Insulin immunoassays: fast approaching 50 years of existence and still calling for standardization.

摘要

Insulin (51 amino acids, 5808 Da) is synthesized from its precursors preproinsulin and proinsulin (hPI; 86 amino acids) in the beta cells of the pancreatic islets of Langerhans. A human insulin molecule is chemically homogeneous, consisting of 2 polypeptide chains, the A chain (21 amino acids) and B chain (30 amino acids), connected by 2 disulfide bonds (A7-B7 and A20-B19); a 3rd disulfide bond links the A6 and All residues. In serum, insulin circulates in a free form (not bound to carrier proteins) together with small quantities of its precursors, mainly intact hPI and des (31,32) split hPI (hPI cleaved at the junction between the B chain and C-peptide linking the A and B chains in the hPI molecule). Des (64,65) split hPI (hPI cleaved at the junction between the A chain and C-peptide) is a minor component of hPIs in serum (1).