Abstract
In the current study, we report for the first time that grain components of barley, rice, wheat and maize can inhibit the activity of Aspergillus ficuum phytase. The phytase inhibition is dose dependent and varies significantly between cereal species, between cultivars of barley and cultivars of wheat and between Fusarium graminearum infected and non-infected wheat grains. The highest endpoint level of phytase activity inhibition was 90%, observed with grain protein extracts (GPE) from F. graminearum infected wheat. Wheat GPE from grains infected with F. graminearum inhibits phytase activity significantly more than GPE from non-infected grains. For four barley cultivars studied, the IC50 value ranged from 0.978 ± 0.271 to 3.616 ± 0.087 mg×ml-1. For two non-infected wheat cultivars investigated, the IC50 values were varying from 2.478 ± 0.114 to 3.038 ± 0.097 mg×ml-1. The maize and rice cultivars tested gaveIC50 values on 0.983 ± 0.205 and 1.972 ± 0.019 mg×ml-1, respectively. After purifying the inhibitor from barley grains via Superdex G200, an approximately 30–35 kDa protein was identified. No clear trend for the mechanism of inhibition could be identified via Michaelis-Menten kinetics and Lineweaver-Burk plots. However, testing of the purified phytase inhibitor together with the A. ficuum phytase and the specific protease inhibitors pepstatin A, E64, EDTA and PMSF revealed that pepstatin A repealed the phytase inhibition. This indicates that the observed inhibition of A. ficuum phytase by cereal grain extracts is caused by protease activity of the aspartic proteinase type.
Highlights
Phytases are phosphatases that initiate the sequential liberation of orthophosphate groups from phytate
We report for the first time that grain components of barley, rice, wheat and maize can inhibit the activity of Aspergillus ficuum phytase
More than 90% inhibition of phytase activity was obtained in the presence of 2 mg ml-1 proteins from Fusarium infected grains whereas grain protein extracts (GPE) from the non-infected grains only reduced the phytase activity of about 35%
Summary
Phytases (myoinositol hexakisphosphate phosphohydrolase; EC 3.1.3.26 and EC 3.1.3.8) are phosphatases that initiate the sequential liberation of orthophosphate groups from phytate (myoinositol 1, 2,3,4,5, 6-hexakisphosphate). Phytate is the major storage form of phosphorous in plant seeds contributing up to 70% of the total phosphorus reserve [1] and 1–5% (dry w/w) of cereal grains, legume seeds, oilseeds, pollen and nuts [2]. In mature seeds, it exists as a mixed salt of K+, Ca2+, Mg2+ and Zn2+, called phytate/ phytin. Monogastric animals like pigs and poultry have basically no phytase activity in their digestive tract, and the phytase level of the mature plant seed is most often inadequate for efficient phytate
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