THE ROLE OF FREE AMINO ACIDS IN MAINTAINANCE OF THE OSMOTIC HOMEOSTASIS OF FISHES

Abstract

This article is a review of the data presented in literature describing the roles of free amino acids and dipeptides in maintenance of osmotic homeostasis in following groups of lower vertebrates: hagfishes, lampreys, elasmobranchs, sturgeons and bony fishes. The emphasis is made on the role of former metabolites as “compatible osmolytes”. Those are small organic metabolites preserving biological function and native structure of macromolecules such as proteins and nucleic acids under action of high ionic power. Mechanism of this biological action is briefly described. 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 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, sarcosine, followed by glycine, alanine and proline. In freshwater elasmobranchs beta-alanine, taurine, proline, glycine, glutamine and glutamate can be used. In sturgeons main amino acids in osmoregulation can be glycine, beta-alanine, taurine, glutamate, glutamine. Finally, bony fishes use mainly taurine, proline, glycine, alanine, in particular species such as Monopterus albus – glutamine as compatible osmolytes. Bony fishes also use sarcosine, alanine, methionine, glutamate, 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.