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Insulin (from Latin insula, "island", as it is produced in the Islets of Langerhans in the pancreas) is an anabolic polypeptide hormone that regulates carbohydrate metabolism. Apart from being the primary agent in carbohydrate homeostasis, it has effects on fat metabolism and it changes the liver's activity in storing or releasing glucose and in processing blood lipids, and in other tissues such as fat and muscle. The amount of insulin in circulation has extremely widespread effects throughout the body.
Insulin is used medically to treat some forms of diabetes mellitus. Patients with type 1 diabetes mellitus depend on external insulin (most commonly injected subcutaneously) for their survival because of the absence of the hormone. Patients with type 2 diabetes mellitus have insulin resistance, relatively low insulin production, or both; some type 2 diabetics eventually require insulin when other medications become insufficient in controlling blood glucose levels.
Insulin is composed of 51 amino acid residues and has a molecular weight of 5808 Da.
Insulin's structure varies slightly between species of animal. Insulin from animal sources differs somewhat in regulatory function strength (ie, in carbohydrate metabolism) in humans because of those variations. Porcine (pig) insulin is especially close to the human version.
Insulin is required for all animal (including human) life. Its mechanism of action is almost identical in nematode worms (e.g. C. elegans), fish, and mammals, and it is a protein that has been highly conserved throughout evolution. In humans, insulin deprivation due to the removal or destruction of the pancreas leads to death in days or, at most, weeks. Insulin must be administered to patients who experience such a deprivation. Clinically, this condition is called diabetes mellitus type 1.
The initial sources of insulin for clinical use in humans were cow, horse, pig or fish pancreases. Insulin from these sources is effective in humans as it is nearly identical to human insulin (three amino acid difference in bovine insulin, one amino acid difference in porcine). Differences in suitability of beef, pork, or fish derived insulin for individual patients have been primarily due to low preparation purity resulting in allergic reactions to the presence of non-insulin substances. Though purity has improved steadily since the 1920s, less severe allergic reactions still occur. Insulin production from animal pancreases was widespread for decades, but very few patients today rely on insulin from animal sources.
Human insulin is now manufactured for widespread clinical use using genetic engineering techniques, which significantly reduces the presence of impurities. Eli Lilly marketed the first such insulin, Humulin, in 1982. Humulin was the first medication produced using modern genetic engineering techniques in which actual human DNA is inserted into a host cell (E. coli in this case). The host cells are then allowed to grow and reproduce normally, and due to the inserted human DNA, they produce actual human insulin.
Genentech developed the technique Lilly used to produce Humulin. Novo Nordisk has also developed a genetically engineered insulin independently. Most insulins used clinically today are produced this way, as they are less likely to produce an allergic reaction.
Since January 2006, all insulins distributed in the U.S. and some other countries are human insulins or their analogs. A special FDA importation process is required to obtain bovine or porcine derived insulin for use in the U.S., though there may be some remaining stocks of porcine insulin made by Lilly in 2005 or earlier.
There are several conditions in which insulin disturbance is pathologic:
Diabetes mellitus – general term referring to all states characterized by hyperglycemia.
Type 1 – autoimmune-mediated destruction of insulin producing beta cells in the pancreas resulting in absolute insulin deficiency.
Type 2 – multifactoral syndrome with combined influence of genetic susceptibility and influence of environmental factors, the best known being obesity, age, and physical inactivity, resulting in insulin resistance in cells requiring insulin for glucose absorption. This form of diabetes is strongly inherited.
Other types of impaired glucose tolerance.
Insulinoma or reactive hypoglycemia.
Metabolic syndrome – a poorly understood condition first called Syndrome X by Gerald Reaven, Reaven's Syndrome after Reaven, CHAOS in Australia (from the signs which seem to travel together), and sometimes prediabetes. It is currently not clear whether these signs have a single, treatable cause, or are the result of body changes leading to type 2 diabetes. It is characterized by elevated blood pressure, dyslipidemia (disturbances in blood cholesterol forms and other blood lipids), and increased waist circumference (at least in populations in much of the developed world). The basic underlying cause may be the insulin resistance of type 2 diabetes which is a diminished capacity for insulin response in some tissues (eg, muscle, fat) to respond to insulin. Commonly, morbidities such as essential hypertension, obesity, Type 2 diabetes, and cardiovascular disease (CVD) develop.
Polycystic ovary syndrome – a complex syndrome in women in the reproductive years where there is anovulation and androgen excess commonly displayed as hirsutism. In many cases of PCOS insulin resistance is present.