Primary Structures and Multiplicity of Pancreatic Hormone in Fish

The pancreas contains several peptide hormones including insulin and glucagon which regulate nutrition metabolism and consequently growth. The primary structures and biochemical characters of these hormones are important information for the artificial synthesis of these hormones and the development of the immunoassay system. In fish, several species possess plural molecular forms of insulin and glucagons, and this phenomenon complicates the understanding of the physiological function of these hormones. The present study aimed to review the current state of knowledge concerning biochemical characters of those hormones and the systems producing multiple molecular forms. The production systems of multiple molecular forms of insulin and glucagon are different among species. Purifying the hormones and analyzing of primary structures and gene structures elucidated this phenomenon. The production systems are classified into the following types: (1) arising from a single prohormone by proteolytic cleavage at different sites of the signal peptide (flounder insulins) or (2) C-terminus regions (ratfish insulins, barfin flounder glucagons), (3) arising from two distinct genes amplified by chromosome duplication (kaluga sturgeon insulins, paddlefish insulins and glucagons) or (4) irrespective of chromosome duplication (bonito insulins, kaluga sturgeon glucagons). It is not clear whether there are any functional differences between each molecular form in fish. However, immunoreactive differences have been demonstrated in insulins from flounders. These differences are significant, but both molecular forms are crossreactive to the antibody. This suggests that it is difficult to measure their blood levels distinctly by immunological assays in each fish species. Thus, multiplicity of insulin and glucagon is an obstacle to develop measurement techniques for these hormones due to immunological similarities between each molecular form, but science needs to overcome this obstacle to understand the physiological function of each hormone.