Peptides in this context are short chains of amino acids that have a therapeutic biological activity. Many naturally-occurring substances in the body are short chains of peptides, for example, the endorphin and enkephalins in the central nervous system. Peptide drug discovery has led to the development of therapeutic agents that mimic the effects of these naturally-occurring substances.
The endorphins and enkephalins are endogenous opioids that function mainly in the central nervous system where they control responses to pain and stress. They play important roles in attachment, emotion, motivation and control of appetite. The word, endogenous, means that the substances are made in the body. Opioids are wholly synthetic, while the opiate drugs (morphine, opium, codeine and heroin) come from the sticky substance found in the seed pods of a certain species of poppy.
Another major group of molecules of this class are the tachykinins. These include Elediosin, Substance P, Kassinin and Neurokinins B and A. They are around five amino acids long and cause smooth muscle tissue in the digestive system to contract. Another group, the vasoactive intestinal peptides, are 28 AAs long. In this group are growth hormone releasing hormone, secretin, peptide histidine isoleucine, glucagon and pituitary adenylate cyclase activating peptide.
The calcitonin peptides form a fourth group of this type of molecule. These include calcitonin itself, along with AGG01 and Amylin. Within the human body, calcitonin is manufactured in the thyroid gland by the parafollicular cells. It counters the effects of parathyroid hormone by decreasing the amount of circulating calcium ions. Calcitonin derived from salmon is used as a treatment in postmenopausal women for osteoporosis. It is also used in the general population to treat phantom limb pain, bone metastases, Paget’s disease and hypercalcemia (elevated plasma calcium levels).
The pancreatic polypeptide-related peptides include neuropeptide Y (NPY), PYY (peptide YY), APP (avian pancreatic polypeptide) and PPY (pancreatic polypeptide). Pancreatic polypeptide consists of about 36 amino acids and has a molecular weight of 4200 Daltons (a dalton is a unit of molecular mass and is equivalent to 1 g/mol).
Pancreatic polypeptide controls hepatic glycogen levels, modulates secretions in the digestive tract and controls the secretory functions in the pancreas. These secretions may be endocrine, acting on the pancreas itself, or exocrine, acting on distant organs. PP is secreted by the pancreas in response to a protein meal, fasting, exercising or during a bad attack of hypoglycemia. Ingesting glucose can reverse these effects.
Two very small groups of therapeutic polypeptides are the lactotripeptides and basic natriuretic peptide (BNP). The lactotripeptides consist of two polypeptides, each only three amino acid residues in length. They occur naturally in milk and have been credited with a possible activity in reducing blood pressure. BNP, also called B-type natriuretic peptide, is made up of 32 amino acids.
Peptide drug discovery is a promising area of medicine because it is relatively simple to manufacture short amino acids in the laboratory. Calcitonin that is used medically is derived from salmon. Because the molecules are so short, they are not long enough to generate an immune response and so it is possible to use animal-derived versions. So far, we have been able to synthesize drugs that operate on the thyroid, pancreas and central nervous system. The lactotripeptides may one day help to control hypertension.
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