GPx activity is usually measured following the disappearance of NADPH (decrease in OD) in the reaction mixture at 340 nm in a spectrophotometer over a period of time (usually 8-10 min). It is calculated based on extinction coefficient of NADPH.
GPx activity is calculated as NADPH oxidized/min/mg protein =(Change in absorbance at 340nm/min for the sampleTest - Change in absorbance at 340nm/min for the blank) x final volume/ 0.0062 x concentration of the sample in mg/ml protein x sample volume; where 0.0062 = micromolar extinction coefficient of NADPH at 340 nm for 1 cm path length of the cuvette.