Insulin is synthesized as preproinsulin in the ribosomes of the rough endoplasmic reticulum. The preproinsulin is then cleaved to proinsulin, which is transported to the Golgi apparatus where it is packaged into secretory granules located close to the cell membrane. Most of the proinsulin is cleaved into equimolar amounts of insulin and C-peptide in the secretory granules.
