Distinct polypeptide hormon

INSULIN PRODUCTION The islet of Langerhans contains four major ceU types, each of which synthesizes and secretes a distinct polypeptide hormone insulin in the j3 (B) cell, glucagon in the a (A) cell, somatostatin in the S (D) cell, and pancreatic polypeptide in the PP or F cell. The P cells make up 60-80% of the islet and form its central core with the a, 6 and F cells in a discontinuous mantle around this core. The geographical organization of islet cells facilitates coordinated control of groups of cells. Blood in the islet flows from the j3 cells to a and S cells. Thus, the j3 cell is the primary glucose sensor for the islet, and the other cell types presumably are exposed to particularly high concentrations of insulin. 

A major portion of the pancreas essentially eomprises of glandular tissue whieh speeially eontains acinar cells that predominantly gives rise to the secretion of certain digestive enzymes. Besides, there also exist some isolated gronps of pancreatic cells commonly known as the islets of Langerhans which usually made up of four cell types, each of which generates a distinct polypeptide hormone, namley ... 

Intracellular metabolic activity may be modulated in response to extracellular conditions by hormones. Hormones are substances which act as chemical messengers between cells in different locations to alter the activity of the recipient cell. Three classes of hormones have been identified protein (polypeptide) hormones, steroid hormones and tyrosine-derivative hormones of which there are two distinct groups, the catecholamines and the thyroid hormones. The catecholamine hormones (adrenaline and noradrenaline) are produced by and secreted from the adrenal medulla. The thyroid hormones (thyroxine and triiodothyronine) are elaborated by and secreted from the thyroid gland. 

The hormone pancreatic polypeptide (PP) is a 36 amino acid peptide, which is closely related to neuropeptide Y and peptide YY PP is mainly found in pancreatic cells distinct from those storing insulin, glucagon or somatostatin. It acts on receptors that belong to the family of neuropeptide Y receptors, particularly on the Y4 subtype. 

Parathyroid hormone is an 84 amino acid, li ear polypeptide secreted by the parathyro gland. Early structure activity studies showt that the amino-terminal region consisting 34 residues was sufficient for full biologic activity (96, 97). This fragment could furtb be divided into three distinct regions (1)T1... 

A particularly interesting case is that seen when a single polypeptide sequence contains two or more distinct hormones. The most complex example is a pituitary multihormone precursor that contains sequences for and T-lipotropin, ot-, / -, and -melanocyte-stimulating hormone (MSH), endorphin, enkephalin, and ACTH (Figure 23.10). This precursor, called pro-opiomelanocortin, derives its name from its role as precursor to endogenous opiates, melanocyte-stimulating hormone, and corticotropin. A remarkable fact about pro-opiomelanocortin is that it is cleaved at different sites in different cells, so that different cell types produce different ensembles of hormones derived from this one precursor. 

Since almost all peptides were identified initially on the basis of bioassays, their names reflect these biologically assayed junctions (e.g., thyrotropin-releasing hormone and vasoactive intestinal polypeptide). A parsimonious view is that each peptide has unique messenger roles at the cellular level that are used repeatedly in functionally similar pathways within functionally distinct systems. 

---