To 800 ll of reaction mixture (90 lM xanthine, 16 mM Na2CO3,
22.8 lM NBT, and 18 mM phosphate buffer; pH 7.0) were added
either 100 ll of phosphate buffer (50 mM; pH 7.0) (0% scavenging
tube) or 100 ll of an extract. After mixing, the reaction was started
by the addition of 100 ll of xanthine oxidase (168 U/L).