STRUCTURE AND EVOLUTION OF INSULIN AND INSULIN-LIKE GROWTH-FACTORS IN CHORDATES

The hormone insulin is one member of a superfamily of structurally related peptide hormones found in vertebrates and invertebrates, and members of this superfamily include insulin, insulin-like growth factor (IGF)-I, IGF-II, and the ovarian hormone, relaxin. This chapter focuses on the structure and evolution of the insulin and IGF genes among the chordates and describes the evidence, which demonstrates that insulin and the IGFs have maintained separate gene lineages throughout vertebrate evolution. The primary structure of insulin consists of two polypeptide chains (B and A chain), which are linked by two disulfide bonds; the A chain also contains an intrachain disulfide bond. A comparison of insulin sequences from various species reveals that there are residues in the molecule, which are highly conserved, whereas others are more variable. The results are consistent with the possibility that a single gene of the insulin/IGF family is present in amphioxus, and this subsequently expanded into the three insulin and IGF genes found in humans and other higher vertebrates. However, the details of this evolutionary expansion remain obscure. Further work will be necessary to more fully characterize amphioxus insulin-like peptide (ILP), and it will be of particular interest to determine whether ILP has biological activities similar to insulin and/or IGF. © 1992, Academic Press Inc.