Introduction

Insulin is a two-chain disulfide-linked polypeptide hormonen which is essential for normal growth and metabolic regulation in higher organisms. It is a member of a superfamily of structurally related peptides which includes relaxin (a two-chain polypeptide hormone that functions in vertebrate reproductive physiology (Bryant-Greenwood, 1982)) and insulin-like growth factors (IGF) I and II. IGF-I and -II are single-chain polypeptides which stimulate growth in vitro and in vivo and are probably required for normal fetal and postnatal growth and development (Froesch & Zapf, 1985). Insulin is also related to two invertebrate neuropeptides that have recently been isolated and characterized: prothoracicotrophic hormone of the silkworm Bombyx mori, which regulates the secretion of the molting hormone ecdysone (Nagasawa et al, 1986; Jhoti et al, 1987); and molluscan insulin-related peptide (MIP) (Smit et al, 1988), a pond snail (Lymnaea stagnalis) hormone, which is synthesized by the cerebral light-green cell and is believed to regulate, at least in part, growth of soft body parts and shell and glycogen metabolism. These invertebrate insulin-like proteins are two-chain polypeptide hormones like insulin and relaxin and in contrast to the single-chain IGFs. Although the origins and relationships of the various members of the insulin gene family have not been clarified, it seems likely that they evolved from a common ancestral protein through a process of gene duplication and diversification. Furthermore, the presence of two non-allelic insulin or relaxin genes in some vertebrate species suggests that this process is still ongoing.