C-peptide

C peptide|thumb C-peptide, also known as the connecting peptide, is a short 31-amino-acid protein integral in the proinsulin molecule's formation. It acts as a bridge, connecting insulin's A-chain to its B-chain. The peptide plays a pivotal role in insulin synthesis and has subsequently been recognized for its own bioactive properties.

Insulin Synthesis Pathway

During insulin production, preproinsulin is transported to the endoplasmic reticulum within the beta cells of the pancreas. This structure comprises an A-chain, C-peptide, a B-chain, and a signal sequence. An enzyme called signal peptidase cleaves the signal sequence from the N-terminus, resulting in proinsulin formation. Once in the Golgi apparatus, proinsulin is packaged into vesicles where the C-peptide gets detached, resulting in the insulin molecule, a combination of the A-chain and B-chain.

History

C-peptide, in association with the biosynthesis of insulin, was first unveiled in 1967. It functions as a linker between insulin's A and B chains, ensuring effective assembly, folding, and processing within the endoplasmic reticulum. Both insulin and C-peptide are stored in the beta cells of the pancreas, and they are released simultaneously. Historically, the primary interest in C-peptide was as a determinant of insulin secretion. It has significantly advanced our understanding of the pathophysiology of both type 1 and type 2 diabetes. The inaugural use of the C-peptide test occurred in 1972. However, over the last decade, C-peptide has been recognized as a bioactive peptide with distinct effects on microvascular blood flow and tissue well-being.

Function

Cellular effects of C-peptide

Studies have shown that C-peptide can bind to various cell surfaces, including neuronal, endothelial, fibroblast, and renal tubular cells. This binding likely happens through a G-protein-coupled receptor. This initiates intracellular signaling pathways such as MAPK, PLCγ, and PKC, which in turn lead to the activation of various transcription factors, eNOS, and Na+K+ATPase activities. The last two enzymes are especially relevant since their activity diminishes in type I diabetes patients, contributing to complications such as neuropathy.

Animal studies have illustrated that administering C-peptide can result in significant improvements in nerve and kidney functionality. Additionally, C-peptide appears to have anti-inflammatory effects and aids the repair of smooth muscle cells.

Clinical uses of C-peptide testing

C-peptide tests are crucial for:

• Distinguishing between type 1 and type 2 diabetes or Maturity Onset Diabetes of the Young (MODY).
• Assessing a patient's natural insulin production.
• Identifying the root cause of hypoglycemia.
• Gauging the potential of gastrinomas linked with Multiple Endocrine Neoplasm syndromes (MEN 1).
• Checking C-peptide levels in Polycystic Ovarian Syndrome (PCOS) patients to determine the degree of insulin resistance.

Therapeutics

Phase 1 and exploratory Phase 2 studies involving nearly 300 type 1 diabetes patients lacking endogenous C-peptide have identified multiple physiological effects of the peptide. Improvements were noticed in conditions like diabetic peripheral neuropathy, nephropathy, and other long-term complications associated with type I diabetes.

• C-peptide
  C-peptide