Protein Fragments in Urine Have Been Considerably Underestimated by Various Protein Assays

It has recently been discovered that filtered proteins in the kidney are not excreted in their intact forms (1)(2). During renal passage, high-molecular mass proteins, including albumin, transferrin, and IgG, undergo degradation. The products of this degradation are then excreted in urine as heavily degraded fragments. In rats, ∼90% of the excreted albumin is heavily degraded to fragments with molecular masses <10 000 Da, and in humans this percentage is even higher (1)(2). The excretion of filtered protein in the form of protein fragments that are not detected by conventional protein assays has not been recognized in the clinical chemistry literature. Previous studies have shown that several techniques used routinely to measure urinary total protein and more specific techniques used to measure urinary albumin concentrations are insensitive to albumin fragments (3). Such techniques include immunochemical albumin assays, the benzethonium chloride method, the sulfosalicylate assay, the pyrogallol red assay, and the Coomassie blue assay for proteins (1)(3). This means that analysis of the total amount of a particular protein (intact plus fragments) excreted has never been accomplished and that total urinary protein/peptide excretion has been severely underestimated. The problem is particularly evident for normal and microalbuminuric urine: Whereas normal human excretion of intact albumin (as measured by RIA) is <25 mg/day, the excretion of accompanying derived fragments can be >1300 mg/day (1).