Comparison of radiochemical purity and tissue binding of labelled insulin prepared by lactoperoxidase and chloramine T iodination.

The radiochemical purity and tissue binding of 125I-insulin labelled by conventional chloramine T method were compared with those of (A14)-monoiodoinsulin prepared by lactoperoxidase iodination. In the first case the labelled insulin was purified on a cellulose column, while (A14)-monoiodoinsulin prepared with the aid of lactoperoxidase was purified on a column of QAE Sephadex A-25. After the digestion of labelled insulin by pronase about 15 percent of diiodoinsulin was found after chloramine T iodination, while a negligible amount (i.e. less than 1 percent) was detected after the use of lactoperoxidase. In addition, no damage of insulin molecules was found after the use of the latter method and the purification on QAE Sephadex A-25 yielded a homogenous preparation of insulin. Specific binding to isolated rat fat cells, rat liver plasma membranes and human erythrocytes was consistently higher in the case of (A14)-monoiodoinsulin compared to the insulin labelled by chloramine T method.