Some aspects of plasma protein metabolism as compared with intracellular protein breakdown.

Many plasma proteins are cleared according to first-order kinetics. Half-lives of individual plasma proteins are widely different, just like those of intracellular proteins. Plasma proteins with molecular weights up to about 50000 are often mainly cleared by glomerular filtration, especially if the protein has a high isoelectric point. An other important clearance mechanism is endocytosis. Fluid endocytosis might contribute significantly to the catabolism of plasma proteins with long-lives like serum albumin. Adsorptive endocytosis is responsible for the rapid clearance of complexes of some plasma proteins with other molecules, like antigen-antibody complexes. Other plasma proteins like certain glycoproteins and lipoproteins are bound to be specific receptors on the surface of endocytosing cells without previous complex formation. Experiments with sucrose-containing labels suggest that certain tissues are specifically involved in endocytosis of some plasma proteins. If endocytosis and renal filtration constitute the main mechanisms for the clearance of plasma proteins, one cannot expect extensive homologies between this process and catabolism of intracellular proteins.