PURITY OF RADIOIODIDE-I131 ASSESSED BY IN VIVO AND IN VITRO METHODS

Abstract

Appreciable amounts of non-iodide radioactivity was found in I131 preparations supplied without cysteine preservative. Commercial I131 preparations treated with cysteine were relatively free of impurities. Incubation of rat thyroid homogenates with tyrosine, cupric chloride, and impure I131 resulted in the formation of monoiodotyrosine-I131. Iodination which occurred under these conditions was largely non-enzymatic. However, when cysteine-treated I131 was employed, the expected enzymatic formation of monoiodotyrosine-I131 took place. The thyroidal uptake of I131 after administration I131 treated with cysteine was consistently higher than that found when impure I131 was employed. Conversion of non-iodide radioactivity in commercial samples of I131 to iodide-I131 was accomplished by stable iodide and by cysteine, but not by incubation at pH 2.