The Role of Free Amino Acids in Maintaining the Osmotic Homeostasis of Fishes

This article is a review of data presented in the literature describing the roles of free amino acids and dipeptides in the maintenance of osmotic homeostasis in the following groups of lower vertebrates: hagfishes, lampreys, elasmobranchs, sturgeons, and bony fishes. The role of former metabolites as “compatible osmolytes” is emphasized: small organic metabolites preserving the biological function and native structure of macromolecules such as proteins and nucleic acids under the action of high ionic power. The mechanism of this biological action is briefly described. The evolution of the main osmoregulatory strategies in lower vertebrates is briefly considered. The role of amino acids as energy sources helping to cover elevated demands of the organism during osmotic stress and the role of building blocks for synthesis of protein ionic channels, hormones, enzymes etc., participating in the salinity coping strategies of the organism are also discussed. Amino acids playing the main role in osmotic adjustments of marine elasmobranchs are taurine, beta-alanine, and sarcosine, followed by glycine, alanine, and proline. In freshwater elasmobranchs, beta-alanine, taurine, proline, glycine, glutamine, and glutamate can be used. In sturgeons, the main amino acids in osmoregulation can be glycine, beta-alanine, taurine, glutamate, and glutamine. Finally, bony fishes use mainly taurine, proline, glycine, and alanine; in particular, species such as Monopterus albus use glutamine as compatible osmolyte. Bony fishes also use sarcosine, alanine, methionine, glutamate, and aspartate as metabolic precursors of amino acid osmolytes. The role of dipeptides such as carnosine, anserine, and dipeptide lysine-proline is controversial and requires further investigations.