Isolation of Iodinated Bovine Parathyroid Hormone Using Ion Exchange: Demonstration of Its Immunological Characteristics and Biological Activity*

We have shown that iodination of the sole tyrosine residue of bovine parathyroid hormone (bPTH) increases its acidity, enabling the iodinated form (iodo-bPTH) to be separated from noniodinated precursor on the basis of the difference in charge. After labeling a large excess of bPTH with lactoperoxidase (hormone to iodide molar ratio, 48:1), essentially pure monoiodinated bPTH could be completely separated from unreacted hormone by anion exchange chromatography on QAE Sephadex at pH 9.4 in the presence of urea (8 M). The same methods were used to prepare larger amounts of iodo-bPTH after the hormone (1 mg) had been iodinated with an equimolar quantity of iodide. The isolated iodinated product was shown to contain both monoiodinated tyrosine (80%) and diiodinated tyrosine (20%). Four percent of the radioactivity was coincidentally incorporated onto histidine. This material gave dilution curves which were similar to noniodinated bPTH with two different antisera, predominantly directed to the middle and carbox terminus of the hormone, respectively. The product was found to possess full biological potency in an in vitro assay system using rat kidney cortical adenylate cyclase. Thus, we have shown directly, for the first time, that the immunological and biological potencies of bPTH are retained after enzymic iodination. This investigation validates the use of the iodinated product in studies of the mode of action and metabolism of the hormone.