C –Peptide

Introduction

Insulin is produced from the pancreatic islet beta cells, initially in the form of a large molecule called the preproinsulin molecule. This s then cleaved into a smaller molecule called the proinsulin, which has a chain of amino acids connecting the A and B chains of the insulin. This part, which is later cleaved off to produce the final insulin is called the c-peptide or the connecting peptide.

Chemical structure

It is a chain of amino acids, which is cleaved off and released into the blood along with the insulin, from the pancreas.

Metabolism in the body

The c-peptide, released in equal amounts as that of the insulin, is excreted unchanged in the urine.

Physiological action in the body

Till recently, it was believed that this peptide is an inactive product, that is formed purely as byproduct in the insulin production and that it had no physiological role. But recently it has been shown that it has a protective function on both the nerves and the eyes, through some unknown mechanism.

Uses

This is mainly used in the diagnosis of varying conditions like

1. Factitious hyperinsulinemia due to exogenous insulin administration
2. Hyper insulinemia due to insulinoma
3. Differentiate the type of diabetes, when diagnosed newly etc

Factitious hyperinsulinemia

This is a reason of hypoglycemia when the insulin preparations are taken in excess, either intentionally or otherwise. Since the insulin preparations have no c-peptide component and since the exogenously administered insulin inhibits the natural insulin’s secretion, c-peptide is low in this instant

Insulinoma

This is a non-cancerous tumour of the insulin producing cells of the pancreas, resulting in an abnormally high production of insulin and hence an abnormally high level of c-peptide, resulting in hypoglycemia again.

Type 1 diabetes mellitus

On being newly diagnosed, c-peptide concentrations are very low, since the basic defect in this type of diabetes is absolute deficiency in insulin production. After some years, it may be undetectable since there is no insulin production now.

Type 2 diabetes mellitus

Type 2 has normal or greater than normal c-peptide concentration, since the defect here is the deficiency of insulin action not its production.

Patient preparation for the test.

Patient is kept fasting for about 10-12 hours. Patient is also asked not to take his regular Insulin or any other medications since they may interfere with the results.

Procedure

The blood required is drawn from a peripheral vein (venepuncture) using all sterile precautions and then sent immediately for the assay.

Confounding factors

These are the factors, which give false positive or false negative results

A. Drugs :like insulin, oral diabetic drugs, corticosteroids, birth control pills, etc.

B. Renal failure : since the kidney eliminates c-peptide, it gives a false high positive result in renal failure.

C. Obesity also is associated with high levels of c-peptide on its own.

Update.

This peptide previously thought to be an inactive peptide, is now supposed to have protective role on the nerves and the eyes.