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Studies on the Metabolism of Radioactive Albumin in Tumor-bearing Rats^*

( Departments of Biochemistry and Pharmacology, Baylor University College of Medicine, Houston, Texas)

In experiments designed to test the possibility that albumin enters the cells of the Walker 256 carcinosarcoma as the whole molecule, rather than as free amino acids or peptides, albumin was labeled with each of a variety of C¹⁴-labeled amino acids. The specific activity of the proteins of the whole homogenate was found to be higher than that of the other tissues studied and was found to be the same regardless of the amino acid employed as a tracer. Evidence for differences in the intratissue metabolism of labeled albumin and labeled amino acids emerged from studies on the intracellular distribution of the label. In the tumor, the specific activities of the proteins of the cytoplasmic sap were found to be 2–3 times those of the mitochondria and microsomes, following the injection of albumin labeled with tritiated lysine. The specific activity of the histones was only half that of the proteins of the microsomes and mitochondria. These data were, contrasted with the results obtained following the injection of labeled lysine into tumor-bearing rats. The N-terminal sequences of rat and human serum albumin were investigated. The data obtained support the concept that albumin is absorbed into the neoplastic cell as a whole molecule.

^* These studies were supported in part by grants from the American Cancer Society, the U.S. Public Health Service, and the Jane Coffin Childs Fund.

Received 9/23/60.