insulin is synthesized as an inactive precursor molecule, a 110 amino acid-long protein called "preproinsulin". preproinsulin is translated directly into the rough endoplasmic reticulum (rer), where its signal peptide is removed by signal peptidase to form "proinsulin". as the proinsulin folds, opposite ends of the protein, called the "a-chain" and the "b-chain", are fused together with three disulfide bonds. folded proinsulin then transits through the golgi apparatus and is packaged into specialized secretory vesicles. in the granule, proinsulin is cleaved by proprotein convertase 1/3 and proprotein convertase 2, removing the middle part of the protein, called the "c-peptide". finally, carboxypeptidase e removes two pairs of amino acids from the protein's ends, resulting in active insulin – the insulin a- and b- chains, now connected with two disulfide bonds. the resulting mature insulin is packaged inside mature granules waiting for metabolic signals (such as leucine, arginine, glucose and mannose) and vagal nerve stimulation to be exocytosed from the cell into the circulation. insulin and its related proteins have been shown to be produced inside the brain, and reduced levels of these proteins are linked to alzheimer's disease. insulin release is stimulated also by beta-2 receptor stimulation and inhibited by alpha-1 receptor stimulation. in addition, cortisol, glucagon and growth hormone antagonize the actions of insulin during times of stress. insulin also inhibits fatty acid release by hormone sensitive lipase in adipose tissue. structure