Small Water-soluble Compounds Research Articles

Homeostasis in the Gut Microbiota in Chronic Kidney Disease.

The gut microbiota consists of trillions of microorganisms, fulfilling important roles in metabolism, nutritional intake, physiology and maturation of the immune system, but also aiding and abetting the progression of chronic kidney disease (CKD). The human gut microbiome consists of bacterial species from five major bacterial phyla, namely Firmicutes, Bacteroidetes, Actinobacteria, Proteobacteria, and Verrucomicrobia. Alterations in the members of these phyla alter the total gut microbiota, with a decline in the number of symbiotic flora and an increase in the pathogenic bacteria, causing or aggravating CKD. In addition, CKD-associated alteration of this intestinal microbiome results in metabolic changes and the accumulation of amines, indoles and phenols, among other uremic metabolites, which have a feedforward adverse effect on CKD patients, inhibiting renal functions and increasing comorbidities such as atherosclerosis and cardiovascular diseases (CVD). A classification of uremic toxins according to the degree of known toxicity based on the experimental evidence of their toxicity (number of systems affected) and overall experimental and clinical evidence was selected to identify the representative uremic toxins from small water-soluble compounds, protein-bound compounds and middle molecules and their relation to the gut microbiota was summarized. Gut-derived uremic metabolites accumulating in CKD patients further exhibit cell-damaging properties, damage the intestinal epithelial cell wall, increase gut permeability and lead to the translocation of bacteria and endotoxins from the gut into the circulatory system. Elevated levels of endotoxins lead to endotoxemia and inflammation, further accelerating CKD progression. In recent years, the role of the gut microbiome in CKD pathophysiology has emerged as an important aspect of corrective treatment; however, the mechanisms by which the gut microbiota contributes to CKD progression are still not completely understood. Therefore, this review summarizes the current state of research regarding CKD and the gut microbiota, alterations in the microbiome, uremic toxin production, and gut epithelial barrier degradation.

MO277: Effect of P-Cresyl Sulfate in Cardiorenal Axis in Mice

Abstract BACKGROUND AND AIMS Uremic toxins (UT) are extremely relevant for the understanding of pathologies of the cardiovascular system once kidney and heart have a close relationship and share different functions. UT are divided into three groups according to physicochemical characteristics: small water-soluble compounds, medium compounds, and protein-bound uremic toxins (PBUT). The PBUT are cytotoxic molecules that have shown deleterious effects in various tissues, including the cardiovascular system. P-Cresyl (PCS), a toxin derived from tyrosine, is one of most important UT once it has a strong affinity for serum proteins, which makes their removal by dialysis difficult. Although some deleterious effects of PCS are known, systematic studies that address the cellular and molecular mechanisms of action of PCS, in the kidney and heart, are still scarce [1, 2]. Moreover, many works in the literature diverge as to the experimental model and the dose of PCS to be used. In this sense, the present study aimed define the dose of PCS capable of leading to renal impairment in mice and also evaluate the effect of PCS alone (without renal injury installed) in kidney and heart. METHODS C57BL/6J male mice 6–8 weeks old (22–28 g) were obtained from the Animal Facility of Federal University of ABC (SP, Brazil). All procedures and protocols were approved by the Ethics Committee on Animal Use of the Federal University of ABC (no. 6 548 291 020). A dose response curve was performed, and the mice were divided into four groups and treated for 21 days, i.p.: Vehicle (saline), PCS 60 mg/L, PCS 40 mg/L and 20 mg/L. Doses were based on uremic maximum concentration available on EUTox-European Uremic Toxins Work Group [3]. After treatment, kidneys, heart, blood, tibia were collected to morphometric and biochemical analysis. Gene expression was evaluated using real-time PCR. RESULTS The EUTox suggested that ∼40 mg/L is the maximum uremic dose for humans. However, the dose–response curve showed that only PCS 20 mg/L was able to increase the mRNA levels of renal injury markers such as Cystatin (3x), KIM-1 (6x) and Vimentin (12x) in relation to control animals (P < .05). Besides, it was possible to observe a decrease on mRNA gene expression of IL-1β levels in kidney (40%, P < .05) when compared with control group. No alterations were observed in MCP-1 levels. In relation to cardiac changes, heart weight/tibia length ratio, showed a cardiac atrophy about 13% after PCS treatment. The OAT 1 and 3 expression did not alter after different doses of PCS treatment. CONCLUSION The results indicated that lowest dose of PCS was able to promote renal injury and cardiac remodeling, despite the changes in OAT1 and OAT3 in kidney and heart. In addition, the establishment of critical dose in mice model is very important since the use of knockout mice models could help to understand the role of UT in organ–organ crosstalk. Finally, this study indicates that the PCS has an impact in kidney/heart axis independent of previous renal injury. The molecular mechanisms involved in cardiac atrophy and inflammatory status are being investigated by our group.

Impact of gelatine coating on the performance of tannin-loaded pectin microbeads obtained through external gelation

One of the limitations of external gelation for microencapsulation is that small water-soluble compounds tend to diffuse out of the microbeads, resulting in low encapsulation efficiencies. In this work we propose a one-step approach for hydrogel microbead formation and simultaneous coating using external gelation. We explored amidated pectin as the encapsulation matrix for two different tannin-rich extracts (from chestnut and quebracho). The inclusion of tannin extracts contributed to the improvement of the structure of the microbeads through their interactions with pectin. By adding gelatine to the gelling bath, the microbeads were coated with the protein. This led to a significant increase in microencapsulation efficiency, which in some cases almost doubled compared to non-coated microbeads. Thanks to the binding of tannins with gelatine, coated microbeads loaded with the greatest amount of tannin extracts (20% w/w) presented the best retention of the bioactive compounds. A 14-days storage release study showed that these microencapsulation systems only experienced a slight loss of tannins during this period, with quebracho extract exhibiting greater retention than chestnut extract. Overall, by exploiting interactions in the pectin/tannins/gelatine ternary system, the proposed strategy for microbead production and coating in a single step allowed the development of a simple and more efficient microencapsulation approach for tannin extracts through external gelation.

Extraction of natural moisturizing factor from the stratum corneum and its implication on skin molecular mobility

The natural moisturizing factor (NMF) is a mixture of small water-soluble compounds present in the upper layer of the skin, stratum corneum (SC). Soaking of SC in water leads to extraction of the NMF molecules, which may influence the SC molecular properties and lead to brittle and dry skin. In this study, we investigate how the molecular dynamics in SC lipid and protein components are affected by the removal of the NMF compounds. We then explore whether the changes in SC components caused by NMF removal can be reversed by a subsequent addition of one single NMF component: urea, pyrrolidone carboxylic acid (PCA) or potassium lactate. Samples of intact SC were investigated using NMR, X-ray diffraction, infrared spectroscopy and sorption microbalance. It is shown that the removal of NMF leads to reduced molecular mobility in keratin filaments and SC lipids compared to untreated SC. When the complex NMF mixture is replaced by one single NMF component, the molecular mobility in both keratin filaments and lipids is regained. From this we propose a general relation between the molecular mobility in SC and the amount of polar solutes which does not appear specific to the precise chemical identify of the NMF compounds.

Immune Dysfunction in Uremia 2020.

Cardiovascular disease and infections are major causes for the high incidence of morbidity and mortality of patients with chronic kidney disease. Both complications are directly or indirectly associated with disturbed functions or altered apoptotic rates of polymorphonuclear leukocytes, monocytes, lymphocytes, and dendritic cells. Normal responses of immune cells can be reduced, leading to infectious diseases or pre-activated/primed, giving rise to inflammation and subsequently to cardiovascular disease. This review summarizes the impact of kidney dysfunction on the immune system. Renal failure results in disturbed renal metabolic activities with reduced renin, erythropoietin, and vitamin D production, which adversely affects the immune system. Decreased kidney function also leads to reduced glomerular filtration and the retention of uremic toxins. A large number of uremic toxins with detrimental effects on immune cells have been identified. Besides small water-soluble and protein-bound compounds originating from the intestinal microbiome, several molecules in the middle molecular range, e.g., immunoglobulin light chains, retinol-binding protein, the neuropeptides Met-enkephalin and neuropeptide Y, endothelin-1, and the adipokines leptin and resistin, adversely affect immune cells. Posttranslational modifications such as carbamoylation, advanced glycation products, and oxidative modifications contribute to uremic toxicity. Furthermore, high-density lipoprotein from uremic patients has an altered protein profile and thereby loses its anti-inflammatory properties.

Single needle hemodialysis: is the past the future?

Whereas the usual way to gain access to the vascular bed for hemodialysis is by inserting two needles, an alternative option based on the introduction of only one needle has been available for several decades. Although single needle hemodialysis gradually lost popularity in the early nineties of last century, this option now seems to make a come-back, with the current change in patient mix towards more elderly and cardio-vascular disease and the appearance of more flexible hardware. Single needle hemodialysis offers several advantages, such as the possibility to puncture small or maturing access systems, a decrease in number of punctures with less potential access damage and subsequent complications, the avoidance of central vein catheter use, and an improved quality of life by reducing puncture-related pain, stress and complications. The main drawback is recirculation which however can be overcome (if considered necessary) by making dialysis somewhat longer and in addition has more impact on removal of small water soluble compounds than on clearance of the more toxic difficult to remove solutes (middle molecules and protein bound compounds). Effective dialyzer blood flow with single needle dialysis cannot be much higher than 300mL/min, which however also offers advantages by making short dialysis sessions less feasible and thus reducing the likelihood of intradialytic blood pressure falls, organ stunning and other negative outcomes of shorter dialysis. Direct outcome comparisons between single and double needle dialysis are not available but indirect data suggest no differences, in as far as efficient enough access perfusion can keep dialyzer blood flow adequate. The single needle method seems especially suited for the elderly and for home hemodialysis. Recent technological improvements have made the system more accessible and adequate, but further studies are needed to assess with modern methodologies the clearance kinetics of these systems, which could emanate in further technological fine-tuning.

Introduction to the Toxins Special Issue on “Novel Issues in Uremic Toxicity”

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Dialysis membranes: A 2018 update.

Dialysis membranes are the basic element of a hemodialyzer. Synthetic and natural materials characterized by various fiber arrangements are used in their production. The most up-to-date ones are made of synthetic polymers such as polyamide, phosphatidylserine (PS), polyacrylonitrile-based fiber (PAN), polyarylethersulfone, polyethersulfone, or polymethylmethacrylate. Dialysis membranes are characterized by the ability to remove uremic molecules, which can be divided into small water-soluble compounds, protein-bound compounds and larger "middle molecules". Newer membranes such as medium cut off membranes (MCO) allow the removal of a wider spectrum of uremic molecules, which reduces the risk of late complications of dialysis. Dialysis membranes are used in therapy methods such as low flux, high flux or HDx therapy. An important aim in dialysis membrane development is to increase their biocompatibility. Insufficient biocompatibility can result in complement activation or platelet activation, which can lead to an increased risk of cardiovascular complications. The aim of the study is to discuss the latest reports on dialysis membranes.

Biochemical and Clinical Impact of Organic Uremic Retention Solutes: A Comprehensive Update.

In this narrative review, the biological/biochemical impact (toxicity) of a large array of known individual uremic retention solutes and groups of solutes is summarized. We classified these compounds along their physico-chemical characteristics as small water-soluble compounds or groups, protein bound compounds and middle molecules. All but one solute (glomerulopressin) affected at least one mechanism with the potential to contribute to the uremic syndrome. In general, several mechanisms were influenced for each individual solute or group of solutes, with some impacting up to 7 different biological systems of the 11 considered. The inflammatory, cardio-vascular and fibrogenic systems were those most frequently affected and they are one by one major actors in the high morbidity and mortality of CKD but also the mechanisms that have most frequently been studied. A scoring system was built with the intention to classify the reviewed compounds according to the experimental evidence of their toxicity (number of systems affected) and overall experimental and clinical evidence. Among the highest globally scoring solutes were 3 small water-soluble compounds [asymmetric dimethylarginine (ADMA); trimethylamine-N-oxide (TMAO); uric acid], 6 protein bound compounds or groups of protein bound compounds [advanced glycation end products (AGEs); p-cresyl sulfate; indoxyl sulfate; indole acetic acid; the kynurenines; phenyl acetic acid;] and 3 middle molecules [β2-microglobulin; ghrelin; parathyroid hormone). In general, more experimental data were provided for the protein bound molecules but for almost half of them clinical evidence was missing in spite of robust experimental data. The picture emanating is one of a complex disorder, where multiple factors contribute to a multisystem complication profile, so that it seems of not much use to pursue a decrease of concentration of a single compound.

Quantitative analysis of mycosporine-like amino acids in marine algae by capillary electrophoresis with diode-array detection

Marine species have evolved a variety of physical or chemical strategies to diminish damage from elevated environmental ultraviolet radiation. Mycosporine-like amino acids, a group of widely distributed small water soluble compounds, are biologically relevant because of their photo-protective potential. In addition, presumed antioxidant and skin protective strategies raise the interest for possible medicinal and cosmetic applications. In this study the first CE method for the quantification of mycosporine-like amino acids in marine species is presented. A borate buffer system consisting of 30mM sodium tetraborate in water at a pH-value of 10.3 enabled the baseline separation of five MAAs, namely palythine, mycosporine-serinol, asterina-330, shinorine and porphyra-334, in 27min. Separation voltage, temperature and detection wavelength were 25kV, 25°C and 320nm, respectively. The optimized method was fully validated and applied for the quantitative determination of MAAs in the marine macroalgae Palmaria palmata, Porphyra umbilicalis, and Porphyra sp., as well as the lichen Lichina pygmaea.

Degradation of polystyrene and selected analogues by biological Fenton chemistry approaches: Opportunities and limitations

Conventional synthetic polymers typically are highly resistant to microbial degradation, which is beneficial for their intended purpose but highly detrimental when such polymers get lost into the environment. Polystyrene is one of the most widespread of such polymers, but knowledge about its biological degradability is scarce. In this study, we investigated the ability of the polymer-degrading brown-rot fungus Gloeophyllum trabeum to attack polystyrene via Fenton chemistry driven by the redox-cycling of quinones. Indications of superficial oxidation were observed, but the overall effects on the polymer were weak. To assess factors constraining biodegradation of polystyrene, the small water-soluble model compounds ethylbenzene and isopropylbenzene (cumene) were also subjected to biodegradation by G. trabeum. Likewise, ethylbenzene sulfonate, cumene sulfonate and the dimer 1,3-diphenylbutane sulfonate were used as model compounds for comparison with polystyrene sulfonate, which G. trabeum can substantially depolymerise. All model compounds but cumene were degraded by G. trabeum and yielded a large variety of oxidised metabolites, suggesting that both the very poor bioavailability of polystyrene and its inert basic structure play important roles constraining biodegradability via biologically driven Fenton chemistry.

Successful entrapping of liposomes in glucan particles: an innovative micron-sized carrier to deliver water-soluble molecules.

Glucan particles (GPs) are monodisperse microspheres derived from baker's yeast and represent an interesting class of microcarriers for theranostic applications as they show a high affinity toward immune system cells. The typical loading strategy was to harness the ability of the molecule to be loaded to interact with nano-/microassembled systems through electrostatic or hydrophobic forces. However, small water-soluble chemicals could not be steadily retained by the leaky shell of GPs. In this work, we propose an alternative loading approach for small water-soluble compounds that is based on their entrapment in the aqueous core of liposomes that are directly formed into the microparticles through the reverse phase evaporation method (REV). The construct obtained may act as biocompatible carrier to deliver and release, even in a triggerable way, bioactive compounds.

Brown Carbon Formation by Aqueous-Phase Carbonyl Compound Reactions with Amines and Ammonium Sulfate

Reactions between small water-soluble carbonyl compounds, ammonium sulfate (AS), and/or amines were evaluated for their ability to form light-absorbing species in aqueous aerosol. Aerosol chemistry was simulated with bulk phase reactions at pH 4, 275 K, initial concentrations of 0.05 to 0.25 M, and UV-vis and fluorescence spectroscopy monitoring. Glycolaldehyde-glycine mixtures produced the most intense absorbance. In carbonyl compound reactions with AS, methylamine, or AS/glycine mixtures, product absorbance followed the order methylglyoxal > glyoxal > glycolaldehyde > hydroxyacetone. Absorbance extended into the visible, with a wavelength dependence fit by absorption Ångstrom coefficients (Å(abs)) of 2 to 11, overlapping the Å(abs) range of atmospheric, water-soluble brown carbon. Many reaction products absorbing between 300 and 400 nm were strongly fluorescent. On a per mole basis, amines are much more effective than AS at producing brown carbon. In addition, methylglyoxal and glyoxal produced more light-absorbing products in reactions with a 5:1 AS-glycine mixture than with AS or glycine alone, illustrating the importance of both organic and inorganic nitrogen in brown carbon formation. Through comparison to biomass burning aerosol, we place an upper limit on the contribution of these aqueous carbonyl-AS-amine reactions of ≤ 10% of global light absorption by brown carbon.

Removal of water-soluble and protein-bound solutes with reversed mid-dilution versus post-dilution haemodiafiltration

Convective dialysis strategies are superior in the removal of protein-bound uraemic retention solutes. Mid-dilution and mixed-dilution haemodiafiltration (HDF), both combining pre-dilution and post-dilution, are promising options to further improve removal capacity and have been shown of additional benefit for large middle molecules. In this study, we compared the removal of small water-soluble and protein-bound solutes in post-dilution versus mid-dilution HDF. Fourteen chronic haemodialysis (HD) patients were included in this crossover study. Patients were kept for 4 weeks on high-flux HD. On the mid-week session of Weeks 3 and 4, either post-dilution or reversed mid-dilution HDF were applied, in random order. Blood and dialysate flows were maintained at 300 and 800 mL/min, while the substitution flow was 75 mL/min in post-dilution and 150 mL/min in mid-dilution HDF. Based on the data collected during the sessions under study, extraction ratio (ER) and reduction ratio (RR) of small water-soluble and protein-bound solutes were calculated, as well as total solute removal (TSR) based on spent dialysate. No differences were observed for TSR, ER and RR for protein-bound solutes. For small water-soluble solutes, ER in post-dilution HDF was significantly higher than in mid-dilution HDF: 0.92 ± 0.02 versus 0.87 ± 0.04 for urea (P < 0.001), 0.92 ± 0.02 versus 0.88 ± 0.02 for creatinine (P < 0.001) and 0.84 ± 0.02 versus 0.82 ± 0.03 for uric acid (P = 0.009). TSR and RR were, however, not different due to the lower inlet concentrations with post-dilution HDF. TSR of mid-dilution and post-dilution HDF was not different for both small water-soluble and protein-bound compounds. Both strategies in the setting as applied in this study are as adequate for the removal of these solutes.

Comparison of removal capacity of two consecutive generations of high-flux dialysers during different treatment modalities

Innovative modifications have been introduced in several types of dialyser membranes to improve adequacy and permselectivity. Which aspects of removal are modified and how this relates to different diffusive or convective strategies has, however, been insufficiently investigated. In a prospective cross-over study, 14 chronic kidney disease (Stage 5D) patients were dialysed with a second-generation high-flux dialyser (Polynephron) in comparison to a first-generation type (DIAPES-HF800). Both dialysers were assessed in haemodialysis, in online pre-dilution and in post-dilution haemodiafiltration. Reduction ratio (RR, %) of small water-soluble compounds (urea and uric acid), low-molecular weight proteins (LMWPs) (β(2)-microglobulin, cystatin C, myoglobin and retinol-binding protein) and protein-bound solutes (hippuric acid, indole acetic acid, indoxylsulphate and p-cresylsulphate) was assessed, together with albumin losses into the dialysate. Comparing the two types of membranes, the second-generation dialyser demonstrated a higher RR for LMWPs, whilst at the same time exhibiting lower albumin losses but only during post-dilution haemodiafiltration. No differences in RR were detected for both the small water-soluble and the protein-bound compounds. Comparing dialysis strategies, convection removed the same amount of solute or more as compared to diffusion. The second-generation membrane resulted in a higher removal of LMWPs compared to the first-generation membrane, but for the other solutes, differences were less prominent. Convection was superior in removal of a broad range of uraemic retention solutes especially with the first-generation membrane.

An approach to speed up the isolation of hydrophilic metabolites from natural sources at semipreparative level by using a hydrophilic–lipophilic balance/mixed-mode strong cation exchange–high-performance liquid chromatography/mass spectrometry system

An approach to speed up the isolation of hydrophilic metabolites in complex natural matrixes by using a HLB/MCX–HPLC/MS system based on the retention properties of hydrophilic–lipophilic and cation exchange polymeric cartridges was developed. This methodology was successfully applied to the re-isolation of small water soluble compounds with completely different structures from two different natural extracts such as a dipeptide (vanchrobactin) from a bacterium culture broth and a pyrrolidine bearing a carboxylic acid moiety (clionapyrrolidine A) from a sponge. This method improved not only the efficiency of the isolation methodology but also the isolation time in relation to the existing methods.

What is new in uremic toxicity?

Uremic syndrome results from a malfunctioning of various organ systems due to the retention of compounds which, under normal conditions, would be excreted into the urine and/or metabolized by the kidneys. If these compounds are biologically active, they are called uremic toxins. One of the more important toxic effects of such compounds is cardio-vascular damage. A convenient classification based on the physico-chemical characteristics affecting the removal of such compounds by dialysis is: (1) small water-soluble compounds; (2) protein-bound compounds; (3) the larger “middle molecules”. Recent developments include the identification of several newly detected compounds linked to toxicity or the identification of as yet unidentified toxic effects of known compounds: the dinucleotide polyphosphates, structural variants of angiotensin II, interleukin-18, p-cresylsulfate and the guanidines. Toxic effects seem to be typically exerted by molecules which are “difficult to remove by dialysis”. Therefore, dialysis strategies have been adapted by applying membranes with larger pore size (high-flux membranes) and/or convection (on-line hemodiafiltration). The results of recent studies suggest that these strategies have better outcomes, thereby clinically corroborating the importance attributed in bench studies to these “difficult to remove” molecules.

Complex Compartmental Behavior of Small Water-Soluble Uremic Retention Solutes: Evaluation by Direct Measurements in Plasma and Erythrocytes

Although scanty data suggest that large solutes show kinetic behavior different from urea, there are virtually no data comparing the kinetics of urea with those of other small water-soluble uremic compounds, which are believed to behave similarly. Cross-sectional study of kinetics of urea and guanidino compounds in plasma and erythrocyte compartments during a single hemodialysis session. Six stable hemodialysis patients on standard low-flux dialysis therapy. Reduction ratios (RRs) of urea calculated from plasma and erythrocyte concentrations. RRs for guanidino compounds calculated from measurements of both plasma and erythrocyte concentrations. Blood samples were collected from the dialyzer inlet and outlet at 0, 5, 15, 30, and 120 minutes and at the end of the session. Plasma and erythrocyte concentrations of urea and guanidino compounds (creatinine [CTN], guanidinosuccinic acid [GSA], guanidinoacetic acid [GAA], guanidine [G], and methylguanidine [MG]) were determined. Postdialysis plasma RR was higher for GSA (82% +/- 3%) compared with urea (77% +/- 2%; P < 0.01), whereas CTN (69% +/- 4%), GAA (49% +/- 14%), G (55% +/- 7%), and MG (55% +/- 7%) showed smaller RRs (P < 0.01). In erythrocytes, GSA (45% +/- 1%), G (10% +/- 13%), and MG (27% +/- 10%) showed markedly smaller RRs than urea (59% +/- 6%; P < 0.05). Finally, significant differences were found between plasma and erythrocyte RRs for urea, GSA, G, and MG (P < 0.01). Discrepancies were found between the biochemical and mathematical approaches. Hence, the erythrocyte compartment does not necessarily conform to the kinetic nonperfused compartment. Our data indicate by means of direct estimations that the compartmental behaviors of guanidino compounds and urea are substantially different. Hence, we should consider that not all changes in concentrations in uremia and dialysis are representatively reflected by urea kinetics, even when considering other small water-soluble substances, such as the guanidino compounds.

Biochemical and clinical evidence for uremic toxicity.

The uremic syndrome is a mix of clinical features resulting from multiple organ dysfunctions which develop when kidney failure progresses, and is attributed to the retention of solutes, which under normal conditions are excreted by the healthy kidneys into the urine. The most practical classification of uremic solutes is based on their physicochemical characteristics that influence their dialytic removal, in (1) small water soluble compounds, (2) the larger "middle molecules," and (3) the protein bound compounds. Hence, uremic retention is much more complex than originally believed. Among the small water soluble compounds, urea exerts not much toxic activity and is not very representative in its kinetic behavior for many other uremic solutes. Among the middle molecules, many have been recognized to exert biological activity and hence to contribute to the uremic syndrome. Specific dialysis strategies apply large pore membranes to remove those middle molecules and have a beneficial impact on uremic morbidity and mortality. A substantial number of uremic solutes are protein bound. Only recently, a relation between their concentration and clinical status could be demonstrated. Likewise, it was only recently possible to demonstrate more than standard removal with super-flux dialysis membranes. To further improve characterization of uremic solutes and to develop directed therapeutic approaches, further concerted action among various groups of researchers will be needed.

An Overview of Uremic Toxicity

About 100 uremic retention solutes have been identified at present, but not all of these compounds are necessarily toxic. They can be defined as uremic toxins if they exert biochemical/biological actions. Based on their physicochemical characteristics, there are three major groups of uremic retention solutes: 1) the small water-soluble compounds (<500 Da), which are easily removed by standard low-pore-size dialyzer membranes; 2) the protein-bound solutes (also mostly <500 Da), whose dialytic removal is hampered by their protein binding, irrespective of the membrane type; and 3) the so-called middle molecules (>500 Da), which can be removed only by membranes with a large pore size and/or adsorptive capacity. In the present review, we will summarize the currently known information about the toxicity of the uremic retention solutes. Although removal of small water-soluble urea has been recognized for many years as a current measure of dialysis adequacy, data on its toxicity are very scanty. Almost 50 other water-soluble compounds are known to be retained in uremia, but only a few exert biological effects. Most of the toxic water-soluble moieties, such as the guanidines, phosphate, xanthine, and hypoxanthine show an intra-dialytic compartmental behavior, which is different from urea. A substantial number of uremic solutes are protein bound, and most of them exert biological action. Among them are the phenols, indoles, homocysteine, and carboxy-methyl-propyl-furanpropionic acid. Recent data suggest that protein binding acts as a buffer against the toxic effects of these compounds, and that hypoalbuminemia increases both their free fraction and their toxicity. In addition, many middle molecules, such as ss(2)-microglobulin, leptin, and advanced glycation end products, have been related to biological/clinical effects. Our current knowledge of the biological impact of the middle molecules is very likely incomplete. It is concluded that many of the water-soluble compounds exert little or no toxicity, and that urea removal pattern per se is not identical to that of many biologically active molecules. Hence, in dialysis, more than urea removal alone should be pursued.