In this article we will discuss about Parathormone (PTH) of Parathyroid Gland. After reading this article you will learn about: 1. Meaning of Parathormone 2. Synthesis of Parathormone 3. Catabolism 4. Regulation.

Meaning of Parathormone (PTH):

PTH is the principal hormone of parathyroid gland. It is secreted from chief cells of par­athyroid gland. This hormone is a monomeric peptide, without any prosthelic group. Its molecular weight is about 9.5 Kdt. It is made up of 84 amino acid residues. The primary structure of human PTH, as well as those of the pig, cow and other mam­mals, has been determined.

Synthesis of Parathormone (PTH):

PTH is derived from a precursor molecule of 115 amino acid residues. The translation of the precursor PTH takes place in the GER. Then with the help of enzyme signal pepti­dase in the lumen of ER a cleavage takes place and leads to the formation of signal peptide (contains 25 amino acid residues) and pro PTH (contains 90 amino acid resi­dues). Signal peptide is cleaved off from the N-terminal end of pre-pro PTH.

Pro PTH is an intermediate form it is then transferred to Golgi cisternae. The N-terminal basic hexapeptide segment of pro PTH is hydrolytically cleaved off by the enzyme clipase B (a trypsine like protease) and leads to 84 amino acids containing PTH and 6 amino acids containing inactive peptide (Fig. 6.21).

Biosynthesis of PTH

In the peripheral circulation, the intact hormone is cleaved into fragments, one of which, PTH (1-34), retains some biological activity. Studies using radiolabeled bovine PTH in calves showed on initial rapid disap­pearance of most of hormone with a half-life of 3-4 minutes. In humans, PTH has approxi­mately the same half-life as measured after Para thyroidectomy.

Catabolism of Parathormone (PTH):

Most of PTH is degraded in the parathyroid itself by cathepsins and other peptidases into a C-terminal 48-amino acid fragment and small N-terminal fragment. In the kidney, it is cleaved into N and C- terminal fragments. Degradation also occurs in hepatic Kupffer cells.

Regulation of Parathormone (PTH):

The secretion of PTH is controlled by circu­lating levels of Ca++. Low concentration of plasma Ca++ increases PTH secretion and high concentration of Ca++ level decreases PTH release.