Tests on Three Antisera and Subsequent Development of Radioimmunoassay for Different Regions of Human Parathyrin

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Abstract
Three antisera (Ab), 2 raised in sheep and 1 in a goat, with determinants for different parts of the human parathyrin (hPTH, parathyroid hormone) molecule were tested. One sheep antibody (Ab S-469 VI) recognized only C-regional fragments of hPTH (C-hPTH), the other (Ab S-478 VI) both C- and N-regional fragments (C + N + hPTH), while the goat antibody, raised against 1-34 hPTH (Ciba), was specific for N-regional fragments (N-hPTH). Kinetic studies showed that the differing affinities of Ab S-469 VI and Ab S-478 VI antibodies for bovine (b) PTH and C-hPTH made it possible to develop sensitive assays giving results in as little as 8 h after receipt of blood, and a routine assay giving results within 30 h. Although the numerical PTH levels from the assay were different, their ability to distinguish between normals and patients with hyperparathyrinemia (HPT) was similar. Studies on the labeling and separation of 125I-bPTH led to a tracer with a shelf life exceeding 3 mo. Correlation between assays using Ab S-469 and Ab S-478 was good, although exceptions occurred, which may have been due to the differing specificity of the antisera. Comparison of the original 7 day assay with the 24 h assay using Ab S-469 VI in 109 patients gave a correlation coefficient [r] = 0.957, and using Ab S-478 in the 24 h assay in 81 patients a r = 0.934. The results in the 24 h assay with Ab S-469 VI were 3 times higher and those from the 24 h assay with Ab S-478 VI 20% higher than in the 7 day assay. The normal ranges in the 7 day and both 24 h assays were also different. The clinical value of the N-hPTH assay has not been fully established but is useful in explaining some anomalous results obtained in the C- and C + N hPTH assays, in which the nonbiologically active PTH fragments were within the normal range for the assay, but in which the N-hPTH level, i.e., the biologically active part of the PTH molecule, was significantly elevated.

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