Preparation and characterization of radioactive monoiodotyrosine and diiodotyrosine derivatives of parathyroid hormone

Highly purified native parathyroid hormone was iodinated by the enzymatic method and separated from unlabeled hormone by isocratic HPLC. The separation system used also resolved iodohistidine, monoiodotyrosine, and diiodotyrosine forms of the hormone from one another. A simplified procedure for direct bioassay of the carrier-free, high specific activity, mono- and diiodinated parathyroid hormone (PTH) by the renal membrane adenylyl cyclase method was also developed. Both labeled forms of the hormone are very potent in this assay, but the iodinated forms appeared to give a lower V_max than the native hormone. The methods for iodination, separation and biological characterization of this PTH tracer are exceptionally facile, inexpensive, and convenient.