• Chinese: (yídǎosù)
• Czech: inzulín
• French: insuline
• Finnish: insuliini
• German: Insulin
• Italian: insulina
• Japanese: インシュリン (inshurin)
• Korean: 인슐린
• Portuguese: insulina
• Spanish: insulina

confuse inulin Insulin is a hormone with extensive effects on both metabolism and several other body systems (eg, vascular compliance). When present, it 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.

Mechanism

Insulin is produced in the pancreas, and released when any of several stimuli are detected. These include protein ingestion, and glucose in the blood (from food which produces glucose when digested -- characteristically this is carbohydrate, though not all types produce glucose and so an increase in blood glucose levels). In target cells, they initiate a signal transduction which has the effect of increasing glucose uptake and storage. Finally, insulin is degraded, terminating the response.

Production

In mammals, insulin is synthesized in the pancreas within the beta cells (β-cells) of the islets of Langerhans. One million to three million islets of Langerhans (pancreatic islets) form the endocrine part of the pancreas, which is primarily an exocrine gland. The endocrine portion only accounts for 2% of the total mass of the pancreas. Within the islets of Langerhans, beta cells constitute 60–80% of all the cells.

In beta cells, insulin is synthesized from the proinsulin precursor molecule by the action of proteolytic enzymes, known as prohormone convertases (PC1 and PC2), as well as the exoprotease carboxypeptidase E. These modifications of proinsulin remove the center portion of the molecule (ie, C-peptide), from the C- and N- terminal ends of proinsulin. The remaining polypeptides (51 amino acids in total), the B- and A- chains, are bound together by disulfide bonds/disulphide bonds. Confusingly, the primary sequence of proinsulin goes in the order "B-C-A", since B and A chains were identified on the basis of mass, and the C peptide was discovered after the others.

Regulation of production

The endogenous production of insulin is regulated in several steps along the synthesis pathway:

• At transcription from the insulin gene
• In mRNA stability
• At the mRNA translation
• In the posttranslational modifications

Release

Beta cells in the islets of Langerhans release insulin mostly in response to increased blood glucose levels through the following mechanism (see figure to the right):

• Glucose enters the beta cells through the glucose transporter GLUT2
• Glucose goes into the glycolysis and the respiratory cycle where multiple high-energy ATP molecules are produced by oxidation
• Dependent on ATP levels, and hence blood glucose levels, the ATP-controlled potassium channels (K+) close and the cell membrane depolarizes
• On depolarization, voltage controlled calcium channels (Ca2+) open and calcium flows into the cells
• An increased calcium level causes activation of phospholipase C, which cleaves the membrane phospholipid phosphatidyl inositol 4,5-bisphosphate into inositol 1,4,5-triphosphate and diacylglycerol.
• Inositol 1,4,5-triphosphate (IP3) binds to receptor proteins in the membrane of endoplasmic reticulum (ER). This allows the release of Ca2+ from the ER via IP3 gated channels, and further raises the cell concentration of calcium.
• Significantly increased amounts of calcium in the cells causes release of previously synthesised insulin, which has been stored in secretory vesicles

This is the main mechanism for release of insulin and regulation of insulin synthesis. In addition some insulin synthesis and release takes place generally at food intake, not just glucose or carbohydrate intake, and the beta cells are also somewhat influenced by the autonomic nervous system. The signalling mechanisms controlling these linkages are not fully understood.

Other substances known to stimulate insulin release include amino acids from ingested proteins, acetylcholine, released from vagus nerve endings (parasympathetic nervous system), cholecystokinin[citation needed], released by enteroendocrine cells of intestinal mucosa and glucose-dependent insulinotropic peptide (GIP). Three amino acids (alanine, glycine and arginine) act similarly to glucose by altering the beta cell's membrane potential. Acetylcholine triggers insulin release through phospholipase C, while the last acts through the mechanism of adenylate cyclase.

The sympathetic nervous system (via Alpha2-adrenergic stimulation as demonstrated by the agonists clonidine or methyldopa) inhibit the release of insulin. However, it is worth noting that circulating epinephrine will activate Beta2-Receptors on the Beta cells in the pancreatic Islets to promote insulin release. This is important since muscle cannot benefit from the raised blood sugar resulting from adrenergic stimulation (increased gluconeogenisis and glycogenolysis from the low blood insulin:glucogon state) unless insulin in present to allow for GLUT-4 translocation in the tissue. So in summary, first through direct innervation, NE inhibits insulin release via alpha2-receptors, then later, circulating Epi from the adrenal medulla will stimulate beta2-receptors thereby promoting insulin release.

When the glucose level comes down to the usual physiologic value, insulin release from the beta cells slows or stops. If blood glucose levels drop lower than this, especially to dangerously low levels, release of hyperglycemic hormones (most prominently glucagon from Islet of Langerhans' alpha cells) forces release of glucose into the blood from cellular stores, primarily liver cell stores of glycogen. By increasing blood glucose, the hyperglycemic hormones correct life-threatening hypoglycemia. Release of insulin is strongly inhibited by the stress hormone norepinephrine (noradrenaline), which leads to increased blood glucose levels during stress.

Oscillations

Over the next two decades, several attempts were made to isolate whatever it was the islets produced as a potential treatment. In 1906 George Ludwig Zuelzer was partially successful treating dogs with pancreatic extract but was unable to continue his work. Between 1911 and 1912, E.L. Scott at the University of Chicago used aqueous pancreatic extracts and noted a slight diminution of glycosuria but was unable to convince his director of his work's value; it was shut down. Israel Kleiner demonstrated similar effects at Rockefeller University in 1919, but his work was interrupted by World War I and he did not return to it. Nicolae Paulescu, a professor of physiology at the University of Medicine and Pharmacy in Bucharest was the first one to isolate insulin, which he called at that time pancrein, and published his work in 1921 that had been carried out in Bucharest. Use of his techniques was patented in Romania, though no clinical use resulted.

In October 1920 Canadian Frederick Banting was reading one of Minkowski's papers and concluded that it is the very digestive secretions that Minkowski had originally studied that were breaking down the islet secretion(s), thereby making it impossible to extract successfully. He jotted a note to himself Ligate pancreatic ducts of the dog. Keep dogs alive till acini degenerate leaving islets. Try to isolate internal secretion of these and relieve glycosurea.

The idea was that the pancreas's internal secretion, which supposedly regulates sugar in the bloodstream, might hold the key to the treatment of diabetes.

He travelled to Toronto to meet with J.J.R. Macleod, who was not entirely impressed with his idea – so many before him had tried and failed. Nevertheless, he supplied Banting with a lab at the University of Toronto, an assistant (medical student Charles Best), and 10 dogs, then left on vacation during the summer of 1921. Their method was tying a ligature (string) around the pancreatic duct, and, when examined several weeks later, the pancreatic digestive cells had died and been absorbed by the immune system, leaving thousands of islets. They then isolated an extract from these islets, producing what they called isletin (what we now know as insulin), and tested this extract on the dogs. Banting and Best were then able to keep a pancreatectomized dog alive all summer because the extract lowered the level of sugar in the blood.

Macleod saw the value of the research on his return but demanded a re-run to prove the method actually worked. Several weeks later it was clear the second run was also a success, and he helped publish their results privately in Toronto, ON that November. However, they needed six weeks to extract the isletin, which forced considerable delays. Banting suggested that they try to use fetal calf pancreas, which had not yet developed digestive glands; he was relieved to find that this method worked well. With the supply problem solved, the next major effort was to purify the extract. In December 1921, Macleod invited the biochemist James Collip, to help with this task, and, within a month, the team felt ready for a clinical test.

On January 11, 1922, Leonard Thompson, a 14-year-old diabetic who lay dying at the Toronto General Hospital, was given the first injection of insulin. However, the extract was so impure that Thompson suffered a severe allergic reaction, and further injections were canceled. Over the next 12 days, Collip worked day and night to improve the ox-pancreas extract, and a second dose was injected on the 23rd. This was completely successful, not only in having no obvious side-effects, but in completely eliminating the glycosuria sign of diabetes.

Children dying from diabetic keto-acidosis were kept in large wards, often with 50 or more patients in a ward, mostly comatose. Grieving family members were often in attendance, awaiting the (until then, inevitable) death. In one of medicine's more dramatic moments Banting, Best and Collip went from bed to bed, injecting an entire ward with the new purified extract. Before they had reached the last dying child, the first few were awakening from their coma, to the joyous exclamations of their families.

However, Banting and Best never worked well with Collip, regarding him as something of an interloper, and Collip left the project soon after.

Over the spring of 1922, Best managed to improve his techniques to the point where large quantities of insulin could be extracted on demand, but the preparation remained impure. The drug firm Eli Lilly and Company had offered assistance not long after the first publications in 1921, and they took Lilly up on the offer in April. In November, Lilly made a major breakthrough, and were able to produce large quantities of highly refined, 'pure' insulin. Insulin was offered for sale shortly thereafter.

Nobel Prizes

The Nobel Prize committee in 1923 credited the practical extraction of insulin to a team at the University of Toronto and awarded the Nobel Prize to two men; Frederick Banting and J.J.R. Macleod. They were awarded the Nobel Prize in Physiology or Medicine in 1923 for the discovery of insulin. Banting, insulted that Best was not mentioned, shared his prize with Best, and Macleod immediately shared his with Collip. The patent for insulin was sold to the University of Toronto for one dollar.

Surprisingly, Banting and Macleod received the 1923 Nobel Prize in Physiology or Medicine for the discovery of insulin, while Paulescu's pioneering work was being completely ignored by the scientific and medical community. International recognition for Paulescu's merits as the true discoverer of insulin came only 50 years later.

The primary structure of insulin was determined by British molecular biologist Frederick Sanger. It was the first protein to have its sequence be determined. He was awarded the 1958 Nobel Prize in Chemistry for this work.

In 1969, after decades of work, Dorothy Crowfoot Hodgkin determined the spatial conformation of the molecule, the so-called tertiary structure, by means of X-ray diffraction studies. She had been awarded a Nobel Prize in Chemistry in 1964 for the development of crystallography.

Rosalyn Sussman Yalow received the 1977 Nobel Prize in Medicine for the development of the radioimmunoassay for insulin.

Timeline of insulin research

• 1922 Banting, Best, Collip use bovine insulin extract in human
• 1923 Eli Lilly produces commercial quantities of much purer bovine insulin than Banting et al had used
• 1923 Farbwerke Hoechst, one of the forerunner's of today's Sanofi Aventis, produces commercial quantities of bovine insulin in Germany
• 1923 Hagedorn founds the Nordisk Insulinlaboratorium in Denmark – forerunner of today's Novo Nordisk
• 1926 Nordisk receives a Danish charter to produce insulin as a non-profit
• 1936 Canadians D.M. Scott, A.M. Fisher formulate a zinc insulin mixture and license it to Novo
• 1936 Hagedorn discovers that adding protamine to insulin prolongs the duration of action of insulin
• 1946 Nordisk formulates Isophane porcine insulin aka Neutral Protamine Hagedorn or NPH insulin
• 1946 Nordisk crystallizes a protamine and insulin mixture
• 1950 Nordisk markets NPH insulin
• 1953 Novo formulates Lente porcine and bovine insulins by adding zinc for longer lasting insulin
• 1955 Frederick Sanger determines the amino acid sequence of insulin
• 1965 Synthesized by total synthesis by C.L. Tsou and coworkers
• 1969 Dorothy Crowfoot Hodgkin solves the crystal structure of insulin by x-ray crystallography
• 1973 Purified monocomponent (MC) insulin is introduced
• 1973 The U.S. officially "standardized" insulin sold for human use in the U.S. to U-100 (100 units per milliliter). Prior to that, insulin was sold in different strengths, including U-80 (80 units per milliliter) and U-40 formulations (40 units per milliliter), so the effort to "standardize" the potency aimed to reduce dosage errors and ease doctors' job of prescribing insulin for patients. Other countries also followed suit.
• 1978 Genentech produces synthetic 'human' insulin in Escheria coli bacteria using recombinant DNA techniques, licenses to Eli Lilly
• 1981 Novo Nordisk chemically and enzymatically converts porcine to human insulin
• 1982 Genentech synthetic 'human' insulin (above) approved
• 1983 Eli Lilly and Company produces synthetic 'human' insulin with recombinant DNA technology, Humulin
• 1985 Axel Ullrich sequences a human cell membrane insulin receptor.
• 1988 Novo Nordisk produces recombinant human insulin
• 1996 Lilly Humalog "lispro" insulin analogue approved.
• 2000 Sanofi Aventis Lantus "glargine" insulin analogue approved for clinical use in the US and Europe.
• 2004 Sanofi Aventis insulin glulisine insulin analogue approved for clinical use in the US.
• 2006 Novo Nordisk Levemir "detemir" insulin analogue approved for clinical use in the US.

See also

• Insulin analog
• Anatomy and physiolology
  • Glucagon
  • Pancreas
  • Islets of Langerhans
  • Endocrinology
• Forms of diabetes mellitus
  • Diabetes mellitus
  • Diabetes mellitus type 1
  • Diabetes mellitus type 2
• Treatment
  • Diabetic coma
  • Intensive insulinotherapy
  • Insulin pump
  • Conventional insulinotherapy
• Other medical / diagnostic uses
  • Insulin tolerance test
  • Triple bolus test

References

• Insulin Resistance: The Metabolic Syndrome X

• Insulin Therapy

• Insulin Resistance: Insulin Action and Its Disturbances in Disease

• Medical Terminology for Health Professions

• Molecular Biology of Diabetes: Autoimmunity and Genetics; Insulin Synthesis and Secretion
• Famous Canadian Physicians: Sir Frederick Banting at Library and Archives Canada
• McKeage K, Goa KL. (2001) Insulin glargine: a review of its therapeutic use as a long-acting agent for the management of type 1 and 2 diabetes mellitus. Drugs.61:1599-624

Footnotes

External links

• The Insulin Protein
• Inspired by Insulin article by parent of a diabetic child
• Frederick Sanger, Nobel Prize for sequencing Insulin Freeview video with John Sanger and John Walker by the Vega Science Trust.
• Insulin: entry from protein databank
• The History of Insulin
• Insulin Lispro
• CBC Digital Archives - Banting, Best, Macleod, Collip: Chasing a Cure for Diabetes
• Cosmos Magazine: Insulin mystery cracked after 20 years
• National Diabetes Information Clearinghouse
• Discovery and Early Development of Insulin, 1920–1925
• Secretion of Insulin and Glucagon
• Insulin Types Comparison Chart