Table I

Sources of urinary proteins

Note that epithelial cells include all epithelial cells along urinary tract starting from podocytes to urethral epithelia.

Sources of urinary proteinsComments
Soluble proteins
    Glomerular filtration of plasma proteinsNormally present (<150 mg/day).
Defects in glomerular filter increase high molecular weight protein (e.g. albumin) excretion.
Defects in proximal tubule reabsorption or abnormal production of low molecular weight plasma proteins increase low molecular weight protein (e.g. β2-microglobulin, immunoglobulin light chains, retinol-binding protein, and amino acids) excretion.
    Epithelial cell secretion of soluble proteinsVia exocytosis (e.g. epidermal growth factor) or glycosylphosphatidylinositol-anchored protein detachment (e.g. Tamm-Horsfall protein).
Solid phase components
    Epithelial cells
        Whole cell sheddingIncreased cell number compatible with several diseases including acute tubular necrosis (e.g. renal tubule cell shedding) and glomerular diseases (e.g. podocyte shedding).
        Plasma membrane and intracellular component sheddingCould be due to nonspecific, nephrotoxic, or apoptotic processes.
        Exosome secretionNormal process, see “Proteomics of Urinary Exosomes.”
    Other cellsIn certain diseases, red blood cells, white blood cells, or tumor cells (e.g. bladder cancer and lymphoma) can be present in urine.