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

Irreversible post-translational modifications - Emerging cardiovascular risk factors

Despite the introduction of lipid-lowering drugs, antihypertensives, antiplatelet and anticoagulation therapies for primary prevention of cardiovascular and heart diseases (CVD), it remains the number one cause of death globally, raising the question for novel/further essential factors besides traditional risk factors such as cholesterol, blood pressure and coagulation. With continuous identification and characterization of non-enzymatic post-translationally modified isoforms of proteins and lipoproteins, it is becoming increasingly clear that irreversible non-enzymatic post-translational modifications (nPTMs) alter the biological functions of native proteins and lipoproteins thereby transforming innate serum components into CVD mediators. In particular renal insufficiency and metabolic imbalance are major contributors to the systemically increased concentration of reactive metabolites and thus increased frequency of nPTMs, promoting multi-morbid disease development centering around cardiovascular disease. nPTMs are significantly involved in the onset and progression of cardiovascular disease and represent a significant and novel risk factor. These insights represent potentially new avenues for risk assessment, prevention and therapy. This review chapter summarizes all forms of nPTMs found in CKD and under metabolic imbalance and discusses the biochemical connections between molecular alterations and the pathological impact on increased cardiovascular risk, novel nPTM-associated non-traditional cardiovascular risk factors, and clinical implication of nPTM in cardiovascular disease.

Avenues for post-translational protein modification prevention and therapy

Non-enzymatic post-translational modifications (nPTMs) of proteins have emerged as novel risk factors for the genesis and progression of various diseases. We now have a variety of experimental and established therapeutic strategies to target harmful nPTMs and potentially improve clinical outcomes. Protein carbamylation and glycation are two common and representative nPTMs that have gained considerable attention lately as favorable therapeutic targets with emerging clinical evidence. Protein carbamylation is associated with the occurrence of cardiovascular disease (CVD) and mortality in patients with chronic kidney disease (CKD); and advanced glycation end products (AGEs), a heterogeneous group of molecules produced in a series of glycation reactions, have been linked to various diabetic complications. Therefore, reducing the burden of protein carbamylation and AGEs is an appealing and promising therapeutic approach. This review chapter summarizes potential anti-nPTM therapy options in CKD, CVD, and diabetes along with clinical implications. Using two prime examples-protein carbamylation and AGEs-we discuss the varied preventative and therapeutic options to mitigate these pathologic nPTMs in detail. We provide in-depth case studies on carbamylation in the setting of kidney disease and AGEs in metabolic disorders, with an emphasis on the relevance to reducing adverse clinical outcomes such as CKD progression, cardiovascular events, and mortality. Overall, whether specific efforts to lower carbamylation and AGE burden will yield definitive clinical improvement in humans remains largely to be seen. However, the scientific rationale for such pursuits is demonstrated herein.

A study of the binding between radicicol and four proteins by means of spectroscopy and molecular docking

The interactions between radicicol and four proteins (catalase, trypsin, pepsin, and human serum protein) are investigated by spectroscopic techniques and molecular docking. A static quenching process is confirmed. The binding constant value between radicicol and human serum protein is the largest among the four proteins. Results reveal changes in the micro-environment of the protein by the addition of radicicol. It is found that radicicol shows an inhibitory effect on the activity of proteins (catalase, trypsin, and pepsin). Molecular docking results are consistent with the thermodynamic experimental results. This work provides clues to the elucidation of the mechanisms of the interactions between radicicol and proteins.

Protein Carbamylation in Chronic Kidney Disease and Dialysis

Protein carbamylation is a nonenzymatic posttranslational protein modification that can be driven, in part, by exposure to urea's dissociation product, cyanate. In humans, when kidney function is impaired and urea accumulates, systemic protein carbamylation levels increase. Additional mediators of protein carbamylation have been identified including inflammation, diet, smoking, circulating free amino acid levels, and environmental exposures. Carbamylation reactions on proteins are capable of irreversibly changing protein charge, structure, and function, resulting in pathologic molecular and cellular responses. Carbamylation has been mechanistically linked to the biochemical pathways implicated in atherosclerosis, dysfunctional erythropoiesis, kidney fibrosis, autoimmunity, and other pathological domains highly relevant to patients with chronic kidney disease. In this review, we describe the biochemical impact of carbamylation on human proteins, the mechanistic role carbamylation can have on clinical outcomes in kidney disease, the clinical association studies of carbamylation in chronic kidney disease, including patients on dialysis, and the promise of therapies aimed at reducing carbamylation burden in this vulnerable patient population.

New Insights About Albumin and Liver Disease

Decompensated liver cirrhosis has a dismal prognosis, with an overall survival of 2-4 years, which is worse than for many oncological diseases. Albumin is an important tool in the management of patients with cirrhosis, since it decreases for less than half the risk for post-paracentesis cardiocirculatory dysfunction and mortality associated with spontaneous bacterial infection, as well as, it triplicates the response to terlipressin in patients with hepatorenal syndrome. Recently, research on albumin has been a hot topic, with important new insights such as the characterization of the pleiotropic effects of albumin (which surpass its oncotic properties) and the concept of effective albumin concentration. In fact, patients with liver cirrhosis present posttranslational modifications on albumin that compromises its function. Those modified albumin forms were proved to have prognostic value and its knowledge may change the paradigm of albumin treatment. In this review, we critically summarize the latest evidence on the potential benefits of albumin in patients with end-stage liver disease.

Protein Carbamylation: Chemistry, Pathophysiological Involvement, and Biomarkers

Protein carbamylation refers to a nonenzymatic modification, which consists in the binding of isocyanic acid on protein functional groups. This reaction is responsible for the alteration in structural and functional properties of proteins, which participate in their molecular aging. Protein molecular aging is now considered a molecular substratum for the development of chronic and inflammatory diseases, including atherosclerosis, chronic kidney disease, or rheumatoid arthritis. As a consequence, carbamylation-derived products have been proposed as interesting biomarkers in various pathological contexts and appropriate analytical methods have been developed for their quantification in biological fluids. The purpose of this review is (i) to describe the biochemical bases of the carbamylation reaction, (ii) to explain how it contributes to protein molecular aging, (iii) to provide evidence of its involvement in aging and chronic diseases, and (iv) to list the available biomarkers of carbamylation process and the related analytical methods.

Binding of bromocresol green and bromocresol purple to albumin in hemodialysis patients

Background:

Colorimetric albumin assays based on binding to bromocresol purple (BCP) and bromocresol green (BCG) yield different results in chronic kidney disease. Altered dye binding of carbamylated albumin has been suggested as a cause. In the present study, a detailed analysis was carried out in which uremic toxins, acute phase proteins and Kt/V, a parameter describing hemodialysis efficiency, were compared with colorimetrically assayed (BCP and BCG) serum albumin.

Methods:

Albumin was assayed using immunonephelometry on a BN II nephelometer and colorimetrically based on, respectively, BCP and BCG on a Modular P analyzer. Uremic toxins were assessed using high-performance liquid chromatography. Acute phase proteins (C-reactive protein and α1-acid glycoprotein) and plasma protein α2-macroglobulin were assayed nephelometrically. In parallel, Kt/V was calculated.

Results:

Sixty-two serum specimens originating from hemodialysis patients were analyzed. Among the uremic toxins investigated, total para-cresyl sulfate (PCS) showed a significant positive correlation with the BCP/BCG ratio. The serum α1-acid glycoprotein concentration correlated negatively with the BCP/BCG ratio. The BCP/BCG ratio showed also a negative correlation with Kt/V.

Conclusions:

In renal insufficiency, the BCP/BCG ratio of serum albumin is affected by multiple factors: next to carbamylation, uremic toxins (total PCS) and α1-acid glycoprotein also play a role.

Quantification of carbamylated albumin in serum based on capillary electrophoresis

Protein carbamylation, a nonenzymatic posttranslational modification promoted during uremia, is linked to a poor prognosis. In the present study, carbamylation of serum albumin was assayed using the symmetry factor on a capillary electrophoresis instrument (Helena V8). The symmetry factor has been defined as the distance from the center line of the peak to the back slope, divided by the distance from the center line of the peak to the front slope, with all measurements made at 10% of the maximum peak height. Serum albumin, creatinine, and urea concentrations were assayed using routine methods, whereas uremic toxins were determined using HPLC. In vitro carbamylation induced a marked albumin peak asymmetry. Reference values for the albumin symmetry factor were 0.69-0.92. In kidney patients, albumin peak asymmetry corresponded to the chronic kidney disease stage (p < 0.0001). The symmetry factor correlated well with serum urea (r = -0.5595, p < 0.0001) and creatinine (r = -0.5986, p < 0.0001) concentrations. Several protein-bound uremic toxins showed a significant negative correlation with the symmetry factor. Morphology of the albumin fraction was not affected by presence of glycated albumin and protein-bound antibiotics. In conclusion, the presented method provides a simple, practical way for monitoring protein carbamylation.

Longitudinal Changes in Protein Carbamylation and Mortality Risk after Initiation of Hemodialysis

BACKGROUND AND OBJECTIVES

Carbamylation describes a post-translational protein modification associated with adverse outcomes in ESRD, but the risk implications of changes in carbamylation over time are not well understood.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We investigated the 1-year natural history of protein carbamylation in patients initiating maintenance hemodialysis and determined the prognostic value of longitudinal carbamylation changes in relation to mortality. In a nested patient-control study, we measured serial carbamylated albumin concentrations in select participants from a large incident dialysis cohort followed from 2004 to 2005 (n=10,044); 122 individuals who survived at least 90 days but died within 1 year of initiating hemodialysis (patients) were randomly selected along with 244 individuals who survived for at least 1 year (controls; matched for demographics). Carbamylated albumin concentration was measured using plasma collected at dialysis initiation and every subsequent 90-day period until 1 year or death.

RESULTS

Baseline carbamylated albumin concentration was similar between controls and patients (mean±SD; 18.9±0.7 and 19.8±1.1 mmol/mol, respectively; P=0.94). From dialysis initiation to day 90, carbamylated albumin concentration markedly fell in all patients, with controls -9.9±0.8 mmol/mol (P<0.001) and patients -10.0±1.2 mmol/mol (P<0.001). Adjusted repeated measures analysis of carbamylated albumin concentration from dialysis initiation to 1 year or death showed that the mean change (95% confidence interval) in carbamylated albumin concentration from baseline to final measure differed significantly between groups (-9.3; 95% confidence interval, -10.8 to -7.7 for controls and -6.3; 95% confidence interval, -7.7 to -2.8 for patients; P<0.01). There were no such between-group differences in blood urea levels, Kt/V, or normalized protein catabolic rate. Mortality prediction assessed using c statistics showed that carbamylated albumin concentration, when modeled continuously as the difference from baseline to final, improved a fully adjusted model from 0.76 to 0.87 (P=0.03).

CONCLUSIONS

Protein carbamylation decreased with dialysis initiation, and a greater reduction over time was associated with a lower risk for mortality. Carbamylation changes were able to predict individuals' mortality risk beyond traditional variables, including markers of dialysis adequacy and nutrition.

Effects of unbound mycophenolic acid on inosine monophosphate dehydrogenase inhibition in pediatric kidney transplant patients

BACKGROUND

Mycophenolic acid (MPA) is a key immunosuppressive drug that acts through inhibition of inosine monophosphate dehydrogenase (IMPDH). MPA is commonly measured, as part of therapeutic drug monitoring, as the total concentration in plasma. However, it has been postulated that the free (unbound) fraction of MPA (fMPA) is responsible for the immunosuppressive effects. In this study, a sensitive low volume high-performance liquid chromatography (HPLC) assay was developed to measure fMPA concentrations to explore the relationship between fMPA and IMPDH activity.

METHODS

To obtain fMPA concentrations, plasma samples were filtrated using Centrifree ultrafiltration devices. The ultrafiltrate was analyzed by HPLC using a Kinetex C18 column (2.6 μm, 3.0 × 75 mm). fMPA concentrations were compared with the total MPA concentrations available in 28 pediatric kidney transplant patients at 3 consecutive occasions after transplantation. The relationship between fMPA and IMPDH activity was analyzed using an Emax model.

RESULTS

The HPLC assay, using 25 μL of the ultrafiltrates, was validated over a range from 2.5 to 1000 μL with good accuracy, precision, and reproducibility. Total and free MPA concentrations were well correlated (R = 0.85, P < 0.0001), although large intraindividual and interindividual variability in the bound MPA fractions was observed. The overall relationship between fMPA concentrations and IMPDH inhibition using the Emax model was comparable with that of total MPA, as previously reported. The model estimated EC50 value (164.5 μL) is in good agreement with reported in vitro EC50 values.

CONCLUSIONS

This study provides a simple HPLC method for the measurement of fMPA and a pharmacologically reasonable EC50 estimate. The good correlation between the total and free MPA concentrations suggests that routine measurement of fMPA to characterize mycophenolate pharmacokinetic and pharmacodynamic does not seem warranted, although the large variability in the bound fractions of MPA warrants further study.

Protein carbamylation in kidney disease: pathogenesis and clinical implications

Carbamylation describes a nonenzymatic posttranslational protein modification mediated by cyanate, a dissociation product of urea. When kidney function declines and urea accumulates, the burden of carbamylation naturally increases. Free amino acids may protect proteins from carbamylation, and protein carbamylation has been shown to increase in uremic patients with amino acid deficiencies. Carbamylation reactions are capable of altering the structure and functional properties of certain proteins and have been implicated directly in the underlying mechanisms of various disease conditions. A broad range of studies has demonstrated how the irreversible binding of urea-derived cyanate to proteins in the human body causes inappropriate cellular responses leading to adverse outcomes such as accelerated atherosclerosis and inflammation. Given carbamylation's relationship to urea and the evidence that it contributes to disease pathogenesis, measurements of carbamylated proteins may serve as useful quantitative biomarkers of time-averaged urea concentrations while also offering risk assessment in patients with kidney disease. Moreover, the link between carbamylated proteins and disease pathophysiology creates an enticing therapeutic target for reducing the rate of carbamylation. This article reviews the biochemistry of the carbamylation reaction, its role in specific diseases, and the potential diagnostic and therapeutic implications of these findings based on recent advances.

Post-translational modification derived products (PTMDPs): toxins in chronic diseases?

In living organisms, proteins are progressively modified by spontaneous non-enzymatic reactions generating many post-translational modification derived products (PTMDPs) which exert deleterious effects and may be considered endogenous toxins in diabetes mellitus and chronic renal failure. Non-enzymatic glycation, which refers to the spontaneous binding of reducing sugars to free amino groups, is increased in diabetes mellitus because of hyperglycemia and is amplified by oxidative processes ('glycoxidation'). Glycoxidation leads to the formation of 'advanced glycation end products' (AGEs), together with products of other oxidative pathways. AGEs alter tissue organization and cell-protein interactions, mainly in the case of long-lived extracellular matrix proteins, and interact with membrane receptors, among which RAGE (receptor of AGEs), a multiligand receptor which triggers intracellular signaling pathways stimulating inflammatory functions. Another major protein modification, carbamylation, is increased in chronic renal failure, which may occur during the course of diabetes mellitus. Carbamylation is due to the binding of isocyanic acid on the α-NH2 extremity of proteins or amino acids, or on ε-NH2 lysine groups, generating homocitrulline, a potential biomarker in atherosclerosis. Isocyanic acid is formed in vivo either by spontaneous dissociation of urea or by myeloperoxidase action on thiocyanate. Carbamylated proteins exhibit altered properties. For example, carbamylated collagen is unable to stimulate oxidative functions of polymorphonuclear neutrophils but increases matrix metalloproteinase-9 production by monocytes. Lipoprotein functions are altered by carbamylation and may contribute to atherogenesis. Thus, the numerous PTMDPs may be considered both hallmarks of protein damage in chronic diseases and endogenous toxins acting at the molecular and cellular levels.

Carbamylation and antibodies against carbamylated proteins in autoimmunity and other pathologies

Carbamylation is a non-enzymatic post-translational modification in which cyanate binds to molecules containing primary amine or thiol groups and forms carbamyl groups. Cyanate is in equilibrium with urea in body fluid and increased carbamylation was first reported in patients with increased urea levels such as patients suffering renal diseases. Next, increased carbamylation related to inflammation has also been described in other conditions such as cardiovascular disease. Recently, a new consequence of carbamylation has been observed: induction of an autoantibody response. We identified anti-carbamylated protein (anti-CarP) antibodies in rheumatoid arthritis (RA) patients and in patients having 'pre-RA' symptoms, arthralgia. The presence of anti-CarP antibodies in arthralgia patients is associated with an increased risk of developing RA. The presence of anti-CarP antibodies in RA patients is associated with more severe joint damage in RA patients who do not have anti-citrullinated protein antibodies. It is currently unknown to what extent carbamylation and/or the formation of anti-CarP antibodies contributes to the disease processes of chronic diseases such as renal diseases, cardiovascular diseases and RA. This review summarizes the current knowledge on carbamylation and the formation of anti-CarP antibodies and discusses their possibly important implications.

Albumin-binding capacity (ABiC) is reduced in patients with chronic kidney disease along with an accumulation of protein-bound uraemic toxins

BACKGROUND

Albumin is an important transport protein for non-water-soluble protein-bound drugs and uraemic toxins. Its transport capacity is reduced in patients with advanced chronic kidney disease (CKD) and unbound fractions of uraemic toxins are related to complications of CKD. We investigated whether this reduction could be quantified and how it correlated with the stages of CKD. Albumin-binding capacity (ABiC) is a dye-based method that quantifies the remaining binding capacity of one major binding site (site II) of the albumin molecule.

METHODS

Blood samples from 104 CKD patients were incubated with a binding site-specific fluorescent marker and the amount of unbound marker was determined by means of fluorescence detection after filtration. Measurements in a pooled human plasma were used for reference. Glomerular filtration rate and serum indoxyl sulphate (IS) levels were also determined.

RESULTS

Impairment of renal function was associated with a reduction in ABiC (mean ± SD: 118 ± 12; 111 ± 11; 99 ± 8 and 79 ± 9% for Stages 1/2, 3, 4 and 5, respectively; P < 0.001) and an increase in IS (3.9 ± 1.1; 6.2 ± 3.2; 16.3 ± 14.9 and 56.1 ± 28.6 μmol/L for Stages 1/2, 3, 4 and 5, respectively; P < 0.001). In dialysis patients, ABiC was lower in those with urine outputs <500 mL/day than in those with preserved urine output (73.7 ± 6.0 versus 83.8 ± 8.5%; P < 0.001).

CONCLUSION

Impaired albumin function in CKD patients can be quantified, is related to severity of kidney disease and is associated with an accumulation of uraemic albumin-bound retention solutes.

A review of albumin binding in CKD

Hypoalbuminemia is associated with excess mortality in patients with kidney disease. Albumin is an important oxidant scavenger and an abundant carrier protein for numerous endogenous and exogenous compounds. Several specific binding sites for anionic, neutral, and cationic ligands were described. Overall, the extent of binding depends on the ligand and albumin concentration, albumin-binding affinity, and presence of competing ligands. Chronic kidney disease affects all these determinants. This may result in altered pharmacokinetics and increased risk of toxicity. Renal clearance of albumin-bound solutes mainly depends on tubular clearance. Dialytic clearance by means of conventional hemodialysis/hemofiltration and peritoneal dialysis is limited. Other epuration techniques combining hemodialysis with adsorption have been developed. However, the benefit of these techniques remains to be proved.

Plasma protein homocysteinylation in uremia

Protein homocysteinylation is proposed as one of the mechanisms of homocysteine toxicity. It occurs through various means, such as the post-biosynthetic acylation of free amino groups (protein-N-homocysteinylation, mediated by homocysteine thiolactone) and the formation of a covalent -S-S- bond found primarily with cysteine residues (protein-S-homocysteinylation). Both protein modifications are a cause of protein functional derangements. Hemodialysis patients in the majority of cases are hyperhomocysteinemic, if not malnourished. Protein-N-homocysteinylation and protein-S-homocysteinylation are significantly increased in hemodialysis patients compared to controls. Oral folate treatment normalizes protein-N-homocysteinylation levels, while protein-S-homocysteinylation is significantly reduced. Albumin binding experiments after in vitro homocysteinylation show that homocysteinylated albumin is significantly altered at the diazepam, but not at the warfarin and salicilic acid binding sites.

Clin Chem Lab Med 2007;45:1678–82.

Increased plasma protein homocysteinylation in hemodialysis patients

Hyperhomocysteinemia, an independent cardiovascular risk factor, is present in the majority of hemodialysis patients. Among the postulated mechanisms of toxicity, protein homocysteinylation is potentially able to cause significant alterations in protein function. Protein homocysteinylation occurs through various mechanisms, among which is the post-translational acylation of free amino groups (protein-N-homocysteinylation, mediated by homocysteine (Hcy) thiolactone). Another type of protein homocysteinylation occurs through the formation of a covalent -S-S- bond, found primarily with cysteine residues (protein-S-homocysteinylation). Scant data are available in the literature regarding the extent to which alterations in protein homocysteinylation are present in uremic patients on hemodialysis, and the effects of folate treatment are not known. Protein homocysteinylation was measured in a group of hemodialysis patients (n=28) compared to controls (n=14), with a new method combining protein reduction, gel filtration and Hcy derivatization. Chemical hydrolysis was performed, followed by high-pressure liquid chromatography separation. The effects of folate treatment on protein homocysteinylation, as well as in vitro binding characteristics were evaluated. Plasma Hcy, protein-N-homocysteinylation and protein-S-homocysteinylation were significantly higher in patients vs controls. Plasma Hcy and protein-S-homocysteinylation were significantly correlated. After 2 months of oral folate treatment, protein-N-homocysteinylation was normalized, and protein-S-homocysteinylation was significantly reduced. Studies on albumin-binding capacity after in vitro homocysteinylation show that homocysteinylated albumin is significantly altered at the diazepam-binding site. In conclusion, increased protein homocysteinylation is present in hemodialysis patients, with possible consequences in terms of protein function. This alteration can be partially reversed after folate treatment.

Plasma proteins containing damaged L-isoaspartyl residues are increased in uremia: implications for mechanism

BACKGROUND

Several alterations of protein structure and function have been reported in uremia. Impairment of a transmethylation-dependent protein repair mechanism possibly related to a derangement in homocysteine metabolism is also present in this condition, causing erythrocyte membrane protein damage. Homocysteine may affect proteins via the accumulation of its parent compound S-adenosylhomocysteine (AdoHcy), a powerful in vivo methyltransferase inhibitor. However, since plasma homocysteine is mostly protein bound, a direct influence on protein structures cannot be ruled out. We measured the levels of L-isoaspartyl residues in plasma proteins of uremic patients on hemodialysis. These damaged residues are markers of molecular age, which accumulate when transmethylation-dependent protein repair is inhibited and/or protein instability is increased.

METHODS

L-isoaspartyl residues in plasma proteins were quantitated using human recombinant protein carboxyl methyl transferase (PCMT). Plasma concentrations of homocysteine metabolites were also measured under different experimental conditions in hemodialysis patients.

RESULTS

The concentration of damaged plasma proteins was increased almost twofold compared to control (controls 147.83 +/- 17.75, uremics 282.80 +/- 26.40 pmol of incorporated methyl groups/mg protein, P < 0.003). The major protein involved comigrated with serum albumin. Although hyperhomocysteinemia caused a redistribution of thiols bound to plasma proteins, this mechanism did not significantly contribute to the increase in isoaspartyl residues. The S-adenosylmethionine (AdoMet)/AdoHcy concentration ratio, an indicator of the flux of methyl group transfer, was altered. This ratio was partially corrected by folate treatment (0.385 +/- 0.046 vs. 0.682 +/- 0.115, P < 0.01), but protein L-isoaspartate content was not.

CONCLUSIONS

Plasma protein damage, as determined by protein L-isoaspartyl content, is increased in uremia. This alteration is to be ascribed to an increased protein structural instability, rather than the effect of hyperhomocysteinemia.

Kinetics of carbamylated haemoglobin in acute renal failure

BACKGROUND

Carbamylation of proteins by isocyanic acid, the reactive form of cyanate derived from urea, is increased in uraemia and may contribute to uraemic toxicity. Kinetics of carbamylation that may reflect uraemic toxicity is not clearly defined in acute renal failure (ARF).

METHODS

Twenty-eight patients with ARF and 13 with chronic renal failure (CRF) were included in the study in order to determine changes in carbamylated haemoglobin concentration (CarHb) in ARF. The usefulness of this parameter for differentiating ARF from CRF was also investigated. CarHb was measured by high-performance liquid chromatography after acid hydrolysis.

RESULTS

Mean CarHb level (expressed as microg carbamyl valine per gram (CV/g) Hb) was significantly higher in ARF (54.3+/-5.2) than in normal subjects (31.6+/-1.3). On admission, CarHb level was correlated with duration of ARF prior to hospitalization in the intensive care unit (r(2)=0.723, P<0.001). CarHb was significantly higher at recovery in the subgroup of patients requiring haemodialysis than in the subgroup not requiring haemodialysis (82. 4+/-11.3 vs 46.7+/-5.2, P<0.01). Similarly dialysis patients lost more weight (8.6+/-1.4 vs 2.7+/-0.5 kg, P<0.005) and had higher averaged blood urea levels in the 20 days prior to recovery (17. 6+/-1.9 vs 11.3+/-1.8 mol/l, P<0.05). After recovery, CarHb level decreased at a rate of 0.219 microg CV/g Hb per day in patients with reversible renal insufficiency. CarHb concentration was higher in patients with CRF. A cut-off CarHb value of 100 microg CV/g Hb had a sensitivity of 94% and a positive predictive value of 94% for differentiating ARF from CRF.

CONCLUSIONS

Kinetics of CarHb showed a near normal red blood cell life span in ARF. Changes in CarHb enabled, with a good sensitivity, the distinction to be made between patients who recovered from ARF and those with sustained renal impairment, whether due to prior CRF or resulting from parenchymal sequelae. Measurement of CarHb is valuable at clinical presentation of ARF in patients with an unknown medical history of renal disease.

Carbamylated proteins activate glomerular mesangial cells and stimulate collagen deposition

Carbamylated proteins formed in renal insufficiency from the spontaneous decomposition of urea exert a variety of metabolic effects. Here we examined the effects of carbamylated proteins on glomerular mesangial cells to determine whether urea retention in early renal insufficiency may itself promote glomerular sclerosis and hasten the progression to kidney failure. To this effect we carbamylated fetal bovine serum proteins in vitro and tested their effect on mesangial cell proliferation (by tritiated thymidine uptake), de novo protein synthesis (by tritiated leucine uptake), collagen I and collagen IV accumulation (by avidin-biotin enzyme immunoassay), and gelatinase levels in the medium (by zymography and quantitative fluorescence assay). Carbamylated fetal bovine serum at concentrations present in uremia increased tritiated thymidine incorporation by 50% without altering tritiated leucine incorporation, and it increased collagens I and IV in the monolayer by 150% to 300%. Gelatinase activity was unchanged. We conclude that carbamylated proteins can activate mesangial cells to a profibrogenic phenotype. From a clinical perspective, the carbamylation of proteins by elevated urea levels may accelerate the progression to kidney failure and thus set up a vicious cycle in which the nitrogen retention itself would cause further progression of fibrosis and deterioration of kidney function.

Oxidative stress in chronic renal failure

Cardiovascular disease is the major cause of morbidity and mortality in chronic renal failure. The aim of this review is to summarise current evidence suggesting that there is increased free radical production, antioxidant depletion and changes in lipoprotein composition in renal failure which will lead to oxidation of LDL and hence to accelerated development of atherosclerosis.

Carbamylated hemoglobin and carbamylated plasma protein in hemodialyzed patients

The carbamylation reaction in vivo involves the nonenzymatic, covalent attachment of isocyanic acid, the spontaneous dissociation product of urea, to proteins. Carbamylated proteins have been proposed as markers of uremia and indicators of uremic control. However, the utility of measuring carbamylated proteins has not been investigated adequately. Therefore, this study was done to determine the relationship between the carbamylation of long-lived protein (hemoglobin) with that of short-lived proteins (plasma proteins) in hemodialyzed patients. Significantly higher carbamylated hemoglobin (CHb; 157 +/- 40 microg valine hydantoin/g Hb) and carbamylated protein (CTP; 0.117 +/- 0.011 absorbance/mg protein) concentrations were found in hemodialyzed patients (N = 13) as compared to normal individuals (N = 9, 53 +/- 20 microg valine hydantoin/g Hb and 0.08 +/- 0.01 absorbance/mg protein, respectively). A high correlation was found between CHb and CTP concentrations (r = 0.87, P < 0.0001), demonstrating a strong relationship between these two different half-lived proteins. A six-month longitudinal study of seven hemodialyzed patients showed that the between subject correlations were significant for CHb versus CTP as well as CHb versus pre-dialysis urea. Correlations were not significant for CTP versus pre-dialysis urea or Kt/V, nor CHb versus Kt/V. Carbamylated hemoglobin fluctuated the most over this time period (30.1% +/- 20.2%), pre-dialysis urea and CTP varied less (18.3% +/- 13.4% and 14.9% +/- 7.5%, respectively), and Kt/V varied the least (6.3% +/- 3.3%). Within subject correlations were not significant between any two tests. It is unclear whether the lack of correlations found is real or a function of the small sample size. However, these data do show that CHb and CTP are positively associated and reflect the degree of urea exposure in the blood, but their usefulness for patients on maintenance hemodialysis is not clear.

Oxidation of low-density lipoprotein and atherosclerosis in chronic renal failure

The major cause of death in patients with end-stage renal failure receiving renal replacement therapy is cardiovascular disease. Oxidation of low-density lipoprotein (LDL) is recognized as a key early stage in the development of atherosclerosis, leading to uptake of LDL by the macrophage scavenger receptor and hence to foam cell formation. However, several studies have suggested that the susceptibility of LDL to oxidation is not increased in chronic renal failure. We propose a number of mechanisms which may lead to increased lipoprotein oxidation in vivo, and hence contribute to increased atherosclerosis in renal failure.

Drug-protein binding sites. New trends in analytical and experimental methodology

In the last few years, continuous progress in instrumental analytical methodology has been achieved with a substantial increase in the number of new, more specific and more flexible methods for ligand-protein assays. In general, the methods used for drug-protein binding studies can be divided into two main groups: separation methods (enabling the calculation of binding parameters, i.e. the number of binding sites and their respective affinity constants) and non-separation methods (describing predominantly qualitative parameters of the ligand-protein complex). This review will be focussed particularly on recent trends in the development of drug-protein binding methods including stereoselective and non-stereoselective aspects using chromatography, capillary electrophoresis and microdialysis as compared to the "conventional approach" using equilibrium dialysis, ultrafiltration or size exclusion chromatography. The advantages and limitations of various methods will be discussed including a focus on "optimal" experimental strategies taking into account in vitro, ex vivo and/or in vivo studies. Furthermore, the importance of some particular aspects concerning the drug binding to proteins (covalent binding of drugs and metabolites, stereoselective interactions and evaluation of binding data) will be outlined in more detail.

Carbamylation of proteins and atherogenesis in renal failure

The incidence of atherosclerosis is greatly increased in patients with chronic renal failure, and this increased risk is only partly explained by conventional risk factors. Carbamylation of proteins occurs in renal failure as a result of reactions between urea-derived cyanate and protein amino groups. A mechanism is proposed whereby oxidation of LDL cholesterol within the arterial wall may be enhanced as a result of carbamylation. This may be accentuated as a result of inhibition of antioxidant enzymes by carbamylation. The combination of these processes may lead to increased atherogenesis.