C-Peptide (C-P)

C-Peptide (C-P), also known as connecting peptide, is secreted by pancreatic β-cells and contains 31 amino acids; it shares a common precursor with insulin, insulinogen. In the insulinogen molecule connects the A and B chains of insulin. Insulinogen cleaves into one molecule of insulin and one molecule of C-peptide, so the two are co-secreted in equimolar amounts. However, its clearance from the blood is slower and less variable than that of insulin. Unlike insulin, which has a hepatic degradation rate of about 50%, C-peptide is not heavily degraded by the liver and its level is not affected by exogenous insulin levels or antibodies to insulin, so measuring C-peptide is a measure of insulin levels, which accurately reflects the functioning of pancreatic islet cells. However, in the case of chronic kidney disease, or when using less specific assays, beta cell function may be overestimated if levels of proinsulin or its conversion intermediates are elevated.

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Biological function of C-peptide

C peptide test and insulin test are usually done together, both are used to reflect the function of insulin secretion by β-cells. Human C peptide (C peptide) consists of 31 amino acids, and insulinogen contains a complete insulin molecule and a connecting peptide. This peptide is metabolized by removing two basic amino acids from each end of the peptide and becomes the C peptide. 1 molecule of insulinogen is cleaved by a special protease in the secretory granules of β-cells to form 1 molecule of insulin and 1 molecule of C peptide, with 4 free amino acids, and then β-cells secrete insulin and C peptide into the bloodstream in an equal number of molecules. C peptide plays an important role in the formation of molecular structure, forming and maintaining the stability and integrity of the insulin molecule. At the same time, C peptide is connected with the A chain and B chain of insulin, which reduces the biological activity of insulin and prevents insulin from direct decomposition by insulinase.C peptide is the secretion product of pancreatic β-cells and is produced by the same molecule as insulin, because C peptide is not destroyed by the liver, has a longer half-life than insulin and is not affected by the influence of exogenous insulin, the determination of C peptide level can more accurately reflect the function of pancreatic β-cells.

C-peptide measurement method

For the determination of human C-peptide, specific anti-human C-peptide antiserum must be prepared. Due to the common antigenic restriction between C-peptide and insulinogen, all C-peptide antibodies cross-react with insulinogen. In order to obtain accurate and specific C-peptide values, trypsinogen can be removed with antibodies to trypsin conjugated to dextran, and then C-peptide can be measured. Currently, radioimmunoassay or enzyme-linked immunosorbent assay is generally used to determine blood C-peptide concentration.

Value of C-peptide measurement

C-peptide value can be used to evaluate the endogenous insulin secretion capacity, which is more accurate than insulin measurement. Basal state and postexcitation C peptide levels are generally required to evaluate beta cell function.

(1) Peptide assays are used to differentiate between type 1 and type 2 diabetes; (2) fasting C peptide levels may overlap in normal individuals and type 2 diabetes; and (3) they are used in patients who receive exogenous insulin or who have developed antiglucagonistic antibodies to insulin. Immunologically, C-peptide does not cross-react with insulin antibodies, C-peptide measurement is not interfered by insulin, and plasma C-peptide levels can fully reflect endogenous insulin levels. C-peptide is more reliable than insulin as an indicator for evaluating the ability of β-cells to secrete insulin; (4) C-peptide measurement is used for evaluating the need of insulin treatment in type 2 diabetes mellitus; (5) C-peptide measurement is used in the differentiation of various hypoglycemic etiologies and monitoring the endocrine function of pancreatic grafts.

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