<P> The primary structure of bovine insulin was first determined by Frederick Sanger in 1951 . After that, this polypeptide was synthesized independently by several groups . The 3 - dimensional structure of insulin was determined by X-ray crystallography in Dorothy Hodgkin's laboratory in 1969 (PDB file 1ins). </P> <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>

Where is insulin stored in the human body