Isolation and characterization of an endogenous drug-binding inhibitor present in uremic serum

Interaction of Human Serum Albumin with Uremic Toxins: The Need of New Strategies Aiming at Uremic Toxins Removal

Chronic kidney disease (CKD) is acknowledged worldwide to be a grave threat to public health, with the number of US end-stage kidney disease (ESKD) patients increasing steeply from 10,000 in 1973 to 703,243 in 2015. Protein-bound uremic toxins (PBUTs) are excreted by renal tubular secretion in healthy humans, but hardly removed by traditional haemodialysis (HD) in ESKD patients. The accumulation of these toxins is a major contributor to these sufferers’ morbidity and mortality. As a result, some improvements to dialytic removal have been proposed, each with their own upsides and drawbacks. Longer dialysis sessions and hemodiafiltration, though, have not performed especially well, while larger dialyzers, coupled with a higher dialysate flow, proved to have some efficiency in indoxyl sulfate (IS) clearance, but with reduced impact on patients’ quality of life. More efficient in removing PBUTs was fractionated plasma separation and adsorption, but the risk of occlusive thrombosis was worryingly high. A promising technique for the removal of PBUTs is binding competition, which holds great hopes for future HD. This short review starts by presenting the PBUTs chemistry with emphasis on the chemical interactions with the transport protein, human serum albumin (HSA). Recent membrane-based strategies targeting PBUTs removal are also presented, and their efficiency is discussed.

Furan fatty acids - Beneficial or harmful to health?

Furan fatty acids are found in plants, algae, and fish, and reported to have some positive health benefits, including anti-oxidant and anti-inflammatory activities, and inhibition of non-enzymatic lipid peroxidation. A major metabolite of furan fatty acids, 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF), has been reported to be increased in patients who progress from prediabetes to type 2 diabetes, although CMPF is not necessarily associated with impaired glucose metabolism. Other studies report that CMPF levels are lower in subjects with diabetes than control subjects. Plasma CMPF levels increase in subjects who consume fish or fish oil, and in patients with renal failure. It is not known where furan fatty acids are converted to CMPF and it is speculated that this might be a result of microbiome activity. The plasma levels reported for CMPF in healthy, diabetic and patients with renal disease vary by factors of more than 100-fold within each of these three groups, so measurement error appears to be limiting the ability to interpret studies. This review explores these controversies and raises questions about whether CMPF is a marker for healthy diets or indeed associated with diabetes and renal health. The review concludes that, on balance, furan fatty acids are beneficial for health.

Variability in teicoplanin protein binding and its prediction using serum albumin concentrations

The impact of lower serum albumin levels on teicoplanin pharmacokinetics has not been previously determined. The authors assessed the relationship between total and free concentrations of teicoplanin in serum samples obtained from patients receiving teicoplanin therapy for Gram-positive bacterial infections. In addition, the authors determined the contribution of serum albumin concentrations to the unbound fraction of teicoplanin. One hundred ninety-eight serum samples were obtained from 65 patients undergoing routine therapeutic drug monitoring of teicoplanin. Free serum teicoplanin was separated by ultrafiltration, and total and free serum concentrations of teicoplanin were determined by a fluorescence polarization immunoassay. Regression analysis was then performed to build a prediction model for the free serum teicoplanin concentration from the total serum teicoplanin concentration and the serum albumin level using the first 132 samples. The predictive performance of this model was then tested using the next 66 samples. Free serum teicoplanin concentrations (Cf) (mug/mL) were predicted using a simple model constructed using total serum teicoplanin (Ct) (mug/mL) and albumin concentrations (ALB) (g/dL): Cf = Ct/(1 + 1.78 * ALB). This model could estimate free serum teicoplanin concentrations with a small bias and an acceptable error. The measured free level of teicoplanin will lie between 0.63 and 1.38 times the predicted concentration in 95% of cases. Serum albumin level plays a major role in the variability of the fraction unbound of teicoplanin. This model can reliably estimate free serum teicoplanin concentrations more easily than by using direct measurements.

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

The ability of albumin to bind drugs and other lipophilic organic acids is decreased in chronic renal failure by the accumulation of albumin-bound uraemic toxins such as hippuric acid, indoxyl sulphate and 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF). This furan acid is the most highly bound and is not removed by haemodialysis. The inhibitory effects of these three uraemic toxins on the interaction of three marker ligands sodium octanoate (for medium chain fatty acids), salicylic acid and phenol red (bilirubin site/site I) with albumin have been investigated by differential scanning microcalorimetry and flow microcalorimetry. CMPF was the most potent inhibitor and its binding site coincided with that of bilirubin (site I). Indoxyl sulphate binds to the site for medium-chain fatty acids and tryptophan (site II) and hippuric acid, the weakest inhibitor, inhibited binding to the salicylic acid site.

Correlation and Prediction of Phenytoin Protein Binding Using Standard Laboratory Parameters in Patients After Renal Transplantation

Renal transplant recipients provide a unique model for protein-binding studies in that patients experience hypoalbuminemia and renal dysfunction, both of which alter protein binding. The purposes of this investigation were to model the relationship between serum creatinine, blood urea nitrogen (BUN), albumin, and the unbound fraction of phenytoin (FU, as a percentage) in patients who had undergone renal transplant, and to determine the value of these measurements in predicting FU. Blood from 29 patients was collected at various time points after establishment of graft function. Sera were spiked with phenytoin to a concentration of 15 mg/L, and total/unbound phenytoin concentrations were determined. Correlations between FU and the biochemical indices of serum creatinine, BUN, and albumin were determined using multiple regression. The algorithm with the highest correlation at all times after the transplant became the method to predict future FU. This algorithm was applied prospectively in 23 samples from 14 other patients with variable renal function after transplant. Samples were analyzed as above and the corresponding biochemical indices of serum creatinine, BUN, and albumin were used to calculate FU values. Accuracy of the predictions was evaluated using prediction-error analysis. The best relationship between FU and the measured biochemical indices incorporated serum creatinine and albumin [y = 24.3 + 0.6(serum creatinine) - 3.9(albumin)] and served as the method for FU prediction. Prediction-error analysis resulted in a bias of -5.1% and a precision of 5.7%. This method failed to estimate FU with sufficient accuracy to permit clinical utility. The predicted value underestimated the measured value, and some other variable(s) must be affecting the binding even though serum creatinine and albumin are within or approaching the reference range. Consequently, estimating FU in patients with a history of uremia and hypoalbuminemia, based on measures of serum creatinine and albumin alone, should not be used.

Overexpression of Erythrocyte Glutathione S-Transferase in Uremia and Dialysis

Background: Overexpression of glutathione S-transferase (GST; EC 2.5.1.18) has been documented in the erythrocytes of patients with chronic renal failure, and this event may well be of relevance from a clinical standpoint. In fact, it could serve as a marker of uremic toxicity overall, which can contribute to impair the function and survival of the erythrocytes. However, the biochemical details of this phenomenon are poorly understood.

Methods: In this study, we characterized the expression of GST in erythrocytes of 118 uremic patients under different clinical conditions. The mechanisms responsible for the regulation of protein expression and enzyme activity were investigated in light of different dialysis approaches, oxidative stress, uremic toxins, erythrocyte age, and erythropoietin (EPO) supplementation.

Results: Mean GST activity in uremic patients was highly overexpressed with respect to controls, and this phenomenon was exclusively attributable to an increased expression of GST. Overexpression of GST did not appear to be dependent on oxidative stress and was not influenced by vitamin E supplementation. In the same manner, both erythrocyte age and EPO supplementation apparently did not interfere with the GST concentrations, which were the same in controls and patients. Preliminary experiments suggested that high-molecular weight or protein-bound toxins could play some role in the overexpression of GST.

Conclusions: GST expression may be a useful marker for the individual accumulation of uremic toxins as well as of the efficiency of new dialysis strategies in removing them.

Serum substances that interfere with thyroid hormone assays in patients with chronic renal failure

OBJECTIVE

Serum thyroid hormone concentrations in patients with chronic renal failure are usually low, despite normal serum TSH levels. We investigated the effect on thyroid hormone assays of serum dialysable organic acids that are elevated in uraemic patients.

PATIENTS

Serum samples from 42 patients with chronic renal failure who were receiving haemodialysis and 37 sex- and age-matched healthy subjects were examined.

DESIGN AND MEASUREMENTS

Serum thyroid hormone concentrations were measured with an analogue radioimmunoassay (RIA), a labelled antibody assay, and an equilibrium dialysis/RIA method. Serum concentrations of organic acids were determined with high performance liquid chromatography.

RESULTS

Serum thyroid hormone levels determined by an analogue RIA and a labelled antibody assay in uraemic patients increased, and serum concentrations of organic acids decreased following haemodialysis. A significant association existed between serum free T3 (FT3) levels determined by an analogue RIA and serum concentrations of indoxyl sulphate (IS) prior to dialysis. There was also a significant association between serum free T4 (FT4) levels determined by an analogue RIA and serum concentration of IS and hippuric acid (HA) prior to dialysis. There was a significant association between the changes of serum concentrations of indole acetic acid (IAA) and FT4 concentrations prior to and following haemodialysis when determined by an analogue RIA. Serum FT3 and FT4 levels significantly decreased after the addition of IS to serum from healthy subjects when determined by an analogue RIA but not by a labelled antibody assay. Serum FT4 levels, but not FT3 levels, decreased after addition of IAA when determined by an analogue RIA. Serum FT4 concentrations determined by an equilibrium dialysis/RIA were significantly higher than those determined by the other two methods. The addition of IS, IAA, and HA to serum samples from healthy subjects significantly increased FT4 concentrations when determined by an equilibrium dialysis/RIA method.

CONCLUSIONS

Increased serum levels of indoxyl sulphate, indole acetic acid and hippuric acid in sera of uraemic patients may interfere with thyroid hormone measurements when an analogue radioimmunoassay is used. In contrast, there was little Interference with a labelled antibody assay. Dialysable organic acids may also interfere with thyroid hormone assays determined by an equilibrium dialysis/radioimmunoassay method.

Mass spectrometry in the search for uremic toxins

This article reviews the literature on the mass spectrometry (MS) that has been used in the research of uremic toxins. Gas chromatography/mass spectrometry (GC/MS) has been most often used for the analysis of low-molecular-weight compounds in uremic blood such as organic acids, phenols, and polyols. However, it cannot be used for the analysis of middle- to high-molecular-weight substances or for involatile compounds. The development of fast atom bombardment (FAB) and liquid secondary ion mass spectrometry (LSIMS) has made possible the analysis of middle-molecules and involatile low-molecular-weight substances such as peptides and nucleosides. The development of atmospheric pressure chemical ionization (APCI) has also lead to the analysis of involatile low-molecular-weight substances. The recent advances in ionization methods, such as electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI), have permitted the MS analysis of high-molecular-weight substances such as beta 2-microglobulin, a major component of dialysis amyloid. Liquid chromatography/mass spectrometry (LC/MS), using ESI, APCI, or FAB as an ionization method, is currently the preferred method for the analysis of low- to high-molecular-weight substances in uremic blood. ESI-LC/MS and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOFMS) are useful for elucidating the structure of post-translationally modified proteins obtained from the blood and tissues of uremic patients. Post-translational modification such as the formation of advanced glycation end-products and carbamoylation is enhanced in uremic patients, and is considered to be responsible for some uremic symptoms. Laser microprobe MS is unique in its capability for the two-dimensional detection of atoms such as aluminum in a tissue section obtained from uremic patients. This review focuses on the mainstream research for discovering uremic toxins, specific uremic toxins identified or quantified using MS, and the MS analysis of post-translationally modified proteins in uremia. These studies have provided ample evidence that MS has played an important role in the search for uremic toxins.

Serum and urine albumin: a progress report on their measurement and clinical significance

For about 25 years, bromcresol green and bromcresol purple have been the basis for most of the measurements of serum albumin in the US and perhaps in the world. The longevity of the methods is due to their being simple, sensitive, specific, inexpensive and relatively free from interferences. The lack of change in the serum albumin methodology is balanced by two important developments. First, the recognition of the importance of serum albumin in the maintenance of good health, and the association of decreased concentrations with increased risk of morbidity and mortality. Second, the association of albuminuria with diabetic nephropathy, which without medical intervention could lead to end-stage renal disease. The development of accurate and precise methods for urinary albumin has provided a tool to physicians to extend the length and improve the quality of life of many diabetic individuals.

Free and protein-bound tryptophan in serum of untreated patients with chronic renal failure

In fasting sera from 46 untreated patients with chronic renal failure and in 22 normal subjects, non-protein-bound tryptophan, F, was separated by pH-controlled equilibrium dialysis. Total tryptophan, T, and F were measured by HPLC. Results in patients were related to gender, severity of chronic renal failure (as measured by radioisotopic glomerular filtration rate), protein intake (as measured by 24-hr urinary urea N excretion), and protein nutrition (as measured by serum concentrations of albumin and transferrin). T was subnormal in 59% of the patients. In seven hypoalbuminemic patients, F/T was markedly increased (approaching unity) but F was normal. In 39 non-hypoalbuminemic patients, F was again normal but F/T was often increased at glomerular filtration rates below 30 ml/min/3 m2 of height2, especially in females. T was significantly correlated with estimated protein intake (r = 0.54, P = 0.0004), even though neither F nor serum protein levels were correlated with it. We conclude that the serum level of free tryptophan is well-maintained in chronic renal failure, being uninfluenced by severity of renal insufficiency, voluntary protein intake, or serum protein concentrations. On the other hand, protein-bound tryptophan varies with protein intake, decreases markedly in hypoalbuminemic patients, and also decreases in many nonhypoalbuminemic patients (especially females) when the glomerular filtration rate falls below approximately 30 ml/min/3 m2 of height2.

A naturally occurring furan fatty acid enhances drug inhibition of thyroxine binding in serum

We studied the thyroxine (T4)-displacing effects of a naturally occurring, highly albumin-bound furanoid acid that accumulates in serum in renal failure to concentrations in excess of 0.2 mmol/L. This substance, 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF), has been shown to displace acidic drugs from albumin binding. The effects of CMPF on ligand binding were assessed in the following systems: (1) T4 binding to T4-binding globulin (TBG) and transthyretin (TTR), (2) T4 binding in undiluted serum, (3) T4-displacing potency of fenclofenac, furosemide, diflunisal, and aspirin in undiluted serum, (4) serum binding of [14C]-drug preparations, and (5) serum binding of [14C]-oleic acid. CMPF had a minor direct effect on T4 binding to TBG comparable in relative affinity to that of aspirin, ie, almost 7 orders of magnitude less than T4 itself. CMPF alone at a concentration of 0.3 mmol/L, which produced only a 10% to 14% increase in free T4 augmented the T4-displacing effects of high therapeutic concentrations of the various drugs in undiluted serum as follows: furosemide by 180%, fenclofenac by 160%, diflunisal by 130%, and aspirin by 40%. In the presence of fenclofenac, increments of CMPF from 0.075 to 0.3 mmol/L progressively augmented the T4-displacing effect of this drug, associated with a progressive increase in its calculated free concentration. CMPF also inhibited the binding of [14C]-oleic acid, suggesting that in some situations CMPF could also indirectly influence thyroid hormone binding by increasing the unbound concentration of nonesterified fatty acids (NEFA), as previously described.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural analysis of oxidation products of urofuran acid by hypochlorous acid

The oxidation of urofuran acid derivatives (1-2) by hypochlorous acid (HOCl) was investigated with the goal to possibly simplify the detection of their metabolites in biological materials. The oxidation products of 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid (1) were obtained as an isomeric mixture and confirmed to exist as cis (3a) and trans (3b) isomers, based on their 13C nuclear magnetic resonance (NMR) spectra. Similarly, the products of 5-H substituted acid 2 obtained by oxidation with HOCl were identified as 4a and 4b by 13C and 1H NMR which indicated the presence of cis and trans hemiacetal hydrogens at C-5 in a ratio of 2.11:1. The oxidation was found to proceed in a manner different from that of the F-acid, because of the presence of the electron withdrawing COOCH3 group at C-3 which favored the nucleophilic attack on the carbonyl group to afford cis- and trans-2,5-dihydroxy-2,5-dihydrofurans (3a-b, 4a-b).

Microcalorimetric studies on uraemic plasma

The heat production by human plasma in healthy and uraemic subjects has been measured by direct isothermal microcalorimetry. The plasma from uraemic subjects displayed an increased heat production compared to that of normal plasma. The heat production by plasma from healthy subjects, but not by that from uraemic patients, was both proportional to the amount of thiol groups in the plasma and also to the oxidation of the thiol groups. The oxygen consumption of uraemic plasma was proportional to the heat production. The heat production by samples from uraemic patients was significantly correlated to the plasma concentration of creatinine, whereas no such correlation was found between the concentration of urea and heat production of such samples.

Albumin binding in uraemia: quantitative assessment of inhibition by endogenous ligands and carbamylation of albumin

The binding capacity of human serum albumin (HSA) for small acidic molecules is known to be reduced in chronic renal failure (CRF). The contribution of competitive inhibition by accumulated endogenous ligands and of structural changes in HSA has now been evaluated. In a fluorimetric in vitro assay using HSA and two dansylated amino acids the inhibitory properties of various endogenous ligands were determined in concentration-effect studies. The effect of carbamylation of HSA on binding was also examined. The mode of inhibition, including binding parameters n and Ka, was determined. Finally, HSA binding in sera from controls and dialysis patients was compared in a modified assay. Thirty three substances were tested and were placed in 3 groups: strong inhibitors (IC50 < 3*10(-5) mol.l-1, e.g. indolyl acids, furanoic acids), medium inhibitors (IC50 > 3*10(-5), eg. vanillic acid), and no inhibition (e.g. urea, creatinine, guanidino compounds). Complete (> 80%) carbamylation of HSA reduced binding by 67% in a non-competitive mode. There was a significant reduction in the binding capacity of HSA from the dialysis patients (approximately 24%), irrespective of medication. It is concluded that the uraemic binding defect of HSA is caused by competitive inhibition by the many physiological ligands accumulated in CRF and structural modifications of HSA. The assay presented proved useful for the rapid analysis of possible HSA binding inhibitors and for testing large groups of patients, e.g. comparison of dialysis treatments, and pharmacological binding studies.

Hippuric acid and 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid in serum and urine. Analytical approaches and clinical relevance in kidney diseases

Hippuric acid (HA) and 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid (FA) were determined in serum, plasma, ultrafiltrate and urine by gas chromatography (GC), high-performance liquid chromatography and GC with mass-selective detection, and the methods were compared. As determined by affinity chromatography and analysis of serum and ultrafiltrate, 0.5% of FA in serum occurs free and 99.5% is bound to albumin. In haemodialysed patients with chronic renal failure, the plasma levels of HA and FA are elevated in comparison with normal controls and hospital patients without kidney diseases: HA, 11.1 +/- 5.7 mg/dl (n = 86); FA, 1.9 +/- 1.2 mg/dl (n = 86). Gradual increases in HA in serum, depending on the creatinine concentrations, are found in non-dialysed patients with chronic renal failure. By haemodialysis and haemofiltration the HA levels are lowered (53-66 and 30-36%, respectively), whereas FA is not dialysable.

The effect of renal transplantation on a major endogenous ligand retained in uremic serum

The effects of renal transplantation on serum concentrations of 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF) and indole-3-acetic acid (IAA), which are endogenous ligands retained in uremic serum, and on phenytoin binding to serum protein were investigated. IAA, a weakly bound ligand, was rapidly excreted by the transplanted kidney during the first one to three days after renal transplantation, but CMPF, a strongly bound ligand, was slowly excreted. The binding defect of phenytoin was partially corrected by transplantation during the period of study. The results suggested that the prolonged drug binding defect observed despite successful renal transplantation is caused by a slower decrease of strongly bound ligands such as CMPF retained in uremic serum; hypoalbuminemia, usually observed after transplantation, may also contribute to this phenomenon.

Accumulation of furancarboxylic acids in uremic serum as inhibitors of drug binding

3-carboxy-4-methyl-5-propyl-2-furanpropionic acid, 3-carboxy-4-methyl-5-pentyl-2-furanpropionic acid, 3-carboxy-4-methyl-5-ethyl-2-furanpropionic acid and 3-carboxy-5-propyl-2-furanpropionic acid were detected in uremic serum using gas chromatography-mass spectrometry. Mass chromatography revealed that the serum concentrations of the furancarboxylic acids especially 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid, were increased in the chronic hemodialysis patients and that the acids could not be removed by conventional hemodialysis due to their strong binding to plasma protein. 3-Carboxy-4-methyl-5-propyl-2-furanpropionic acid was also quantitated in uremic serum by high-performance liquid chromatography. Serum level of the acid in uremic patients showed significant but weak correlation with serum level of urea and duration on hemodialysis. Equilibrium dialysis demonstrated that 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid and 3-carboxy-4-methyl-5-pentyl-2-furanpropionic acid inhibited the bindingof salicylate and 5,5-diphenylhydantoin to albumin. In conclusion, the furancarboxylic acids especially 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid were accumulated in uremic serum as inhibitors of drug binding.

Underestimation of serum albumin by the bromcresol purple method and a major endogenous ligand in uremia

In hemodialyzed patients, the serum albumin concentration determined by the bromcresol purple (BCP) method was lower than that determined by an immunological method. The degree of underestimation appeared to be well correlated to the serum concentration of 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF), a major endogenous ligand substance present in uremic serum. CMPF inhibited in vitro the binding of BCP to serum protein and human serum albumin. Our results suggest that CMPF is a major interferent in the underestimation of the serum albumin concentration by the BCP method in uremia.