The free amino acid contribution to osmotic regulation in <i>Acanthamoeba castellanii</i>

Abstract

In Acanthamoeba, a sudden rise in the medium osmolality from 240 to 500 mosmol/kg water was found to produce 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 more than 10-fold to constitute 49% of the cell FAA content. Taken together, 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 to 40 mosmol/kg water resulted in a "leveling" reduction of the FAA pool, with decreases in Ala, γ-aminobutyric acid, and Pro accounting for 71% of the sixfold 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.