Iodoinsulin used to determine specific activity of iodine-131.

Anodal mobility of iodoinsulin on starch-gel electrophoresis increases progressively as the number of iodine atoms substituted in the molecule increases. The iodine content of iodine-131 solutions is determined by comparison of autoradiographic patterns of insulin heavily labeled with iodine-131 and of insulins lightly labeled with iodine-131 and known quantities of iodine-127.