- Insulin is a peptide hormone and is thnus composed of a linear amino acid polymer. Uniquely, in its mature secreted form, insulin contains two separate polypeptide chains linked together by disulfide bonds. Insulin is synthesized from a precursor, Proinsulin, which is a single, long polypeptide with internal disulfide bonds. Proinsulin is expressed like any other cellular protein and is transported to the endoplasmic reticulum where a segment of the protein is cleaved, generating the mature insulin protein and C-peptide fragment. Mature Insulin and C-peptide are then stored in secretory granules in beta cells and await regulated release into the blood through exocytosis, as discussed in insulin regulation. Importantly, C-peptide is released together with mature insulin during secretion and can be detected by laboratory assays.