Retention of biological activity following radioiodination of human interleukin 2: comparison with biosynthetically labeled growth factor in receptor binding assays.

Human interleukin 2 (IL-2) was radioiodinated by a modified form of the chloramine-T reaction. Comparison with biosynthetically radiolabeled IL-2 demonstrated that the iodinated molecule retained similar receptor binding characteristics and proliferation-inducing ability. The iodinated molecule also possessed the distinct advantages of a higher specific radioactivity and a reduced processing time for the assay samples. The majority of the iodine was incorporated at the tyrosine in position 45 of the polypeptide chain. Evidently, this residue is unimportant for the molecule's association with its receptor. The development of active radioiodinated IL-2 should facilitate routine measurement of the high-affinity IL-2 binding sites which mediate the physiological response to this lymphokine.