Oxygen-dependent low-temperature Delta 12 (n6)-desaturase induction and alteration of fatty acid composition in Acanthamoeba castellanii

The influence of dissolved oxygen on the synthesis and activity of Delta 12-desaturase in Acanthamoeba castellanii was investigated. A decline in oxygen concentration during batch growth at 30 degrees C was correlated with a decline in the degree of cellular fatty acid unsaturation, Chilling of early-stationary-phase cultures to 15 degrees C led to increased dissolved oxygen levels (from < 1 mu M to 305 mu M) and increased fatty acid unsaturation, which has been shown previously [Avery, S. V., Harwood, J. L. & Lloyd, D. (1994) Microbiology 140, 2423-2431] to be due mainly to Delta 12-desaturase induction. In contrast, chilling of mid-exponential-phase cultures, where the dissolved oxygen concentration prior to chilling was high (> 160 mu M), gave no change in cellular fatty acid unsaturation, Measurement of [1-C-14]acetate incorporation by oxygen-limited A. castellanii revealed that labelling of the Delta 12-desaturase product, linoleate (18:2), increased with oxygen concentration. Microsomal levels of the Delta 12-desaturase enzyme were found to increase by up to 10-fold during aeration of A. castellanii cultures; a transient elevation in oxygen was sufficient to induce Delta 12-desaturase synthesis that was still fully detectable 1 h later. In addition, the activity of pre-existing Delta 12-desaturase, measured in isolated microsomal membranes, increased by up to fivefold with increases in the oxygen concentration of assay mixtures. These results demonstrate for the first time that (i) oxygen availability alone can regulate de novo Delta 12-desaturase synthesis in A. castellanii, and that (ii) oxygen can limit the activity of preexisting Delta 12-desaturase. These responses can occur independently of temperature changes.