C-Peptide ELISA

Solid phase enzyme immunoassay (ELISA) for the determination of C-Peptide in serum, plasma and urine. FDA exempt, can be used for in-vitro diagnostics.<br><br>

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).<br><br>

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 cpeptide in the circulation is between two and five times longer than that of insulin (7). Therefore, c-peptide 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.<br><br>

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).<br><br>

C-peptide has also been measured as an additional means for evaluating glucose tolerance and glibenclamide glucose tests (2, 3, 9, 10).<br><br>

C-peptide levels are in many ways a better measurement of endogenous insulin secretion than peripheral insulin levels. C-peptide 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 differentiation 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.