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Phytic acid (myo-inositol-hexakis-dihydrogenphosphate) is an organic form of phosphorus, which is abundantly present in plants comprising 1-5% (by weight) of edible legumes, cereals, oilseeds, pollens and nuts. It is largely unavailable to monogastrics such as poultry, pigs, fish and humans, due to the lack of adequate levels of phytic acid hydrolyzing enzyme i.e phytase. The phytic acid present in the plant derived food acts as an anti-nutritional factor, since it causes mineral deficiency due to efficient chelation of metal ions such as Ca2+, Mg2+, Zn2+and Fe2+, forms complexes with proteins, thus affecting their digestion and also inhibits some digestive enzymes like a-amylase, trypsin, acid phosphatase and tyrosinase (Boling et al., 2000). Due to the lack of adequate level of phytase in humans and other monogastric animals, phytic acid is excreted in faeces, which is degraded by soil microorganisms, releasing phosphorus in the soil. This phosphorus reaches aquatic bodies, leading to eutrophication. Phytic acid can be removed by some physical (autoclaving, cooking) and chemical (ion exchange, acid hydrolysis) methods, but these methods decrease the nutritional value of the food. Therefore, the reduction of phytic acid content in foods and feed is desirable as it improves the nutritional value of the food.
PHYTATE AND ITS ROLE IN NUTRITION
Phytate can exist in a metal-free form or in metal-phytate complex, depending on the pH of the solution and the concentration of metal cations (Fig. 1A). At acidic pH, protonation of the phosphate groups of phytate generates the metal-free form. At neutral pH, in contrast, deprotonation of the phosphate groups of phytate enhances the affinity for divalent metal cations and thus phytate forms metal-phytate complexes with divalent metal cations, mostly Mg2+ and Ca2+ (Cheryan, 1980; Maenz et al., 1999). The binding pattern of metals depends upon the ionic radii of the metal ions. The divalent metal cations with large ionic radii (Ca2+ and Sr2+) bind two oxianions from the phosphate groups of phytate in a bidentate fashion (Martin and Evans, 1986). However, divalent metal cations with small radii, such as Mg2+ (0.65 Å), Fe2+ (0.74 Å), and Zn2+ (0.71 Å), bind in a monodentate fashion within two oxygen atoms from the phosphate groups of phytate (Fig. 1B). Therefore, bidentate metal-complex formation prefers divalent cations with large ionic radii (Cheryan, 1980). In addition to its role in phosphate storage, phytate acts as a strong chelator for divalent metal cations and exists as a stable metal-phytate complex with metal ions in plants (Asada et al., 1969; Reddy et al., 1982).
Due to the interaction of phytic acid with other compounds, it acts as an anti-nutritional factor in several ways as described below:
Six reactive groups in the molecules of IP6 make it a strong chelating agent, which binds cations such as Ca2+, Mg2+, Fe2+, Zn2+. Under gastrointestinal pH conditions, insoluble metal phytate complexes are formed which make the metal unavailable for absorption in the intestinal tract of animals and humans (Maga, 1982).
Phytates reduce digestibility of proteins, starch and lipids. Phytate complexes with proteins, and thus, making them insoluble. There is an evidence for the fact that phytate-protein complexes are less subject to proteolytic digestion than the same protein alone (Harland and Morris, 1995).
The action of certain enzymes such as amylase, trypsin, acid phosphatase and tyrosinase has been shown to be inhibited by phytic acid and also by inositol pentaphosphate (Harland and Morris, 1995).
Fig.1 Effect of divalent metal cations and pH on physiological nature of phytate
METHODS OF PHYTATE REDUCTION IN THE FEEDS
A number of physical, chemical and enzymatic methods have been used for reducing the phytate content. Phytates can be partly degraded by extrusion processing (Sandberg et al., 1987), and by soaking to activate endogenous phytase in wheat bran (Morris and Elli, 1980). Toma and Tabekhia (1979) reported that cooking milled rice led to a large reduction in phytate content, while Satoh et al. (1998) recorded 30% reduction in phytate due to extrusion cooking of canola meal. A reduction in phytate content of soybean meal by treatment with alkaline solution (Hartmann 1979) and ion exchange resin (Niiyama, 1992) had also been shown to be possible. The soaking of phytase-supplemented diet in water at room temperature for 8-15 h improved P digestibility (Kemme and Jongbloed, 1993). According to a study of Näsi et al. (1995), soaking of a diet with whey for 3 h at 40 oC also ameliorated the apparent P absorption. The steeping of a diet for 9 h at room temperature reduced the phytate content by 45% (Skoglund et al., 1997). A similar observation was also recorded by Larsen et al. (1999). Microwave heating had, however, little effect on the phytate reduction in soybean meal (Hafez et al., 1989).
Disadvantages of physical and chemical methods of phytate reduction
Feed pre-treatment methods like dry/wet heating and solvent extraction for removing anti-nutrients has been successfully used, but sometimes they cause adverse effects on nutritional quality of the feed. Heat treatment alters the chemical nature of the feed and decreases the nutritional quality of proteins and carbohydrates. Excessive heating of oilseed meal during processing led to loss in the content and digestibility of amino acids (Rackis, 1974). Mwachireya et al. (1999) compared the effect of physical, chemical and enzymatic processing on the digestibility of commercial canola meal. Aqueous-methanol washing of sieved canola meal increased the phytate levels and neutral detergent fibre (insoluble), whereas further treatment with phytase in acidified aqueous media followed by filtration increased levels of crude protein and concurrently decreased levels of phytate (33%) as compared to the commercial canola meal.
1. MITIGATING PHYTIC ACID LEVELS WITH PHYTASE SUPPLEMENTATION
Phytases (myo-inositol hexaphosphate phosphohydrolase EC 188.8.131.52 & EC 184.108.40.206) hydrolyze phytic acid to myo-inositol and inorganic phosphates through a series of myo-inositol phosphate intermediates, and eliminate anti-nutritional characteristics of phytic acid. There are two phytases as classified by Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (NC-IUBMB) in consultation with the IUPAC-IUBMB Joint Commission on Biochemical Nomenclature (JCBN):
3-phytase (EC 220.127.116.11), which first hydrolyses the ester bond at the 3 position of myo-inositol hexakisphosphate. It is typical of microorganisms.
6-phytase (EC 18.104.22.168), which first hydrolyses the ester bond at the 6 position of myo-inositol hexakisphosphate. It is typical of plants. But recently it has been reported from some Basidiomycetous fungi (Lassen et al., 2001).
Besides this classification, recently phytases have also been classified as HAP (Histidine acid phosphatase), BPP (b-Propeller phytase) and PAP (purple acid phosphatase) depending upon their catalytic properties (Mullaney and Ullah, 2003).
Amelioration of nutritional status of foods
The major food supplements in animal feeds are derived from plant sources such as cereals, legumes, soybean etc. The presence of phytate in plant food stuffs (De Boland et al., 1975) is well known where they act as anti-nutritional factors and cause mineral deficiency. Canola meal contains 4-6% phytic acid, which reduces nutrition value of meal. This phytic acid has been shown to bind with multivalent cations and hence reduce their bioavailability. Following are the important roles of phytases in ameliorating the nutritional status of foods:
Effect of phytase on bioavailability of minerals
As phytate forms complexes with minerals, hydrolysis of phytate increases the mineral bioavailability. The phytase supplementation significantly improved the digestibility of Ca, Mg, Mn, total-P, phytate-P, and gross energy (Cheng and Hardy, 2002). The studies on human who were given phytate rich diets lead to zinc deficiency. Decreasing the levels of phytate in the diet is one possible way of improving zinc absorption and this can be achieved by the supplementation of phytase or by the food preparation methods that activate endogenous phytase (e.g. baking, fermentation and soaking). Phytase hydrolyses the hexaphosphate into inositol phosphates with lower degrees of phosphorylation. Anti-nutritional effect of phytic acid did not inhibit copper absorption, but has a significant effect on manganese absorption.
Effect of dietary phytase on protein digestibility
Phytase treatment of soy-protein concentrate was found to improve protein digestibility and retention in Atlantic salmon (Storebakken et al., 1998). In addition, phytate binds trypsin in vitro and thus reduces protein digestibility. Digestibility of dry matter (Papatryphon et al., 1999) and crude protein (Storebakken et al., 1998) were also improved by dietary phytase supplementation. The negative effect of phytate on protein utilization has been observed in fish. Phytase supplementation in plant-based practical diets has been reported to increase protein digestibility (Vielma et al., 1998). In poultry, phytase was reported to improve protein and amino acid utilization through breakdown of phytin-protein complexes (Kornegay, 1995).
2. MITIGATING LEVELS OF PHYTIC ACID USING TRANSGENIC TECHNOLOGY
The advancement in molecular biology techniques, dramatically improved many fields including food science and agriculture. Molecular approaches were carried out to reduce the phytic acid levels in crops mainly cereals such as maize (Zea mays), barley (Hordeum vulgare) and rice (Oryza sativa). These crops are called as low-phytate crops. The mutations, alterations in gene, in the genes that involve in the phytic acid synthetic pathway leads to reduced synthesis of phytic acid. Reduction in phytate in the range of 50% to 80% reduction has been achieved using these mutant lines (Fig.2).
The utilization of low-phytate crops for food has many disadvantages. First of all, the mineral content is much lower than the wild type varieties. Phytic acid forms complexes with metal ions and plants store the minerals in the form of these chelates in the endosperm. Low levels of phytic acid in the seed lead to reduced levels of mineral content. Hence, these crops may not be a good source of minerals and the consumption of food that is made using low phytate seeds may lead to mineral deficiency. However, Hambridge et al. (2004) reported that zinc absorption from low phytate corn was significantly higher than that with wild-type varieties.
Another disadvantage of low-phytate crops is yield. The yields from low-phytate crops are very low compared to their wild type counterparts. This may lead to increase in per-capita expenditure on food.
In order to address these problems, scientists developed transgenic plants that express phytase. The phytase hydrolyses the phytic acid that is formed during seed development and releases the mineral content and proteins. This hydrolysis also leads to formation of inositol phosphates that act as anti-cancer agents (Ishizuka, 2011). These transgenic plants with phytase may catch the recent trends in food consumption i.e. increasing interest in functional foods and nutraceuticals.
Fig. 2 Targets for reducing phytic acid levels in seeds (Raboy, 2007).
Phytic acid acts as an anti-nutritional factor in several ways. Modern biotechnological approaches lead to development of phytase and transgenic plants with lower phytate levels. Supplementation of phytase to the diets improves the bioavailability of proteins and minerals.
Genetically modified low phytic acid crops have the potential to serve as a choice for the replacement of phytate rich food. However, some more research should be carried out to further our understanding the molecular clues of phytic acid accumulation during seed development and also to know the negative and positive effects of dietary phytic acid on human health (Mendoza, 2002)