A pancreatic polypeptide of about 110 amino acids, depending on the species, that is the precursor of insulin. proinsulin, produced by the PANCREATIC BETA cells, is comprised sequentially of the N-terminal B-chain, the proteolytically removable connecting c-peptide, and the C-terminal A-chain. It also contains three disulfide bonds, two between A-chain and B-chain. After cleavage at two locations, insulin and c-peptide are the secreted products. Intact proinsulin with low bioactivity also is secreted in small amounts.