Identification of the Sites of IgG Catabolism in the Rat.

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Rat immunoglobulins IgG2a and IgG1 were purified and labeled with [G-3H]raffinose, a radioactive tag which has been shown to accumulate in cells following protein degradation. After injection into rats, neither the immunoglobulins' circulating half-lives nor equilibration into extravascular spaces were affected by labeling with raffinose. The tissue distribution of total and acid-soluble radioactivity was measured for each IgG at times when 30 and 50% of injected dose were catabolized. In all experiments peripheral tissue, hide and muscle, accounted for the highest proportion (40–50%) of catabolized dose in the body. Liver, spleen, and other organs accounted for smaller amounts of total IgG catabolism. However, liver and spleen were more active in IgG catabolism, on a weight basis, than were peripheral tissues. The overall patterns of catabolism of IgG and albumin (J. W. Baynes and S. R. Thorpe (1981) Arch. Biochem. Biophys.206, 372–379) were remarkably similar. For both proteins catabolism appears to occur primarily at sites distributed diffusely throughout the body.