Enzymatic radioiodination of porcine thyroid-stimulating hormone

Abstract

Summary Enzymatic radioiodination of highly purified porcine thyroid-stimulating hormone (37 I.U./mg) was performed using lactoperoxidase, hydrogen peroxide and Na125I. Preparations of the radioiodinated hormone with specific radioactivities of 54 to 240 μCi/μg (1.2 to 5.6 iodine atoms per mole thyrotropin) were obtained. For these iodine contents, biological activity as shown by the property of the iodinated hormone to stimulate isolated thyroid cell reorganization into follicles was preserved (80 to 100 p. cent activity). In contrast, immunoreactivity of the radioiodinated hormone preparations decreased with increasing amounts of incorporated iodine. Purity of the preparations was controlled by polyacrylamide gel electrophoresis in sodium dodecylsulfate. In this analytical system radioiodinated thyrotropin showed an apparent molecular weight of about 42,000 as compared to about 60,000 for the native hormone suggesting that molecular rearrangements consecutive to radioiodination and/or electrophoresis in sodium dodecylsulfate occured. Ion exchange chromatography of the enzymatic digest of radioiodinated thyrotropin preparations disclosed 125I-labelled mono- and di-iodotyrosines representing about 80 p. cent and 12 p. cent of the total hormonal radioactivity respectively whatever the amount of iodine incorporated up to 4.7 atoms per mole.