insulin

insulin hormone secreted by the β cells of the islets of Langerhans, specific groups of cells in the pancreas . Insufficiency of insulin in the body results in diabetes . Insulin was one of the first products to be manufactured using genetic engineering .

Action

In general, insulin acts to reduce extracellular (including blood plasma) levels of glucose by interacting in some way yet unknown with various cell membranes. In adipose (fatty) tissue it facilitates the cellular uptake of glucose and its subsequent conversion to fatty acids , and it inhibits the breakdown of fatty acids to simpler compounds. In muscle it again facilitates the transport of glucose into cells and in addition stimulates its conversion to glycogen . It also increases protein synthesis in muscle. In the liver, insulin facilitates glucose catabolism and its conversion to glycogen and inhibits its synthesis from simpler compounds.

Isolation and Structure

Canadians Frederick G. Banting and Charles H. Best were the first to obtain, from extracts of pancreas (1921-22), a preparation of insulin that could serve to replace a deficiency of the hormone in the human body. The complete amino acid sequence of the insulin molecule was described in the early 1950s; insulin was the first protein to be sequenced entirely. This pioneering work was confirmed from 1963 to 1966, when several groups reported laboratory synthesis of biologically active insulin. The three-dimensional structure of the crystalline hormone was published in 1969.

Insulin has been shown to be a protein consisting of two polypeptide chains (see peptide ), one of 21 amino acid residues and the other of 30, joined by two disulfide bridges (see cysteine ). The two chains are synthesized in the β cells as part of one continuous polypeptide chain called proinsulin; a 32-amino acid sequence (the connecting peptide) is subsequently split out of the proinsulin molecule by an enzyme resembling trypsin to yield active insulin.

Insulin in Diabetes Treatment

Many, but not all, of the symptoms of diabetes can be controlled by the administration of insulin. The forms of insulin available early in the 20th cent. had to be injected frequently because they were quick-acting. Later modifications gave the insulin solution a more prolonged action so that hypodermic injections could be made less frequently. Some now control their insulin levels via a small, portable insulin pump. In certain cases of mild diabetes, oral medications that stimulate production of insulin can be taken in lieu of insulin. See glucagon .

insulin Hormone secreted by the islets of Langherhans in the pancreas and concerned with the control of blood-glucose levels. Insulin lowers the blood-glucose level by helping the uptake of glucose into cells, and by causing the liver to convert glucose to glycogen. In the absence of insulin, glucose accumulates in the blood and urine, resulting in diabetes. Insulin was isolated in 1921 by Canadian physician Frederick Banting and Canadian physiologist Charles Best. Its structure was discovered in the 1940s by English biochemist Frederick Sanger.

insulin A protein hormone, secreted by the β (or B) cells of the islets of Langerhans in the pancreas, that promotes the uptake of glucose by body cells, particularly in the liver and muscles, and thereby controls its concentration in the blood. Insulin was the first protein whose amino-acid sequence was fully determined (in 1955). Underproduction of insulin results in the accumulation of large amounts of glucose in the blood (hyperglycaemia) and its subsequent excretion in the urine in abnormally high concentrations (glycosuria). This condition, known as diabetes mellitus, can be treated successfully by insulin injections.