Polar Lipids from Oat Kernels

Cereal Chem. 87(5):467–474 Oat (Avena sativa L.) kernels appear to contain much higher polar lipid concentrations than other plant tissues. We have extracted, identified, and quantified polar lipids from 18 oat genotypes grown in replicated plots in three environments to determine genotypic or environmental variation in these lipids. Validation experiments indicated a solid phase silica gel extraction step elution provided excellent and clean separation of extracted lipids into neutral lipid, glycolipid, and phospholipid fractions. Analysis of phospholipids by HPLC (normal phase, diol column) indicated phosphatidylethanolamine, phosphatidylcholine, phosphatidic acid, phosphatidylglycerol, phosphatidylinositol, and lysoforms but very little genotypic or environmental variation. Di, tri and tetragalactosyl-diacylglycerols were quantified in the glycolipids, along with their mono-, di-, and triacyl estolides. Most of these exhibited significant genotypic variation. Molecular species analysis of the glycolipids in the Morton cultivar by direct infusion electrospray ionization tandem mass spectrometry confirmed the enormous diversity of galactosyl-lipids in oats. Analyses indicated total lipid of ≈8.3% (dry weight basis), of which ≈10% was phospholipid and 11% was glycolipids. These results indicate that oats are a rich source of polar lipids and contain an extremely rich diversity of galactosyl-lipids. Oats (Avena sativa L.) are a cereal rich in oil, which can provide a high energy diet for animals. However the high levels of oil in oats have been a concern for interests in human food because of an emphasis on fat-conscious diets and because of stability (oxidation) problems with lipids that can reduce shelf life in processed foods. Yet the composition of oat oil is relatively healthful, being comparatively high in monounsaturated fatty acids such as oleic acid, and containing significant levels of tocopherols (Peterson and Wood 1997). Of particular interest in the study of oat lipids are the high levels of polar lipids. Previous studies have indicated that oat oil may contain 5–15% glycolipids and 5–26% phospholipids (de la Roche et al 1977; Youngs et al 1977; Sahasrabudhe 1979). Based on 8% total oil concentration, these values indicate that oat kernels may contain ≈0.8–2.8% polar lipids (phospholipid and glycolipid combined) on a dry weight basis (dwb). These values would appear to be remarkably high levels compared to survey results presented by Sugawara and Miyazawa (1999), who reported a range of 0.005–0.6% for tissue glycolipid concentration among 48 edible plant species. Characterization of oat polar lipids has largely been performed by thin-layer chromatography. Studies have identified monogalactosylmonoacylglycerol, monogalactosyldiacylglycerol, digalactosylmonoacylglycerol, digalactosyldiacylglycerol (DGDG), sulfatides, sterol glucosides and glucocerebrosides among glycolipids in oats (Price and Parsons 1976; Youngs et al 1977; Sahasrabudhe 1979). Of these, DGDG was the most abundant glycolipid (Price and Parsons 1976; Youngs et al 1977; Sahasrabudhe 1979; Aro et al 2007). Indeed, Andersson et al (1997) reported yields of 6 g/kg of a highly purified DGDG preparation from oat bran by a serial extraction involving supercritical CO2 and acetone. Among the phospholipids, phosphatidylethanolamine, phosphatidylcholine, phosphatidic acid, phosphatidylglycerol, phosphatidylinositol and lysoforms of these have been identified, with phosphatidylcholine the most abundant (Price and Parsons 1976; Youngs et al 1977; Sahasrabudhe 1979; Aro et al 2007). In addition, some more unusual lipids have been reported. Holmback et al (2001) reported N-acylphosphatidylethanolamine and acylphosphatidylglycerol in concentrations of ≤15% of total phospholipid content. Hamberg et al (1998) reported an estolide of DGDG, where additional linoleic acid was esterified to DGDG at the 15 position of a hydroxyl-linoleic acid. More recently, Moreau et al (2008a), using liquid chromatography-mass spectrometry (LC-MS) and direct infusion electrospray ionization tandem mass spectrometry (tandem MS/MS) have described a series of natural estolides of galactosyl lipids in oats (Fig. 1) including monodiand tri-acyl derivatives of ditriand tetra-galactosyldiacylglycerols. These results indicated that oats contain a hitherto undescribed diversity of forms of galactosyl-lipids. Unfortunately, because of a lack of replication in this study, its quantitative results cannot be considered reliable. Knowledge of abundance of polar lipids in oats has stimulated some studies into the possible use in commercial applications. Several studies have investigated the application of polar oat lipids in breadbaking, primarily as emulsifiers (Forssel et al 1998; Erazo-Castrejon et al 2001). Blom et al (1996) described the use of a DGDG preparation in oats in the emulsification of fat soluble pharmaceuticals for more efficient absorption. Successful development of applications for these unique oat lipids may depend on more complete knowledge of them. It is well-known that various solvents will extract different lipids based mostly on solvent polarity (Sahasrabudhe 1979; Moreau et al 2003). Application of modern HPLC techniques coupled with mass spectroscopy and 1 USDA-ARS Wheat Quality Laboratory, Harris Hall, North Dakota State University, Dept 7640, P.O. Box 6050, Fargo, ND. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable. 2 Corresponding author. E-mail: [email protected] 3 USDA-ARS, Eastern Regional Research Center, Wyndmoor, PA. 4 Kansas Lipidomics Research Center, Division of Biology, Kansas State University, Manhattan, KS. 5 Department of Plant Sciences, North Dakota State University, Dept 7670, P.O. Box 6050, Fargo, ND. doi:10.1094 / CCHEM-04-10-0060 This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. AACC International, Inc., 2010. Fig. 1. Structure for diacyl-digalactosyldiacylglycerol, a natural estolide occurring in oat oil, as proposed by Moreau et al (2008).