Metabolism of Labeled Parathyroid Hormone. I. Preparation of Biologically Active125I-Labeled Parathyroid Hormone11

By modification of an electrolysis procedure for the iodination of proteins (controlled voltage to prevent oxidation), it was found possible to produce a highly radioactive 125Ilabeled parathyroid hormone with ease and good yield. The labeled hormone was found to be at least 95% homogenous by chromatoelectrophoresis, density gradient ultracentrifugation, gel filtration, and acrylamide gel electrophoresis. Moreover, by these last three techniques the labeled hormone was found to be indistinguishable from the hormone used as starting material. The labeled hormone was tested by three established bioassay procedures involving liver mitochondria, embryonic rat bone in tissue culture, and mobilization of calcium from bone in thyroparathyroidectomized rats. With all three assays, the labeled hormone was identical to untreated hormone within experimental error. The extent and nature of the iodination is such that it can be concluded that hormone molecules so labeled have undergone no measurable loss in biological activity. Therefore, the tagged hormone produced by this procedure should be a valid marker for the study of the metabolism of the hormone. (Endocrinology92: 1596, 1973)