Renal Excretion

The kidneys play an important role in the clearance of proteins and amino acids. Many proteins with molecular weight less than 30 kDa are filtered by the glomerulus and excreted (11). Peptides and proteins less than 5 kDa are filtered efficiently, and their glomerular rate equals the glomerular filtration rate observed in humans (120 mL/min). As the molecular weight of proteins increases (>30 kDa), the capacity for glomerular filtration decreases. Besides molecular weight, charge and size of proteins are also important for glomerular filtration (12). After glomerular filtration, peptides can be excreted unchanged in the urine or degraded to the products that are excreted in the urine (9). Poly-peptides and proteins can also be actively reabsorbed by the proximal tubules through a process known as luminal endocytosis and then hydrolyzed by the digestive enzymes in the lysosomes to peptide fragments and amino acids (8). The amino acids are then reabsorbed by a carrier-mediated, energy-dependent transport mechanism. The net result is that only a small fraction of intact protein is eliminated unchanged in the urine (9).

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