Parathyroid hormone--quo vadis?

Parathyroid hormone is the most important calcium regulating hormone, especially in the process of aging. As a compensation for progressive calcium deficiency in aging, PTH secretion progressively rises. Heterogeneity of PTH in peripheral blood indicates the importance of understanding the degradation mechanism throughout the life cycle of the hormone. In addition to cathepsin B and D which degrades PTH in the liver, kidney and parathyroid gland, a new neutral PTH ase was found in the cytosolic fraction of rat kidney cells. PTH responsive bone and kidney cell lines, UMR-106 and OK cells, were found to degrade PTH by a chymotrypsin-like enzyme on the plasma membrane through a receptor mediated mechanism. This process was inhibited by cyclic AMP-A-kinase system and augmented by C-kinase system, suggesting an intimate relationship between PTH action and degradation and also a new physiological significance of PTH degradation.