C-peptide is a peptide which is made when proinsulin is split into insulin and C-peptide. They split before proinsulin is released from endocytic vesicles within the pancreas -- one C-peptide for each insulin molecule.

C-peptide is the abbreviation for "connecting peptide", although its name was probably also inspired by the fact that insulin is also composed of an "A" chain and a "B" chain. C-peptide was discovered in 1967. It should not to be confused with c-reactive protein or Protein C. The first documented use of the C-peptide test was in 1972.

Function

C-peptide functions in repair of the muscular layer of the arteries.

C-peptide also exerts beneficial therapeutic effects on many complications associated with diabetes mellitus, such as diabetic neuropathy and other diabetes-induced ailments. In the kidneys, C-peptide prevents diabetic nephropathy, and in the heart blood flow is improved in diabetic patients.

In spite of these physiological functions, C-peptide is not present in pharmaceutical preparations of insulin sold by drug companies that are in wide-scale clinical usage today, a practice seen as unethical in light of more research suggesting the peptide's utility.

Ironically, back in 1997, insulin manufacturer Eli Lilly and Company jointly funded research into C-Peptide as a possible therapeutic. In the research undertaken by researchers at Washington University School of Medicine in St. Louis, they determined that C-Peptide may effectively prevent and even reverse cardiovascular disease and nerve damage in people with diabetes, although their studies were only on rodent models of the disease. However, the company never pursued commercialization of the product. But in 2007 a letter to the Indianapolis Star, company executive John C. Lechleiter did indicate that the company was pursuing development of drugs to treat diabetes-induced complications.

Uses

• Newly diagnosed diabetes patients often get their C-peptide levels measured, to find if they have type 1 diabetes or type 2 diabetes. The reason that the C-peptide levels are measured instead of the insulin levels themselves is because insulin concentration in the portal vein ranges from two to ten times higher than in the peripheral circulation. The liver extracts about half the insulin reaching it (the plasma), but this varies with the nutritional state. The pancreas of patients with type 1 diabetes is unable to produce insulin and they will therefore usually have a decreased level of C-peptide, while C-peptide levels in type 2 patients is normal or higher than normal. Measuring C-peptide in patients injecting insulin can help to determine how much of their own natural insulin these patients are still producing.

C-peptide is also easily detected because antibodies that are sensitive to it are readily available, whereas antibodies to insulin are much more difficult to obtain.