Immunoreactivity of human parathyroid hormone (28-48): attempt to develop an assay for intact human parathyroid...

The postsecretory catabolism of parathyroid hormone (PTH) is known to involve proteolytic cleavages in the 28-48 region. In an effort to develop assays that would detect the intact human PTH molecule but not its catabolic products, I have developed radioimmunoassays specific for the 28-48, or cleavage, region. A synthetic fragment of the human PTH molecule, substituted with tyrosine, [27Tyr]hPTH-(27-48), was radioiodinated to serve as tracer. Four of four high titer antisera against native human PTH bound significant amounts of this tracer. Binding of this radioligand was inhibited by unlabeled synthetic human PTH-(28-48) or native human PTH-(1-84) but not by the following synthetic hormone fragments which resemble those thought to be formed in vivo: hPTH-(1-34), hPTH-(44-68), or hPTH-(53-84). The most sensitive assay would detect about 20 fmol of hPTH-(1-84) or 10 fmol of hPTH-(28-48). The bovine PTH 28-48 region crossreacted poorly with the human 28-48 region. Neither bovine PTH-(1-84) nor PTH-(28-48) would inhibit the binding of the labeled human PTH 27-48 peptide to the antisera against native human PTH, nor would the labeled human 27-48 peptide bind to any of nine antisera against bovine PTH. These results confirm that the steric configuration of the 28-48 region of PTH differs greatly between human and bovine PTH. The use of assays specific for the 28-48 region of hPTH appears to be valid approach to the problem of measuring the intact hormone. Improvements in sensitivity may allow detection of intact PTH in human serum.