Peripheral metabolism of PTH: fate of biologically active amino terminus in vivo

Abstract

Clearance of intact parathyroid hormone (PTH) from blood is associated with rapid uptake by liver and kidney, limited proteolysis by tissue endopeptidases and, within minutes, appearance of circulating carboxyl-(COOH)-terminal PTH fragments. The fate of the corresponding amino(NH2)-terminal portion of the hormone during this peripheral metabolism is still unknown, however. To determine this, we have employed [35S]bovine PTH (bPTH) labeled to high specific activity at NH2-terminal methionines, which permits direct monitoring of the fate of the PTH NH2-terminus during metabolism in vivo. The [35S]PTH was administered by bolus or continuous intravenous infusion to anesthetized normal rats, to rats subjected to acute ablation of the liver, the kidneys, or both, and to rats receiving co-infusions of excess synthetic bPTH(1-34) NH2-terminal fragments. Analysis by high-resolution chromatographic techniques sensitive to 10(-13) M [35S]PTH peptides in plasma yields no evidence that peripheral metabolism of PTH generates circulating NH2-terminal fragments, even when special measures are taken to block clearance of such putative fragments from blood. We find that the NH2-terminus of PTH is rapidly degraded in situ by the liver but that both liver and especially kidney nevertheless contain low levels of NH2-terminal PTH fragments that, although not released into the blood, are large enough to be potentially active. Thus, the peripheral metabolism of PTH in normal animals does not normally lead to the formation of circulating amino terminal fragments of the hormone that might act independently of intact PTH on peripheral target tissues.

Origin of parathyroid hormone (PTH) fragments detected by intact-PTH assays

“…The difference from prior results is probably best explained by the fact that the hPTH(1-84) preparation used here was biologically active while the 125 I-bPTH(1-84) used previously was not (9, 10, 29, 30) because of chloramine-T iodination. Prior studies have demonstrated a slightly different tissue distribution (more in the liver, less in the kidneys) of bioactive 125 I-bPTH(1-84) (31) and of bioactive internally labeled bPTH(1-84) with a somehow shorter half-life (32). Finally, more PTH fragments were observed in NPX rats at all time points, reflecting the absence of kidney clearance and increased production in the presence of more available substrate.…”

“…In the first case, gel chromatography was used and was probably unable to separate these large 125 I-PTH fragments from 125 IbPTH(1-84), and/or 125 I-PTH fragments were not sequenced long enough to identify 125 I-non(1-84) PTH (9,10,29). In the second case (32), HPLC was performed on gel chromatography fractions and not on untouched samples, leading to the same conclusion.…”

Parathyroid Hormone Monitoring during Total Parathyroidectomy for Renal Hyperparathyroidism: Pilot Study of the Impact of Renal Function and Assay Specificity

“…PTH(1-84) and its various fragments are eliminated by the liver and by glomerular filtration in the kidneys (1)(2)(3)(4)(5). In patients with impaired renal function, plasma clearance of PTH and other PTH fragments is diminished.…”

Circulating Forms of Parathyroid Hormone: Peeling Back the Onion

“…This event generates a series of N-truncated "CPTH" fragments (the corresponding N-terminal segments are destroyed in situ) that reenter the circulation and are cleared mainly by glomerular filtration, an important route for PTH clearance as well (4 ). Chemical analysis of PTH fragments isolated from rat blood after administration of radiolabeled bovine PTH established that hepatic metabolism yields a family of CPTH fragments with NH 2 termini ranging between residues 34 and 43, although the methods used likely would not have detected substantially larger CPTH fragments (5 ). Interestingly, chemically similar CPTH fragments are secreted directly from the parathyroid glands as well, and the ratio of secreted CPTH fragments to intact PTH increases as blood calcium increases (6,7 ).…”