High Specific Activity Labeling of Protein with I 131 by the Iodine Monochloride Method. ∗

Abstract

Summary

A modified iodine monochloride method suitable for preparing I¹³¹-labeled proteins of a high degree of radioactivity is described, and results are given. Hydrogen peroxide present in high level I¹³¹ preparations is destroyed with catalase. Then IC1 is added to a mixture of the I¹³¹ as iodide and the protein to be iodinated. Total iodine content of I¹³¹ preparations sets a limit on the specific activity of I¹³¹-labeled proteins that can be achieved with a low degree of iodination. In the two commonly used methods for producing I¹³¹ (fission of U²³⁵ and thermal neutron irradiation of natural tellurium) stable I¹²⁷ and long-lived I¹²⁹ are also formed. Analysis showed the total iodine content of fission product I131, as received from Oak Ridge National Laboratory soon after processing, to average 2.4 μg per 100 mc, 3 times the amount present as I¹³¹ (0.8 μg/100 mc). For I¹³¹ produced from tellurium it was substantially greater. Since the ratio of total iodine to I¹³¹ increases with time after processing, freshly produced I¹³¹ is necessary to make very high level labeled preparations. Precautions to prevent protein damage as a result of high level labeling procedures are described.