proinsulin , a single-chain precursor of insulin , has been detected in biosynthetic studies in vitro , ( 1-4 ) and in crystalline preparations of insulin from several species . ( 1 , 5-7 ) it appears that proinsulin normally serves neither as a major storage nor secretory form of the hormone . ( 8 ) its primary functions seems to be to facilitate the efficient formation of the disulfide bonds of insulin . ( 9 ) although several species of fish have two insulins differing slightly in primary structure , ( 10-12 ) the rat is the only mammal where this is known to be the case . ( 13 , 14 ) as the two rat insulins are nonallelic ( 14 ) and presumably coded by two distinct genes , a separate proinsulin would be expected to precede each insulin .
C0033362
proinsulin
C0021641
insulin