Wikipedia - Insulin
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.
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[edit] 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.
[edit] 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.
[edit] Production
In mammals, insulin is synthesized in the pancreas within the beta cells (ß-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.
[edit] Regulation of production
- Further information: Insulin gene
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
[edit] Release
- See also: Blood glucose regulation
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 adrenaline 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 gluconeogenesis and glycogenolysis from the low blood insulin: glucagon state) unless insulin is present to allow for GLUT-4 translocation in the tissue. Therefore, beginning with direct innervation, norepinephrine inhibits insulin release via alpha2-receptors, then subsequently, circulating adrenaline 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.
[edit] Oscillations
Even during digestion, generally one or two hours following a meal, insulin release from pancreas is not continuous, but oscillates with a period of 3–6 minutes, changing from generating a blood insulin concentration more than ~800 pmol/l to less than 100 pmol/l.[1] This is thought to avoid downregulation of insulin receptors in target cells and to assist the liver in extracting insulin from the blood.[1] This oscillation is important to consider when administering insulin-stimulating medication, since it is the oscillating blood concentration of insulin release which should, ideally, be achieved, not a constant high concentration.[1] This may be achieved by delivering insulin rhythmically to the portal vein or by islet cell transplantation to the liver.[1] Future insulin pumps may include this characteristic. (See also Pulsatile Insulin.)
[edit] Signal transduction
There are special transporter proteins in cell membranes through which glucose from the blood can enter a cell. These transporters are, indirectly, under blood insulin's control in certain body cell types (e.g., muscle cells). Low levels of circulating insulin, or its absence, will prevent glucose from entering those cells (e.g., in Type 1 diabetes). However, more commonly there is a decrease in the sensitivity of cells to insulin (e.g., the reduced insulin sensitivity characteristic of Type 2 diabetes), resulting in decreased glucose absorption. In either case, there is 'cell starvation', weight loss, sometimes extreme. In a few cases, there is a defect in the release of insulin from the pancreas. Either way, the effect is, characteristically, the same: elevated blood glucose levels.
Activation of insulin receptors leads to internal cellular mechanisms that directly affect glucose uptake by regulating the number and operation of protein molecules in the cell membrane that transport glucose into the cell. The genes that specify the proteins that make up the insulin receptor in cell membranes have been identified and the structure of the interior, cell membrane section, and now, finally after more than a decade, the extra-membrane structure of receptor (Australian researchers announced the work 2Q 2006).
Two types of tissues are most strongly influenced by insulin, as far as the stimulation of glucose uptake is concerned: muscle cells (myocytes) and fat cells (adipocytes). The former are important because of their central role in movement, breathing, circulation, etc, and the latter because they accumulate excess food energy against future needs. Together, they account for about two-thirds of all cells in a typical human body.
[edit] Effects
The actions of insulin on the global human metabolism level include:
- Control of cellular intake of certain substances, most prominently glucose in muscle and adipose tissue (about ? of body cells).
- Increase of DNA replication and protein synthesis via control of amino acid uptake.
- Modification of the activity of numerous enzymes.
The actions of insulin on cells include:
- Increased glycogen synthesis – insulin forces storage of glucose in liver (and muscle) cells in the form of glycogen; lowered levels of insulin cause liver cells to convert glycogen to glucose and excrete it into the blood. This is the clinical action of insulin which is directly useful in reducing high blood glucose levels as in diabetes.
- Increased fatty acid synthesis – insulin forces fat cells to take in blood lipids which are converted to triglycerides; lack of insulin causes the reverse.
- Increased esterification of fatty acids – forces