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      Nephrotic-Like Proteinuria in Experimental Diabetes

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          Aims/Hypothesis: Streptozotocin (STZ) diabetic rats are characterized by the development of albuminuria. It is not known, however, whether the excess excretion of protein is primarily due to intact protein or protein fragments or whether it is specific for albumin or occurs for all high-molecular-weight plasma proteins. To test this we have measured the excretion rates and fractional clearances of [<sup>14</sup>C]albumin, [<sup>3</sup>H]immunoglobulin G and [<sup>3</sup>H]transferrin in diabetic rats. Methods: The radiolabeled proteins were delivered to the circulation of conscious diabetic (STZ induced for 6 weeks) and control rats by ALZET osmotic pumps. The plasma level of the radiolabeled proteins reached steady-state levels by day 7. Urine and plasma samples from day 7 were used to determine the excretion rates of the proteins by radioactivity and radioimmunoassay. Results: When excretion rates were determined by radioactivity it was apparent that only the albumin excretion rate increased significantly with STZ diabetes to a value of 354 ± 166 µg/min which agrees with proteinuria determined by Biuret assay of 299.9 ± 52.4 µg/min. The major proportion of protein being excreted was in the form of protein fragments which are not detected by conventional immmunochemical assays. Conclusion: The previously unrecognized nephrotic-like levels of proteinuria in experimental diabetes appears to be associated with an albumin-specific mechanism responsible for the increase in albumin peptides in urine. There was significant lowering of plasma albumin concentration but plasma concentrations of transferrin and immunoglobulin G remained unchanged. There was also no significant appearance of intact protein in urine that is normally found in nephrotic states.

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          Renal handling of albumin: a critical review of basic concepts and perspective.

          Biochemical and physiological processes that underlie the mechanism of albuminuria are completely reassessed in this article in view of recent discoveries that filtered proteins undergo rapid degradation during renal passage and the resulting excreted peptide fragments are not detected by conventional urine protein assays. This means that filtered protein and/or albumin levels in urine have been seriously underestimated. The concept that albuminuria is a result of changes in glomerular permeability is questioned in light of these findings and also in terms of a critical examination of charge selectivity, shunts, or large-pore formation and hemodynamic effects. The glomerulus appears to function merely in terms of size selectivity alone, and for albumin, this does not change significantly in disease states. Intensive albumin processing by a living kidney occurs through cellular processes distal to the glomerular basement membrane. Failure of this cellular processing primarily leads to albuminuria. This review brings together recent data about urinary albumin clearance and current knowledge of receptors known to process albumin in both health and disease states. We conclude with a discussion of topical and controversial issues associated with the proposed new understanding of renal handling of albumin. Copyright 2002 by the National Kidney Foundation, Inc.
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            Ramipril and aminoguanidine restore renal lysosomal processing in streptozotocin diabetic rats

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              Production and characterization of [14C]protein A, a long-lived immunological reagent

               Ludeman Eng (1985)
              A procedure for the production of [14C]protein A is described which involves reductive methylation of lysine residues with [14C]formaldehyde and NaCNBH3. The binding of [14C]protein A to IgG is apparently unaltered, as determined by competitive binding studies. The use of [14C]protein A may be preferred to that of 125I-protein A when a radioactive label with a long half-life is desirable.

                Author and article information

                Am J Nephrol
                American Journal of Nephrology
                S. Karger AG
                October 2002
                23 October 2002
                : 23
                : 1
                : 38-46
                aDepartment of Biochemistry and Molecular Biology, Monash University, Clayton and bEndocrine Unit, Austin and Repatriation Medical Center, Heidelberg, Vic., Australia
                66297 Am J Nephrol 2003;23:38–46
                © 2003 S. Karger AG, Basel

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                Page count
                Figures: 5, Tables: 5, References: 38, Pages: 9
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                Original Article: Basic Sciences


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