ISOLATION AND STRUCTURE OF A NEW GALACTOLIPID FROM OAT SEEDS

Seeds of oat (Avena sativa L.) were recently shown to contain signific ant quantities of a new hydroxy acid, (15 R)-hydroxy-(9Z),(12Z)-octade cadienoic acid (trivial name, avenoleic acid). In the present work, av enoleate was found to be mainly (63%) localized in the glycolipid frac tion of oat seed lipids. Fractionation of the glycolipids by thin-laye r chromatography and reversed-phase high-performance liquid chromatogr aphy revealed the presence of a main molecular species which accounted for 20% of the total avenoleate content of oat seeds. Structural stud ies by chemical methods and mass spectrometry demonstrated that the av enoleate-containing glycolipid was a galactolipid assembled of one mol ecule of avenoleic acid, two molecules of linoleic acid, two molecules of D-galactose, and one molecule of glycerol. Degradation of the new galactolipid by chemical and enzymatic methods demonstrated the locali zation of acyl chains, i.e., linoleate at sn-1 and linoleoyl-avenoleat e at sn-2. Nuclear magnetic resonance spectroscopy gave independent su pport for this structure and also demonstrated that the two galactoses formed an pha-D-galactopyranosyl-1-6-beta-D-galactopyranosyl moiety w hich was bound to the sn-3 position. Based on these experiments, the n ew galactolipid could be formulated as 1-[(9'Z),(12'Z)-octadecadienoyl ]-2-[(15 '' R)-{(9'''Z),(12'''Z)-octadecadienoyloxy)-(9 '' Z),(12 '' c topyranosyl-1-6-beta-D-galactopyranosyl)glycerol. Quantitatively, the amount of the avenoleate-containing galactolipid was of the same order of magnitude as those of individual molecular species of digalactosyl diacylglycerol containing nonoxygenated acyl chains. The content of th e new galactolipid in oat seeds was 0.5-0.6 mg per g of seed.