Preparation, purification, and stability of high specific activity ¹²⁵I - labeled thyronines

Abstract

A modified chloramine -T method is described for the preparation of several radioiodothyronines by an exchange reaction, which results in low specific activity preparations, or by an addition reaction, which yields radioiodothyronines with specific activities up to 7 mCi/μg. Purification by paper chromatography is shown to be more convenient than by LH - 20 chromatography and provides better resolution among the various thyronines. Radioiodothyronines with only a single iodine atom in the outer ring (3,5,3'-triiodothyronine and 3,3'- diiodothyronine) are stable for several months when stored in organic solvents. The least stable radioiodothyronines are those with two ¹²⁵I atoms in the outer ring (3,3',5'- triiodothyronine (rT₃)) produced from 3-iodothyronine and thyroxine (T₄) from 3,5-diiodothyronine). The stability of rT₃ and T₄ stored in human plasma at 4 C is much greater than when stored in buffer at the same pH. The use of high specific activity [¹²⁵I]rT₃ has permitted the development of a radioimmunoassay with a sensitivity of 1 pg rT₃/ml incubation volume.