Typification of proteinuria
To determine the type of proteinuria, it is necessary to know the spectrum of proteins excreted in the urine. Electrophoretic methods are used for this purpose. Electrophoretic partitioning of urinary proteins according to their molecular weight allows semiquantitative evaluation of individual diagnostically significant proteins and classification of proteinuria. Electrophoresis in agarose or polyacrylamide gel has gradually become the method of choice for urinary protein analysis.
In order to separate the proteins by size (rather than charge), a polyacrylamide gel can be used, the density of which increases from the cathode to the anode (i.e., the mesh or pore size in the gel gradually decreases). Small molecules in such a gel travel further than large molecules.
Another, more commonly used option, is to treat the sample with the detergent sodium lauryl sulfate (sodium dodecyl sulfate – SDS), which "surrounds" the protein and replaces its own charge with its negative charge. The resulting complexes have approximately the same charge (more precisely: they have the same surface charge density). If electrophoresis is then carried out in a relatively dense gel, it is separated according to relative molecular weight: smaller molecules travel through the gel faster than large ones (molecular sieve technique). It moves the fastest β2-microglobulin, albumin lies about midway along the partition pathway; between the start and albumin are proteins with Mr greater than 70 000.
Urinary protein electrophoresis evaluation[edit | edit source]
In glomerular proteinuria we find proteins in the electrophoreogram between start and albumin (i.e. Mr > 70 000).
Proteins observed in glomerular proteinuria Mr Albumin 68 000 selective non-selective Transferrin 77 000 selective non-selective IgG 150 000 non-selective IgA 160 000 non-selective Haptoglobins 85 000–1 000 000 non-selective
Tubular proteinuria are characterized by the presence of protein between albumin and the anodic end of the electrophoreogram (i.e. Mr < 70 000).
Proteins observed in tubular proteinuria Mr β2-microglobulin 11 800 Lysozyme 15 000 Retinol binding protein (RBP) 21 000 Ig free light chains 25 000 α1-microglobulin 33 000 Dimeric Ig free light chains 50 000 Albumin 68 000
Proteins observed in tubular proteinuria
Presence of α2-macroglobulin (Mr = 800 000) with other findings is similar to mixed proteinuria suggest postrenal proteinuria.
