The free amino acid contribution to osmotic regulation in Acanthamoeba castellanii

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Abstract
In Acanthamoeba, a sudden rise in the medium osmolality from 240-500 mosmol/kg water produced within 5 h a 2.7-fold increase in the cell free amino acid (FAA) content which accounted for 25% of the change in the solute content of the cell. While in general, modifications of the FAA pool were essentially completed within 1 h. Pro was accumulated linearly with time. Over the 5-h experimental period, the cell content in Pro increased by > 10-fold to constitute 49% of the cell FAA content. The variations of Pro, Ala and Glu accounted for 100% of the increase in the FAA pool after 5 h of acclimation. A sudden drop in the medium osmolality from 240-40 mosmol/kg water resulted in a leveling reduction of the FAA pool, with decreases in Ala, .gamma.-aminobutyric acid and Pro accounting for 71% of the 6-fold change observed after 5 h. The preferential use of FAA and particularly of Pro as osmoregulatory solutes can be explained in terms of energetic considerations and of compatibility with enzyme function.