Insulin is a hormone with intensive effects on both metabolism and several other body systems (eg, vascular compliance). Insulin causes most of the body's cells to take up glucose from the blood (including liver, muscle, and fat tissue cells), storing it as glycogen in the liver and muscle, and stops use of fat as an energy source. When insulin is absent (or low), glucose is not taken up by most body cells and the body begins to use fat as an energy source (ie, transfer of lipids from adipose tissue to the liver for mobilization as an energy source). As its level is a central metabolic control mechanism, its status is also used as a control signal to other body systems (such as amino acid uptake by body cells). It has several other anabolic effects throughout the body. When control of insulin levels fail, diabetes mellitus results.

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 the hormone is no longer produced internally. Patients with Type 2 diabetes mellitus are insulin resistant, have relatively low insulin production, or both; some patients with Type 2 diabetes may eventually require insulin when other medications fail to control blood glucose levels adequately.

Insulin is a peptide hormone composed of 51 amino acid residues and has a molecular weight of 5808 Da. It is produced in the Islets of Langerhans in the pancreas. The name comes from the Latin insula for "island".

Insulin's structure varies slightly between species of animal. Insulin from animal sources differs somewhat in 'strength' (i.e., in carbohydrate metabolism control effects) in humans because of those variations. Porcine (pig) insulin is especially close to the human version.

Structure

Within vertebrates, the similarity of insulins is extremely close. Bovine insulin differs from human in only three amino acid residues, and porcine insulin in one. Even insulin from some species of fish is similar enough to human to be clinically effective in humans. Insulin in some invertebrates (eg, the c elegans nematode) is quite close to human insulin, has similar effects inside cells, and is produced very similarly. Insulin has been strongly preserved over evolutionary time, suggesting its centrality in animal metabolic control. The C-peptide of proinsulin (discussed later), however, differs much more amongst species; it is also a hormone, but a secondary one.