Assessment of Insulin Secretion in the Mouse

Insulin is synthesized by the β cells of the pancreatic islets as part of a single 110-amino acid precursor, preproinsulin ( see Fig. 1 ). Processing is initiated by removal of the amino terminal, 24-amino acid signal sequence ( 1 ). The resulting 86-amino acid product folds through the formation of three disulfide bridges between Cys ⁷ -Cys ⁷² , Cys ¹⁹ -Cys ⁸⁵ , and Cys ⁷¹ -Cys ⁷⁶ to produce the prohormone, proinsulin. Insulin and C-peptide are produced when endopeptidases, prohormone convertases 2 and 3 (PC2 and PC3, respectively), cleave proinsulin at two paired basic amino acid sites, Lys ⁶⁴ -Arg ⁶⁵ and Arg ³¹ -Arg ³² ( see Fig. 1 ). The basic amino acid pairs are then removed from each site by carboxypeptidase H ( 3 ). Proinsulin amino acids 66–86 and 1–30 comprise the A- and B-chains, respectively, of mature insulin ( see Fig. 1 ). “Split” proinsulin 65–66 and 32–33 are produced when cleavage is incomplete and the basic amino acid pairs are not removed from the cleavage site. “Des” proinsulin 64–65 and 31–32 are produced when cleavage is incomplete and the basic amino acid pairs are removed from the cleavage site ( 4 ). In the rat, two separate 110-amino acid preproinsulins are transcribed from two nonallelic preproinsulin genes, from which two forms of insulin and C-peptide are subsequently cleaved ( 1 ) ( see Fig. 1 ). The mouse synthesizes two molecular forms of insulin and C-peptide, which are identical to their respective rat counterparts ( 5 ). The two rodent insulins, designated insulin I and II, are present at a ratio of 1∶3 in the mouse and 4∶1 in the rat (insulin I∶II) ( 6 ).

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