What is phytate?
Phytic acid (myoinositol 1,2,3,4,5,6-hexakis dihydrogen phosphase) is the main storage form of phosphorous in plant-based feed ingredients (Gupta, et al., 2015). Phytate, also known as IP6, is the salt form of phytic acid (Dersjant-Li et al., 2015). Phytin is the mixed salt, IP6 with potassium, magnesium and calcium deposited as a complex within the plant, form of phytic acid (Selle and Ravindran, 2007 and Dersjant-Li et al., 2015).Approximately 50-85% of the total phosphorous found in the seeds of plant based feed sources (legumes, cereals, nuts, oil seed) is in the form of Phytic acid (Dersjant-Li et al., 2015 and Gupta, et al., 2015).
Table 1: phytic acid content of common feed ingredients used in monogastric diets (Gupta, et al., 2015)
Phytate is a ployanionic molecule, Figure 1, which has 12 negative charges (Selle, et al., 2009).
Figure 1: Structure of phytic acid (phytate) (Dersjant-Li et al., 2015)
Phytate dissociates at a pH less than 2 and becomes increasingly more negatively charged as pH increases. An increase in negative charge is associated with an increase in solubility of phytate. Thus as the phytate passes from the gizzard/stomach (low pH) to the lower digestive tract (neutral pH) its negative charge increases, increasing its interaction with cations (Santos, 2012 andDersjant-Li et al., 2015).
Cations are minerals, such as calcium, zinc and copper, which have dissociated at the low pH and become positively charged (Selle, et al., 2009). Calcium and phytate have a linear relationship; a diet high in calcium will reduce phytic phosphorous absorption and a diet high in phytate will increase the animals’ requirement for calcium from 0.60% to 0.95%. In the same way deficiencies in zinc and copper in piglets are also seen as zinc and copper have a high inclusion rate in the pre-starter and starter diets for the purpose of improved growth (Santos, 2012). This interaction results in the formation of stable, insoluble salts in the small intestine which are excreted resulting in mineral deficiency even though the animal is supplemented with minerals at high levels (Santos, 2012 and Dersjant-Li et al., 2015). Additionally Ca-phytate complex reduces saturated fat digestion as it results in the formation of metallic soaps in the lumen of the gut (Dersjant-Li et al., 2015).
Phytate attracts water molecules. This reduces the amount of water molecules surrounding protein molecules resulting in reduced protein solubility (Santos, 2012). At an acidic pH phytate non-selectively binds to protein forming insoluble protein-phytate complexes. Phytate can also bind protein through cations forming protein-mineral-phytate complexes when the pH is above the isoelectric point of the protein (Dersjant-Li et al., 2015). Between pH 0.8 and 2.8 phytate suppresses the activation of pepsin in the stomach/ gizzard resulting in reduced protein digestion in the upper digestive tract and increasing the amount of protein entering the lower digestive tract to be fermented (Santos, 2012).
Undigested protein reaching the duodenum results in increased gastrin, HCl and pepsinogen secretions. These secretions result in increased mucin production because they irritate the gut mucosa. This results in increased endogenous losses and energy expended by the animal. To compensate for this increase in expended energy the animal will consume more feed which increases total feed costs of the operation (Santos 2012).
The pancreas increases sodium bicarbonate secretion as digesta enters the lower digestive tract in order to increase the pH of the digesta. The sodium deficiency compromises the NaK-ATPase pump which is responsible for the absorption of amino acids and other nutrients (Santos, 2012).
Overall the antinutritional effects of phytate results in decreased protein digestibility and nutrient utilization, increased maintenance energy and decreased energy available for production resulting in reduced performance and increased production costs (Dersjant-Li et al., 2015).
Gut microflora in monogastrics have the ability to breakdown phytate in the large intestine by secreting endogenous phytase enzymes resulting in the release of bound phosphorous to be absorbed and utilized by the animal. However the insoluble complexes formed due to the interaction between phytate and dietary nutrients reduces the efficiency of the endogenous phytase enzymes resulting in 70% of the total phosphorous being excreted (Selle et al., 2009 and Gupta, et al., 2015). Leaching or surface run-off of the excreted phosphorous results in eutrophication of surface water, algae blooms, hypoxia, death of aquatic species and production of nitrous oxide (Gupta, et al., 2015).
What is phytase
Phytases (myo-inositol hexakisphosphate phosphohydrolase) are protein enzymes that catalyze the stepwise removal of phosphate from phytic acid/phytate releasing the inorganic phosphorous (Dersjant-Li et al., 2015). Classification of phytases are based on different properties, table 2 (Gupta, et al., 2015).
Table 2: Classification of phytase based on different properties (Gupta, et al., 2015).
Exogenous Phytases commercially produced are HAP (can initiate hydrolyses at C3 or C6 position) phytases (Gupta, et al., 2015 and Dersjant-Li et al., 2015). Phytase activity is expressed as FTU. FTU is defined as the amount of phytase that liberates 1mmol of inorganic phosphate per minute from 0.0051mol/L sodium phytate at pH 5.5 and temperature 37oC. The main problem with FTU is that the pH in the stomach is less than 2 thus the true activity of the phytase enzyme in the animal is different from the activity measured in the lab (Dersjant-Li et al., 2015).
Variation between different commercial Phytases activity is due to the different enzyme origin, ie fungal, bacterial or plant origin. The characteristics of an ideal phytase enzyme are (Gupta, et al., 2015 and Dersjant-Li et al., 2015):
- Active over a broad pH range
- Catalytically efficient (degrade phytate as quickly as possible to minimize formation of insoluble complexes)
- Thermostable (as feed processing can denature the enzyme)
- Resistant to protease enzymes
- Easily incorporated into feed by the feed manufacturer
Factors that influence Phytase activity
Figure 2: Factors influencing phytase activity within the animal (Dersjant-Li et al., 2015).
- Phytase-related factors: phytase is a protein molecule thus can by digested by endogenous proteases. Bacterial phytases have greater proteolytic resistance and affinity for IP6 compared to fungal phytases. Phytases from different sources have different molecular structures resulting in differences in activity, optimal pH, catalytic efficiency and thermostability (Gupta, et al., 2015). For example Phytase from E.coli has better activity against soy protein compared to A.niger. Location of first hydrolysed phosphate influencing the binding capacity of the phytate molecule thus its ability to form insoluble complexes. IP3 has only 11% of the binding capacity of IP6 thus enzymes catalyzing phosphate removal stepwise from IP6 to IP1-2 will be the most efficient (Dersjant-Li et al., 2015).
- Animal-related factors: endogenous phytase activity occurs in the colon of pigs. Supplementing diets with exogenous phytase shifts phytase activity to stomach (crop in poultry) and upper small intestine. The pH in a pig’s stomach is 2-2.5 while the pH in the crop is 5.2-5.8. This difference in pH of the main phytase activity sites between species results in different catalytic efficiencies of the phytase enzyme (Dersjant-Li et al., 2015).
- Dietary-related factors: phytate level and its interaction with cations and proteins determines the amount and rate that insoluble mineral-phytate or protein-mineral-phytate complexes are formed (Dersjant-Li et al., 2015).
Benefit of Exogenous Phytase
Exogenous phytase are added to feed to increase the digestibility of phytate. This increases the bioavailability and absorption of inorganic phosphorous as well as minerals (calcium, zinc and copper) and amino acids. Phosphorous excretion is reduced which reduces the negative impact that phosphorous has on the environment. Phytase supplementation has the added benefit of reducing feed supplementation costs as less minerals need to be supplemented. This is an important economic aspect as inorganic phosphorous prices have steadily been increasing due to the depletion of global phosphorous reserves (Selle, et al., 2009).
References available on request