insulin

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insulin,

hormonehormone,
secretory substance carried from one gland or organ of the body via the bloodstream to more or less specific tissues, where it exerts some influence upon the metabolism of the target tissue.
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 secreted by the β cells of the islets of Langerhans, specific groups of cells in the pancreaspancreas
, glandular organ that secretes digestive enzymes and hormones. In humans, the pancreas is a yellowish organ about 7 in. (17.8 cm) long and 1.5 in. (3.8 cm) wide. It lies beneath the stomach and is connected to the small intestine at the duodenum (see digestive system).
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. Insufficiency of insulin in the body results in diabetesdiabetes
or diabetes mellitus
, chronic disorder of glucose (sugar) metabolism caused by inadequate production or use of insulin, a hormone produced in specialized cells (beta cells in the islets of Langerhans) in the pancreas that allows the body to use and store
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. Insulin was one of the first products to be manufactured using genetic engineeringgenetic engineering,
the use of various methods to manipulate the DNA (genetic material) of cells to change hereditary traits or produce biological products. The techniques include the use of hybridomas (hybrids of rapidly multiplying cancer cells and of cells that make a
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.

Action

In general, insulin acts to reduce extracellular (including blood plasma) levels of glucoseglucose,
 dextrose,
or grape sugar,
monosaccharide sugar with the empirical formula C6H12O6 . This carbohydrate occurs in the sap of most plants and in the juice of grapes and other fruits.
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 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 acidsfatty acid,
any of the organic carboxylic acids present in fats and oils as esters of glycerol. Molecular weights of fatty acids vary over a wide range. The carbon skeleton of any fatty acid is unbranched. Some fatty acids are saturated, i.e.
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, 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 glycogenglycogen
, starchlike polysaccharide (see carbohydrate) that is found in the liver and muscles of humans and the higher animals and in the cells of the lower animals. Chemically it is a highly branched condensation polymer of glucose; it is readily hydrolyzed to glucose.
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. It also increases protein synthesis in muscle. In the liver, insulin facilitates glucose catabolismcatabolism
, subdivision of metabolism involving all degradative chemical reactions in the living cell. Large polymeric molecules such as polysaccharides, nucleic acids, and proteins are first split into their constituent monomeric units, such as amino acids, after which the
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 and its conversion to glycogen and inhibits its synthesis from simpler compounds.

Isolation and Structure

Canadians Frederick G. BantingBanting, Sir Frederick Grant,
1891–1941, Canadian physician, M.D. Univ. of Toronto, 1922. From 1923 he was professor of medical research at Toronto. Working with C. H. Best under the direction of J. J. R.
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 and Charles H. BestBest, Charles Herbert,
1899–1978, Canadian physiologist, b. West Pembroke, Maine. With F. G. Banting he discovered (1921) the use of insulin in the treatment of diabetes. He was appointed professor of physiology at the Univ.
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 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 peptidepeptide,
organic compound composed of amino acids linked together chemically by peptide bonds. The peptide bond always involves a single covalent link between the α-carboxyl (oxygen-bearing carbon) of one amino acid and the amino nitrogen of a second amino acid.
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), one of 21 amino acid residues and the other of 30, joined by two disulfide bridges (see cysteinecysteine
, organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l-stereoisomer participates in the biosynthesis of mammalian protein.
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). 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 trypsintrypsin,
enzyme that acts to degrade protein; it is often referred to as a proteolytic enzyme, or proteinase. Trypsin is one of the three principal digestive proteinases, the other two being pepsin and chymotrypsin.
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 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 glucagonglucagon
, hormone secreted by the α cells of the islets of Langerhans, specific groups of cells in the pancreas. It tends to counteract the action of insulin, i.e., it raises the concentration of glucose in the blood.
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.

Insulin

Produced and secreted by the beta cells of the islets (insulae) of Langerhans of the pancreas, the hormone which regulates the use and storage of foodstuffs, especially the carbohydrates. Chemically insulin is a small, simple protein. Insulins from various species differ in the composition; these differences account for the fact that diabetics treated with animal insulins develop antibodies which may sometimes interfere with the action of the hormone. The structure has been verified by synthesis of insulin from pure amino acids in the laboratory. See Carbohydrate metabolism, Immunology, Pancreas

Insulin, being a polypeptide, can also be broken down by many proteolytic enzymes to its constituent amino acids. Because of these breakdown systems, the turnover of insulin in the body is rapid; its “half-life” has been estimated to be 10–30 min. The liver alone is capable of destroying about 50% of the insulin passing through it on its way from the pancreas to the bodily tissues.

The role played by insulin in the body is most clearly approached by considering the abnormalities resulting from removing insulin from an organism by surgical excision of the pancreas or by the chemical destruction of the insulin-producing cells: A state of severe diabetes is produced. Normally the blood glucose level is about 100 mg/100 ml. A carbohydrate meal raises the blood sugar to about 150 mg and the premeal value is reached again within 1.5 h. The normal organism manages to dispose of food by storage and oxidation within this period because insulin is present. When food (carbohydrate and protein) reaches the upper intestine, a substance is liberated which in turn stimulates the beta cells to secrete extra insulin. Insulin acts on most tissues to speed the uptake of glucose. In the cells the glucose is burned for energy, stored as glycogen, or transformed to and stored as fat. The human pancreas probably produces 1–2 mg of the hormone per day. This is sufficient to regulate the metabolism of more than 250 g of carbohydrate, 70 g of protein, and 75 g of fat, the usual composition of an ordinary 2000-calorie diet.

In diabetes the rate of glucose uptake is slowed, the level of circulating blood sugar rises, and sugar spills over into the excreted urine. Calories are wasted, more water is excreted, and there is muscular weakness and weight loss; hence urinary frequency, hunger, thirst, and fatigue. Whenever glucose metabolism is defective, stored fat is broken down to fatty acids because of the actions of adrenaline and the pituitary growth hormone. Insulin is able to reverse all these phenomena by favoring storage and swift intake of glucose into the tissues, by decreasing the breakdown of stored fat, and by promoting protein synthesis.

When insulin is secreted or given in excess, it may lower the blood sugar level much below its normal value, causing hypoglycemia. Hypoglycemia is dangerous because the metabolism in the brain cells depends primarily upon an adequate supply of glucose.

The precise molecular mechanisms of insulin action are still not known. The initial step is the binding of the hormone to a specific receptor on the cell membrane. This event somehow activates a set of transport molecules, so that glucose, potassium, and amino acids enter cells more freely. At the same time, fat breakdown is slowed and glycogen storage increased. All these actions depend upon the integrity of the outer cell membrane. See Cell permeability

Not all the cells of the body require or respond to insulin. The insulin-responsive tissues are the liver, skeletal muscle, the heart, and the adipose tissue. Sensitivity to insulin is affected by many conditions. Obesity, antibodies to the hormone or its receptor, oversecretion of growth hormone or adrenal steroids, ketosis, and unknown genetic factors all cause insulin resistance. Muscular exercise, correction of obesity, and a deficiency of pituitary or adrenal hormones are associated with an increased sensitivity to the hormone.

Insulin

 

a protein hormone formed by the beta cells of the islets of Langerhans in the pancreas. It was first isolated by the Canadian scientists F. Banting and C. Best (1921–22). The structural unit of insulin is a monomer with a molecular weight of about 6,000. Under varying conditions, the molecular weight may be 12,000 or 36,000 because the insulin molecule combines different numbers of monomers, depending on the experimental conditions. Every monomer contains 51 amino acids arranged in two peptide chains, A and B, linked by two disulfide bridges (—S—S—). The presence of these bridges is needed for the hormonal activity of insulin to be manifested. If they are destroyed, insulin becomes inactive. Insulin differs from one animal species to another solely in the position of certain amino acids in the chain. The structure of the insulin monomer, that is, the sequence in which the amino acid residues are arranged, was elucidated by the British biochemist F. Sanger (1945–56). This led to the chemical synthesis of insulin.

Insulin lowers blood sugar levels, delaying the breakdown of glycogen and the synthesis of glucose in the liver. At the same time it increases the permeability of the cell membranes to glucose, assisting its passage into the tissues. It also increases the utilization of glucose in reactions of the pentose phosphate cycle and accelerates the glycogen synthesis in the muscles. The presence of insulin is responsible for the dominance of the synthesis of proteins and fatty acids over their decomposition and promotes the conversion of carbohydrates to fatty acids and the formation of fats.

An insulin insufficiency causes a metabolic disorder—diabetes mellitus. Insulin preparations obtained from the pancreas of slaughtered cattle and other animals are used in the treatment of diabetes. Insulin activity is determined biologically (from the ability to lower blood sugar levels in rabbits). A quantity of 0.04082 mg of pure crystalline insulin is taken as a unit of activity (international unit, IU). Insulin is injected subcutaneously or intramuscularly (it is destroyed by gastric juice when taken orally). Free insulin is quickly inactivated by the enzyme insulinase. Various insulin preparations have more prolonged action than insulin, for example, a suspension of amorphous zinc insulin, a solution of protamine zinc insulin, and a suspension of protamine insulin. Low doses of insulin (also in the form of injections) are used for general exhaustion, weight loss, and some other disorders. In psychiatric practice, insulin is injected to induce hypoglycemia.

G. A. SOLOV’EVA

insulin

[′in·sə·lən]
(biochemistry)
A protein hormone produced by the beta cells of the islets of Langerhans which participates in carbohydrate and fat metabolism.

insulin

a protein hormone, secreted in the pancreas by the islets of Langerhans, that controls the concentration of glucose in the blood. Insulin deficiency results in diabetes mellitus