On the Biosynthesis of Insulin in Anglerfish Islets

An intermediate in the biosynthesis of insulin has been isolated from anglerfish islets and shown to have the structure of insulin with the additional amino acid residues Gly-Thr-Lys attached to the NH2 terminus of the A chain. This intermediate can be produced in vitro by a rapid tryptic enzymic cleavage of anglerfish proinsulin; more extensive trypsin treatment converts this intermediate to insulin. We propose that the biosynthesis of insulin in anglerfish islets in vivo proceeds via two sequential steps: a rapid enzymic hydrolysis of proinsulin to yield the insulin intermediate, followed by a slow hydrolysis of this intermediate to produce insulin. It is possible that one tryptic-like enzyme is operative in both steps.