In the process, the red NAD plus molecules will be converted to the green NADH molecules. And as this process is taking place, we're measuring how much light is being absorbed. And if that is increasing, then we know that our enzyme is present because only this enzyme can catalyze this reaction. If nothing took place, if we measured no absorbance of light, then that means that enzyme is not present in our mixture. So what exactly is the enzyme activity? Well, the enzyme activity is the number of moles of products that are produced.