Circulating Serum C-Peptide

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Abstract
The discovery of proinsulin by Steiner and Oyer1 established that insulin is not synthesized as such, but as a larger precursor molecule in which the insulin A and B chains are connected by an additional peptide of 30 to 35 amino acids, depending on the species. Transformation of proinsulin to insulin begins in the Golgi apparatus and continues in the secretion granules of the pancreatic beta cells.2 A major portion of the connecting chain of amino acids remains intact during the conversion process and is released into the circulation along with insulin. This polypeptide which has been designated the "connecting-peptide" . . .