Low Molecular Weight Proteinuria

Low molecular weight (LMW) proteinuria is defined in this article as the excretion in the urine of proteins of a size smaller than albumin [MW 69,000 ; molecular radius (MR) 37.A]. The LMW proteins may enter the urine either from the plasma by filtration through the normal glomerular basement membrane (GBM) or by secretion or loss from the nephron. However, most of the proteins secreted by the nephron that have been identified to date are of high molecular weight (HMW) e.g. Tamm Horsfall protein, secretory IgA. At present, therefore, the LMW proteins found in the urine seem to originate by the filtration of LMW plasma proteins at the glomerulus. The LMW proteins present normally include enzymes (lyso­ zyme, amylase), polypeptide hormones (insulin and growth hormone), polyclonal immunoglobulin fragments, fibrin-fibrinogen degradation prod­ ucts, and LMW proteins of unknown function «(12 and f32 microglobulin, post-y proteins). In some disease states LMW proteins not normally present in the plasma (myoglobin, hemoglobin, monoclonal immunoglobulin frag­ ments) may appear and may enter the urine, and thus provide us with valuable diagnostic markers.