Radioimmunoassay for the middle region of human parathyroid hormone: studies with a radioiodinated synthetic peptide.

We radioiodinated a synthetic fragment representing residues 44-68 from the middle region of human parathyroid hormone (hPTH). At least 90% of the purified [125I]-hPTH-(44-68) was able to bind to anti-hPTH serum. Antibody-bound [125I]hPTH-(44-68) could be rapidly and efficiently separated from nonbound radioligand by dextran-coated charcoal. [125I]hPTH-(44-68) was not degraded after a 72-h incubation in undiluted plasma at 7 C, and it was stable for many weeks at -20 C in a 1% albumin buffer. [125I]hPTH-(44-68) was used to develop midregion specific PTH RIAs. The immunoreactive PTH concentration in plasma was above the upper limit of the normal range in 39 of 43 patients with primary hyperparathyroidism. Values from the midregion assay and an established carboxy-terminus assay correlated using peripheral plasma from 17 patients with primary hyperparathyroidism (r = 0.84; P less than 0.0001) or using parathyroid gland venous effluent plasma from the same 17 patients (r = 0.79; P less than 0.0005). Gel filtration analysis of peripheral plasma from 2 patients with primary hyperparathyroidism and azotemia suggested peptides possessing midregion immunoreactivity but deficient in carboxyterminus immunoreactivity. Similar peptides were present at higher concentrations in parathyroid gland venous effluent plasma than in peripheral plasma, indicating release from the parathyroid gland. In conclusion, [125I]hPTH-(44-68) had properties favorable for the development of RIAs reactive solely with the midregion of PTH. Fragments secreted in vivo by two human parathyroid glands were reactive in midregion assays but nonreactive in a carboxy-terminus assay.