High Specific Activity Iodination and Displacement Radioimmunoassay of Fructose-1,6-diphosphatase

Displacement radioimmunoassay of fructose-1,6-diphosphatase is described, capable of measuring quantitatively 100 mμg of enzyme in biological fluids. The preparation of radioiodinated FDPase with high specific activity, a necessary prerequisite for the radioimmunoassay, was complicated by the lability of the enzyme and was, therefore, investigated in detail. Low specific activities of radioiodination in the range of 1-8 mCi/mg of protein were obtained by the IC1 method. Using chloramine-T as an oxidant resulted in radioiodinated FDPase with specific activities in the range of 20-80 mCi/mg of protein. However, the enzyme proved to be sensitive against exposure to chloramine-T; best yields were obtained at short reaction times of less than 10 sec.