Identification of carboxyl-terminal peptide fragments of parathyroid hormone in human plasma at low-picomolar levels by mass spectrometry

For decades, researchers have tried to identify the primary structures of circulating carboxyl-terminal parathyroid hormone (C-PTH) peptide fragments that may be present at only picomolar levels in human plasma. Although immunoassays and radiosequencing techniques have provided valuable fragment characterizations, no analysis has successfully determined their exact primary structures. In this work, for the first time, four human C-PTH peptide fragments, hPTH(34-84), hPTH(37-84), hPTH(38-84), and hPTH(45-84), have been identified from human plasma using MS-based methods. C-PTH peptide fragments were isolated from plasma samples by immunoaffinity extraction. The eluate was analyzed by capillary LC fractionation followed by MALDI-TOF-MS or by online coupling of nano-LC with ESI-TOF-MS. Both the MALDI- and the ESI-based approaches were capable of detecting C-PTH peptide fragments in human plasma at < 10 pmol/L. The MALDI-TOF approach was effective in preliminary searches for C-PTH peptide fragments, but the use of high laser power limited the resolution necessary for accurate C-PTH peptide identification. The high mass resolution (10 000) and accuracy (10 ppm) attained by the ESI-TOF approach enabled unambiguous identification of these peptides. The four C-PTH peptide fragments identified in plasma samples from patients with chronic renal insufficiency were also found in the plasma of healthy women receiving recombinant human PTH either by subcutaneous injection or by intravenous infusion. This newly developed analytical capability should greatly enhance the understanding of PTH metabolism and parathyroid gland function.