C-Peptide
The C-Peptide ELISA is an enzyme immunoassay for the quantitative in vitro diagnostic measurement of CPeptide in serum, plasma and urine
Insulin is synthesized in the pancreatic beta cells as a 6000 MW component of an 86 amino acid polypeptide called proinsulin (1, 2, 3). Proinsulin is subsequently cleaved enzymatically, releasing insulin into the circulation along with a residual 3000 MW fragment called connection (“C”) peptide, so-named because it connects A and B chains of insulin within the proinsulin molecule (1, 2, 3, 4). Human C-Peptide, a 31 amino acid residue peptide, has a molecular mass of approximately 3000 daltons. C-Peptide has no metabolic function. However, since C-Peptide and insulin are secreted in equimolar amounts, the immunoassay of C-Peptide permits the quantitation of insulin secretion (4, 5, 6). This is the reason for the clinical interest of serum and urinary determinations of C-Peptide. Moreover, C-Peptide measurement has several advantages over immunoassays of insulin. The half-life of C-Peptide in the circulation is between two and five times longer than that of insulin (7). Therefore, CPeptide levels are a more stable indicator of insulin secretion than the more rapidly changing levels of insulin. A very clear practical advantage of C-Peptide measurement arising from its relative metabolic inertness as compared to insulin is that C-Peptide levels in peripheral venous blood are about 5-6 times greater than insulin levels (3). Also, relative to an insulin assay, the C-Peptide assay’s advantage is its ability to distinguish endogenous from injected insulin. Thus, low C-Peptide levels are to be expected when insulin is diminished (as in insulin-dependent diabetes) or suppressed (as a normal response to exogenous insulin), whereas elevated C-Peptide levels may result from the increased β-cell activity observed in insulinomas (3, 6, 9). C-Peptide has also been measured as an additional means for evaluating glucose tolerance and glibenclamide glucose tests (2, 3, 9, 10). C-Peptide levels are in many ways a better measurement of endogenous insulin secretion than peripheral insulin levels. CPeptide may be measured in either blood or urine (9). With improved sensitive C-Peptide immunoassays, it is now possible to measure C-Peptide values at extremely low levels. The clinical indications for C-Peptide measurement include diagnosis of insulinoma and differentation from factitious hypoglycemia, follow-up of pancreatectomy, and evaluation of viability of islet cell transplants (11, 12, 13). Recently, these indications have been dramatically expanded to permit evaluation of insulin dependence in maturity onset diabetes mellitus.
The C-Peptide ELISA Kit is a solid phase enzyme-linked immunosorbent assay (ELISA), based on the principle of competitive binding. The microtiter wells are coated with anti-mouse antibody, which binds a monoclonal antibody directed towards a unique antigenic site on the C-Peptide molecule. Endogenous C-Peptide of a patient sample competes with a C-Peptidehorseradish peroxidase conjugate for binding to the coated antibody. After incubation the unbound conjugate is washed off. The amount of bound peroxidase conjugate is inversely proportional to the concentration of C-Peptide in the sample. After addition of the substrate solution, the intensity of colour developed is inversely proportional to the concentration of C-Peptide in the patient sample.
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- Beischer, W.: Proinsulin and C-Peptide in Humans. Hormones in Normal and Abnormal Human Tissues. Volume 3K, Fotherby and Pal, S., ed. (Berlin: Walter DeGruyter). pp. 1-43, 1983
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- Blix, P. Boddie-Wills, C., Landau, R., Rochman, H. Rubenstein, A.: Urinary C-Peptide: An Indicator of Beta-Cell Secretion under Different Metabolic Conditions. Journal of Clinical Endocrinology and Metabolism. 54:574, 1982.
- Rendell, M.: C-Peptide Levels as a Criterion in Treatment of Maturity-Onset Diabetes. Journal of Clinical Endocrinology and Metabolism. 57 (6): 1198, 1983
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- Canivet, B., Harter, M., Viot, G., Balgrac, N., Krebs, B.: Residual ß-Cell Function in Insulin-Dependent Diabetes: Evaluation by Circadian Determination of C-Peptide Immuno reactivity. Journal of Endocrinological Investigation. 3:107, 1980.
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- Rubenstein, A., Kuruya, H., Horwitz, D.: Clinical Significance of Circulating C-Peptide in Diabetes Mellitus and Hypoglycemic Disorders. Archives of Internal Medicine. Vol. 137:625, May 1977.
- Yalow, R., Berson, S.: Introduction and General Considerations. Principles of Competitive Protein Binding Assays. Ch. 2, Eds. Odell, W. and Daugheday, W., J.B. Lippincott Co., Philadelphia, 1971