Parathyroid Hormone-Action and Degradation

Abstract

Peptide hormone hydrolysis has two main purposes. One is processing, and the other is degradation. In most of the hormones, these two processes are clearly distinguished, because the former takes place in the secretory gland and the latter in the target organ. The former is a metabolically controlled limited hydrolysis yielding certain active fragment, whereas the latter is a random hydrolysis to generate small, in-active fragments with no other purpose but getting rid of the biological effect of the hormone. While the processing and generation of active fragment is directly connected with the action of the hormone, degradation into small fragments may be involved only indirectly in the control of hormone action, by controlling its rate of disappearance and consequently, the half-life of the hormone in blood. Parathyroid hormone is unique in its inhibition of secretion by calcium ion along with renin, and this was explained by calcium stimulation of its degradation already in the secretory gland, from which only inactive fragments are secreted during hypercalcemia, because of the augmentation of PTH degradation in the parathyroid glands. It is thus difficult to separate processing and degradation of PTH. In the target cells of PTH, both processing and degradation may take place.