Myxine insulin : amino-acid sequence, three dimensional structure, biosynthesis, release, physiological role, receptor binding affinity, and biological activity

Sammanfattning: The Atlantic hagfish, Myxlne. glutinosa,is the most primitive vertebrate extant, and it diverged from the main vertebrate evolutionary chain some 500 mi 11 ion years ago.The primary sequence of hagfish insulin shows that it contains the residues implemented for expression of activity and the residues stabi­lizing the insulin monomer and dimer, but not the hexamer. The primary sequence of hagfish preproinsulin, deduced from the mRNA-cDNA sequence shows little homology in sequence of the precursor parts of the molecule. However, the sequence contains the structural requirements for the tenta­tive functions, jL.z. vectorial discharge of the prohormone and a minimum over-all size of the precursor. The proinsulin converting enzyme(s) seems to have a specificity similar to that of all other vertebrates studied. The tertiary structure of hagfish insulin in the crystal is almost super­imposable on pig insulin's structure.The biological  activity of hagfish insulin is 5%   of that  of piginsulin and its receptor binding affinity   is 23% in isolated   rat fat  cells.Hagfish insulin was the first partial insulin antagonist on the rat fat cell insulin receptor. The change(s) in structure responsible for the reduction of acti­vity and binding are not known.Biosynthesis of hagfish insulin, In vXJyto, follows the pattern observed in higher vertebrates, although at a much slower rate. Unlike the situation in mammals, hagfish insulin biosynthesis is not stimulated by glucose.A radioimmunoassay for hagfish insulin was developed and the antiserum cross-reacted with       bovine insulin to only 0.01%. Theassay was used to study insulin release in vitro. Glucose  stimulates insulin release but, unlike the situation in higher vertebrates, amino acids do not.In vivo, hagfish insulin stimulated the incorporation of 14C-glucose and 14C-leucine into hagfish skeletal muscle glycogen and protein.The observed similari ties,between hagfish and higher vertebrates,with regards to insulin's structure, biosynthesis, release, receptor binding, and biological activity support the conclusion that, insulin and its processing and effector machineries were structurally and biologically well defined some 500 million years ago.