearance of protein-bound uremic toxins using cationic liposomes as an adsorbent in dialysate

Anionic and protein-bound uremic toxins, represented by indoxyl sulfate (IS), may be associated with cardiovascular outcomes and the progression of chronic kidney disease in cases of injured kidney function and are not easily cleared by traditional dialysis therapy. We fabricated cationic liposomes that were modified with polyethyleneimine (PEI), octadecylamine (Oct), and hexadecyl trimethyl ammonium bromide (CTAB), and evaluated the effects on the clearance of the representative protein-bound uremic toxins (PBUTs). The binding rate was obtained by ultrafiltration and in vitro dialysis was performed in a Rapid Equilibrium Dialysis (RED) device to assay the clearing efficiency of the dialysate supported by three types of cationic liposomes. The cationic liposomes showed a higher binding rate with IS (1.24-1.38 fold higher) and p-cresol (1.07-1.09 fold higher) than in the unmodified plain liposomes. The dialysate supported by cationic liposomes also exhibited better clearing efficiency for IS (PEI-20: 57.65 ± 1.74 %; Oct-5: 62.80 ± 0.69 %; CTAB-10: 66.54 ± 0.91 %; p < 0.05) and p-cresol (PEI-20: 67.05 ± 3.09 %; Oct-5: 79.26 ± 0.43 %; CTAB-5: 68.45 ± 1.72 %; p < 0.05) than for phosphate buffer saline (IS: 29.70 ± 2.38 %; p-cresol: 33.59 ± 3.44 %) or dialysate supported by bovine serum albumin (IS: 50.00 ± 4.01 %; p-cresol: 53.06 ± 0.97 %). In conclusion, cationic liposomes are efficient in the clearance of anionic PBUTs, and these modified liposomes suggest a potential application in blood purification.

Increasing the removal of protein-bound uremic toxins by liposome-supported hemodialysis

Protein-bound uremic toxins (PBUTs) accumulate at high plasma levels and cause various deleterious effects in end-stage renal disease patients because their removal by conventional hemodialysis is severely limited by their low free-fraction levels in plasma. Here, we assessed the extent to which solute removal can be increased by adding liposomes to the dialysate. The uptake of liposomes by direct incubation in vitro showed an obvious dose-response relationship for p-cresyl sulfate (PCS) and indoxyl sulfate (IS) but not for hippuric acid (HA). The percent removal of both PCS and IS but not of HA was gradually increased with the increased concentration of liposomes in a rapid equilibrium dialysis setup. In vitro closed circulation showed that adding liposomes to the dialysate markedly increased the dialysances of PBUTs without greatly altering that of urea and creatinine. In vivo experiments in uremic rats demonstrated that adding liposomes to the dialysate resulted in higher reduction ratios (RRs) and more total solute removal (TSR) for several PBUTs compared to the conventional dialysate, which was approximately similar to the addition of bovine serum albumin to the dialysate. These findings highlight that as an adjunct to conventional hemodialysis, addition of liposomes to the dialysate could significantly improve the removal of protein-bound uremic solutes without greatly altering the removal of small, water-soluble solutes.

Improved dialytic removal of protein-bound uraemic toxins with use of albumin binding competitors: an in vitro human whole blood study

Protein-bound uraemic toxins (PBUTs) cause various deleterious effects in end-stage kidney disease patients, because their removal by conventional haemodialysis (HD) is severely limited by their low free fraction in plasma. Here we provide an experimental validation of the concept that the HD dialytic removal of PBUTs can be significantly increased by extracorporeal infusion of PBUT binding competitors. The binding properties of indoxyl sulfate (IS), indole-3-acetic acid (IAA) and hippuric acid (HIPA) and their binding competitors, ibuprofen (IBU), furosemide (FUR) and tryptophan (TRP) were studied in uraemic plasma. The effect of binding competitor infusion on fractional removal of PBUT was then quantified in an ex vivo single-pass HD model using uraemic human whole blood. The infusion of a combination of IBU and FUR increased the fractional removal of IS from 6.4 ± 0.1 to 18.3 ± 0.4%. IAA removal rose from 16.8 ± 0.3 to 34.5 ± 0.7%. TRP infusion increased the removal of IS and IAA to 10.5 ± 0.1% and 27.1 ± 0.3%, respectively. Moderate effects were observed on HIPA removal. Pre-dialyzer infusion of PBUT binding competitors into the blood stream can increase the HD removal of PBUTs. This approach can potentially be applied in current HD settings.

Guanidinylations of albumin decreased binding capacity of hydrophobic metabolites

AIM

As post-translational modifications of proteins may have an impact on the pathogenesis of diseases such as atherosclerosis, diabetes mellitus and chronic kidney disease (CKD), post-translational modifications are currently gaining increasing interest. In this study, a comprehensive method for analysis of these post-translational modifications is established for the clinical diagnostic routine.

METHODS

Here, we analysed albumin - the most abundant plasma protein in human - isolated from patients with CKD and healthy controls by chromatographic steps and identified by MALDI mass spectrometry. Post-translational modifications of albumin were identified after digestion by analysing mass signal shifts of albumin peptides using pertinent mass databases.

RESULTS

Albumin isolated from plasma of patients with CKD but not from healthy control subjects was specifically post-translationally modified by guanidinylation of lysines at positions 249, 468, 548, 565 and 588. After identification of guanidinylations as post-translational modifications of albumin isolated from patients with CKD, these modifications were quantified by mass spectrometry demonstrating a significant increase in the corresponding mass signal intensities in patients with CKD compared to healthy controls. The relative amount of guanidinylation of lysine at position 468 in patients with CKD was determined as 63 ± 32% (N = 3). Subsequently, we characterized the pathophysiological impact of the post-translational guanidinylation on the binding capacity of albumin for representative hydrophobic metabolic waste products. In vitro guanidinylation of albumin from healthy control subjects caused a decreased binding capacity of albumin in a time-dependent manner. Binding of indoxyl sulphate (protein-bound fraction) decreased from 82 ± 1% of not post-translationally modified albumin to 56 ± 1% after in vitro guanidinylation (P < 0.01), whereas the binding of tryptophan decreased from 20 to 4%. These results are in accordance with the binding of indoxyl sulphate to albumin from healthy control subjects and patients with CKD (88 ± 3 vs. 74 ± 10, P < 0.01). Thus, in vitro post-translational guanidinylation of albumin had a direct effect on the binding capacity of hydrophobic metabolites such as indoxyl sulphate and tryptophan.

CONCLUSION

We used a mass spectrometry-based method for the characterization of post-translational modification and demonstrated the pathophysiological impact of a representative post-translational modification of plasma albumin. The data described in this study may help to elucidate the pathophysiological role of protein modifications.

Investigation of the prerequisites for the optimization of specific plasma protein binding as a strategy for the reduction of first-pass hepatic metabolism

1. It is hypothesized that the deliberate structural tailoring of compounds designed for drug use to increase the specific plasma protein binding can be used to reduce first-pass hepatic metabolism. To test the feasibility of this hypothesis, a dataset of drugs with plasma protein binding of 90% or above divided into three classes including 50 acids, 44 bases and 69 neutrals was analyzed. 2. Among the drugs with ≥99% plasma protein binding, the fraction of the total dose existing in free form in vivo (free dose fraction) decreased in the following order: acids (0.55%) > neutrals (0.16%) > bases (0.08%). The order was different for the fraction of the total dose that existed in plasma protein bound form (plasma protein bound dose fraction): acids (58%) > neutrals (17%) = bases (18%). 3. The free fraction was poorly correlated with the partition coefficient (Log P). The lower aqueous solubility associated with high plasma protein binding was explained by differences in Log P and not by the plasma protein binding per se. The logarithm of the extrarenal clearance was correlated with Log P. For acids and bases, extrarenal clearance was also correlated with fu. For neutrals, plasma protein binding had no protective effect.

Elimination of endogenous toxin, creatinine from blood plasma depends on albumin conformation: site specific uremic toxicity & impaired drug binding

Uremic syndrome results from malfunctioning of various organ systems due to the retention of uremic toxins which, under normal conditions, would be excreted into the urine and/or metabolized by the kidneys. The aim of this study was to elucidate the mechanisms underlying the renal elimination of uremic toxin creatinine that accumulate in chronic renal failure. Quantitative investigation of the plausible correlations was performed by spectroscopy, calorimetry, molecular docking and accessibility of surface area. Alkalinization of normal plasma from pH 7.0 to 9.0 modifies the distribution of toxin in the body and therefore may affect both the accumulation and the rate of toxin elimination. The ligand loading of HSA with uremic toxin predicts several key side chain interactions of site I that presumably have the potential to impact the specificity and impaired drug binding. These findings provide useful information for elucidating the complicated mechanism of toxin disposition in renal disease state.

Hepatic clearance, but not gut availability, of erythromycin is altered in patients with end-stage renal disease

Nonrenal clearance of drugs can be significantly lower in patients with end-stage renal disease (ESRD) than in those with normal renal function. Using erythromycin (ER) as a probe compound, we investigated whether this decrease in nonrenal clearance is due to reduced hepatic clearance (CL(H)) and/or gut metabolism. We also examined the potential effects of the uremic toxins 3-carboxy-4-methyl-5-propyl-2-furan propanoic acid (CMPF) and indoxyl sulfate (Indox) on ER disposition. Route-randomized, two-way crossover pharmacokinetic studies of ER were conducted in 12 ESRD patients and 12 healthy controls after oral (250 mg) and intravenous (125 mg) dosing with ER. In patients with ESRD, CL(H) decreased 31% relative to baseline values (0.35 +/- 0.14 l/h/kg vs. 0.51 +/- 0.13 l/h/kg, P = 0.01), with no change in steady-state volume of distribution. With oral dosing, the bioavailability of ER increased 36% in patients with ESRD, and this increase was not related to changes in gut availability. As expected, plasma levels of CMPF and Indox were significantly higher in the patients than in the healthy controls. However, no correlation was observed between CL(H) of ER and the levels of uremic toxins.

New approaches to the removal of protein-bound toxins from blood plasma of uremic patients

The article is devoted to the theoretical aspects of the development of the effective method for the removal of protein-bound uremic toxins. It is shown that the methods of flow and differential scanning microcalorimetry are sufficient enough for the evaluation of the degree of ligand loading of human serum albumin with protein-bound uremic toxins. The molecules of albumin isolated from blood plasma of the patients being kept on chronic dialysis are demonstrating significant alterations of conformation and complex-forming properties, the correction of which by conventional methods of extracorporeal detoxification (exhaustive dialysis, treatment on synthetic SCN carbons) are practically ineffective. Deliganding of uremic albumin may be successfully performed on conventional carbon haemosorbents upon preliminary separation of blood plasma and its dilution with acetate buffer 1:1 at pH = 5.08. Treatment of the whole blood of patients onto new mass-fractal deliganding carbon, i.e., hemosorbents of HSGD trademark. These HSGD haemosorbents quite effectively could be used for restoration of main parameters of uremic HAS molecules conformation and ligand-binding activity simultaneously with hemodialysis upon the protection by locally performed citrate anticoagulation as an easier and cheaper method for the removal of protein-bound uremic toxins.

Endogenous Uremic Substances are not Involved in the Reduced Hepatic Extraction of Metoprolol in Bilateral Ureter-Ligated Rats†

The hepatic extraction of metoprolol is reduced in rats with bilateral ureter ligation (BUL)-induced renal failure. The aim of the present study was to evaluate the effect of uremic substances on the hepatic metabolism of metoprolol in rats with BUL. The metabolic rate in the liver microsomes of BUL rats was similar to that in sham rats, and there was no significant difference between sham and BUL rats in the effect of the supernatant of liver homogenates on the metabolism. The rate of metabolism in the liver microsomes in the presence of the plasma of BUL rats was also similar to that in the presence of the plasma of sham rats. These findings indicated that uremic substances which accumulate in BUL rats do not directly inhibit the activity of CYP2D2, which is responsible for the metabolism of metoprolol in the rat liver.

EFFECTS OF UREMIC TOXINS ON HEPATIC UPTAKE AND METABOLISM OF ERYTHROMYCIN

Hepatic clearance of erythromycin (Ery) is significantly reduced in patients with end stage renal disease. Since Ery is primarily eliminated via excretion of unchanged drug in the bile, we suspect that this change could be due to the effect of uremic toxins on hepatic uptake and/or efflux transporters. Using rat hepatocytes and microsomes as model proof of concept systems, we examined six uremic toxins, 3-carboxy-4-methyl-5-propyl-2-furan-propanoic acid (CMPF), indoxyl sulfate (IS), hippuric acid (HA), indole acetic acid (IA), guanidinosuccinic acid (GSA), and indoxyl-beta-D-glucuronide (IG), for their effects on Ery uptake and metabolism. Ery and the metabolite N-demethyl-Ery were measured by liquid chromatography/tandem mass spectrometry. The uptake of Ery by rat hepatocytes was markedly inhibited by rifampin and digoxin, but not by quinidine, suggesting that Oatp2 plays a major role in the uptake of Ery. At 50 microM, CMPF significantly (p < 0.05) reduced hepatocyte accumulation of Ery and N-demethyl-Ery. At higher concentrations (>200 microM), CMPF appears to also inhibit the enzymatic metabolism of Ery. In contrast, IS did not significantly inhibit the hepatocyte uptake of Ery, even at the highest concentration (800 microM) tested, but reduced metabolite generation (p < 0.001). The other uremic toxins, HA, IA, IG, and GSA, did not affect either hepatic uptake or microsomal metabolism of Ery. CMPF, IS, and HA were shown not to inhibit differential P-glycoprotein transport of Ery in cellular systems. Our results suggest that CMPF can directly inhibit the uptake of Ery by inhibiting Oatp2, whereas IS is more likely to inhibit the enzymatic metabolism of Ery.

Protein-bound uremic retention solutes

Protein-bound uremic retention solutes are molecules with low molecular weight (MW) but should be considered middle or high MW substances. This article describes the best known substances of this group, which include p-cresol, indoxyl sulfate, hippuric acid, 3-carboxy-4-methyl-5-propyl-2-furan-propionic acid (CMPF), and homocysteine. At concentrations encountered during uremia, p-cresol inhibits phagocyte function and decreases leukocyte adhesion to cytokine-stimulated endothelial cells. CMPF has been implicated in anemia and neurologic abnormalities of uremia. CMPF could alter the metabolism of drugs of inhibiting their binding to albumin and their tubular excretion. Indoxyl sulfate administrated to uremic rats increases the rate of progression of renal failure. Hippuric acid inhibits glucose utilization in the muscle, and its serum concentration is correlated with neurologic symptoms of uremia. Homocysteine predisposes uremic patients to cardiovascular disease through impairment of endothelial and smooth muscle cell functions. The removal of protein-bound compounds by conventional hemodialysis is low. Other strategies to decrease their concentrations include increase in dialyze pore size, daily hemodialysis, peritoneal dialysis, reduction of production or acceleration of degradation, and preservation of residual renal function.