Metabolism of parathyroid hormone by Kupffer cells: analysis by reverse-phase high-performance liquid chromatography

Abstract

Parathyroid hormone (PTH) undergoes rapid proteolysis in the liver, which results in the appearance of multiple COOH-terminal fragments in plasma. Using reverse-phase high-performance liquid chromatographic (HPLC) techniques, we have shown that biologically active bovine PTH (bPTH) internally labeled with [3H]tyrosine is, like 125I-labeled bPTH, rapidly metabolized by isolated rat Kupffer cells in vitro to multiple COOH-terminal fragments that are chemically identical with those previously found in plasma after metabolism in vivo. Quantitation of specific carboxyl fragments in crude mixtures is achieved rapidly by direct HPLC analysis and is as precise as that achieved by Edman degradation. In addition, several different carboxyl fragments with identical NH2 termini were resolved, revealing a complexity not apparent in previous studies employing direct Edman degradation of such mixtures. Parallel studies with [[35S]Met]bPTH show the generation, by the Kupffer cells in vitro, of several labeled NH2-terminal fragments which undergo rapid further degradation in vitro. Thus, hepatic metabolism of PTH by Kupffer cells proceeds by an initial endopeptidase cleavage within the hormonal sequence in a manner compatible with the generation of biologically active NH2 fragments.

International Union of Basic and Clinical Pharmacology. XCIII. The Parathyroid Hormone Receptors—Family B G Protein–Coupled Receptors

“…It is also interesting to consider that the differences in the C-terminal regions of the two ligands account, to some extent, for the distinct modalities by which the two ligands carry out their biologic functions-i.e., the endocrine modality by which PTH controls Ca and Pi homeostasis versus the paracrine modality by which PTHrP controls cell differentiation programs. For PTH, it has been shown that C-terminal peptide fragments derived from the secreted hormone can be detected in the circulation (Bringhurst et al, 1982;D'Amour et al, 2005), and synthetic C-terminal fragments can bind with moderate affinities to some non-PTHR1, but as yet unidentified binding sites in cultured cells (Divieti et al, 2005). There is thus possibility that the C-terminal domains of PTH and PTHrP can function autonomously and independently of the PTHR1.…”

Reference Range for Serum Parathyroid Hormone

“…Most early assays were directed against the middle or C-terminus of the PTH peptide (2). Investigators soon found that several fragments of PTH in serum were derived from the parathyroid glands and from peripheral metabolism of PTH(1-84) (3)(4)(5)(6)(7)(8)(9)(10)(11). The later development of a 2-site radioimmunometric assay allowed measurement of primarily intact PTH(1-84) (12)(13)(14)(15)(16)(17)(18)(19).…”

PTH secretion of “manipulated” parathyroid adenomas

“…In the Kupffer cells of the liver, the whole PTH molecule undergoes a fast metabolism. The whole (1-84)PTH molecule is split up into various fragments [13][14][15][16]. These fragments are afterwards eliminated by the kidney.…”