Hello, I have problem with SOD activity

Normally, this method in control should be higher absorbance than sample. However, Control was always lower than sample's absorbance..

I followed these protocol.

For the determination of enzyme activities, sugar beet leaf sample was ground to a fine powder in liquid nitrogen using a pre-cooled mortar and pestle and then freeze dried. 50 mg of freeze dried sample was homogenized in 5 mL 50 mM PBS (pH 7.8) using a pre-chilled mortar and pestle, then centrifuged at 15000 × g for 20 minutes at 4 ºC. The supernatant was designated as crude enzyme extract and stored at 4 °C for the assays of various antioxidant enzyme activities (Wu et al., 2003). The SOD activity was determined according to Zhang (1992). SOD activity was assayed using nitroblue tetrazolium (NBT). The reaction mixture (3 mL) contained 50 mM PBS (pH 7.8), 13 mM methionine, 75 µM NBT, 10 µM EDTA, 2 mM riboflavin, and 25 µL enzyme extract.

Blank : reaction mixture(2.75 ml) + H2O (0.25 ml) (dark, 20minutes)

Control : reaction mixture(2.75 ml) + H2O (0.25 ml) (light, 20minutes)

sample : reaction mixture(2.725ml) + enzyme extracts(0.025 mL) + H2O (0.25 ml) (light, 20minutes)

Absorbance measured in 560nm

control OD : 1.0554

Blank OD: 0.063

sample OD : 1.58, 1.76~ (All of samples Higher than control OD...!!)

Reaction tubes were kept under two 15W fluorescent lamps for 20 min. One unit of SOD represented the amount that inhibited NBT photoreduction by 50% at 25C.

Can you help me how to solve this problem?