Development of sequence specific radioimmunoassay of human parathyroid hormone and its use in the diagnosis of hyperparathyroidism.

Two antisera which were raised against bovine parathyroid hormone (bPTH), and which cross-reacted with the human hormone, have been characterized. The antisera which originated from rooster and guinea-pig, were found to contain several populations of antibodies directed against both N-terminal and C-terminal sequences of the hormone. However, at proper dilutions the rooster antiserum did not bind the N-terminal fragment nor could this fragment displace the [125I] bPTH (1--84 amino acid residue) from binding to the antiserum. Furthermore, preincubation experiments with excess N-terminal fragment showed only a negligible reduction in maximal binding of the iodinated intact hormone using the rooster antiserum. In contrast, the guinea-pig antiserum reacted equally well with the N-terminal fragment and the intact hormone, and preincubation with this fragment reduced the binding of the [125I]bPTH (1--84 amino acid residues) by 75%. Gel filtration of hyperparathyroid serum on Bio-Gel P-60 showed immunoreactive material which was measured with both antisera, eluting at a position similar to the intact hormone. However, in the C-terminal specific, but not in the N-terminal specific radioimmunoassay the major component eluted together with or somewhat earlier than the N-terminal bPTH fragment (1--34 amino acid residue), and this peak represented more than 90% of total immunoreactive PTH (iPTH) in serum. This major iPTH component must therefore represent fragment(s) with intact carboxy-terminal sequences. The N-terminal specific radioimmunoassay was unable to measure iPTH in about 80--90% of healthy individuals while the C-terminal specific assay detected iPTH in about 88% of these sera (equal to or above 0.1 micrograms/l). Similarly, the N-terminal specific antiserum measured consistently lower serum iPTH concentrations in patients with primary hyperparathyroidism. In thirty-four out of forty-one patients with surgically verified primary hyperparathyroidism, serum iPTH concentrations equal to or above 0.60 micrograms/l were demonstrated using the C-terminal, specific radioimmunoassay.