Why is proinsulin so called ? How is insulin different from it?

Insulin is a small protein hormone produced by the pancreas to regulate the sugar concentration. The human insulin protein is composed of 51 amino acids. It is a dimer of an A-chain and a B-chain, which are linked together by disulfide bonds. In humans, pancreatic cells first make pro-insulin, which then undergoes post-translational modification to make the final, functional insulin. Proinsulin contains an extra stretch called the C-peptide. The C-peptide is not present in mature insulin and is removed during the maturation into insulin.