Metabolism of Plasma Proteins

Evidence has been accumulating for the selective localization of radioactive plasma proteins in tumors. A number of clinical studies have shown that plasma proteins labeled with Il31 can be utilized for localization of hepatic metastases as well as primary cerebral neoplasms."'0"' Studies on localization of radioactive plasma proteins in animal tumors have been carried out by Babson and Winnick,' Bauer et al.2 Gavrilova,7 and Busch and Greene.8 The last studies8 indicated that the specific activity of tumor proteins increased at a significantly greater rate and to a greater extent than proteins of other tissues following injection of plasma biosynthetically labeled with radioactive amino acids, L-lysine or glycine-C14. Several questions arose from these studies which are germane to most of the studies carried out thus far."2 8 5' First, plasma was used as the tracer material and, accordingly, the role of the specific plasma proteins in labeling tumor proteins was not determined. Secondly, there was no assurance that the labeling of the tumor proteins was not due to a non-dialyzable constituent of plasma other than plasma proteins. Finally, the values for the specific activity of the proteins of the lung varied markedly from considerably lower to considerably greater than the corresponding values for the tumor; the possibility was suggested that the lung might be concentrating denatured proteins or protein aggregates. In the present studies, some of these questions were approached by fractionation of the radioactive plasma with the techniques of Cohn et al. 7; the uptake of isotope was then determined in intracellular particulate fractions prepared by differential centrifugation of homogenates of tissues. The most significant difference found between the Walker 256