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The Pro-Insulin ELISA kit is used for the quantitative measurement of insulinomas in human serum. Proinsulin is a 9390 MW polypeptide of 86 amino acids that is synthesized in the ß cells of the pancreas and is the precursor molecule for insulin. Most proinsulin is converted to insulin and C-Peptide, which are secreted in equimolar amounts into the blood. About 15 % is not converted and is released as proinsulin. The biological activity of proinsulin is only about 10% of Insulin, but the half life of proinsulin is three times as long as insulin. The level of proinsulin in serum can be a reflection of ß cell status. Both IDDM and NIDDM are characterized by dysfunction of the pancreatic ß cells. Elevated proinsulin levels have been noted at the onset of IDDM and in healthy siblings of IDDM patients. Proinsulin levels may also be increased in patients with established NIDDM. Increased levels of circulating proinsulin are found in older patients, pregnant or obese diabetics, patients with insulinomas, functional hypoglycemia and hyperinsulinemia, a rare syndrome. Because the structure of proinsulin is similar to insulin, proinsulin may be detected as immunoreactive insulin in the insulin assay. Immunoreactive insulin levels are generally determined in conventional RIA's, which overestimate the insulin level because the methods use antibodies which crossreact with proinsulin. By calculating the molar ration of proinsulin to true insulin (P/I), a better assessment of ß cell function can be made.