Peptide hormone
Peptide hormones are a class of peptides that are secreted into the blood stream, sometimes from a specialised endocrine gland, and which act on target cells that express specific receptors for them. For example, oxytocin is released from the posterior pituitary gland of the mother when an infant is suckling at the breast; oxytocin acts on the myoepithelial cells of the mammary gland to cause milk to be let down. Often a hormone has several physiological functions; oxytocin for example also regulates uterine contractions during parturition. In recent years many new peptide hormones have been identified, secreted from tissues that were not previously thought to be endocrine; for example, in 1994, an important new appetite-regulating hormone, leptin was identified as a peptide hormone secreted from adipose tissus (fat cells}, and in 2000, another important hormone ghrelin, was identified as a peptide hormone secreted from the stomach.
Many neuropeptides are also released within the brain, and although they are not carried by the blood but by the extracellular fluid, they also often have hormone-like actions in the brain; as some of these neuropeptides are also classical peptide hormones s(like oxytocin), the distinction between a peptide hormone and a neuropeptide is often ill-defined.
Synthesis
Like all other proteins, peptide hormones are synthesized from amino acids according to an mRNA template, which is itself synthesized from a DNA template inside the cell's nucleus. Peptide hormone precursors (pre-prohormones) are then processed in stages, typically in the endoplasmic reticulum, including removal of the N-terminal signal sequence and sometimes glycosylation, resulting in prohormones. The prohormones are then packaged into membrane-bound secretory vesicles, which can be secreted from the cell by calcium-dependent exocytosis in response to specific stimuli.
These prohormones often contain superfluous amino acid residues that were needed to direct folding of the hormone molecule into its active configuration but have no function once the hormone folds. For example, the precursor to oxytocin codes for a very large molecule called neurophysin which seems to be important for packaging oxytocin into the secretory vesicles, but which has no known biological activity. Specific endopeptidases in the cell cleave the prohormone just before it is released into the blood stream, generating the mature hormone form of the molecule. Mature peptide hormones then diffuse through the blood to all of the cells of the body, where they interact with specific receptors on the surface of their target cells.
Notable peptide hormones
Several important peptide hormones are secreted from the pituitary gland. The anterior pituitary secretes luteinizing hormone and follicle stimulating hormone, which act on the gonads, prolactin, which acts on the mammary gland, adrenocorticotrophic hormone (ACTH), which acts on the adrenal cortex to regulate the secretion of glucocorticoids[1], and growth hormone, which acts on bone, muscle and the liver. The posterior pituitary gland secretes antidiuretic hormone, also called vasopressin, and oxytocin. Peptide hormones are produced by many different organs and tissues, however, including the heart (atrial-natriuretic peptide (ANP) or atrial natriuretic factor (ANF)) and pancreas (insulin and somatostatin), the gastrointestinal tract (cholecystokinin, gastrin and ghrelin[2], and fat stores (leptin)[3].