INSULIN

hormone that regulates the level of sugar (glucose) in the blood and is produced by the beta cells of the islets of Langerhans in the pancreas. Insulin is secreted when the level of blood glucose risesas after a meal. When the level of blood glucose falls, secretion of insulin stops, and the liver releases glucose into the blood. Insulin is a simple protein in which two polypeptide chains of amino acids are joined by disulfide linkages. Insulin helps transfer glucose into cells so that they can oxidize the glucose to produce energy for the body. In adipose (fat) tissue, insulin facilitates the storage of glucose and its conversion to fatty acids. Insulin also slows the breakdown of fatty acids. In muscle it promotes the uptake of amino acids for making proteins. In the liver it helps convert glucose into glycogen (the storage carbohydrate of animals) and it decreases gluconeogenesis (the formation of glucose from noncarbohydrate sources). The action of insulin is opposed by glucagon, another pancreatic hormone, and by epinephrine. Inadequate production of insulin is responsible for the condition called diabetes mellitus. Severe diabetics require periodic injections of insulin, which is extracted from the pancreas of pigs, sheep, and oxen. Insulin was first isolated as a pancreatic extract in 1921 by the Canadian scientists Sir Frederick G. Banting and Charles H. Best. By the early 1980s certain strains of bacteria had been genetically modified to produce human insulin.