The effect of oxygen availability on the osmoregulatory contribution of free amino acids in Acanthamoeba castellanii

In Acanthamoeba, the availability of oxygen markedly affects the response to hyperosmotic (250 to 500 mosmol/kg water) or hypoosmotic (250 to 125 mosmol/kg water) stress. The short-term recovery of cell hydration after hyperosmotic shock was more rapid at a [Formula: see text] of 54 Torr (1 Torr = 133.322 Pa) than at 5 Torr, despite similar increases in the free amino acid content of the cell (2.3- to 2.4-fold). Under anoxia, the early response to both hyper- and hypo-osmotic stress resulted in osmometer-like cells and the osmoregulatory contribution of the free amino acid pool following hyperosmotic shock was totally abolished. After a hypoosmotic shock under anoxia, however, the decrease of the free amino acid pool, though much slower, was of the same magnitude as under aerobic conditions (1.8- to 2.5-fold). This was apparently the only osmoregulatory mechanism operative in the absence of oxygen. The free amino acid pool was qualitatively affected by the oxygenation level, with proline being more abundant at high [Formula: see text] and alanine being more important under low [Formula: see text] and anoxia. The oxygen consumption of the cells was essentially unaffected by osmotic stress and by the [Formula: see text] level. The osmotically induced changes in the free amino acid content of the cell involve flows of energy into and out of the free amino acid pool that appear considerable when compared with the energy available from oxidative metabolism. This suggests that de novo synthesis and complete oxidation have only a limited role in the supply and disposal of the free amino acids involved in osmotic regulation.