Frozen rice plants (approximately 200 mg) were homogenized
in 10 ml of 50 mM potassium phosphate
buffer (pH 7.0) containing 1 mM ethylenediaminetetraacetic
acid and 1 % polyvinylpyrrolidone for SOD
and CAT assay or the combination with the addition of
1 mM ASC in the case of APX assay. The homogenate
was centrifuged at 12,000×g for 20 min at 4 °C and the
supernatant was immediately desalted by Sephadex G-
25 gel filtration to remove interfering materials and
used as the crude enzyme extract.
Total SOD activity was measured on the basis of its
ability to reduce nitroblue tetrazolium (NBT) by superoxide
anion generated by the riboflavin system
under illumination. One unit of SOD was defined as
the amount of crude enzyme extract required to inhibit
the reduction rate of NBT by 50 % (Beauchamp and
Fridovich 1971). APX activity was measured by monitoring
the decrease in absorbance at 290 nm as ASC
was oxidized (ε=2.8 mM−1
cm−1
) for at least 1 min in
3 ml reaction mixture, as described by Nakano and
Asada (1981). CAT activity was spectrophotometrically
measured by monitoring the consumption of hydrogen
peroxide (H2O2; ε=39.4 mM−1
cm−1
) at 240 nm
for at least 3 min (Huang et al. 2006).