C-peptide is a small protein that is produced when proinsulin is cleaved during the process of insulin biosynthesis in the pancreas. 

Proinsulin is the precursor to insulin, and during biosynthesis, it is split into insulin and C-peptide.

Once thought to be biologically inert, it is now recognized as an important molecule with several functions. 

C-peptide serves as a biomarker for insulin secretion and can be used to assess beta-cell function in the pancreas. 

An indirect measure of viable beta cell function.

Measuring C-peptide levels in the blood can differentiate between endogenous insulin production and insulin from exogenous sources, such as insulin injections.

Measurements of C-peptide responses to glucose stimulation are helpful to determine the decision to use insulin in both elderly and middle-aged patients with diabetes.

Blunted insulin responses identify those patients who require insulin treatment, whereas high C-peptide values in insulin-treated patients suggest the possibility of discontinuing insulin therapy.

The connecting peptide, or C-peptide, is a short 31-amino-acid protein that connects insulin’s A-chain to its B-chain in the proinsulin molecule.

In the insulin synthesis pathway: first preproinsulin is translocated into the endoplasmic reticulum of beta cells of the pancreas with an A-chain, a C-peptide, a B-chain, and a signal sequence.

The signal sequence is cleaved from the N-terminus of the peptide by a signal peptidase, leaving proinsulin.

After proinsulin is packaged into vesicles in the Golgi apparatus, the C-peptide is removed, leaving the A-chain and B-chain, bound together by disulfide bonds, that constitute the insulin molecule.

Proinsulin C-peptide serves as a linker between the A- and the B- chains of insulin and facilitates the efficient assembly, folding, and processing of insulin in the endoplasmic reticulum.

Equimolar amounts of C-peptide and insulin are stored in secretory granules of the pancreatic beta cells and both are eventually released to the portal circulation.

C-peptide is a marker of insulin secretion.

C-peptide is a bioactive peptide with effects on microvascular blood flow and tissue health.

C-peptide also has physiological effects on its own, separate from those of insulin. It has been shown to improve microvascular blood flow in people with type 1 diabetes and may have protective effects on nerves and kidneys. 

Following autologous nonmyeloblative hematopoietic stem cell transplantation in patients with type I diabetes mellitus a follow-up at 29.8 months C-peptide levels increased significantly and majority of patients achieved insulin independence with good glycemic control (Couri E).

Late microvascular complications are inversely related to C-peptide levels.

Verapamil increased C-peptide levels.

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