Two homologous radioimmunoassays for parathyrin compared and applied to disorders of calcium metabolism.

These 48-h homologous equilibrium radioimmunoassays for human parathyrin (hPTH) are based on the use of two antisera, MS 6 and MS 7, raised in guinea pigs against human parathyroid adenoma extract. For the assay with MS 6, Tyr43-hPTH(44-68) and hPTH(44-68) were radioiodinated for use as the assay tracer. Labeled peptides were separated from free iodine by passage through Sep-Pak C18 cartridges. This RIA appears to be mid-region specific: hPTH(28-48) and hPTH(64-84) were not recognized, whereas hPTH(53-84) was 100% cross reactive with hPTH(44-68). Thus, the PTH recognition site of antiserum MS 6 must be between amino-acid residues 53 and 63. With antiserum MS 7, which recognized the PTH molecule between amino-acid residues 69 and 84, we used hPTH(53-84) for preparing the standard curve and 125I-labeled Tyr52-hPTH(53-84) as the assay tracer. Because this RIA recognized PTH fragments containing residues 69-84 of hPTH, we termed it a C-terminal assay. Both assays were useful for diagnosis of primary and secondary hyperparathyroidism.