<P> Insulin is produced and stored in the body as a hexamer (a unit of six insulin molecules), while the active form is the monomer . The hexamer is an inactive form with long - term stability, which serves as a way to keep the highly reactive insulin protected, yet readily available . The hexamer - monomer conversion is one of the central aspects of insulin formulations for injection . The hexamer is far more stable than the monomer, which is desirable for practical reasons; however, the monomer is a much faster - reacting drug because diffusion rate is inversely related to particle size . A fast - reacting drug means insulin injections do not have to precede mealtimes by hours, which in turn gives people with diabetes more flexibility in their daily schedules . Insulin can aggregate and form fibrillar interdigitated beta - sheets . This can cause injection amyloidosis, and prevents the storage of insulin for long periods . </P> <P> Insulin is produced in the pancreas and the Brockmann body (in some fish), and released when any of several stimuli are detected . These stimuli include ingested protein and glucose in the blood produced from digested food . Carbohydrates can be polymers of simple sugars or the simple sugars themselves . If the carbohydrates include glucose, then that glucose will be absorbed into the bloodstream and blood glucose level will begin to rise . In target cells, insulin initiates a signal transduction, which has the effect of increasing glucose uptake and storage . Finally, insulin is degraded, terminating the response . </P> <P> In mammals, insulin is synthesized in the pancreas within the beta cells . One million to three million pancreatic islets form the endocrine part of the pancreas, which is primarily an exocrine gland . The endocrine portion accounts for only 2% of the total mass of the pancreas . Within the pancreatic islets, beta cells constitute 65--80% of all the cells . </P> <P> Insulin consists of two polypeptide chains, the A - and B - chains, linked together by disulfide bonds . It is however first synthesized as a single polypeptide called preproinsulin in beta cells . Preproinsulin contains a 24 - residue signal peptide which directs the nascent polypeptide chain to the rough endoplasmic reticulum (RER). The signal peptide is cleaved as the polypeptide is translocated into lumen of the RER, forming proinsulin . In the RER the proinsulin folds into the correct conformation and 3 disulfide bonds are formed . About 5--10 min after its assembly in the endoplasmic reticulum, proinsulin is transported to the trans - Golgi network (TGN) where immature granules are formed . Transport to the TGN may take about 30 min . </P>

Where is insulin produced in the human body
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