Structure of non-(1-84) PTH fragments secreted by parathyroid glands in primary and secondary hyperparathyroidism.

BACKGROUND: Non-(1-84) parathyroid hormone (PTH) fragments are large circulating carboxyl-terminal (C) fragments with a partially preserved amino-terminal (N) structure. hPTH (7-84), a synthetic surrogate, has been demonstrated to exert biologic effects in vivo and in vitro which are opposite to those of hPTH (1-34) on the PTH/PTHrP type I receptor through a C-PTH receptor. We wanted to determine the N structure of non-(1-84) PTH fragments. METHODS: Parathyroid cells isolated from glands obtained at surgery from three patients with primary hyperparathyroidism and three patients with secondary hyperparathyroidism were incubated with 35S-methionine to internally label their secretion products. Incubations were performed for 8 hours at the patient-ionized calcium concentration and in the presence of various protease inhibitors. The supernatant was fractionated by high-performance liquid chromatography (HPLC) and fractions were analyzed with PTH assays having (1 to 4) and (12 to 23) epitopes, respectively. The serum of each patient was similarly analyzed. Peaks of immunoreactivity identified were submitted to sequence analysis to recover the 35S-methionine residues in positions 8 and 18. RESULTS: Three regions of interest were identified with PTH assays. They corresponded to non-(1-84) PTH fragments (further divided in regions 3 and 4), a peak of N-PTH migrating in front of hPTH (1-84) (region 2) and a peak of immunoreactivity corresponding to the elution position of hPTH (1-84) (region 1). The last corresponded to a single sequence starting at position 1. Region 2 gave similar results in all cases (a major signal starting at position 1) but also sometimes minor sequences starting at position 4 or 7. Regions 3 and 4 always identified a major sequence starting at positions 7 and minor sequences starting at positions 8, 10, and 15. Surprisingly, a major signal starting at position 1 was also present in region 3. The HPLC profile obtained from a given patient's parathyroid cells was qualitatively similar to the one obtained with his/her serum in each case. CONCLUSION: These results indicate that non-(1-84) PTH fragments are composed of a family of fragments which may be generated by specific or progressive cleavage at the N region. The longest fragment starts at position 4 and the shortest at position 15. A peptide starting at position 7 appears as the major component of non-(1-84) PTH fragments. The generation process is similar to the one described for smaller C-PTH fragments a number of years ago, suggesting a similar production mechanism and source for all C-PTH fragments.

Similar predictive value of bone turnover using first- and second-generation immunometric PTH assays in pediatric patients treated with peritoneal dialysis.

BACKGROUND: Accurate measurements of the concentration of parathyroid hormone (PTH) in serum or plasma are essential for the proper assessment of renal osteodystrophy. The first-generation immunometric PTH assay (1st PTH-IMA) not only detects the intact hormone, but also additional PTH fragments truncated at the amino N-terminally truncated PTH-derived fragments [ntPTH(1-84)]. A second-generation immunometric PTH assay (2nd PTH-IMA) recognizes only PTH(1-84) and possibly PTH fragments that are truncated at the carboxyl-terminus but not PTH(7-84). Whether estimates of the ratio between PTH(1-84) and ntPTH(1-84) fragments are a better predictor of bone turnover remains controversial. METHODS: Thirty-three patients aged 12.8 +/- 4.4 years treated with continuous cycling peritoneal dialysis (CCPD) for 13 +/- 9 months underwent iliac crest bone biopsy. PTH levels were measured by two newly developed first-generation and second-generation PTH-IMA. The ntPTH(1-84) fragments were calculated by subtracting PTH values determined using the 2nd PTH-IMA from values obtained using 1st PTH-IMA that detects both PTH(1-84) and relatively large ntPTH(1-84). RESULTS: Determinations of PTH levels by both assays were highly correlated (r = 0.89, P < 0.001). The relationships between first-generation and second-generation PTH-IMA and bone formation were similar (r = 0.67, P < 0.0001 and r = 0.64, P < 0.0001, respectively). When patients were grouped according to the presence or absence of secondary hyperparathyroidism, the ratio PTH(1-84) to ntPTH(1-84) did not differ between groups. CONCLUSION: PTH concentrations determined by either the first- or the second-generation PTH-IMA were found to be better predictors of bone formation than the PTH(1-84) to ntPTH(1-84) fragments ratio.