Urinary excretion of glycated albumin in insulin-dependent diabetic patients with normal urinary albumin excretion

Clinical, pathophysiological and structure/function consequences of modification of albumin by Amadori-glucose adducts

BACKGROUND

The nonenzymatic condensation of glucose with albumin results in the formation of albumin modified by Amadori glucose adducts, the principal form in which glycated albumin exists in vivo.

SCOPE OF REVIEW

This review focuses on (a) the utility of measurement of Amadori-modified glycated albumin (AGA) as a biomarker in diabetes, where elevated levels attend the hyperglycemic state; (b) the role of AGA as a causal factor in the pathogenesis of complications of diabetes; (c) effects on transport properties; and (d) structural and functional consequences of the modification of albumin by Amadori glucose adducts. It does not discuss counterparts with respect to Advanced Glycation Endproducts (AGE), which may be found in other publications.

MAJOR CONCLUSIONS

Nonenzymatic glycation of albumin, which is increased in diabetes, has clinical relevance and pathophysiologic importance, with ramifications for the management of this disease, the development of its complications, and the transport of endogenous and exogenous ligands.

GENERAL SIGNIFICANCE

Appreciation of the manifold consequences of AGA has afforded new avenues for assessing clinical management of diabetes, awareness of the impact of nonenzymatic glycation on albumin biology, insights into the pathogenesis of vascular complications of diabetes, and avenues of investigation of and intervention strategies for these complications. This article is part of a Special Issue on albumin. This article is part of a Special Issue entitled Serum Albumin.

Advanced glycation endproducts stimulate renal epithelial cells to release chemokines that recruit macrophages, leading to renal fibrosis

Diabetic nephropathy is a major complication of diabetes and tubulointerstitial fibrosis is one of its manifestations. This study aimed to clarify the pathogenicity of advanced glycation endproducts (AGEs) toward NRK-52E, a tubular epithelial cell line. The AGE-exposed cells significantly increased gene expression of transforming growth factor beta, plasminogen activator inhibitor-1, and tissue transglutaminase, and a medium conditioned by them showed strong potential to recruit macrophages, partly through a chemokine, monocyte chemoattractant protein-1. Albumin denatured by maintenance at 37 °C for 120 d exhibited similar activities, but they were lower than those of the AGEs. Thus, AGEs generated in diabetic patients might exacerbate fibrosis in the kidneys directly through renal epithelial cell stimulation, and indirectly by recruitment of macrophages.

Letter to the Editor: “Urine Glycosyl Albumin Formation: Relation to Severe Nephropathy in the Poor Control Diabetes Mellitus Patients?”

Diabetes mellitus (DM) is a frequent disorder affecting individuals of all ages. Urine glycosyl albumin is an important finding in the patients with severe diabetic nephropathy. In addition, accumulation of glycated albumin in end-stage renal failure is noted. Here, the author performed a reappraisal on the bonding energy based on quantum chemical analysis. The author calculated the bonding energy of the reaction and found that the reaction is a type of "energy providing reaction." The author hereby proposed that the nature of energy providing reaction in formation of glycosyl albumin can be an insult to the surrounding renal tissues in poorly controlled DM patients. Giving out the energy from the nearby cellular compartment during formation of glycosyl albumin might be an important pathological process leading to severe diabetic nephropathy and renal failure.

Post-translational non-enzymatic modification of proteins. II. Separation of selected protein species after glycation and other carbonyl-mediated modifications

There are two strategies applicable to revealing non-enzymatic post-translational modifications of proteins; while assaying of the hydrolytically stable adducts was the subject of our previous communication [1], here we attempted to review separation technologies for the unfragmented modified proteins. There are a few standard procedures used for this purpose, namely Laemmli gel electrophoresis, different modes of gel permeation chromatography and boronate affinity chromatography. The latter approach makes use of the vicinal hydroxy groups present in glycated proteins. Some (but not all) arising adducts exhibit typical fluorescence which can be exploited for detection. In most cases fluorescence is measured at 370/440 nm for the so-called advanced glycation products or at 335/385 nm for the only so far well characterized glycation marker (pentosidine). Some indication exists that, e.g., synchronous fluorescence detection will probably in the future add to the selectivity and allow the distinction of the different adducts arising during non-enzymatic post-translational modifications (glycation). The proteins reviewed are serum albumin, collagen and lens proteins while glycation of hemoglobin is the subject of another review within the present volume.

Damage of charge-dependent renal tubular reabsorption causes diabetic microproteinuria

More negatively-charged proteins are harder to pass through the glomerular charge barrier (GCB) and to be reabsorbed by renal tubules. Although the glycation of albumin increases its negative charge compared to non-glycated albumin, the glycation of transferrin does not change its charge. This difference enabled us to examine the charge-dependent renal function in diabetic proteinuria. The percentage of urinary glycated transferrin (serum %G-transferrin) positively correlated with serum fructosamine concentrations and the percentage of serum glycated albumin (serum %G-albumin) in all subjects. Urinary concentrations of transferrin and beta 2-microglobulin strongly correlated in diabetic patients with microproteinuria, while no significant correlation was observed in subjects with diabetic macroproteinuria or non-diabetic proteinuria. Urine/serum (U/S) ratio of %G-albumin in the patients with diabetic proteinuria was significantly lower than that in subjects with non-diabetic proteinuria, while no difference of the U/S ratio of %G-transferrin was observed between any groups. Furthermore, U-%G-transferrin/U-%G-albumin ratio was highest in the diabetic patients with microproteinuria. These results lead to the conclusion that the initial damage in diabetic kidney causing microproteinuria starts with the dysfunction of charge-dependent tubular reabsorption prior to a loss of GCB.

Diabetic damage of selective renal reabsorption assessed by albumin negative charge

To clarify the pathogenesis of diabetic albuminuria at its onset, the percentage of glycated albumin (%G-albumin) in excreted urinary albumin and its negative charge were assayed. In non-diabetic albuminuria (control-A) group, the high ratio of urinary %G-albumin to serum %G-albumin (urine/serum ratio of %G-albumin) suggested a selective renal excretion of glycated albumin (G-albumin) over albumin as a result of normally-functioning selective reabsorption of albumin over G-albumin in renal tubules. Urinary %G-albumin and albumin negative charge indexing the degree of glycation, which was assayed by the binding capacity of positively-charged Alcian Blue (ABBC), thus negatively correlated with serum %G-albumin. In non-insulin dependent diabetic subjects with albuminuria (DM-A), however, urinary %G-albumin and ABBC positively correlated with serum %G-albumin, and the urine/serum ratio of %G-albumin was low and gradually increased toward 1 as serum %G-albumin increased. Although the strict glycemic control for 3 weeks reduced the increased urinary %G-albumin and ABBC, the decreased urine/serum ratio of %G-albumin remained unaltered. It is concluded that hyperglycemia-induced renal damage starts with the loss of selective tubular reabsorption of albumin over G-albumin, and that this damage cannot be recovered by strict glycemic control for 3 weeks.

Accumulation of glycated albumin in end-stage renal failure: evidence for the principle of "physiological microalbuminuria"

In light of the growing understanding of the toxic effects of glycated albumin and of the preferential excretion of this substance, the excretion of glycated albumin could be considered a physiologic function of the kidney. Furthermore, if the increased load of glycated albumin in diabetic patients results in glycated albumin excretion rates in the range of 20 to 200 microg/min, might this not be considered "physiologic microalbuminuria"? The hypothesis is presented that microalbuminuria composed of glycated albumin is a homeostatic renal function. Although some proteins are glycosylated for their normal physiologic function, many proteins are glycated nonenzymatically according to ambient blood glucose. Albumin is subject to nonenzymatic glycation in all humans, but at increased rates in diabetic patients. Glycated albumin induces changes in the microvasculature and glomerulus that may lead to endothelial dysfunction and diabetic nephropathy, respectively. Renal excretion of glycated albumin is enhanced compared with native albumin. To explore this potential homeostatic function of the kidney, patients with impaired renal function were studied to determine whether glycated albumin accumulates. Plasma levels of glycated albumin were determined in diabetic and nondiabetic patients on hemodialysis. Hemoglobin A1c was used as an index of the rate of nonenzymatic glycation of proteins. Hemoglobin A1c was increased in the diabetic subjects but was normal in the nondiabetic group (7.9% +/- 0.5% v 6.2% +/- 0.2%, respectively; P < 0.01). On the other hand, the glycated albumin was elevated in both groups and was not significantly different between them (1.95% +/- 0.15% in the diabetic patients v 1.75% +/- 0.14% in the nondiabetic patients; P = NS). The results of this study provide the first clinical evidence supporting the hypothesis that the excretion of glycated albumin is a homeostatic renal function.