About superoxide dismutase (SOD) activity
Superoxide dismutase (SOD) is an antioxidant enzyme that catalyzes the dismutation of the superoxide radical (O₂·⁻) into molecular oxygen and hydrogen peroxide: 2 O₂·⁻ + 2 H⁺ → O₂ + H₂O₂. Because superoxide cannot be measured directly with a simple plate reader, SOD activity is almost always determined indirectly through an inhibition assay. A superoxide-generating system (for example xanthine + xanthine oxidase) feeds an indicator dye that changes color when it reacts with superoxide — commonly nitroblue tetrazolium (NBT), pyrogallol, cytochrome c, or a WST tetrazolium salt. Added SOD competes for the superoxide, so it slows the indicator reaction. The more SOD present, the greater the inhibition of the color change.
The formula
This calculator uses the standard percent-inhibition method. First, percent inhibition is calculated from the control reaction (no SOD) and the sample reaction (with SOD):
% inhibition = (control − sample) / control × 100
The "reading" is the rate of the indicator reaction — typically the change in absorbance per minute (ΔA/min), but any consistent measure of reaction rate works as long as the control and sample use the same units. One unit of SOD is defined as the amount of enzyme that produces 50% inhibition of the indicator reaction under the assay conditions, so units follow directly:
SOD units in the assay = % inhibition ÷ 50
Multiplying by the dilution factor recovers the activity of the original, undiluted sample, and dividing by the milligrams of protein in the assay gives the specific activity (U/mg protein), the value most often reported when comparing tissues, treatments, or purification steps.
Why SOD activity is measured
- Oxidative-stress research: SOD is a front-line defense against reactive oxygen species, so its activity is a common marker in studies of aging, ischemia-reperfusion, inflammation, and neurodegeneration.
- Enzyme purification: specific activity (U/mg) rises as SOD is purified away from other proteins, so the assay tracks purification progress.
- Plant and microbial stress physiology: SOD activity often rises under drought, salinity, cold, or pathogen stress and is used as an indicator of a plant's antioxidant response.
- Food and supplement science: SOD content is measured in extracts (e.g., melon, wheat sprout) marketed for antioxidant activity.
Reference points
Because the "unit" is defined by 50% inhibition, an assay giving exactly 50% inhibition contains 1 unit; 25% inhibition corresponds to 0.5 unit; 75% inhibition to 1.5 units. Assays are most accurate when the sample is diluted so inhibition lands near 50% — the dose-response curve is steepest and most linear there. Purified bovine erythrocyte Cu/Zn-SOD, the classic reference enzyme, has a specific activity on the order of 3,000 U/mg protein.