I was using xanthine oxidase as superoxide generator and cytochrome c as superoxide detector in performing SOD assay. I keep getting negative absorbance for the results. Based on literature, there should be the rate of increase in absorbance at measured wavelength.

So should the absorbance be negative or positive since few of my absorbance readings resulted in positive absorbance whilst majority of them resulted in negative absorbance? so I am a bit confuse whether I did something wrong with the assay for getting inconsistent positive and negative absorbency.

Hope someone can answer.