Radioimmunoassay for the middle region of human parathyroid hormone: comparison of two radioiodinated synthetic peptides.

Two synthetic peptides were evaluated to develop radioligands for midregion-specific radioimmunoassay (RIA) of human parathyroid hormone (hPTH). Both contained the 44-68 sequence of hPTH (no tyrosine residues); one contained a tyrosine residue added to the amino-terminus, (Tyr43)hPTH(43-68). The purified radioligands showed similar chemical properties (stability to storage, efficient phase separation with dextran-coated charcoal, low adsorption to glassware). Both radioligands were tested using three anti-PTH sera of proven clinical utility. While each of these midregion-directed antisera showed unique specificity, they all reacted with high affinity with both radioligands and none of them discriminated significantly between the two synthetic midregion peptides. 125I-(Tyr43)hPTH(43-68) gave RIAs that were 15-50% more sensitive to hPTH(1-84) and the unlabelled synthetic midregion peptides than RIAs using 125I-hPTH(44-68) with all three antisera examined. 125I-(Tyr43)hPTH(43-68) was more susceptible than 125I-hPTH(44-68) to degradation from plasma or serum; this susceptibility was reduced by the peptidase inhibitor aprotinin (500 KIU/ml). Simultaneous RIAs of a series of patient plasmas using either of the two radioligands with antiserum NG5/5 produced indistinguishable discrimination between samples (r = 0.984). Analysis of data on the relation of serum calcium and hPTH midregion immunoreactivity showed a useful separation hyperparathyroidism, primary hypoparathyroidism and secondary hypoparathyroidism.