Lipid Peroxides in the Free Radical Pathophysiology of Brain Diseases

1. Polyunsaturated fatty acids are essential for normal neural cell membrane functioning because many membrane properties, such as fluidity and permeability, are closely related to the presence of unsaturated and polyunsaturated side chains. Lipid peroxidation results in loss of membrane polyunsaturated fatty acids and oxidized phospholipids as polar species contributing to increased membrane rigidity.2. Polyunsaturated fatty acids are released from membrane phospholipids by a number of enzymic mechanisms involving the receptor-mediated stimulation of phospholipase A₂ and phospholipase C/diacylglycerol lipase pathways.3. The overstimulation of excitatory amino acid (EAA) receptors stimulates the activities of lipases and phospholipases, and this stimulation produces changes in membrane phospholipid composition, permeability, and fluidity, thus decreasing the integrity of plasma membranes.4. Alterations in properties of plasma membranes may be responsible for the degeneration of neurons seen in neurodegenerative diseases. Two major processes may be involved in neuronal injury caused by the overstimulation of EAA receptors. One is a large Ca²⁺ influx and the other is an accumulation of free radicals and lipid peroxides as a result of neural membrane phospholipid degradation. It is suggested that calcium and free radicals act in concert to induce neuronal injury in acute trauma (ischemia and spinal cord injury) and in neurodegenerative diseases.