Overproduction of a gastrointestinal hormone, secretin, in Escherichia coli cells and its chemical characterization.

A cloned synthetic DNA fragment coding for 27-desamidosecretin (DAS), which differs from natural porcine secretin by the lack of the amide group at the carboxyl-terminal valine residue, was fused to the beta-galactosidase gene (lacZ) at the EcoRI site near the 3'-terminus of the gene. The fusion gene was efficiently expressed in Escherichia coli, yielding 1.15 X 10(6) fused-protein molecules per cell. After the treatment of the fused protein with cyanogen bromide, DAS was purified by a combination of ion-exchange column chromatography and reverse-phase high-performance liquid chromatography (HPLC). The structure of bacterial DAS was confirmed by tryptic mapping and amino acid composition analyses. The bacterial DAS was not amidated at its carboxyl-terminal valine residue. Nevertheless, it stimulated pancreatic secretion in rats as does authentic porcine secretin, indicating that the carboxyl-terminal amide is not essential to secretin activity.