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[edit | edit source]
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[edit | edit source]
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[edit | edit source]
Cellular effects of C-peptide[edit | edit source]
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[edit | edit source]
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[edit | edit source]
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.
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Contributors: Prab R. Tumpati, MD