C-Peptide and Vascular Complications in Type 2 Diabetic Subjects

C-peptide is a small peptide comprised of 31 amino acids, with a short half-life of approximately 30 minutes. It was first identified by Steiner et al. [1] as a by-product of proinsulin and its main role is in assisting in the arrangement of the correct structure of insulin. Proinsulin consists of an A chain, connecting peptide (C-peptide), and B chain. C-peptide has a central glycine-rich region that allows the correct positioning of the A and B chains for insulin to achieve its tertiary structure [1]. It is secreted into the bloodstream in equimolar amounts together with insulin in response to glucose stimulation. C-peptide has been long considered an inactive peptide; however, over the last two decades, numerous studies have revealed that C-peptide displays a physiological role in different cell types [2,3]. The C-terminal pentapeptide of C-peptide obtains the full activity of intact C-peptide in stimulating Na+/K+-ATPase [4]. The amino acid sequence of C-peptide can vary by species, although it has several conserved sequences, such as its N-terminal acidic region, glycine-rich central segment, and C-terminal pentapeptide [5]. Moreover, evidence indicates that C-peptide is not merely an inactive by-product of insulin biosynthesis but also a hormonally-active peptide itself [3,4].