[Current concept of the mechanism of proteinuria].

Plasma proteins cross the glomerular basement membrane in inverse proportion to their molecular dimension; molecules larger than serum albumin are completely excluded from the glomerular filtrate. This filtration, which is purely passive, also depends of the electrical charge of the proteins; negatively charged proteins are indeed repelled by the negatively charged layer of sialogylcoproteins present along the outer surface of the epithelial cell membrane and extending to the glomerular basement membrane. The filtered proteins are selectively absorbed by the proximal tubular cells and are hydrolyzed by lysosomal enzymes. This results in a renal catabolism of proteins the importance of which depends on their ability to cross the glomerular barrier. In renal disease, proteinuria results either from an increased permeability of the glomerular basement membrane (selective or non-selective glomerular proteinuria) or from a diminished tubular reabsorption of normally filtered proteins of low molecular weight (tubular proteinuria).