Membranes as Structural Antioxidants: RECYCLING OF MALONDIALDEHYDE TO ITS SOURCE IN OXIDATION-SENSITIVE CHLOROPLAST FATTY ACIDS.

Genetic evidence suggests that membranes rich in polyunsaturated fatty acids (PUFAs) act as supramolecular antioxidants that capture reactive oxygen species, thereby limiting damage to proteins. This process generates lipid fragmentation products including malondialdehyde (MDA), an archetypal marker of PUFA oxidation. We observed transient increases in levels of endogenous MDA in wounded Arabidopsis thaliana leaves, raising the possibility that MDA is metabolized. We developed a rigorous ion exchange method to purify enzymatically generated (13)C- and (14)C-MDA. Delivered as a volatile to intact plants, MDA was efficiently incorporated into lipids. Mass spectral and genetic analyses identified the major chloroplast galactolipid: α-linolenic acid (18:3)-7Z,10Z,13Z-hexadecatrienoic acid (16:3)-monogalactosyldiacylglycerol (18:3-16:3-MGDG) as an end-product of MDA incorporation. Consistent with this, the fad3-2 fad7-2 fad8 mutant that lacks tri-unsaturated fatty acids incorporated (14)C-MDA into 18:2-16:2-MGDG. Saponification of (14)C-labeled 18:3-16:3-MGDG revealed 84% of (14)C-label in the acyl groups with the remaining 16% in the head group. 18:3-16:3-MGDG is enriched proximal to photosystem II and is likely a major in vivo source of MDA in photosynthetic tissues. We propose that nonenzymatically generated lipid fragments such as MDA are recycled back into plastidic galactolipids that, in their role as cell protectants, can again be fragmented into MDA.