Proinsulin is synthesized in the β-cells of the endocrine pancreas, one of the four cell types
found in the islets of Langerhans. Specific enzymatic cleavage of proinsulin results in the
formation of equimolar amounts of insulin and C-peptide, via several intermediate split-
proinsulin forms. Most mammals produce a single insulin, but in rodents two non-allelic
insulin genes are expressed. There is an inverse ratio between the two insulins in rats and
mice, the reason for this being unknown. It has been suggested that differences in …