Preparation and immunological characteristics of biologically active radioiodinated human calcitonin

Synthetic human calcitonin (hCT) was radioiodinated by electrolysis and chloramine-T oxidation. The electrolytic method was adapted for practicable quantities of hormone and isotope with no loss in efficiency of labeling. The immunological characterisitcs of the two hormone preparations were nearly identical using four calcitonin antisera with specificities for the regions 17-21, 17-29, 28-29, and 29-32 of hCT, respectively. However, only the electrolytically-labeled preparation retained full ability to produce hypocalcemia in vivo and stimulate cyclic AMP production in kidney homogenates of the rat. In addition, the electrolytically-iodinated hormone remained nearly homogeneous during 104 days of storage and the specific activity as measured by radioimmunoassay remained constant during one half-life of the isotope. These studies demonstrate that electrolytic iodination of hCT results in a tracer which is immunochemically stable and biologically active.