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.

Immunochemical evaluation of radioiodinated protein A.

The [125I]iodoprotein A iodinated by the chloramine-T method using carrier-free 125I- to various specific activities (0.02-2.0 125I/mole protein A) displayed 85% binding activity to excess solid-phase rabbit IgG. On storage, however, the solid-phase IgG binding activity of the high specific activity [125I]iodoprotein A (approximately 2 125I/protein A) decreased rapidly while that of the low specific activity [125I]iodoprotein A (approximately 0.2 125I/protein A) remained essentially unchanged. Analysis of aged high specific activity [125I]iodoprotein A by thin-layer chromatography revealed that free 125I- was released from the [125I]iodoprotein A during storage. Removal of the released 125I- was accomplished by using anion exchange resin. High specific activity [125I]iodoprotein A was found to be more sensitive than the low specific activity [125I]iodoprotein A in quantitating aggregated IgG while the latter was as useful as the former in hybridoma screening. When protein A was labeled with the Bolton-Hunter reagent, the [125I]iodoprotein A had an IgG binding property and stability similar to the [125I]iodoprotein A labeled with the chloramine-T method. Since the chloramine-T method can yield high specific activity [125I]iodoprotein A and repurification of the aged preparation can easily be achieved by using anion exchange resin, we concluded that chloramine-T is more efficient than Bolton-Hunter reagent and that both methods yield materials with comparable immunochemical properties.