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Rat serum albumin has been labeledw ith dilactitollZ5I- tyramine,( 12‘I-DLT) a radioactive tracer which remains entrappedw ithin lysosomes following cellular uptake and degradation of the carrier protein. Similar kinetics of clearance from the rat circulation were observed for albumin labeled conventionally with lZsI or 12‘I-DLT-albumin, both proteinhsa ving circulating half-lives of -2.2 days. In contrast, the recovery of whole body radioactivity had half-lives of -2.2 and 5.1 days, respectively, for the two protein preparations, indicating substantial retention of degradation products derived from catabolism of ”‘I-DLT-albumin. Measurement of total and acid-soluble radioactivity in tissues 2 or 4 days after injection of ”‘1-DLTalbumin revealed that skin and muscle accounted for the largest fraction( 50-60%)o f degradation products in the body. Fibroblaswtse re identified by autoradiography as the major cell type containing radioactive degradation products in skin and muscle. Fibroblasts were isolated from skin by collagenase digestion, followed by density gradient centrifugation. The amount of acid-soluble radioactivity recovered in these cells was in excellent agreement with that predicted based on acid precipitation of solubilized wholsek in preparations. These studies demonstrate for the first time that fibroblasts are a major cell type involved in the degradation of albuminin vivo.

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