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.