2.4. Measurements of phytase activity
2.4.1. Enzyme extraction
Yeast strains were grown in flasks containing 100 ml of Delft Phy
medium. The medium was inoculated with yeast inocula as previously
described (OD600 = 0.1), and cultivated at 30 °C for 48 h. Each strain
was inoculated in triplicates. Samples of extracellular and intracellular
enzyme extracts were collected after 2, 8, 24 and 48 h after inoculation.
Enzyme extracts were prepared as described by Nuobariene et al. (2011).
To confirm that the enzyme is repressed by inorganic phosphate, the
same protocol was followed for yeast strains grown on Delft+medium.
2.4.2. Phytase activity
Phytase activity was measured as previously described (Nuobariene
et al., 2011). Briefly, 0.2 ml of enzyme extract was added to 0.8 ml of
phytic acid dipotassium solution (3 mmol/l phytic acid dipotassium
salt in 0.2 mol/l sodium acetate/HCl buffer, pH 5.5) pre-incubated at
30 °C per 5min. The solutionwas mixed and incubated at 30 °C. Samples
were measured at different time intervals, every time the reaction was
stopped by adding trichloroacetic acid (TCA) to the sample. Blank was
measured with sodium acetate buffer and TCA, together with enzyme
extract. To measure the liberated inorganic phosphate, 0.4 ml of the
samplewas added to 3.2ml of freshly prepared acid molybdate reagent
(1 volume of 10 mmol/l ammonium molybdate, 1 volume of 2.5 mol/l
sulfuric acid and 2 volume of acetone). Absorbance of the yellow at
355 nm was measured in a spectrophotometer, using sodium acetate
buffer with TCA as blank. A standard curve was prepared with KH2PO4
in 0.2 mol/l sodium acetate buffer/HCl buffer (pH 5.5).