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Bovine parathyroid hormone-(41-84), a hormone fragment with desirable properties for use as radioligand.

Radioiodinated bPTH has been widely used as the labeled ligand in the radioimmunoassay of PTH. We now report the properties of a carboxyterminal fragment of bPTH that has several favorable characteristics when used as radioligand. This peptide, the chief component of a commercial preparation of bPTH, was isolated by gel filtration, where it migrated more slowly than did authentic bPTH-(1-84). It yielded lower nonspecific binding values and more sensitive hPTH assays than were seen with the intact hormone. By immunological criteria this peptide lacked the aminoterminal region of PTH, since hPTH-(1-34) did not inhibit its binding to any of 11 different antisera with known ability to recognize the aminoterminal region of PTH. The peptide did not contain most or all of the carboxyterminal region, however, since its binding to anti-PTH sera was inhibited by hPTH-(44-68) or hPTH-(53-84). Sequential Edman degradation of the iodinated peptide released iodotyrosine at the third cycle, suggesting the structure, bPTH-(41-84). The lower nonspecific binding and enhanced assay sensitivity provided by this peptide suggest that the use of other natural or synthetic fragments of PTH as radioligands might enhance the performance of PTH assays.