Changes in Membrane Fatty Acid Composition and Δ12‐Desaturase Activity during Growth of Acanthamoeba castellanii in Batch Culture

Abstract

ABSTRACT. Growth of Acanthamoeba castellanii in batch culture at 30° C was associated with marked changes in cellular fatty acid composition. The largest change occurred in the linoleate to oleate ratio, which was maximal in early‐ to mid‐exponential phase cultures but decreased approximately 10‐fold as cells approached stationary phase. The higher degree of lipid unsaturation in young cultures was accentuated by a greater proportion of 20‐carbon polyunsaturated fatty acids than in stationary phase cultures. The unsaturation index (average number of double bonds per fatty acid) was maximal in mid‐exponential phase cultures after 24 hours growth. Incorporation of [1‐14C]acetate into polyunsaturated fatty acids in short‐term (2 hour) experiments was high in 12 and 24 hour old cultures, where linoleate and eicosadienoate accounted for up to 26% of total labelled fatty acids. Incorporation of [1‐14CJacetate into these fatty acids was negligible in stationary phase cultures. These results were correlated with changes in the specific activity of the Δ12‐desaturase. Δ12‐Desaturase activity was greatest in microsomal membranes isolated from early‐ to mid‐exponential phase cells, but declined by approximately 50% as cultures progressed towards stationary phase. Membrane fractionation studies revealed that although some differences in fatty acid composition between plasma‐membrane, mitochondrial (enriched), and microsomal membrane fractions were evident, the large changes in lipid unsaturation in whole cells of A. castellanii could not be accounted for by differential development of particular subcellular membranes.