Age-dependent modifications in membrane lipids

The membrane lipid composition of Tetrahymena pyriformis NT-I was observed to change in a manner markedly dependent on the progress of culture age. The pellicular, mitochondrial and microsomal membranes were isolated from cell harvested at various growth phases (I, early exponential; II, mid-exponential; III, late exponential; IV, early stationary; V, late stationary) and their lipid composition was analyzed by thin-layer and gas-liquid chromatography. Although the phospholipid composition varied somewhat among membrane fractions, the most general age-dependent alteration was a considerable decrease in the content of phosphatidylethanolamine accompanied by a small increase in phosphatidylcholine. The 2-aminoethylphosphonolipid, enriched in the surface membrane pellicle, did not undergo a consistent change. As for fatty acid composition the most notable variation occurred in unsaturated fatty acids; a great increase in oleic and linoleic acids and a compensatory decrease in palmitoleic acid. This resulted in an augmented unsaturation of the overall phospholipid fatty acid profile of the aged membranes. The age-associated drastic decline in the palmitoleic acid content in membrane phospholipids could be accounted for by the markedly lowered activity of palmitoyl-CoA desaturase. The microsomes from the early exponential phase cells possess a 4-fold higher activity of the desaturase as compared to that of the late stationary phase microsomes. The decreased desaturase activity associated with the culture age was also reflected in the corresponding decrease in the conversion rate of [14C]palmitate to [14C]palmitoleate in cells labelled in vivo. The ESR spectra of the spin-labeled phospholipids extracted from the pellicular and microsomal membranes have led to the suggestion that these types of membrane would become more fluid with the age of growth.