A protein hormone, produced by the pancreas, that controls the level of sugar (glucose) in the blood, and is used in the treatment of diabetes mellitus. The hormone is synthesized in beta cells, which are included in separated groups of hormone-secreting cells of the pancreas known as the islets of Langerhans. The name “insulin” refers to the insular, or “island,” source of the hormone.
Insulin was first extracted from the pancreas in Canada in 1921 by Frederick G. Banting and Charles H. Best, working in cooperation with John J. Macleod. In recognition of their work, Banting and Macleod were awarded the 1923 Nobel Prize in physiology and medicine.
Structure. The insulin molecule consists of two chains of linked amino acids, the A chain containing 21 amino acids, the B chain, 30. The chains are connected by two disulfide bridges (formed of two sulfur atoms each), while a third disulfide bridge stretches across several amino acids on the A chain. The connected chains are partially coiled and twisted into a globular structure, a configuration essential for biological activity.
Insulin is found in fishes, amphibians, reptiles, and birds, as well as in mammals. Pig insulin is often used in the treatment of diabetes, a disease characterized by high levels of blood sugar. It differs from human insulin by only a single different amino acid.
Function. Insulin is the most important hormone in intermediate metabolism. Its most prominent effect is to lower blood sugar, primarily by facilitating the uptake and use of glucose by muscle and fat cells and inhibiting the formation of new glucose by the liver. Insulin increases the storage of excess glucose in the form of glycogen. It also stimulates the storage of other energy forms (fat, protein) and inhibits the breakdown and use of these stored materials by the body.
Some insulin is stored in the pancreas, and glucose in the blood serves as the primary stimulus for its release. Glucose also stimulates the synthesis of additional insulin.
Insulin is secreted continually at varying rates and acts in fine tune with hormones (glucagon, catecholamines) that raise blood sugar to maintain blood sugar levels within very narrow limits (about 80 to 100 mg/100 ml of blood). Circulating insulin is quickly inactivated, mainly in the liver and kidneys, its half-life being only a few minutes.
Pharmaceutical Use. Insulin for the treatment of diabetes is obtained from the pancreases of cattle and pigs. Equally potent human insulin is now available from genetically engineered bacteria and also via enzymatic conversion of pig insulin. Because digestion of the insulin protein inactivates the hormone, insulin cannot be taken orally and must be administered by injection or infusion.