The assay of VDE activity was carried out as described in
references
[5,17,19]
, with some modi
fi
cations. The VDE
extract was brought to a volume of 25
m
l with buffer
containing 0.1% (w/v) Tween 20 or not. To 2.4 ml of 50 mM
sodium acetate
–
HCl buffer (pH 4.9) were added 0.1 ml of
water, 0.1 ml of the suspension of PS2 particles, and 0.3 ml
of VDE extract supplemented with 0.1% Tween 20 or not.
The reaction was started by the addition of 0.08 ml of 0.8 M
sodium ascorbate. The mixture was incubated at 30
8
C for
3 h. The reaction was stopped by the addition of 0.09 ml of
0.2 M dithiothreitol to quench VDE activity
[21]
.The
particles were collected by centrifugation at 27,220
g
for
30 min and analyzed for the contents of XCP by HPLC. The
de-epoxidation state of PS2 particles was described in terms
of the de-epoxidation index (DEI), i.e., (ANT + 2ZEA)/(VIO
+ ANT + ZEA)
[17]
.