Carp Insulin: Amino Acid Sequence, Biological Activity and Structural Properties

Abstract

The amino acid sequence of insulin of carp (Cyprinus carpio) was determined and correlated with its biological activity in a rat fat-cell test and its structural properties as measured by circular dichroism and sedimentation analysis. The amino acid sequence of carp insulin displays some unusual features: the B chain is longer at the N terminus by 2 residues as compared with mammalian insulins and there are substitutions of the charged residues, found in most insulins at positions B21 and B22, by Pro and Thr, respectively. All amino acid residues essential for biological activity and for the association of insulin monomers are the same in carp insulin. Accordingly, the half-maximal response in a fat-cell test is reached with carp insulin at concentrations which are only 3 times higher than with porcine insulin and the maximal response is the same. The circular dichroism spectrum of carp insulin resembles greatly that of bovine insulin indicating that it has a similar spatial structure. Despite amino acid substitutions in the dimer-dimer contact region, carp insulin is able to form hexamers.