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AVENANTHRAMIDE ANTIOXIDANTS IN OATS
AVENANTHRAMIDE ANTIOXIDANTS IN OATS
Lena Dimberg. Department of Food Science, Swedish University of Agricultural Sciences, Uppsala, Sweden. 1 INTRODUCTION
Oats grains are rich in lipids and the unsaturated fatty acids, oleic and linoleic acids dominate (ca 35 % each) which is desirable from a nutritional standpoint. The palmitic acid content (ca 18 %) improves the stability of the oil, but the unsaturated fatty acids are still vulnerable to oxidation. However, oats contain various compounds with antioxidant activity, which presumably protects their own lipids from oxidation and thereby protect oat products from deterioration, but which also might give some health benefits for humans. The use of oat grains as a source of antioxidants was first proposed in the 1 9 3 0 ~ . 'A$ ~specially fineground oat flour was marketed for antioxidant purposes and found to be effective in various food products that are sensitive to oxidation during storage. The best known antioxidants are the tocols and oat grains contain both tocopherols and tocotrienols. Measurement of tocol concentration of hand-dissected oat groats revealed that most of the tocopherols are located in the germ while the tocotrienols are concentrated in the endo~perm.~ Investigations of minor components in oats discovered the presence of antioxidants other than tocopherols. These include the phenolic acids, ubiquitous in the plant kingdom, but also A>avenasterol, reported to have anti-polymerization activity in soybean oil at 180 OC4, and a wide range of hydroxycinnamic esters and a m i d e ~ . ~Some -~ antioxidants have been identified as monoesters comprising hydroxycinnamic acids and long-chain fatty acids or alcohols5 and others as amides, comprising hydroxycinnamic acids and anthranilic acid^.^-^ The latter group of antioxidants are trivially called avenanthramides (derived from Avena, the latin name for oats).
2 AVENANTHRAMIDES 2.1. Oats
The four most common cinnamic acids -p-coumanc acid, caffeic acid, ferulic acid or sinapic acid in combination with four different anthranilic acids (anthranilic acid, 5hydroxyanthranilic acid, 5-hydroxy-4-methoxyanthranilicacid or 4-hydroxyanthranilicacid) give 16 combinations of avenanthramides of which 9 have so far been identified from oats. 6-9 It is possible that further combinations with other substitution patterns may exist. Oat grain contains 500-800 mgkg avenanthramides, but both the total content and the levels of the individual compounds vary between c u l t i v a r ~ . However, ~>~ combinations of 5hydroxyanthranilic acid with ferulic acid (Bf), caffeic acid (Bc) or p-coumanc acid (Bp)
300
Natural Antioxidants and Anticarcinogens in Nutrition, Health and Disease
tend to predominate (Fig. 1).
Bf
Bc
COOH
BP Figure 1 The three most common avenanthramides in oats. Avenanthramides have also been found in oat leaves.10-12There they are reported to act as phytoalexins with anti-fungal activities. Their production in the leaves is induced by fungal infection.ll-12An enzyme, which catalyses the final step in the biosynthesis, that is the fusion of the anthranilic acid and the cinnamic acid moieties, is activated. l 3 Findings by Ishihara et al.14 indicate that Ca2+ is involved in the induction. All of the possible precursors can be used as substrate for the enzyme, but the highest affinities are found for 5hydroxyanthranilic acid and for ferulic acid. l 3 In the kernel, the avenanthramides appear late in the grain maturation phase aAer flowering15 and are presumably located in the outermost parts.8 If they are produced in the kernel is presently not known. Oat samples with a high amount of avenanthramides are evaluated as fresh tasting by a trained sensoric panel while samples with low amounts are considered as rancid.16 Maybe this freshness is due to antioxidant activity of the avenanthramides in the oat samples. The antioxidative activities of the avenathramides are structure related. In principal, the more methoxy or, especially, hydroxy substituents there are in the two aromatic rings, the higher antioxidative activity.8.15 However, it should be pointed out that the test systems used in the measurement of antioxidative activity are of great importance; when a linoleic acid emulsion system is used, the activity of the avenanthramide Bf is higher than for the cinnamic acids,g whereas when a more hydrophilic system is used (diphenyl-picryl-hydracyl radical, DPPH) the results show the opposite, i.e. the cinnamic acids exert higher activities than Bf. To be antioxidants, oat avenanthramides must be quite stable. In fact, neither pH (acid, neutral, basic), high temperature (baking oven or boiling water bath) nor UV-light (254 nm) treatment affect them very much. This is valid both for synthetic compounds and those located within the oat tissue or oil during t r e a t m e n t ~ . 8 ?Avenanthramides ~.~~ are also stable during grain storage (15 months).8,9
Natural Antioxidants or Pro-oxidants in Foods and Nutrition
30 1
2.2. Other sources Besides oats, avenanthramides have been found in carnation leaves ( C a r y ~ p h y l l a c e a e ) ~ ~ * ~ ~ and in eggs of white cabbage butterfly (L~pidoptera).'~ The avenanthramides in the carnation act, as in oat leaves, as phytoale~ins'~.'~ but in eggs of the butterfly as oviposition det~rrents.'~ It seems that butterflies produce avenanthramides in order to prohibit other butterflies fertilizing their eggs at the same place, and thereby ensuring of their own survival. These activities are also structure related, but in this case it seems that less hydroxyl substitution results in higher a ~ t i v i t y , 'which ~ is contradictory to antioxidant activity. Other biological activities reported for avenanthramides include lipoxygenase inhibitionZoand anti-histamic, anti-allergic and anti-asthmatic activities2' 3 CONCLUSION As oat avenanthramides are quite stable antioxidants and probably have a high pass-through in food processing they presumably contribute to the oxidative stability of oat products and might also provide some nutritional health benefits for human beings. References 1. L. Lowen, L. Anderson, and R.W. Harrison. Ind. Eng.Chem. 1937,29,146. 2. F.N. Peters Jr. and S . Musher. Indust. Eng. Chem. 1937,29, 146. 3. D.M. Peterson. Cereal Chem. 1995,72,21. 4. P.J. White and L.S. Armstrong. J. Amv. Oil. Chem. SOC.1986.63, 525. 5. D.G.H. Daniels and H.F. Martin. J. Sci. Fd. Agric. 1968, 19, 710. 6. F.W. Collins. J.Agric. Food Chem. 1989,37,60. 7. F.W. Collins and W.J Mullin. J. Chromatogr. 1988,445,363. 8. L.H. Dimberg 0. Theander and H. Lingnert. Cereal Chem. 1993,70,637. 9. L.H. Dimberg, L.E Molteberg, R. Solheim and W.Frcalich. J. Cerealscience. 1996,24, 263. 10. L. Crombie and J. Mistry. Tetrahedron Letters. 1990,31,2647. 11. H. Miyagawa, A. Ishihara, T. Nishimoto, T. Ueno and S . Mayama. Biosci. Biotech. Biochem. 1995,59,2305. 12. H. Miyagawa, A. Ishihara, Y. Kuwahara, T. Ueno. and S . Mayama. Phytochemistry. 1996,41, 1473. 13. A. Ishihara, T. Matsukawa, H. Miyagawa, T. Ueno, S . Mayama and H. Iwamura Zeitschriftfu'r naturforschung C-Ajournal of biosciences. 1997, 52, 756. 14. A. Ishihara, H. Miyagwa, Y. Kuwahara, T. Ueno and S . Mayama. Plant Science. 1996, 115,9. 15. L.H. Dimberg and K. Sunnerheim, unpublished. 16. E.L. Molteberg, R. Solheim, L.H. Dimberg and W. Fmlich. J. Cereal Science. 1996, 24, 273. 17. G.J. Niemann. Phytochemisty. 1993,34,319. 18. M. Ponchet, J. Favre-Bonvin, M. Hauteville and P. Ricci. Phytochemistry. 1988, 27,
302
Natural Antioxidants and Anticarcinogens in Nutrition, Health and Disease
725. 19. A. Blaakmeer, D. van der Wall, A. Stork, T.A. van Beek, A. de Groot and J.J.A. van Loon. Journal of Natural Products. 1994,57, 1145. 20. T. Wakabayashi, Y . Kumonaka, H. Ichikawa and S. Murota. Japanesepatent. 1986, 60, 152,454. 21. J.P.A. Devlin and K.D Hargrave. “Pulmonary and antialle drugs” (J.P.A. Devlin, ed) John Wiley and sons: Chichester, England, 1985.
Lena Dimberg. Department of Food Science, Swedish University of Agricultural Sciences, Uppsala, Sweden. 1 INTRODUCTION
Oats grains are rich in lipids and the unsaturated fatty acids, oleic and linoleic acids dominate (ca 35 % each) which is desirable from a nutritional standpoint. The palmitic acid content (ca 18 %) improves the stability of the oil, but the unsaturated fatty acids are still vulnerable to oxidation. However, oats contain various compounds with antioxidant activity, which presumably protects their own lipids from oxidation and thereby protect oat products from deterioration, but which also might give some health benefits for humans. The use of oat grains as a source of antioxidants was first proposed in the 1 9 3 0 ~ . 'A$ ~specially fineground oat flour was marketed for antioxidant purposes and found to be effective in various food products that are sensitive to oxidation during storage. The best known antioxidants are the tocols and oat grains contain both tocopherols and tocotrienols. Measurement of tocol concentration of hand-dissected oat groats revealed that most of the tocopherols are located in the germ while the tocotrienols are concentrated in the endo~perm.~ Investigations of minor components in oats discovered the presence of antioxidants other than tocopherols. These include the phenolic acids, ubiquitous in the plant kingdom, but also A>avenasterol, reported to have anti-polymerization activity in soybean oil at 180 OC4, and a wide range of hydroxycinnamic esters and a m i d e ~ . ~Some -~ antioxidants have been identified as monoesters comprising hydroxycinnamic acids and long-chain fatty acids or alcohols5 and others as amides, comprising hydroxycinnamic acids and anthranilic acid^.^-^ The latter group of antioxidants are trivially called avenanthramides (derived from Avena, the latin name for oats).
2 AVENANTHRAMIDES 2.1. Oats
The four most common cinnamic acids -p-coumanc acid, caffeic acid, ferulic acid or sinapic acid in combination with four different anthranilic acids (anthranilic acid, 5hydroxyanthranilic acid, 5-hydroxy-4-methoxyanthranilicacid or 4-hydroxyanthranilicacid) give 16 combinations of avenanthramides of which 9 have so far been identified from oats. 6-9 It is possible that further combinations with other substitution patterns may exist. Oat grain contains 500-800 mgkg avenanthramides, but both the total content and the levels of the individual compounds vary between c u l t i v a r ~ . However, ~>~ combinations of 5hydroxyanthranilic acid with ferulic acid (Bf), caffeic acid (Bc) or p-coumanc acid (Bp)
300
Natural Antioxidants and Anticarcinogens in Nutrition, Health and Disease
tend to predominate (Fig. 1).
Bf
Bc
COOH
BP Figure 1 The three most common avenanthramides in oats. Avenanthramides have also been found in oat leaves.10-12There they are reported to act as phytoalexins with anti-fungal activities. Their production in the leaves is induced by fungal infection.ll-12An enzyme, which catalyses the final step in the biosynthesis, that is the fusion of the anthranilic acid and the cinnamic acid moieties, is activated. l 3 Findings by Ishihara et al.14 indicate that Ca2+ is involved in the induction. All of the possible precursors can be used as substrate for the enzyme, but the highest affinities are found for 5hydroxyanthranilic acid and for ferulic acid. l 3 In the kernel, the avenanthramides appear late in the grain maturation phase aAer flowering15 and are presumably located in the outermost parts.8 If they are produced in the kernel is presently not known. Oat samples with a high amount of avenanthramides are evaluated as fresh tasting by a trained sensoric panel while samples with low amounts are considered as rancid.16 Maybe this freshness is due to antioxidant activity of the avenanthramides in the oat samples. The antioxidative activities of the avenathramides are structure related. In principal, the more methoxy or, especially, hydroxy substituents there are in the two aromatic rings, the higher antioxidative activity.8.15 However, it should be pointed out that the test systems used in the measurement of antioxidative activity are of great importance; when a linoleic acid emulsion system is used, the activity of the avenanthramide Bf is higher than for the cinnamic acids,g whereas when a more hydrophilic system is used (diphenyl-picryl-hydracyl radical, DPPH) the results show the opposite, i.e. the cinnamic acids exert higher activities than Bf. To be antioxidants, oat avenanthramides must be quite stable. In fact, neither pH (acid, neutral, basic), high temperature (baking oven or boiling water bath) nor UV-light (254 nm) treatment affect them very much. This is valid both for synthetic compounds and those located within the oat tissue or oil during t r e a t m e n t ~ . 8 ?Avenanthramides ~.~~ are also stable during grain storage (15 months).8,9
Natural Antioxidants or Pro-oxidants in Foods and Nutrition
30 1
2.2. Other sources Besides oats, avenanthramides have been found in carnation leaves ( C a r y ~ p h y l l a c e a e ) ~ ~ * ~ ~ and in eggs of white cabbage butterfly (L~pidoptera).'~ The avenanthramides in the carnation act, as in oat leaves, as phytoale~ins'~.'~ but in eggs of the butterfly as oviposition det~rrents.'~ It seems that butterflies produce avenanthramides in order to prohibit other butterflies fertilizing their eggs at the same place, and thereby ensuring of their own survival. These activities are also structure related, but in this case it seems that less hydroxyl substitution results in higher a ~ t i v i t y , 'which ~ is contradictory to antioxidant activity. Other biological activities reported for avenanthramides include lipoxygenase inhibitionZoand anti-histamic, anti-allergic and anti-asthmatic activities2' 3 CONCLUSION As oat avenanthramides are quite stable antioxidants and probably have a high pass-through in food processing they presumably contribute to the oxidative stability of oat products and might also provide some nutritional health benefits for human beings. References 1. L. Lowen, L. Anderson, and R.W. Harrison. Ind. Eng.Chem. 1937,29,146. 2. F.N. Peters Jr. and S . Musher. Indust. Eng. Chem. 1937,29, 146. 3. D.M. Peterson. Cereal Chem. 1995,72,21. 4. P.J. White and L.S. Armstrong. J. Amv. Oil. Chem. SOC.1986.63, 525. 5. D.G.H. Daniels and H.F. Martin. J. Sci. Fd. Agric. 1968, 19, 710. 6. F.W. Collins. J.Agric. Food Chem. 1989,37,60. 7. F.W. Collins and W.J Mullin. J. Chromatogr. 1988,445,363. 8. L.H. Dimberg 0. Theander and H. Lingnert. Cereal Chem. 1993,70,637. 9. L.H. Dimberg, L.E Molteberg, R. Solheim and W.Frcalich. J. Cerealscience. 1996,24, 263. 10. L. Crombie and J. Mistry. Tetrahedron Letters. 1990,31,2647. 11. H. Miyagawa, A. Ishihara, T. Nishimoto, T. Ueno and S . Mayama. Biosci. Biotech. Biochem. 1995,59,2305. 12. H. Miyagawa, A. Ishihara, Y. Kuwahara, T. Ueno. and S . Mayama. Phytochemistry. 1996,41, 1473. 13. A. Ishihara, T. Matsukawa, H. Miyagawa, T. Ueno, S . Mayama and H. Iwamura Zeitschriftfu'r naturforschung C-Ajournal of biosciences. 1997, 52, 756. 14. A. Ishihara, H. Miyagwa, Y. Kuwahara, T. Ueno and S . Mayama. Plant Science. 1996, 115,9. 15. L.H. Dimberg and K. Sunnerheim, unpublished. 16. E.L. Molteberg, R. Solheim, L.H. Dimberg and W. Fmlich. J. Cereal Science. 1996, 24, 273. 17. G.J. Niemann. Phytochemisty. 1993,34,319. 18. M. Ponchet, J. Favre-Bonvin, M. Hauteville and P. Ricci. Phytochemistry. 1988, 27,
302
Natural Antioxidants and Anticarcinogens in Nutrition, Health and Disease
725. 19. A. Blaakmeer, D. van der Wall, A. Stork, T.A. van Beek, A. de Groot and J.J.A. van Loon. Journal of Natural Products. 1994,57, 1145. 20. T. Wakabayashi, Y . Kumonaka, H. Ichikawa and S. Murota. Japanesepatent. 1986, 60, 152,454. 21. J.P.A. Devlin and K.D Hargrave. “Pulmonary and antialle drugs” (J.P.A. Devlin, ed) John Wiley and sons: Chichester, England, 1985.