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L Nguyen-Yamamoto, L Rousseau, JH Brossard, R Lepage, P Gao, T Cantor and P D'Amour

BACKGROUND: Intact parathyroid hormone (I-PTH) assays react with non-(1-84)PTH, large carboxyl-terminal (C) fragments with a partially preserved amino-terminal (N) structure. They account for up to 50% of I-PTH in renal failure and may be implicated in PTH resistance. We wanted to know if they were secreted by the parathyroid glands and generated by peripheral metabolism of PTH(1-84). METHODS: Anesthetized normal and nephrectomized (NPX) rats were injected i.v. with 1.5 microg human (h) PTH(1-84). Blood was obtained from 8 rats at 2, 4, 6, 8, 12, 24, 48 and 96 min. I-PTH (Allegro I-PTH) was measured in all samples. Pools of serum were fractionated by HPLC at each time point and the fractions assayed to quantitate hPTH(1-84) and non-(1-84)PTH. Secretion studies were performed with dispersed cells from 5 parathyroid adenomas. The serum of 10 patients with primary hyperparathyroidism and cell supernatants were fractionated by HPLC and were analyzed as described. RESULTS: hPTH(1-84) disappeared from serum biexponentially. The half-life of the first exponential was similar in normal (2.08 min) and NPX (1.94 min) rats, while that of the second was longer in NPX rats (32.4 vs 20.9 min). The residual quantity of hPTH(1-84) under the curve was greater in NPX (6964+/-2392 pmol) than in normal rats (3229+/-561 pmol; P<0.001). Non-(1-84)PTH concentration was maximal at 8 min in both groups and was higher in NPX (92.8+/-13.8 pmol/l) than in normal rats (38.8+/-7.2 pmol/l; P<0.01). The area under the curve of non-(1-84)PTH was also greater in NPX (1904+/-405 pmol) than in normal rats (664+/-168 pmol; P<0.001). All parathyroid adenomas secreted non-(1-84)PTH. It represented 21.1+/-3.9% of secreted and 32.5+/-1.3% of circulating I-PTH in primary hyperparathyroidism. CONCLUSIONS: Non-(1-84)PTH, like other C-PTH fragments, originates from both the peripheral metabolism of hPTH(1-84) and from parathyroid gland secretion. Renal failure influences its concentration by increasing the amount of substrate available and by reducing non-(1-84)PTH clearance. Its higher proportion in serum relative to cell supernatants in primary hyperparathyroidism reflects the added role of peripheral metabolism and the longer half-life of fragments.

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H Yamashita, P Gao, T Cantor, T Futata, T Murakami, S Uchino, S Watanabe, H Kawamoto, M Fukagawa and S Noguchi

OBJECTIVE: It was discovered that an immunoreactive large carboxy-terminal parathyroid hormone (PTH) fragment (large C-PTH), likely 7-84 PTH, is present in the circulation. However, very little is known about the production and metabolism of this large C-PTH. Combining a whole molecule PTH (whole PTH) immunoradiometric assay (IRMA) specifically for 1-84 PTH and an intact PTH (iPTH) IRMA for the sum of 1-84 PTH and large C-PTH, we were able to assess the circulating level of this large C-PTH as well as the glandular secretion and metabolism of this large C-PTH in primary hyperparathyroidism (pHPT). METHODS: This study consisted of two patient groups consisting of 77 pHPT patients with a single adenoma. Of these, 43 comprised the venous sampling study group and 70 comprised the intra-operative PTH study group. (Seven patients belonged only to the former group, 34 patients to only the latter group, and 36 patients to both groups.) Preoperatively, blood samples were drawn from the bilateral internal jugular vein by ultrasonographic guidance and from the peripheral vein (n=43). During surgery, blood samples were drawn after anesthesia (basal level), before excision (pre-excision level) of one enlarged parathyroid gland, and at 5, 10, and 15 min post-excision (n=70). RESULTS: There were 26 patients whose iPTH assay levels differed by more than 10% between the right and left internal jugular. In 24 of the 26 patients, the large C-PTH levels obtained from the adenoma side were significantly higher than those from the contralateral side (117+/-135 vs 43+/-33 pg/ml, P<0.001). The plasma whole PTH values decreased more rapidly than the iPTH values after parathyroidectomy (P<0.001). CONCLUSIONS: Our study has demonstrated that the large C-PTH, likely 7-84 PTH, is directly released from the parathyroid gland in humans. Since the half-life of 1-84 PTH is much shorter than large C-PTH, likely 7-84 PTH, it would be advantageous to use an assay that specifically measures 1-84 PTH for intra-operative monitoring of parathyroidectomy.