There are nearly twenty ways to measure SOD activity, generally they are divided into two categories: one category is direct measurement which measure substrate consumption rate or product production rate in catalytic reaction of SOD. For example, the EPR, pulse radio solution, potassium superoxide and etc, such ways are hard to apply in laboratory limited by the condition of equipment or reagents.
Another category is indirect measurement. The principle of this method: there is a system producing oxygen, this system can induce Ｏ2- regenerate another reaction which is easy to detection, quantitative SOD activity indirectly by measuring inhibition degree of SOD to this reaction. These ways are in common use, such as pyrogallol autoxidation, modified pyrogallol autoxidation, xanthosine-NBT method, NBT-photo reduction method and etc. Such ways have a common drawback, there is a curve relationship between enzyme activity and the inhibition rate. This leads to the following two questions.
1. How can we measure activity accurately
The definition of enzyme activity unit for this method under certain reaction condition, the quantity of enzyme which reach to 50% inhibition rate as an enzyme unit. Therefore, strictly speaking, when we measure the enzyme activity, we have to adjust the quantity of enzyme to let the inhibition rate reach to 50% exactly during test every time. But in this way it will requires a heavy workload. In order to decrease the workload, and have less error, it rules that the inhibition rate must reach to 48-52%, otherwise the quantity of enzyme should increase.
2. Curve Linearization
The unskilled person has to measure for many times if the inhibition rate need to reach 48%-52%, even the skilled person needs to measure for 2-3 times. Then it will require a heavy workload. Solution way is to get a curve by measuring 5-7 gradients of enzyme inhibition rate, then use mathematical to convert it into a straight line, after that, each sample can only measure once (in the range of this line). This method will be not restricted by 50% inhibition rate and won’t introduce large error.