Plasmalogens, phospholipase A2, and docosahexaenoic acid turnover in brain tissue.

Plasmalogens are glycerophospholipids of neural membranes containing vinyl ether bonds. Their synthetic pathway is located in peroxisomes and endoplasmic reticulum. The rate-limiting enzymes are in the peroxisomes and are induced by docosahexaenoic acid (DHA). Plasmalogens often contain arachidonic acid (AA) or DHA at the sn-2 position of the glycerol moiety. The receptor-mediated hydrolysis of plasmalogens by cytosolic plasmalogen-selective phospholipase A2 generates AA or DHA and lysoplasmalogens. AA is metabolized to eicosanoids. The mechanism of signaling with DHA is not known. The plasmalogen-selective phospholipase A2 differs from other intracellular phospholipases A2 in molecular mass, kinetic properties, substrate specificity, and response to glycosaminoglycans, gangliosides, and sialoglycoproteins. A major portion of [3H]DHA incorporated into neural membranes is found at the sn-2 position of ethanolamine glycerophospholipids. Studies with a mutant cell line defective in plasmalogen biosynthesis indicate that the incorporation of DHA is reduced in this RAW 264.7 cell line by 50%. In contrast, the incorporation of AA remains unaffected. This is reversed completely when the growth medium is supplemented with sn-1-hexadecylglycerol, suggesting that DHA can be selectively targeted for incorporation into plasmalogens. We suggest that deficiencies of DHA and plasmalogens in peroxisomal disorders, Alzheimer's disease (AD), depression, and attention deficit hyperactivity disorders (ADHD) may be responsible for abnormal signal transduction associated with learning disability, cognitive deficit, and visual dysfunction. These abnormalities in the signal-transduction process can be partially corrected by supplementation with a diet enriched with DHA.