Insulin (INS; 176730) is produced posttranslationally from its precursor molecule, proinsulin, by site-directed proteolysis in beta-cell granules. Conversion involves cleavage at pairs of basic residues that link both the insulin A and B chains to C-peptide. Human proinsulin conversion has a preferred sequential route, such that cleavage at the B-chain/C-peptide junction occurs first, producing des-31,32 split proinsulin as the major conversion intermediate. Under normal circumstances, proinsulin conversion is largely completed before secretion, and low plasma levels of intact proinsulin and conversion intermediates are found. Structural abnormalities in the proinsulin molecule can impair conversion, leading to the accumulation of proinsulin-like material in the circulation. Such defects show an autosomal dominant mode of inheritance and are the main cause of familial hyperproinsulinemia (summary by Warren-Perry et al., 1997). [from
OMIM]