C-peptide

Overview
C-peptide is a peptide which is made when proinsulin is split into insulin and C-peptide. They split when proinsulin is released from the pancreas into the blood in response to a rise in serum glucose - one C-peptide for each insulin molecule.


 * In proinsulin, C-peptide is an amino-acid chain connecting A and B chains of insulin
 * The level of C-peptide in the body is indicative of how much insulin is being produced in the body
 * Reference range: 0.33-1.20 nmol/l

Etymology and History
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. The first documented use of the C-peptide test was in 1972.

It should not to be confused with c-reactive protein or Protein C.

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 for instance 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 actually removed from pharmaceutical preparations of insulin sold by drug companies when they manufacture the synthetic human insulin that is in widescale clinical usage today.

Uses

 * Newly diagnosed diabetes patients often get their C-peptide levels measured, to find if they are 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 used for determining the possibility of gastrinomas associated with Multiple endocrine neoplasia (MEN). Since a significant amount of gastrinomas also include MEN which include pancreatic, parathyroid, and pituitary adenomas, higher levels of c-protein in addition to a gastrinoma may suggest other organs than just the stomach may include neoplasms.
 * Can be used for identifying malingering: hypoglycemia with low C-peptide level may indicate abuse of insulin.

C-peptide levels are also checked to determine how insulin resistant women with Polycystic Ovarian Syndrome may be.

Decreased

 * Low insulin production
 * With elevated serum insulin: drug-induced hypoglycemia

Increased

 * Insulinoma

Normal Values in Patients with Injections of Insulin

 * Patient is still producing insulin