Insulin is synthesised as a single polypeptide pre-prohormone on rough endo-plasmic reticulum of pancreatic B cells. Cleavage of signal sequence yields 86 amino acid peptide proinsulin, which is the storage form of insulin.
Proinsulin is converted to insulin, immediately before secretion, in clathrin-coated prosecretory vesicles, with the excision of C peptide. Mature secretory vesicles in beta cells of the pancreatic islets contain equimolar amounts of insulin and C peptide. Circulating C peptide levels reflect beta cell activity.
Insulin is released by exocytosis of the secretory vesicles, stimulated by raised plasma glucose levels. The circulating half-life of endogenous insulin is 5-20 minutes.
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