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 gener- The susceptibility of fatty acids to oxidation by reactive oxygen species (ROS) 3 increases with desaturation (1, 2), so that membranes rich in polyunsaturated fatty acids (PUFAs) are potentially vulnerable to oxidative damage. It is therefore remarkable that intra-organellar membranes in mitochondria and chloroplasts, organelles with highly oxidative metabolism, typically contain high percentages of PUFAs (3, 4). Both organelle types are active sites of ROS production (5, 6). For example, thylakoid membranes in chloroplasts are prone to photo-oxidative damage by singlet oxygen ( 1 O 2 ) that is produced through the interaction of excited (triplet-state) chlorophyll with oxygen (6 -8). Moreover, the major site of 1 O 2 generation in chloroplasts, photosystem II (PSII), is surrounded by polyunsaturated fatty acid (PUFA)-rich lipids (9) and, strikingly, twothirds of the FAs in thylakoids are triunsaturated fatty acids (TFAs), chiefly ␣-linolenic acid (10). These fatty acids are highly prone to oxidative fragmentation in vivo (11).Cells counter the threat posed by 1 O 2 and other ROS with multiple protection mechanisms known to involve low molecular mass antioxidants. Among the best characterized of these cellular protectants are tocopherols (reviewed in Ref. 12). Indeed, mutants that lack tocopherol and plastochromanol are less protected against 1 O 2 and show elevated levels of nonenzymatic lipid oxidation (nLPO; 13, 14). Additionally, carotenoids both physically and chemically quench 1 O 2 (15). We proposed recently that a further layer of protection may also exist, one that uses PUFA-rich membranes as structural antioxidants. This hypothesis emerged from the fact that, even when grown under mild and relatively low light conditions, plants that lack TFAs displayed hallmark features of oxidative stress (16). TFAs act to protect proteins by absorbing ROS such as 1 O 2 and hydroxyl radicals produced under both healthy and stressful conditions (7,11,16).The development of the supramolecular antioxidant hypothesis has been facilitated by the rigorous quantitative analysis of a PUFA fragmentation product, the 3-carbon dialdehyde malondialdehyde (MDA) that is generally considered to be harmful to the cells of humans (17, 18) and plants (19 -21). MDA may also have activities in redox signaling (reviewed in Ref. 11). This compound, one of a plethora of PUFA oxidation products, is a robust marker of nonenzymatic lipid oxidation in animals (1) and in plants (e.g. 22). Furthermore, protocol...