Now the question is, how do we measure the activity of this enzyme? So we can measure the activity of this enzyme indirectly by measuring how much of the product is actually formed. So our choice is we can either measure Pyruvate or we can measure NADH. So let's focus on NADH. But before we actually measure NADH, we have to know some type of specific property of NADH that NAD plus doesn't actually have. Well, one major difference between NAD plus and NADH is the fact that NADH can actually absorb a specific type of light with a specific type of wavelength.