Mass spectrometry in the search for uremic toxins

Advances in uremic toxin detection and monitoring in the management of chronic kidney disease progression to end-stage renal disease

Patients with end-stage kidney disease (ESKD) rely on dialysis to remove toxins and stay alive. However, hemodialysis alone is insufficient to completely remove all/major uremic toxins, resulting in the accumulation of specific toxins over time. The complexity of uremic toxins and their varying clearance rates across different dialysis modalities poses significant challenges, and innovative approaches such as microfluidics, biomarker discovery, and point-of-care testing are being investigated. This review explores recent advances in the qualitative and quantitative analysis of uremic toxins and highlights the use of innovative methods, particularly label-mediated and label-free surface-enhanced Raman spectroscopy, primarily for qualitative detection. The ability to analyze uremic toxins can optimize hemodialysis settings for more efficient toxin removal. Integration of multiple omics disciplines will also help identify biomarkers and understand the pathogenesis of ESKD, provide deeper understanding of uremic toxin profiling, and offer insights for improving hemodialysis programs. This review also highlights the importance of early detection and improved understanding of chronic kidney disease to improve patient outcomes.

The Role of Advanced Glycation End Products and Its Soluble Receptor in Kidney Diseases

Patients with chronic kidney disease (CKD) are more prone to oxidative stress and chronic inflammation, which may lead to an increase in the synthesis of advanced glycation end products (AGEs). Because AGEs are mostly removed by healthy kidneys, AGE accumulation is a result of both increased production and decreased kidney clearance. On the other hand, AGEs may potentially hasten decreasing kidney function in CKD patients, and are independently related to all-cause mortality. They are one of the non-traditional risk factors that play a significant role in the underlying processes that lead to excessive cardiovascular disease in CKD patients. When AGEs interact with their cell-bound receptor (RAGE), cell dysfunction is initiated by activating nuclear factor kappa-B (NF-κB), increasing the production and release of inflammatory cytokines. Alterations in the AGE-RAGE system have been related to the development of several chronic kidney diseases. Soluble RAGE (sRAGE) is a decoy receptor that suppresses membrane-bound RAGE activation and AGE-RAGE-related toxicity. sRAGE, and more specifically, the AGE/sRAGE ratio, may be promising tools for predicting the prognosis of kidney diseases. In the present review, we discuss the potential role of AGEs and sRAGE as biomarkers in different kidney pathologies.

Chronic Kidney Disease-Associated Immune Dysfunctions: Impact of Protein-Bound Uremic Retention Solutes on Immune Cells

Regardless of the primary disease responsible for kidney failure, patients suffering from chronic kidney disease (CKD) have in common multiple impairments of both the innate and adaptive immune systems, the pathophysiology of which has long remained enigmatic. CKD-associated immune dysfunction includes chronic low-grade activation of monocytes and neutrophils, which induces endothelial damage and increases cardiovascular risk. Although innate immune effectors are activated during CKD, their anti-bacterial capacity is impaired, leading to increased susceptibility to extracellular bacterial infections. Finally, CKD patients are also characterized by profound alterations of cellular and humoral adaptive immune responses, which account for an increased risk for malignancies and viral infections. This review summarizes the recent emerging data that link the pathophysiology of CKD-associated immune dysfunctions with the accumulation of microbiota-derived metabolites, including indoxyl sulfate and p-cresyl sulfate, the two best characterized protein-bound uremic retention solutes.

Predicting and Defining Steroid Resistance in Pediatric Nephrotic Syndrome Using Plasma Metabolomics

INTRODUCTION

Nephrotic syndrome (NS) is a kidney disease that affects both children and adults. Glucocorticoids have been the primary therapy for >60 years but are ineffective in approximately 20% of children and approximately 50% of adult patients. Unfortunately, patients with steroid-resistant NS (SRNS; vs. steroid-sensitive NS [SSNS]) are at high risk for both glucocorticoid-induced side effects and disease progression.

METHODS

We performed proton nuclear magnetic resonance (1H NMR) metabolomic analyses on plasma samples (n = 86) from 45 patients with NS (30 SSNS and 15 SRNS) obtained at initial disease presentation before glucocorticoid initiation and after approximately 7 weeks of glucocorticoid therapy to identify candidate biomarkers able to either predict SRNS before treatment or define critical molecular pathways/targets regulating steroid resistance.

RESULTS

Stepwise logistic regression models identified creatinine concentration and glutamine concentration (odds ratio [OR]: 1.01; 95% confidence interval [CI]: 0.99-1.02) as 2 candidate biomarkers predictive of SRNS, and malonate concentration (OR: 0.94; 95% CI: 0.89-1.00) as a third candidate predictive biomarker using a similar model (only in children >3 years). In addition, paired-sample analyses identified several candidate biomarkers with the potential to identify mechanistic molecular pathways/targets that regulate clinical steroid resistance, including lipoproteins, adipate, pyruvate, creatine, glucose, tyrosine, valine, glutamine, and sn-glycero-3-phosphcholine.

CONCLUSION

Metabolomic analyses of serial plasma samples from children with SSNS and SRNS identified elevated creatinine and glutamine concentrations, and reduced malonate concentrations, as auspicious candidate biomarkers to predict SRNS at disease onset in pediatric NS, as well as additional candidate biomarkers with the potential to identify mechanistic molecular pathways that may regulate clinical steroid resistance.

Generation, clearance, toxicity, and monitoring possibilities of unaccounted uremic toxins for improved dialysis prescriptions

Current dialysis-dosing calculations provide an incomplete assessment of blood purification. They exclude clearances of protein-bound uremic toxins (PB-UTs), such as polyamines, p-cresol sulfate, and indoxyl sulfate, relying solely on the clearance of urea as a surrogate for all molecules accumulating in patients with end-stage renal disease (ESRD). PB-UTs clear differently in dialysis but also during normal renal function. The kidney clears PB toxins via the process of secretion, whereas it clears urea through filtration. Herein, we review the clearance, accumulation, and toxicity of various UTs. We also suggest possible methods for their monitoring toward the ultimate goal of a more comprehensive dialysis prescription. A more inclusive dialysis prescription would retain the kidney-filtration surrogate, urea, and consider at least one PB toxin as a surrogate for UTs cleared through cellular secretion. A more comprehensive assessment of UTs that includes both secretion and filtration is expected to result in a better understanding of ESRD toxicity and consequently, to reduce ESRD mortality.

Uremic Toxicity: Present State of the Art

The uremic syndrome is a complex mixture of organ dysfunctions, which is attributed to the retention of a myriad of compounds that under normal condition are excreted by the healthy kidneys (uremic toxins). In the area of identification and characterization of uremic toxins and in the knowledge of their pathophysiologic importance, major steps forward have been made during recent years. The present article is a review of several of these steps, especially in the area of information about the compounds that could play a role in the development of cardiovascular complications. It is written by those members of the Uremic Toxins Group, which has been created by the European Society for Artificial Organs (ESAO). Each of the 16 authors has written a state of the art in his/her major area of interest.

Proteins and their modifications in a medieval mummy

Proteins and their modifications of the natural mummy of Cangrande della Scala (Prince of Verona, Northern Italy, 1291-1329) were studied. The nano-LC-Q-TOF analysis of samples of rib bone and muscle from the mummy showed the presence of different proteins including Types I, III, IV, V, and XI collagen, hemoglobin (subunits alpha and beta), ferritin, biglycan, vitronectin, prothrombin, and osteocalcin. The structure of Type I and Type III collagen was deeply studied to evaluate the occurrence of modifications in comparison with Type I and Type III collagen coming from tissues of recently died people. This analysis showed high percentage of asparaginyl and glutaminyl deamidation, carbamylation and carboxymethylation of lysine, as well as oxidation and dioxidation of methionine. The most common reaction during the natural mummification process was oxidation-the majority of lysine and proline of collagen Type I was hydroxylated whereas methionine was oxidated (oxidated or dioxidated). To the best of our knowledge, this is the first study which reports the protein profile of a natural mummified human tissue and the first one which describes the carbamylation and carboxymethylation of lysine in mummified tissues.

Exploration of novel predictive markers in rat plasma of the early stages of chronic renal failure

To identify blood markers for early stages of chronic kidney disease (CKD), blood samples were collected from rats with adenine-induced CKD over 28 days. Plasma samples were subjected to metabolomic profiling by liquid chromatography-mass spectrometry, followed by multivariate analyses. In addition to already-identified uremic toxins, we found that plasma concentrations of N6-succinyl adenosine, lysophosphatidylethanolamine 20:4, and glycocholic acid were altered, and that these changes during early CKD were more sensitive markers than creatinine concentration, a universal indicator of renal dysfunction. Moreover, the increase in plasma indoxyl sulfate concentration occurred earlier than increases in phenyl sulfate and p-cresol sulfate. These novel metabolites may serve as biomarkers in identifying early stage CKD.

Mass spectrometry analysis of nucleosides and nucleotides

Mass spectrometry has been widely utilised in the study of nucleobases, nucleosides and nucleotides as components of nucleic acids and as bioactive metabolites in their own right. In this review, the application of mass spectrometry to such analysis is overviewed in relation to various aspects regarding the analytical mass spectrometric and chromatographic techniques applied and also the various applications of such analysis.

Correlation between Serum Levels of Protein-Bound Uremic Toxins in Hemodialysis Patients Measured by LC/MS/MS

Uremic toxins are involved in a variety of symptoms in advanced chronic kidney disease. Especially, the accumulation of protein-bound uremic toxins in the blood of dialysis patients might play an important role in the development of cardiovascular disease. Serum concentration of protein-bound uremic toxins such as indoxyl sulfate, indoxyl glucuronide, indoleacetic acid, p-cresyl sulfate, p-cresyl glucuronide, phenyl sulfate, phenyl glucuronide, phenylacetic acid, phenylacetylglutamine, hippuric acid, 4-ethylphenyl sulfate, and 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid (CMPF) in hemodialysis patients were simultaneously measured by liquid chromatography/tandem mass spectrometry. Serum levels of these protein-bound uremic toxins were increased in hemodialysis patients. Indoxyl sulfate, p-cresyl sulfate, and CMPF could not be removed efficiently by hemodialysis due to their high protein-binding ratios. Serum level of total indoxyl sulfate did not show any significant correlation with total p-cresyl sulfate. However, free indoxyl sulfate correlated with free p-cresyl sulfate, and reduction rate by hemodialysis of indoxyl sulfate correlated with that of p-cresyl sulfate. Serum levels of total and free indoxyl sulfate showed significantly positive correlation with those of indoxyl glucuronide, phenyl sulfate, and phenyl glucuronide. Serum levels of total and free p-cresyl sulfate showed significantly positive correlation with those of p-cresyl glucuronide, phenylacetylglutamine, and phenylacetic acid. Indoxyl sulfate and indoxyl glucuronide are produced from indole which is produced in the intestine from tryptophan by intestinal bacteria. p-Cresyl sulfate and p-cresyl glucuronide are produced from p-cresol which is produced in the intestine from tyrosine by intestinal bacteria. Thus, intestinal bacteria play an important role in the metabolism of protein-bound uremic toxins.

Update of uremic toxin research by mass spectrometry

Mass spectrometry (MS) has been successfully applied for the identification and quantification of uremic toxins and uremia-associated modified proteins. This review focuses on the recent progress in the MS analysis of uremic toxins. Uremic toxins include low-molecular weight solutes, protein-bound low-molecular weight solutes, and middle molecules (peptides and proteins). Based on MS analysis of these uremic toxins, the pathogenesis of the uremic symptoms will be elucidated to prevent and manage the symptoms. Notably, protein-bound uremic toxins such as indoxyl sulfate, p-cresyl sulfate, and 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid have emerged as important targets of therapeutic removal. Hemodialysis even with a high-flux membrane cannot efficiently remove the protein-bound uremic toxins because of their high albumin-binding property. The accumulation of these protein-bound uremic toxins in the blood of dialysis patients might play an important role in the development of uremic complications such as cardiovascular disease. Indoxyl sulfate is the most promising protein-bound uremic toxin as a biomarker of progress in chronic kidney disease. Novel dialysis techniques or membranes should be developed to efficiently remove these protein-bound uremic toxins for the prevention and management of uremic complications.

Metabolomic search for uremic toxins as indicators of the effect of an oral sorbent AST-120 by liquid chromatography/tandem mass spectrometry

An oral sorbent AST-120 composed of spherical porous carbon particles has superior adsorption ability for certain small-molecular-weight organic compounds known to accumulate in patients with chronic renal failure (CRF). A metabolomic approach was applied to search for uremic toxins as possible indicators of the effect of AST-120. Serum metabolites in normal and CRF rats before and after administration of AST-120 for 3 days were analyzed by liquid chromatography/electrospray ionization-tandem mass spectrometry (LC/ESI-MS/MS) and principal component analysis. Further, serum and urine levels of the indicators were quantified by selected reaction monitoring of LC/ESI-MS/MS. Indoxyl sulfate was the first principal serum metabolite, which could differentiate CRF from both normal and AST-120-administered CRF rats, followed by hippuric acid, phenyl sulfate and 4-ethylphenyl sulfate. CRF rats showed increased serum levels of indoxyl sulfate, hippuric acid, phenyl sulfate, 4-ethylphenyl sulfate and p-cresyl sulfate. Administration of AST-120 for 3 days to the CRF rats reduced the serum and urine levels of these metabolites. In conclusion, indoxyl sulfate is the best indicator of the effect of AST-120 in CRF rats. Hippuric acid, phenyl sulfate and 4-ethylphenyl sulfate are suggested as the additional indicators. 4-Ethylphenyl sulfate is a newly identified uremic substance.

Investigation of uremic analytes in hemodialysate and their structural elucidation from accurate mass maps generated by a multi-dimensional liquid chromatography/mass spectrometry approach

Historically, structural elucidation of unknown analytes by mass spectrometry alone has involved tandem mass spectrometry experiments using electron ionization. Most target molecules for bioanalysis in the metabolome are unsuitable for detection by this previous methodology. Recent publications have used high-resolution accurate mass analysis using an LTQ-Orbitrap with the more modern approach of electrospray ionization to identify new metabolites of known metabolic pathways. We have investigated the use of this methodology to build accurate mass fragmentation maps for the structural elucidation of unknown compounds. This has included the development and validation of a novel multi-dimensional LC/MS/MS methodology to identify known uremic analytes in a clinical hemodialysate sample. Good inter- and intra-day reproducibility of both chromatographic stages with a high degree of mass accuracy and precision was achieved with the multi-dimensional liquid chromatography/tandem mass spectrometry (LC/MS/MS) system. Fragmentation maps were generated most successfully using collision-induced dissociation (CID) as, unlike high-energy CID (HCD), ions formed by this technique could be fragmented further. Structural elucidation is more challenging for large analytes >270 Da and distinguishing between isomers where their initial fragmentation pattern is insufficiently different. For small molecules (<200 Da), where fragmentation data may be obtained without loss of signal intensity, complete structures can be proposed from just the accurate mass fragmentation data. This methodology has led to the discovery of a selection of known uremic analytes and two completely novel moieties with chemical structural assignments made.

Recent progress in the analysis of uremic toxins by mass spectrometry

Mass spectrometry (MS) has been successfully applied for the identification and quantification of uremic toxins and uremia-associated modified proteins. This review focuses on recent progress in the analysis of uremic toxins by using MS. Uremic toxins include low-molecular-weight compounds (e.g., indoxyl sulfate, p-cresol sulfate, 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid, asymmetric dimethylarginine), middle-molecular-weight peptides, and proteins modified with advanced glycation and oxidation. These uremic toxins are considered to be involved in a variety of symptoms which may appear in patients with stage 5 chronic kidney disease. Based on MS analysis of these uremic toxins, the pathogenesis of the uremic symptoms will be elucidated to prevent and manage the symptoms.

Identification of AGE-precursors and AGE formation in glycation-induced BSA peptides

The glycation of BSA leads to protein/peptide modifications that result in the formation of AGEs. AGEs react with the amino groups of N-terminal amino acid residues, particularly arginine and lysine residues. Enhanced AGE formation exists in the blood and tissues of diabetics, as well as in aging and other disorders. The Identification of AGEs is of great importance. Mass spectrometry has been applied to identify and structurally elucidate AGEs. Here, we report on the identification of AGE- peptides and AGE-precursors based on relative mass changes as a result of specific AGE formation. HPLC-ESIMS, ESI-MS/MS, and the Mascot database were used. The relative mass changes due to the specific type of AGE formation were added to the identified peptides followed by a manual search of the glycated samples, which resulted in the identification of seven peptides for the formation of five AGEs, namely CML, pyrraline, imidazolone A, imidazolone B, and AFGP. Four glycated peptides (FPK, ECCDKPLLEK, IETMR, and HLVDEPQNLIK) were identified in the formation of AGE-precursors.

Optical dialysis adequacy sensor: contribution of chromophores to the ultra violet absorbance in the spent dialysate

Several on-line methods have been developed to standardize the assessment of dialysis adequacy. Earlier studies have demonstrated that on-line monitoring of total ultra violet (UV) absorbance in spent dialysate can be utilized to follow continuously a single hemodialysis session. The aim of this study was to investigate the contribution of different compounds, acting as chromophores, to the UV-absorbance in the spent dialysate in order to explain origin of the cumulative and integrated UV-absorbance measured by the optical dialysis adequacy sensor. Four uremic patients, during 12 hemodialysis treatments, were followed by the optical dialysis adequacy sensor using the wavelength of 280 mn. The dialysate samples were taken and analyzed using reversed phase high performance liquid chromatography (HPLC). The total number of detected peaks from the HPLC gradient separation profiles measured at the wavelength 280 nm for the samples collected 10 mm after the start of hemodialysis (Mean +/- SD) was 38 +/- 6. The relative contribution from the area of 10 main peaks to the total area of all detected peaks in percentage was 91.01 +/- 2.52 %. The optical dialysis adequacy sensor provides continuous, on-line hemodialysis measurements and may immediately identify and alert to any deviations in the dialysis. Our study indicates that there exists a number of prevalent compounds that are the main cause of the cumulative and integrated UV- absorbance.

Skin autofluorescence, a novel marker for glycemic and oxidative stress-derived advanced glycation endproducts: an overview of current clinical studies, evidence, and limitations

BACKGROUND

Advanced glycation endproducts (AGEs) predict long-term complications in agerelated diseases. However, there are no clinically applicable markers for measuring AGEs in vivo.

METHODS

We have recently introduced the AGE-Reader (DiagnOptics B.V., Groningen, The Netherlands) to noninvasively measure AGE accumulation in the human skin of the forearm, making use of the characteristic autofluorescence (AF) pattern that AGEs encompass. Skin AF is calculated as a ratio of mean intensities detected from the skin between 420-600 nm and 300-420 nm. It correlates with collagen-linked fluorescence and specific skin AGE levels from skin biopsies in diabetes, renal failure, and control subjects. Skin AF levels are increased in patients with diabetes and renal failure and are associated with the presence of vascular complications. Additionally, skin AF is strongly related to the progression of coronary heart disease and mortality, independently of traditional risk factors. Since skin pigmentation might influence skin AF, we have investigated the relation of relative skin reflectance (R%) to skin AF in subjects with varying skin phototypes (SPT).

RESULTS

The data presented in this article suggest that only in subjects with an SPT of V and VI or R% <12%, no reliable measurement can be performed. Therefore, the current prototype of the AGE-Reader is suitable for subjects with SPT I-IV or R% >12%, and more research is needed for a broader application.

CONCLUSION

The AGE-Reader is useful as a noninvasive clinical tool for assessment of risk for long-term vascular complications in diabetes and in other conditions associated with AGE accumulation.

Mass spectrometry for the study of protein glycation in disease

The structural elucidation of advanced glycation end-product (AGE)-modified proteins and quantitative analysis of free AGEs have been successfully performed, by use of mass spectrometry (MS) in plasma and tissues of patients with AGE-related diseases, such as diabetes mellitus, uremia, cataract, and liver cirrhosis. Matrix-assisted laser desorption/ionization (MALDI)-MS made it possible to directly analyze the AGE-modified proteins such as albumin and IgG. However, because the direct structural analysis of intact AGE-modified proteins is often not easy due to the formation of broad and poorly resolved peaks, peptide mapping after enzymatic hydrolysis was introduced into the analysis of AGE-modified proteins and the site-specific analysis of defined AGEs by MALDI-MS. Liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS) has been employed not only for the structural elucidation of enzymatically hydrolyzed AGEs-modified peptides but also for simultaneous quantification of free AGEs in plasma and tissues of patients. Based on many studies that use MS for the analysis of AGEs, there is no doubt as to the important role of protein-linked AGEs in several diseases.

Accumulation of Advanced Glycation End Products, Measured as Skin Autofluorescence, in Renal Disease

Advanced glycation end products (AGEs) accumulate during renal failure and dialysis. Kidney transplantation is thought to reverse this accumulation by restoring renal function. Using a noninvasive and validated autofluorescence reader, we evaluated AGE levels in 285 transplant recipients (mean age, 52 years; range, 41 to 60 years), 32 dialysis patients (mean age, 56 years; range, 43 to 65 years), and 231 normal control subjects (mean age, 51 years; range, 40 to 65 years). Measurements in transplant recipients were performed for a mean of 73 months (range, 32 to 143 months) after transplantation. Dialysis patients were on dialysis therapy for a mean of 42 months (range, 17 to 107 months). Fluorescence was significantly increased in dialysis patients compared with normal control subjects (2.8 vs. 2.0 arbitrary units [a.u.], P < .0001). Although fluorescence levels were significantly decreased in transplant recipients compared with dialysis patients (2.5 vs. 2.8 a.u., P < .0001), fluorescence in transplant recipients was higher than in controls (2.5 vs. 2.0 a.u., P < .0001). In transplant recipients, fluorescence correlated positively with the duration of dialysis prior to transplantation (R = 0.21, P < .0001), and negatively with creatinine clearance (R = -0.34, P < .0001). No correlation was found between time after transplantation and fluorescence in transplant recipients (R = -0.10, P = .10). Fluorescence in dialysis patients was positively correlated with duration of dialysis (R = 0.36, P = .042). Our results, like those of others, suggest that kidney transplantation does not fully correct increased AGE levels found in dialysis patients. The increased AGE levels in kidney transplant recipients cannot be explained by the differences in renal function alone. The availability of a simple, noninvasive method (AGE-Reader) to measure AGE accumulation may be used to monitor AGE accumulation in a clinical setting as well as in a study setting.

Advanced glycation end products in kidney transplant patients: a putative role in the development of chronic renal transplant dysfunction

Chronic renal transplant dysfunction is one of the leading causes of graft failure in kidney transplantation. A complex interplay of both alloantigen-related and alloantigen-unrelated risk factors is believed to underlie its development. We propose that advanced glycation end products (AGEs) are involved in the development of chronic renal transplant dysfunction. AGE formation is associated with different alloantigen-unrelated risk factors for chronic renal transplant dysfunction, such as recipient age, diabetes, proteinuria, hypertension, and hyperlipidemia. In vitro studies have shown that AGEs induce the expression of various mediators associated with chronic renal transplant dysfunction. Furthermore, AGE-induced renal damage has been found in multiple experimental studies. This renal damage shows similarity to the damage found in chronic renal transplant dysfunction. Together, several lines of evidence support a role of AGEs in the development of chronic renal transplant dysfunction and suggest that preventive therapy with AGE inhibitors may be helpful in preserving renal function in transplant recipients.

Search for peptidic “middle molecules” in uremic sera: isolation and chemical identification of fibrinogen fragments

According to the "middle molecule" (MM) hypothesis, the uremic solutes ranging from 500 to 5,000 Da are insufficiently eliminated by conventional hemodialysis and may act as uremic toxins. However, because of the methodological difficulties of MM purification, their chemical analysis is complicated and the precise structure of these molecules remains obscure. In the present study, a new micro-preparative procedure including SDS electrophoresis and liquid chromatography was applied for isolation of MM peptides from uremic sera. Microsequencing and MS/MS analyses of these peptides showed that most of the identified MM (22 out of 23) represented the N- and C-terminal fragments of the alpha- and beta-chains of fibrinogen. The obtained data provide new information on the precise structure of fibrinogen fragments accumulating in uremic serum as MM.

On-line monitoring of solutes in dialysate using wavelength-dependent absorption of ultraviolet radiation

The aim of the study was to assess the wavelength dependence of the UV absorbance during monitoring of different compounds in the dialysate. UV absorbance was determined by using a double-beam spectrophotometer on dialysate samples taken at pre-determined times during dialysis, over a wavelength range of 180-380 nm. Concentrations of several removed substances, such as urea, creatinine, uric acid, phosphate and beta2-microglobulin, were determined in the blood and in the spent dialysate samples using standard laboratory techniques. Millimolar extinction coefficients, for urea, creatinine, monosodium phosphate and uric acid were determined during laboratory bench experiments. The correlation between UV absorbance and substances both in the dialysate and in the blood was calculated at all wavelengths. A time-dependent UV absorbance was determined on the collected dialysate samples from a single dialysis session over a wavelength range of 200-330 nm. The highest contribution from observed compounds relative to the mean value of the absorbance was found around 300 nm and was approximately 70%. The main contribution to the total absorbance from uric acid was confirmed at this wavelength. The highest correlation for uric acid, creatinine and urea was obtained at wavelengths from 280 nm to 320 nm, both in the spent dialysate and in the blood. The wavelength region with the highest correlation for phosphate and beta2-microglobulin, with a suitable UV-absorbance dynamic range, was from 300 to 330 nm. In the wavelength range of 220-270 nm the highest absorbance sensitivity for the observed substances was obtained. A suitable wavelength range for instrumental design seems to be around 290-330 nm. The relatively high correlation between UV absorbance and the substances in the spent dialysate and in the blood indicates that the UV-absorbance technique can estimate the removal of several retained solutes known to accumulate in dialysis patients.

On-line monitoring of solutes in dialysate using absorption of ultraviolet radiation: technique description

PURPOSE

The aim of this work was to describe a new optical method for monitoring solutes in a spent dialysate using absorption of UV radiation.

METHOD

The method utilises UV-absorbance determined in the spent dialysate using a spectrophotometrical set-up. Measurements were performed both on collected dialysate samples and on-line. During on-line monitoring, a spectrophotometer was connected to the fluid outlet of the dialysis machine, with all spent dialysate passing through a specially-designed cuvette for optical single-wavelength measurements. The concentrations of several substances of various molecular sizes, electrical charge, transport mechanism, etc. were determined in the dialysate and in the blood using standard laboratory techniques. The correlation coefficient between UV-absorbance of the spent dialysate and concentration of the substances in the spent dialysate and in the blood was calculated from data based on the collected samples.

RESULTS

The obtained on-line UV-absorbance curve demonstrates the possibility to follow a single hemodialysis session continuously and to monitor deviations in the dialysator performance using UV-absorbance. The experimental results indicate a very good correlation between UV-absorbance and several small waste solutes removed such as urea, creatinine and uric acid in the spent dialysate and in the blood for every individual treatment at a fixed wavelength of 285 nm. Moreover, a good correlation between the UV-absorbance and substances like potassium, phosphate and beta2-microglobulin was obtained. The lowest correlation was achieved for sodium, calcium, glucose, vitamin B12 and albumin.

CONCLUSIONS

A technique for on-line monitoring of solutes in the spent dialysate utilising the UV-absorbance was developed. On-line monitoring during a single hemodialysis session exploiting UV-absorbance represents a possibility to follow a single hemodialysis session continuously and monitor deviations in dialysis efficiency (e.g. changes in blood flow and clearance). The UV-absorbance correlates well to the concentration of several solutes known to accumulate in dialysis patients indicating that the technique can be used to estimate the removal of retained substances.

Gramicidin S: a peptide model for protein glycation and reversal of glycation using nucleophilic amines

Nonenzymatic glycation of proteins has been implicated in various diabetic complications and age-related disorders. Proteins undergo glycation at the N-terminus or at the epsilon-amino group of lysine residues. Glycation of proteins proceeds through the stages of Schiff base formation, conversion to ketoamine product and advanced glycation end products. Gramicidin S, which has two ornithine residues, was used as a model system to study the various stages of glycation of proteins using electrospray ionization mass spectrometry. The proximity of two ornithine residues in the peptide favors the glycation reaction. Formation of advanced glycation end products and diglycation on ornithine residues in gramicidin S were observed. The formation of Schiff base adduct is reversible, whereas the Amadori rearrangement to the ketoamine product is irreversible. Nucleophilic amines and hydrazines can deglycate the Schiff base adduct of glucose with peptides and proteins. Hydroxylamine, isonicotinic acid hydrazide and aminoguanidine effectively removed glucose from the Schiff base adduct of gramicidin S. Hydroxylamine is more effective in deglycating the adduct compared with isonicotinic acid hydrazide and aminoguanidine. The observation that the hydrazines are effective in deglycating the Schiff base adduct even in the presence of high concentrations of glucose, may have a possible therapeutic application in preventing complications of diabetes mellitus. Hydrazines may be used to distinguish between the Schiff base and the ketoamine products formed at the initial stages of glycation.

Analysis of native proteins from biological fluids by biomolecular interaction analysis mass spectrometry (BIA/MS): exploring the limit of detection, identification of non-specific binding and detection of multi-protein complexes

Biomolecular interaction analysis mass spectrometry (BIA/MS) is a two-dimensional analytical technique that quantitatively and qualitatively detects analytes of interests. In the first dimension, surface plasmon resonance (SPR) is utilized for detection of biomolecules in their native environment. Because SPR detection is non-destructive, analyte(s) retained on the SPR-active sensor surface can be analyzed in a second dimension using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. The qualitative nature of the MALDI-TOF MS analysis complements the quantitative character of SPR sensing and overcomes the shortcomings of the SPR detection stemming from the inability to differentiate and characterize multi-protein complexes and non-specific binding. In this work, the benefit of performing MS analysis following SPR sensing is established. Retrieval and detection of four markers present in biological fluids (cystatin C, beta-2-microglobulin, urinary protein 1 and retinol binding protein) was explored to demonstrate the effectiveness of BIA/MS in simultaneous detection of clinically related biomarkers and delineation of non-specific binding. Furthermore, the BIA/MS limit of detection at very low SPR responses was investigated. Finally, detection of in-vivo assembled protein complexes was achieved for the first time using BIA/MS.

Droplet electrocoupling between capillary electrophoresis and matrix assisted laser desorption/ionization-time of flight-mass spectroscopy and its application

A droplet of an electroconductive solution was put on the sample plate of a matrix-assisted laser desorption/ionization-time of flight-mass spectroscope (MALDI-TOF-MS) and the outlet terminal of a capillary Electrophoresis (CE) capillary was put into this droplet in order to make an electro-connection and to apply high voltage between the metallic sample plate and the counter pole of the CE. This procedure was simple and gave much more stable interfacing than that of the electrospray method. Furthermore, the separated component was collected and concentrated in a droplet. By mixing each separated sample spot with the MALDI matrix on the sample plate, the spots were analyzed in separated sequences to make three-dimensional mass chromatograms, or applied to the enzyme digestion analysis for peptide sequencing.

Detection and quantification of beta-2-microglobulin using mass spectrometric immunoassay

The use of mass spectrometric immunoassay (MSIA) in analyzing beta-2-microglobulin (beta(2)m) present in human biological fluids (tears, saliva, plasma, and urine) is described. Pipettor tips containing porous affinity frits, derivatized with polyclonal anti-beta(2)m immunoglobulin, were manufactured and used to selectively isolate and concentrate beta(2)m from the biofluids, after which matrix-assisted laser desorption/ionization time-of-flight mass spectrometry was used to detect beta(2)m unambiguously at its characteristic molecular mass. The affinity tips were found rapid to use, requiring approximately 15 min per analysis, and exhibited low nonspecific binding properties that yielded essentially interference-free analyses. The beta(2)m MSIA was made quantitative by inclusion of an internal standard into the analysis for signal normalization. The resulting assay had a Linear dynamic range (R(2) = 0.983) covering a beta(2)m concentration range of 0.010-1.0 mg/L with a standard error of approximately 5%. In application, urine samples from healthy individuals were screened and compared with sample from an individual suffering from renal infection. Results indicated an approximately 30-fold increase in beta(2)m levels in samples taken from the infected individual. During the screening, MSIA was able to distinguish between wild-type and glycosylated forms of beta(2)m, which made possible the accurate quantification of wild-type beta(2)m without interference from glycosylated versions of the protein. These results demonstrate a new approach to the rapid and accurate detection/quantification of beta(2)m present in biological fluids.

Mass spectrometric monitoring of albumin in uremia

BACKGROUND

Advanced glycation end products (AGEs) are a novel class of uremic toxins. In plasma, they are present in proteins and also in low molecular mass peptides. AGE-modified peptides are thought to bind and modify plasma proteins. Monitoring of the consequent increase in molecular mass of serum albumin may be used in surveillance of the clinical management of uremia.

METHODS

We investigated molecular mass changes of human serum albumin (HSA) glycated by glucose and methylglyoxal in vitro and of subjects with moderate renal impairment, end-stage renal disease (ESRD), ESRD on hemodialysis, and normal healthy controls by matrix-assisted laser desorption ionization mass spectrometry.

RESULTS

Fatty acid-free HSA had a molecular mass of 66,446 +/- 114 D. Mean (+/-SD) molecular mass increases were HSA minimally glycated by glucose 399 +/- 88 D (N = 5, P < 0.001), HSA highly glycated by glucose 6780 +/- 122 D (N = 5, P < 0.001), HSA minimally glycated by methylglyoxal 73 +/- 121 D (N = 5, P > 0.05), and HSA without fatty acid removal 535 +/- 90 D (N = 5, P < 0.001). For HSA of human subjects, mean (+/- SD) molecular mass increases were normal healthy controls 243 +/- 97 D (N = 5), moderate renal impairment 350 +/- 83 D (P > 0.05 with respect to controls, N = 5), ESRD 498 +/- 128 (P < 0.02 with respect to controls, N = 3), and ESRD on hemodialysis 438 +/- 85 D (P < 0.02 with respect to controls, N = 5). The mean molecular mass of albumin of all groups was increased significantly with respect to that of fatty acid free albumin (P < 0.001).

CONCLUSIONS

Only ESRD was associated with a significant increase in the molecular mass of HSA in vivo. Since this mass increase was very low and much lower than reported for AGE-modified peptides, it may reflect AGE formation on HSA by alpha-oxoaldehydes that accumulate in uremia, rather than modification of albumin by AGE-modified peptides. The molecular mass of HSA in vivo was indicative of a minimal and not a high extent of glycation.

Unexpected suppression of free phenytoin concentration by salicylate in uremic sera due to the presence of inhibitors: MALDI mass spectrometric determination of molecular weight range of inhibitors

Salicylate displaces phenytoin from protein binding leading to an increase in free phenytoin concentration. We observed unexpected decreases in free phenytoin concentration in the presence of salicylate. Serum pools containing no phenytoin or salicylate were supplemented with the same concentrations of phenytoin. Then to the aliquots of the individual pool, no salicylate (control), 150, 300 and 500 microg/ml of salicylate (therapeutic range: 15-300 microg/ml) were added. Specimens were incubated at 37 degrees C for 2 h and after re-equilibration at room temperature for 20 min, total and free phenytoin (in the protein free ultrafiltrates) concentrations were measured using fluorescence polarization immunoassay on the TDx/FLX analyzer. We observed an increase in free phenytoin concentration from 1.91 microg/ml (in the absence of salicylate) to 2.39 microg/ml in the presence of 500 microg/ml salicylate (total phenytoin: 13.3 microg/ml) in the normal pool. In sharp contrast, the free phenytoin concentrations decreased from an initial concentration of 3.82 microg/ml to 2.52 microg/ml in the presence of 500 microg/ml of salicylate (total phenytoin: 13.2 microg/ml) in the uremic pool. We also treated the uremic pool with activated charcoal. In the original uremic pool, the initial free phenytoin concentration was 3.05 microg/ml and the free concentrations then decreased to 2.28 microg/ml in the presence of 300 microg/ml of salicylate. In contrast, in the charcoal treated pool, the initial free phenytoin concentration increased from 1.61 microg/ml to 3.23 microg/ml in the presence of 300 microg/ml of salicylate. More interestingly when uremic toxins were extracted back from charcoal with methanol and the dry residue was added to an aliquot of normal serum, the normal serum behaved like a uremic serum and free phenytoin concentration was significantly decreased in the presence of salicylate. When an aliquot of methanol extract was studied by Matrix-Assisted Laser Desorption Ionization Mass Spectrometry (scan up to 10,000), we observed no peak at molecular weight over 551, indicating that these inhibitors are small molecules. We also identified hippuric acid as one of the inhibitors.