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Parathyroid hormone (PTH) is rapidly metabolized, mainly by liver and kidney, to smaller fragments that are believed to be biologically inactive. The significance of this peripheral metabolism in the overall actions of PTH is unclear. Generation of circulating biologically active amino-terminal PTH fragments during metabolism in vivo has been suggested by certain observations in vitro, and what are believed to be amino-terminal fragments may be detectable in blood under pathological circumstances in vivo (such as renal failure and coexistent hyperparathyroidism) when highly sensitive assays are employed. We recently reported, however, that administration to normal rats of [³⁵S]bovine PTH ([³⁵S]bPTH) directly labelled at amino-terminal methionines, followed by high-resolution chromatographic analysis of extracted [³⁵S]peptides, does not result in appearance of radioactive amino-terminal fragments in blood, even when the tracer is continuously infused to near-physiological plasma concentrations. We have now employed these techniques to address a second question regarding hormonal metabolism: is hormonal metabolism modified during metabolic perturbations such as changing calcium availability or altered levels of calciotrophic hormones? Metabolism of [³⁵S-Met]bPTH (900 Ci/mmol), either alone or together with [³H-Pro]bPTH, however, did riot lead to alterations in the rate of hormonal clearance nor to detectable circulating amino-terminal fragments, either in calcium-deprived or thyroparathyroidectomized rats or when animals were first rendered intoxicated with vitamin D or maintained on a high calcium intake. Likewise, tissue localization and specific cleavage patterns of intact hormone in liver or kidney were all unaltered by these various manoeuvres. We conclude that regulation of peripheral metabolism of PTH does not exert important physiological control over the concentration of circulating biologically active PTH and that active amino-terminal fragments of the hormone are not released into blood even under circumstances of calcium deprivation or hypocalcaemia.

Journal of Endocrinology (1989) 122, 237–245

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