How we determine how fast an enzyme works

•We set up a series of tubes containing graded concentrations of substrate, [S] . At time zero, we add a fixed amount of the enzyme preparation. •  Over the next few minutes, we measure the concentration of product formed. If the product absorbs light, we can easily do this in a spectrophotometer. •     Early in the run, when the amount of substrate is in substantial excess to the amount of enzyme, the rate we observe is the  initial velocity of Vi.