Further Investigations Relative to Studies with I 131 -Labeled Proteins,Insulin in Particular. ∗

Summary

An appreciable portion of inorganic I¹¹³ was found to be precipitated by TCA in the presence of red blood cells. This was not the case with serum or other tissues studied. In evaluating the degradation of I¹³¹-labeled proteins by measuring the TCA precipitability of the blood radioactivity, it should, therefore, be appreciated that the degradation product, inorganic I¹³¹, may show the phenomenon of “re-entry,” and be precipitated by TCA. Findings, somewhat similar to those with inorganic I¹³¹ were also obtained with I¹³¹-labeled diiodotyrosine.