| The objective of this dissertation was to develop and apply a radioactive label which could be used to investigate the sites and mechanism involved in serum protein catabolism. An inert, indigestible label, {lcub}('3)H{rcub}-Raffinose ({lcub}('3)H{rcub}-RAF), was investigated as a tag which accumulates and remains at the site of catabolism of proteins with the long circulating half-lives characteristic of serum proteins. In a model system using proteins rapidly cleared into different liver cell populations, it was established that, in contrast to the ('125)I label, the {lcub}('3)H{rcub}-RAF tag remained for several days in the appropriate liver cell population. In addition, the kinetics of clearance of ('125)I and {lcub}('3)H{rcub}-RAF-labeled proteins were identical for proteins with long or rapid circulating half-lives, demonstrating that the {lcub}('3)H{rcub}-RAF label "goes along for the ride" without influencing catabolism of the labeled protein.; The rat immunoglobulin subclass IgG(,2a) was labeled with {lcub}('3)H{rcub}-RAF and the clearance and sites of catabolism were investigated. IgG(,2a) from a monoclonal (immunocytoma) and a polyclonal (natural) source behaved identically. The kinetics of clearance of both ('125)I and {lcub}('3)H{rcub}-RAF-labeled IgG(,2a) were indistinguishable. IgG(,2a) was catabolized in peripheral tissues, muscle and hide, and in organs of the reticulo-endothelial system (RES). The results imply a major role for the RES in the catabolism of immunoglobulin. Following removal of sialic acid, only minimal ((LESSTHEQ) 10%) alteration in the kinetics of clearance was observed, establishing that sialic acid and galactose have little or no influence on immunoglobulin catabolism. Future investigations to probe the mechanisms of clearance and specific cell types in peripheral tissues responsible for IgG catabolism are discussed. |
