[Turnover of fatty acids in cell membranes]

The turn-over of polyunsaturated fatty acids in membrane is carried out through three differet ways: 1) de novo synthesis of lipids, 2) fatty acyl chains and/or polar head replacement (transacylation processes), 3) exchange with plasma-lipoproteins. De novo synthesis takes place in microsomes. Then phospholipids are exported towards other subcellular structures, carried by exchange proteins or spreadontby lateral diffusion. Polar head groups are modified by transmethylation (PE leads to PC), by decarboxylation (PS leads to PE) or by base-exchange reaction (ethanolamine/serine/inositol) in various subcellular loci. Transacylation process implies three enzymatic steps: A1/A2 phospholipases, acyl CoA synthetase, acyl CoA-lysoderivatives transacylated. It is localized on the cytoplasmic side of the membrane (plasma membranes and microsomes) and in mitochondrial membranes. Lipid exchange occurs primarily with the outer halve of the cytoplasmic membrane and penetrates through the so-called "flip-flop" mechanism or in especially organized (non-bilayer) parts of the membrane. This fatty acids turn-over could play a role in various membrane functions: 1) release of fatty acids acting as precursors of PG or TXA, 2) adaptation of physical parameters of the membrane (fluidity, phases segregation), 3) steady state for membrane composition asymmetry. Their disturbances could account or diseases seen in plasma fatty acids abnormalities.