C-peptide serves as an important linker between A-chain and B-chain of insulin and facilitates the efficient assembly, folding, and processing of insulin in the endoplasmic reticulum. Equimolar amounts of C-peptide and insulin are stored in secretory granules of the pancreatic beta cells and both are eventually released to the portal circulation. The sole interest in C-peptide was as a marker of insulin secretion. Newly diagnosed diabetes patients often get their C-peptide levels measured as a means of distinguishing type 1 and type 2 diabetes. C-peptide is also used for determining the possibility of gastrinomas associated with Multiple Endocrine Neoplasm syndromes (MEN 1).
