Biotechnology and Molecular Biology Review Vol. 1 (1), pp. 21-38, April 2006 Available online at http://www.academicjournals.org/BMBR ISSN 1538-2273 © 2006 Academic Journals Review Phenolic compounds and related enzymes as determinants of sorghum for food use Mamoudou H. Dicko1,2,3*, Harry Gruppen2, Alfred S. Traoré1, Alphons G. J. Voragen2 and Willem J. H. van Berkel3 1Laboratoire de Biochimie, UFR-SVT, CRSBAN, Université de Ouagadougou, 03 BP. 7021, Ouagadougou 03, Burkina Faso, 2Laboratory of Food Chemistry, Department of Agrotechnology and Food Sciences, Wageningen University, PO Box 8129, 6700 EV Wageningen, The Netherlands 3Laboratory of Biochemistry, Department of Agrotechnology and Food Sciences, Wageningen University, PO Box 8128, 6700 ET Wageningen, The Netherlands Accepted 26 January, 2006 Phenolic compounds and related enzymes such as phenol biosynthesizing enzymes (phenylalanine ammonia lyase) and phenol catabolizing enzymes (polyphenol oxidase and peroxidase) are determinants for sorghum utilization as human food because they influence product properties during and after sorghum processing. Phenolic compounds are quality-grade markers for the preparation of several foods because of enzyme inhibitory activities, color, or antioxidant activities. Large inter- varietal differences in contents of phenolic compounds and their antioxidant activities among sorghum varieties exist. Moreover, some red sorghum varieties have higher antioxidant activities than the most important sources of natural antioxidants. Oxidation products of peroxidase and polyphenol oxidase (benzoquinones and polymeric compounds) affect food quality. This paper reviews the current advances in phenolic compounds and phenolic enzymes in sorghum as human food, with emphasis on nutritional and health aspects. This may provide some guidance for researchers in further Investigations and for industries in developing practical health agents and functional foods. Key words: sorghum, phenolic compounds, antioxidants, 3-deoxyanthocyanidins, proanthocyanidins, peroxidase, polyphenol oxidase, phenylalanine ammonia lyase. Table of content I. INTRODUCTION ..........................................................................................................2 II. SORGHUM PHENOLIC COMPOUNDS BIOCHEMISTRY ..............................................2 II. 1. Classes of phenolic compounds in sorghum................................................................2 II. 2. Simple phenols and phenolic acids .............................................................................5 II. 3. Flavonoids ...................................................................................................................5 II. 4. Polyflavans...................................................................................................................7 II. 5. Role of phenolic compounds in plants..........................................................................7 II. 6. Health related properties of phenolic compounds........................................................7 II. 7. Relevance of phenolic compounds for sorghum food quality.......................................8 III. SORGHUM PHENOLIC ENZYMES BIOCHEMISTRY....................................................9 III. 1. Role of phenylalanine ammonia lyase in the biosynthesis of phenolic compounds.…9 III. 2. Polyphenol oxidases..................................................................................................10 III. 3. Peroxidases................................................................................................................10 III. 4. Role of phenolic enzymes in food...............................................................................12 III. 5. Sorghum grain as potential source of bioactive components.....................................13 IV. CONCLUSION................................................................................................................13 V. ACKNOWLEDGEMENTS................................................................................................14 VI. REFERENCES...............................................................................................................14 *Correspondence author. E-mail: [email protected], Tel. +226 70272643, Fax. +226 50307242. 22 Biotechnol. Mol. Biol. Rev. INTRODUCTION Sorghum bicolor (L.) Moench is the fifth most important Polyphenol oxidases and peroxidases are the main cereal crop after wheat, rice, maize, and barley in terms enzymes involved in cereal browning. In sorghum, PPO of production (FAO, 2005). The total world annual is present in the leaves (Stafford and Dresler, 1973; sorghum production is over 60 million tons from a Vaughn and Duke, 1981) and in the grain (Glennie, 1981; cultivated area of 46 millions ha. Sorghum is particularly Dicko et al., 2002a, 2006a). Peroxidases are ubiquitously adapted to drought prone areas: hot, semi-arid tropical present and relatively abundant in sorghum grain both environments with 400-600 mm rainfall-areas that are too before and after germination (Dicko et al., 2002a, 2006a). dry for other cereals. The sorghum genome is currently Polyphenol oxidase and peroxidases being involved in sequenced (Paterson et al., 2003; http://fungen.org/Sorg- the oxidative cross-linking of phenolic compounds are hum.htm). determinants of food quality by influencing product Phenolic sorghum phytochemicals are important for properties during and after processing (Tomas-Barberan human nutrition (Awika et al., 2004a). Phenolic and Espín, 2001). compounds are the most widely distributed secondary metabolites, ubiquitously present in the plant kingdom. Among cereals, sorghum has the highest content of SORGHUM PHENOLIC COMPOUNDS BIOCHEMISTRY phenolic compounds reaching up to 6% (w/w) in some varieties (Deshpande et al., 1986; Beta et al., 1999, Doka Classes of phenolic compounds in sorghum et al., 2004; Awika and Rooney, 2004a, Dicko et al., 2005a). Almost all classes or phenolic compounds are Phenolic compounds, of which more than 8000 are found in sorghum (Chung et al., 1998; Krueger et al., known, embrace a wide range of plants secondary 2003, Awika and Rooney, 2004a). metabolites (Harborne, 1994; Pietta, 2000). Phenolic Proanthocyanidins (condensed tannins), originally compounds are the most widely distributed secondary classified as antinutritional factors, may have health metabolites, ubiquitously present in the plant kingdom. benefits for humans (Waniska, 2000, Parr and Bolwell, Located in the vacuole, they are found in free form or 2000; Clifford, 2000; Awika, 2004a). Some flavanols, e.g. linked to carbohydrates (glucose, galactose, rhamnose, flavan-4-ols have particular therapeutic interest because mannose, rutinose etc.). Among cereals, sorghum has of their antitumor activity (Ferreira and Slade, 2002). the highest content of phenolic compounds reaching up Procyanidins may inhibit the growth of several viruses to 6% (w/w) in some varieties (Deshpande et al., 1986; including the human immunodeficiency virus 1 (HIV-1) Beta et al., 1999, Awika and Rooney, 2004a) (Tables 1 (Chan and Kim, 1998; Okuda et al., 1991, Lu et al., and 2). While all sorghums contain phenolic compounds, 2004). Phenolic compounds, together with other natural its genotype and the environment in which it is grown compounds scavenge free radicals (antioxidant activity). influence the amount present in any particular cultivar. Interestingly, independent of germination, sorghum grains The main classes of phenolic compounds are simple display high antioxidant activities related to their phenolic phenols, hydroxybenzoic acids, hydroxycinnamic acids, content (Dicko et al., 2005a, Dykes et al., 2005). flavonoids (flavanols, flavones, flavanones, isoflavones Moreover, some red sorghum varieties have higher and anthocyanins), chalcones, aurones (hispidol), antioxidant activities than the most important sources of hydroxycoumarins, lignans, hydroxystilbenes and natural antioxidants such as Vaccinium species, e.g. polyflavans (proanthocyanidins and pro- blackberries (Awika et al., 2004a; Dicko et al., 2005a). deoxyanthocyanidins) (Chung et al., 1998; Krueger et al., Since sorghum is a staple food in many African countries, 2003). These compounds are soluble in water or organic it may be the main potential source of natural solvents (methanol, HCl-methanol, acetone, antioxidants. Epidemiological studies suggest that the dimethylformamide, etc.). Sorghum does not contain consumption of whole cereal grains including sorghum tannic acid and hydrolysable tannins (Waniska, 2000, lowers the mortality from cardiovascular disease, which is Awika et al., 2004a). Sorghums with a pigmented testa probably linked to their antioxidant properties (Kushi et and spreader genes (B1B2S) or with purple/red plants al., 1999; Awika and Rooney, 2004a). and thick pericarp genes have the highest levels of The biosynthesis of phenolic compounds in plants phenolic compounds (Dykes et al., 2005). Sorghums with proceeds through the production of phenylalanine, which a black pericarp have higher levels of flavan-4-ols and is subsequently deaminated by the enzyme anthocyanins than the other varieties. This suggests that phenylalanine ammonia lyase (PAL). PAL is indirectly genes for plant color, pericarp thickness, presence of a associated with the synthesis of several phenolic pigmented testa, and spreader genes increase phenolic constituents, including cell wall polymers (Figure 1) (Parr levels (Dykes et al., 2005, Dicko et al., 2005a).