Renal selectivity properties towards endogenous albumin in minimal change nephropathy

State of the art in childhood nephrotic syndrome: concrete discoveries and unmet needs

Nephrotic syndrome (NS) is a clinical entity characterized by proteinuria, hypoalbuminemia, and peripheral edema. NS affects about 2-7 per 100,000 children aged below 18 years old yearly and is classified, based on the response to drugs, into steroid sensitive (SSNS), steroid dependent, (SDNS), multidrug dependent (MDNS), and multidrug resistant (MRNS). Forms of NS that are more difficult to treat are associated with a worse outcome with respect to renal function. In particular, MRNS commonly progresses to end stage renal failure requiring renal transplantation, with recurrence of the original disease in half of the cases. Histological presentations of NS may vary from minimal glomerular lesions (MCD) to focal segmental glomerulosclerosis (FSGS) and, of relevance, the histological patterns do not correlate with the response to treatments. Moreover, around half of MRNS cases are secondary to causative pathogenic variants in genes involved in maintaining the glomerular structure. The pathogenesis of NS is still poorly understood and therapeutic approaches are mostly based on clinical experience. Understanding of pathogenetic mechanisms of NS is one of the 'unmet needs' in nephrology and represents a significant challenge for the scientific community. The scope of the present review includes exploring relevant findings, identifying unmet needs, and reviewing therapeutic developments that characterize NS in the last decades. The main aim is to provide a basis for new perspectives and mechanistic studies in NS.

Urine Proteome Biomarkers in Kidney Diseases. I. Limits, Perspectives, and First Focus on Normal Urine

Urine proteome is a potential source of information in renal diseases, and it is considered a natural area of investigation for biomarkers. Technology developments have markedly increased the power analysis on urinary proteins, and it is time to confront methodologies and results of major studies on the topics. This is a first part of a series of reviews that will focus on the urine proteome as a site for detecting biomarkers of renal diseases; the theme of the first review concerns methodological aspects applied to normal urine. Main issues are techniques for urine pretreatment, separation of exosomes, use of combinatorial peptide ligand libraries, mass spectrometry approaches, and analysis of data sets. Available studies show important differences, suggesting a major confounding effect of the technologies utilized for analysis. The objective is to obtain consensus about which approaches should be utilized for studying urine proteome in renal diseases.

Albumin heterogeneity in low-abundance fluids. The case of urine and cerebro-spinal fluid

BACKGROUND

Serum albumin is a micro-heterogeneous protein composed of at least 40 isoforms. Its heterogeneity is even more pronounced in biological fluids other than serum, the major being urine and cerebrospinal fluid. Modification 'in situ' and/or selectivity of biological barriers, such as in the kidney, determines the final composition of albumin and may help in definition of inflammatory states.

SCOPE OF REVIEW

This review focuses on various aspects of albumin heterogeneity in low 'abundance fluids' and highlights the potential source of information in diseases.

MAJOR CONCLUSIONS

The electrical charge of the protein in urine and CSF is modified but with an opposite change and depending on clinical conditions. In normal urine, the bulk of albumin is more anionic than in serum for the presence of ten times more fatty acids that introduce equivalent anionic charges and modify hydrophobicity of the protein. At the same time, urinary albumin is more glycosylated compared to the serum homolog. Finally, albumin fragments can be detected in urine in patients with proteinuria. For albumin in CSF, we lack information relative to normal conditions since ethical problems do not allow normal CSF to be studied. In multiple sclerosis, the albumin charge in CSF is more cationic than in serum, this change possibly involving structural anomalies or small molecules bindings.

GENERAL SIGNIFICANCE

Massively fatty albumin could be toxic for tubular cells and be eliminated on this basis. Renal handling of glycosylated albumin can alter the normal equilibrium of filtration/reabsorption and trigger mechanisms leading to glomerulosclerosis and tubulo-interstitial fibrosis. This article is part of a Special Issue entitled Serum Albumin.

Oxidized albumin. The long way of a protein of uncertain function

BACKGROUND

Proteins are extremely reactive to oxidants and should represent a potential target of instable reactive oxygen. This may represent a problem for plasma proteins since they may be directly modified in vivo in a compartment where antioxidant enzymatic systems are scarcely represented. On the other hand, it is possible that some plasma components have evolved over time to guarantee protection, in which case they can be considered as anti-oxidants.

SCOPE OF REVIEW

To present and discuss main studies which addressed the role of albumin in plasma antioxidant activity mainly utilizing in vitro models of oxidation. To present some advances on structural features of oxidized albumin deriving from studies carried out on in vitro models as well as albumin purified in vivo from patients affected by clinical conditions characterized by oxidative stress.

MAJOR CONCLUSIONS

There are different interaction with HOCl and chloramines. In the former case, HOCl produces an extensive alteration of (238)Trp and (162)Tyr, (425)Tyr, (47)Tyr, while thiol groups are only partially involved. Chloramines are extremely reactive with the unique free SH group of albumin ((34)Cys) with the formation of sulfenic and sulfinic acid as intermediates and sulfonic acid as end-product. Oxidized albumin has a modified electrical charge for the addition of an acidic residue and presents α-helix and random coil reorganization with subtle changes in domain orientation.

GENERAL SIGNIFICANCE

Albumin, is the major antioxidants in plasma with a concentration (0.8mM) higher than other antioxidants by an exponential factor. Functional and protective roles in the presence of oxidative stress must be defined. This article is part of a Special Issue entitled Serum Albumin.

Repetitive fragmentation products of albumin and alpha1-antitrypsin in glomerular diseases associated with nephrotic syndrome

Even if nephrotic syndrome is characterized by massive urinary loss of major plasma proteins, a clear structural characterization based on proteomics has never been reported. Urine and plasma of 23 patients with different idiopathic nephrotic syndromes (10 steroid-sensitive minimal-change nephropathy, seven steroid-resistant FSGS, and six membranous glomerulonephritis) were analyzed with two-dimensional electrophoresis in soft gel, Western blot, and matrix-assisted laser desorption/ionization time of flight mass spectrometry; 72 urinary components corresponded to fragments of albumin and/or of alpha1-antitrypsin. Several repetitive fragmentation motives and a few differences among different pathologies were found. Several (21 of 72) urinary albumin fragments also were detected in plasma, although in lower concentration, suggesting a preferential excretion. The bulk of components with low molecular weight were detected only in urine, suggesting an in situ formation; zymograms with albumin as substrate showed the presence in urine of specific proteases. A final but not secondary point was the characterization of albumin adducts that harbor both the COOH and NH2 terminal parts of the protein, suggesting the formation of new covalent chemical groups. Altogether, these new findings reveal unexpected structural and functional aspects of proteinuria that may play a key role in pathogenesis. Characterization of urinary fragmentation patterns should be extended to other renal diseases.

Transitions of serum albumin in patients with glomerulosclerosis ‘in vivo’ characterization by electrophoretic titration curves

HSA functions as a physiological transporter of solutes and small molecules that induce structural transitions 'in vitro'. Analysis of these transitions requires prior purification of HSA that could introduce bias due to conformational changes. We utilized electrophoretic titration curves to describe a neutral to acid (N-A) transition of HSA directly in sera of seven patients with active focal segmental glomerulosclerosis (FSGS). The divergent electrophoretic profile of HSA was characterized by a shift in the range of pHs between 4.5 and 7.5 with an average variation of free electrophoretic mobility corresponding to loss of 1 positive charge in the pKa protonation range of histidyl residues and should involve domain I of HSA. 'In-gel' determination by maleimide-PEO2-biotin of free SH 34 of domain I showed inaccessibility of the dye at this site in pathological HSA and alkylation with the same complex induced N-A transition in normal HSA. Potential binders of free imidazoles such as Ca++ and/or of SH 34 such as NO were excluded on the basis of direct titration and studies on binding stimulation. This is the first report describing a transition of HSA directly 'in vivo', and the utilization of electrophoretic titration curves was critical to this purpose. This transition appears to be specific to FSGS and is unrelated to the nephrotic syndrome, Ca++ and NO binding. Spectroscopic analysis will elucidate the structural implication.

Renal glomerular permselectivity and vascular endothelium

The glomerular barrier is the kidney's physical block to the unrestricted flow of molecules from the plasma into the urinary space. Its exquisite selectivity allows solutes and water in the glomerular capillaries to pass through but it prevents the bulk of plasma proteins, most notably albumin, from crossing. Classically the barrier consists of three components: glomerular endothelium, glomerular basement membrane and glomerular epithelium (podocytes) with slit diaphragm. A lot of investigations are experimental but they are sufficient to show the pivotal role of endothelium in glomerular proteinuria. In this study the author discuss glomerular endothelium with particular emphasis on the barrier presumed to be imparted by endothelium-glomerular basement membrane-podocyte interactions. By specialized glomerular endothelial structure (caveolae, tight junctions, endothelium glycocalyx) and by circulating permeability factors (albumin, orosomucoid, apolipoproteins, Amadori's products). Concluding remarks underline the significance to study the glomerular vascular endothelial dysfunction.

Glomerular albumin permeability as an in vitro model for characterizing the mechanism of focal glomerulosclerosis and predicting post-transplant recurrence

The putative mechanisms of proteinuria in idiopathic focal glomerulosclerosis and of its post-transplant recurrence are discussed. It is proposed that a balance between circulating factors with permeability activity on glomeruli and putative inhibitors play a key role. The characterization of inductors is currently in progress; most inhibitors appear to be apolipoproteins (mainly apoJ and apo E) but we cannot exclude other substances. The goal is now to evaluate the concentration of both inducers and inhibitors of glomerular permeability in vivo. Permeability activity in plasma of patients with FSGS with and without recurrence of the disease may be evaluated by an in vitro functional essay with isolated glomeruli. Published data on permeability activity evaluated with this method in different proteinuric states gave, however, controversial results and this test cannot be readily considered of clear clinical utility. Only the definitive characterization and quantification in vivo of the different molecules that play a role in FSGS may furnish adequate answer.

Pathophysiology of proteinuria

Proteinuria is consequence of two mechanisms: the abnormal transglomerular passage of proteins due to increased permeability of glomerular capillary wall and their subsequent impaired reabsorption by the epithelial cells of the proximal tubuli. In the various glomerular diseases, the severity of disruption of the structural integrity of the glomerular capillary wall correlates with the area of the glomerular barrier being permeated by "large" pores, permitting the passage in the tubular lumen of high-molecular-weight (HMW) proteins, to which the barrier is normally impermeable. The increased load of such proteins in the tubular lumen leads to the saturation of the reabsorptive mechanism by the tubular cells, and, in the most severe or chronic conditions, to their toxic damage, that favors the increased urinary excretion of all proteins, including low-molecular-weight (LMW) proteins, which are completely reabsorbed in physiologic conditions. Recent clinical studies showed that in patients with glomerular diseases the urinary excretion of some HMW proteins [immunoglobulins G and M (IgG and IgM)] and of some LMW proteins, alpha1-microglobulin, beta2-microglobulin, correlates with the severity of the histologic lesions, and may predict, better than the quantity of proteinuria, the natural course, the outcome, and the response to treatment. It is suggested that some patients have already, at the time of clinical presentation, a structural damage of the glomerular capillary wall (injury of podocytes) and of the tubulointerstitium, the severity and scarce reversibility of which are reliably indicated by an elevated urinary excretion of HMW and LMW proteins.

Characterization of plasma factors that alter the permeability to albumin within isolated glomeruli

Focal segmental glomerulosclerosis (FSGS) is responsible for intractable proteinuria and has become the leading cause of renal insufficiency in children. Protenuria in FSGS is probably due to the effect of one or more permeability plasma factors which increase the glomerular permeability to proteins. We fractioned serum from children with FSGS using two mixed chromatographic-electrophoretic approaches and have purified ten proteins among several hundreds which maintained the original permeability activity after renaturation, utilizing an isolated rat glomeruli assay. Six proteins were successfully characterized by mass spectometry as fibulin, apolipoprotein J, vitronectin, albumin isoforms, gamma chain fibrinogen and mannan-binding lectin-associated serine protease. Both procedures utilized for purification were based on affinity chromatography with Protein A-Sepharose and ended with two-dimensional electrophoresis, whereas the intermediate steps were different. Cross inhibition with zinc and aprotinin of purified factors and whole FSGS serum indicate strong homology. These are the first data demonstrating permeability activity for serum proteins, an observation with important implications in pathogenesis of proteinuria. Determination of the serum levels of each protein and a careful differentiation of FSGS from normal serum could provide the basis for clarifying the mechanism of proteinuria.

Proteinuria selectivity index based upon alpha 2-macroglobulin or IgM is superior to the IgG based index in differentiating glomerular diseases. Technical note

BACKGROUND

The proteinuria selectivity index (SI) may be used to describe changes of the glomerular permeability for macromolecules in glomerular diseases. Proteins the size of alpha 2-macroglobulin (alpha 2 M) or IgM cannot normally pass the glomerular barrier, whereas IgG can pass through the large pores of glomerular basement membrane. Comparison of the clearance of the three high-molecular-weight proteins to that of albumin may be useful in characterization and diagnosis of different glomerular diseases as well as in understanding of the permeability characteristics of the glomerular filter.

METHODS

Three types of SI, each calculated as a ratio of clearance of either IgG, alpha 2M or IgM to that of albumin, were measured in 199 proteinuric patients. The patients were subdivided into eight different biopsy-verified glomerular diseases.

RESULTS

Two diagnoses could be clearly distinguished using SI based on alpha 2M (alpha 2 M SI) or IgM (IgM SI). Both alpha 2M SI and IgM SI were significantly lower in minimal change nephropathy and higher in crescentic necrotizing glomerulonephritis than in all the other diagnoses. The SI based on IgG (IgG SI) was less useful in determining specific diagnoses, since patients with minimal change nephropathy could not be distinguished from those with other types of primary glomerulonephritis and patients with crescentic necrotizing glomerulonephritis did not differ from those with diabetic nephropathy.

CONCLUSIONS

The findings of this study indicate that alpha 2M SI and IgM SI are superior to IgG SI in characterization of glomerular disorders and might replace the IgG SI for this purpose.

Clinical analysis of human urinary proteins using high resolution electrophoretic methods

The application of isoelectric focusing (IEF), sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), two-dimensional electrophoresis (2-DE) and capillary electrophoresis (CE) for high resolution electrophoretic analysis of human urinary proteins is reviewed. In each case, the information is tabulated chronologically with details of sample preparation, electrophoretic system, detection method and clinical application. The text includes an historical perspective of the use of each method for urinalysis and a detailed review of the application of the methods to the investigation of renal disease, renal transplantation, Bence Jones proteinuria (BJP), diabetes mellitus, cadmium toxicity, nephrolithiasis and cancers of the urogenital tract.

Characterization of proteinuria in primary glomerulonephritides: urinary polymers of albumin

In 142 patients with primary glomerulonephritis (GN), there were polymers of albumin (PAs) in the urine samples of 87% of 15 minimal-change disease (MCD) patients, 52% of 27 focal segmental glomerulosclerosis (FSGS) patients, 51% of 47 membranous glomerulonephritis (MGN) patients, 55% of 20 membranoproliferative glomerulonephritis (MPGN) patients, and 9% of 33 immunoglobulin A nephropathy (IgAN) patients (P = 0.000). In IgAN, only three patients with nephrotic syndrome were PA positive. The PAs were significantly correlated with nephrotic syndrome (NS) (P = 0.000) and with the degree of proteinuria, ranging from 8% in patients with proteinuria less than 0.5 g/d to 58% in patients with proteinuria > or = 15.0 g/d (P = 0.001), but 40% of the nephrotic syndrome patients were PA-negative despite values of proteinuria comparable to those of PA-positive patients, suggesting that the presence of PAs is not simply related to protein loss, but probably to some other unidentified factor or lesion. For 72 patients (43 with NS) (22 FSGS, 36 MGN, and 14 MPGN patients) with normal renal function at entry (serum creatinine, 1.02 +/- 0.23 mg/dL) and a mean follow-up duration of 52 +/- 27 months, for whom PAs were determined and urinary protein characterized by sodium-dodecyl-sulphate polyacrylamide gel electrophoresis (SDS-PAGE) at the beginning of the follow-up period, the functional outcome was correlated with the patterns of proteinuria. Chronic renal failure (CRF) developed in 24% of all 72 patients, in 36% of the PA-positive patients, in 9% of the PA-negative patients (P = 0.007), in 44% of the SDS-PAGE 10-kd mixed glomerulotubular pattern patients, and in 17% of the SDS-PAGE 23-kd mixed-pattern patients (P = 0.001). The association of PAs with the 10-kd pattern enhanced the predictive value for CRF outcome: CRF developed in 62% of the PA-positive patients with the 10-kd pattern compared with 11% of the PA-negative patients with the 23-kd pattern (P = 0.0001). CRF developed in 32% of 43 patients with the nephrotic syndrome, in 48% of the PA-positive patients, and in 11% of the PA-negative patients (P = 0.037); in 50% of the 10-kd patients and in 24% of the 23-kd patients (P = 0.007); and in 70% of the PA-positive patients with the 10-kd pattern and 14% of the PA-negative patients with the 23-kd pattern (P = 0.001). In a retrospective study of 21 treated patients (11 FSGS, nine MGN, and one MPGN patient), a response to therapy with complete or partial remission was observed in 57% of all 21 patients; in 58% of patients with the nephrotic syndrome; in 88% of the PA-negative patients versus 38% of the PA-positive patients (P = 0.027); in 90% of the 23-kd patients versus 27% of the 10-kd patients (P = 0.004); and in 100% of the PA-negative patients with the 23-kd pattern versus 12% of the PA-positive patients with the 10-kd pattern (P = 0.001). In conclusion, urinary PAs are associated with GN characterized by lesions mainly localized in the glomerular capillary wall, with the presence of the nephrotic syndrome, and with the degree of proteinuria. In patients with FSGS, MGN, MPGN, and normal renal function at entry, the presence of polymers has a predictive value for CRF outcome; this value is enhanced by the contemporaneous presence of an SDS-PAGE proteinuric pattern with low molecular weight proteins up to 10-kd, which is known to be associated with diffuse tubulointerstitial lesions. Therefore, the best predictive value for either CRF outcome or for response to therapy was provided by a combination between a marker associated with the degree of proteinuria and the types of GN characterized by lesions mainly localized in the glomerular capillary wall and a marker associated with tubulointerstitial damage (SDS-PAGE mixed glomerulotubular pattern with low molecular weight proteins between 20 and 10 kd).

Effect of glycation of albumin on its binding to renal brush-border membrane vesicles: influence of aging in rats

Aging is associated with the loss of preferential urinary excretion of Amadori-product glycated albumin. We have measured the binding of 125I-labeled glycated albumin to the renal brush-border membrane vesicles from young and old rats to determine whether a specific receptor-mediated endocytosis system may be involved. 125I-Glycated albumin was specifically bound by renal brush-border membrane vesicles in a time- and temperature-dependent manner; the binding was concentration-dependent, saturable and reversible. Scatchard plots gave an apparent dissociation constant Km of 488 +/- 17 nM, and a number of binding sites N of 33.5 +/- 3.4 pmol/mg protein/min in membrane vesicles from young (3 months old) rats; the binding of native [125I]albumin, gave a Km of 1194 +/- 200 nM (P < 2%) and N of 82.4 +/- 16.3 pmol/mg protein/min (P < 3%). Vesicles from 10-month-old rats had a similar Km (619.6 +/- 135.3 nM) and N (21.91 +/- 2.98 pmol/mg protein/min), while those from older (30 months old) rats had significantly increased Km (1344 +/- 237 nM, P < 3%) and N (81.3 +/- 10.9 pmol/mg protein/min, P < 1%) for 125I-glycated albumin binding. 125I-Glycated HSA was not displaced by unlabeled native HSA in less than 100-fold excess and native [125I]HSA was only displaced by a 10-fold excess of unlabeled glycated HSA. The binding of native [125I]HSA was partly inhibited (85%) by unlabeled glycated HSA. Thus, there appear to be two different binding sites, one for glycated and the other for native albumin, lying close together; and the glycation site on albumin is the discriminatory recognition factor.

Glycation of albumin with aging and diabetes in rats: changes in its renal handling

Albumin glycation was investigated in old rats to elucidate the link between the preferential excretion of glycated albumin and age-related microalbuminuria. Postprandial blood glucose and the glycated albumin in the serum and urine of 3-, 10- and 30-month-old Wistar rats and in streptozotocin diabetic rats were determined. Blood glucose increased from 1.46 +/- 0.046 g l-1 in 3-month-old rats to 2.08 +/- 0.06 (10 months) and 1.75 +/- 0.23 (30 months) (P < 0.05). Albumin glycation level in the serum increased from 0.79 +/- 0.07 nmol HCHO/nmol albumin (3 months) to 1.41 +/- 0.14 (10 months) and 1.73 +/- 0.21 (30 months) (P < 0.05); urinary level increased from 1.63 +/- 0.39 nmol HCHO/nmol albumin (3 months) to 2.92 +/- 0.57 (10 months) and 2.39 +/- 0.36 (30 months) (P < 0.01). The percent glycated albumin in serum rose from 3.33 +/- 0.64 to 6.81 +/- 0.63 and 6.99 +/- 1.79% of total albumin (P < 0.05), whereas the urine percentage decreased from 12.81 +/- 3.97 to 12.64 +/- 2.87 and 2.63 +/- 0.97% (P < 0.05) in 3-, 10- and 30-month-old rats, respectively. Editing decreased with aging from 4.28 +/- 0.83 (3 months) to 1.84 +/- 0.32 (10 months) and 0.52 +/- 0.14 (30 months) (P < 0.01). Editing in microproteinuric diabetic rats was lower (0.95 +/- 0.08) than in 3-month-old control rats (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between cationic dyes and erythrocyte membranes in minimal change nephropathy: an electrophoretic approach

A study was undertaken to clarify the usefulness of two cationic dyes, alcian blue (AB) and ruthenium red (RR) in demonstrating the defect in cellular membranes noted in minimal change nephropathy (MCN). The binding of both dyes to RBC membranes purified from normal and nephrotic children was evaluated by electrophoretic titration curves. When examined separately, AB was found to precipitate spontaneously, producing macro-aggregates with no electrophoretic mobility at pH 5. This was presumed to be the result of hydrophobic interaction of the dye with itself. The same phenomenon was observed when this dye was incubated at 37 degrees C with RBC ghost's from normal children, when AB presented a sigmoidal curve with a net positive charge for pHs higher than 5.5 and lower than 5 and no electrophoretic mobility at pH 5. However, incubation of AB with RBC ghosts from children with MCN resulted in an improvement of the solubility of the dye which then migrated with a net positive charge along the whole gradient of pH from 3.5 to 9. The presence of zwitterionic neutral detergents such as CHAPS, but not of a charged substance such as protamine sulphate, inhibited precipitation at pH 5 when incubated with membranes from normal children, supporting the hydrophobic nature of the phenomenon. When RR was used instead of AB, it was fully protonated (i.e. did not precipitate) when analysed alone, but when incubated with normal RBC ghosts, it also revealed no electrophoretic mobility at pH 5.(ABSTRACT TRUNCATED AT 250 WORDS)

Proteinuria: changes and mechanisms in toxic nephropathies

During the last few decades, considerable progress has been made in the understanding of the pathophysiological mechanisms of proteinuria. A great variety of hemodynamic or biochemical mechanisms acting at different sites of the nephron have been shown to alter the renal handling and the urinary excretion of proteins. The finding which perhaps has had most practical implications is that the pattern of protein excretion quantitatively and qualitatively varies with the site and severity of renal damage. This has led to the development of a large array of methods for the identification and quantitation of specific urinary proteins. These methods have been most extensively used by toxicologists in experimental, epidemiological, or clinical studies on potentially nephrotoxic chemicals (e.g., drugs, heavy metals, solvents, etc.). The present review summarizes the current state of knowledge on the mechanisms of proteinuria and the use of urinary proteins as indicators of nephrotoxicity.

Preclinical detection of nephrotoxicity: description of the tests and appraisal of their health significance

The prevention of renal diseases induced by chemicals requires the use of tests with which adverse renal effects can be detected at a stage when removal from exposure to the offending agent(s) may lead to complete recovery or may at least prevent an accelerated decline in renal function. The screening tests used in clinical medicine for assessing the functional integrity of the kidney are not sufficiently sensitive to attain this objective. In this review, we describe the tests currently available for detecting incipient renal damage and attempt to assess their health significance. A minimal battery of tests is recommended for the detection of groups in industry or in the general population who are at risk. If an increased prevalence of abnormal values for these parameters is found repeatedly in a population, efforts should be made to identify the causal agent(s) and, if possible, to reduce the exposure. Follow-up examinations of subjects identified in this way are also indicated, in order to define the predictive value of the detected changes better.

Peroxidative damage of the erythrocyte membrane in children with nephrotic syndrome

The structural composition of erythrocyte ghosts was analysed in children affected by steroid-responsive (SRNS) and unresponsive nephrotic syndrome (SUNS). No variation of either intrinsic or extrinsic ghost proteins was found by discontinuous SDS-electrophoresis associated with a very sensitive double staining technique. By contrast, the composition of inner-layer phospholipids--phosphatidyl ethanolamine (PE) and phosphatidyl serine (PS)--was altered in SRNS with minor changes also involving phosphatidic acid, phosphatidyl inositol and lysophosphatidyl choline. Signs of peroxidative damage were present in both SRNS and SUNS ghosts and inside the cells; these included high levels of fluorescent amino-iminopropene derivates of PE and PS, increased intraerythrocytic amounts of malonyldialdehyde and decreased levels of reduced glutathione. Taken together these results support the concept that in SRNS and SUNS erythrocytes are target cells for peroxidative damage. In SRNS peroxidation of membrane lipids results in a marked alteration of the phospholipid composition of erythrocyte ghosts.

Effect of glycation of albumin on its renal clearance in normal and diabetic rats

Two independent techniques have been used to study the renal clearances of nonenzymatically glycated albumin and nonglycated albumin in normal and streptozotocin-induced diabetic rats, 16 to 24 weeks after the onset of diabetes. In the first technique, serum and urinary endogenous glycated and nonglycated albumin were separated using m-aminophenylboronate affinity chromatography and subsequently quantified by radioimmunoassay. Endogenous glycated albumin was cleared approximately twofold faster than nonglycated albumin in normal and diabetic rats. However, no difference was observed in the glycated albumin/nonglycated albumin clearance ratios (Cga/Calb) in normal and diabetic rats, respectively (2.18 +/- 0.39 vs 1.83 +/- 0.22, P greater than 0.05). The second technique measured the renal clearance of injected 125I-labelled glycated albumin and 125I-labelled albumin. The endogenous results were supported by the finding that 125I-labelled glycated albumin was cleared more rapidly than 125I-labelled albumin in normal (P less than 0.01) and diabetic (P less than 0.05) rats. The Cga/Calb ratio calculated for the radiolabelled albumins was 1.4 and 2.0 in normal and diabetic rats, respectively. This evidence suggests that nonenzymatic glycation of albumin increases its renal clearance to a similar degree in normal and diabetic rats.