Renal handling of proteins and peptides.

The kidney plays an important role in the metabolism of proteins and peptides. Current evidence indicates that only the proximal tubule possesses the mechanisms for absorption, transport and/or degradation of these substances. Large proteins and polypeptide molecules filtered by the glomerulus, are absorbed from proximal tubular fluid by luminal endocytosis into apical vacuoles which fuse with primary lysosomes where hydrolysis occurs followed by diffusion of metabolites out of the cells and into the blood. Recent evidence indicates that small peptides are handled by a different mechanism. It appears that small peptides are degraded at the luminal surface of the brush-border of proximal tubules, which contains many hydrolytic enzymes, by the process of membrane or contact digestion with reabsorption of the breakdown products. Proximal tubular mechanisms for handling of proteins and peptides are probably important biologically to conserve amino acids, inactive toxic substances and help regulate the circulating level of protein and peptide hormones.