Inhibition of COMT with serotonin. ( A) and ( B) show the reaction rates of COMT at different catechol concentrations in the absence and presence of 100 μM serotonin, respectively. Legend: 0.5 μM (black), 1 μM (red), 3.3 μM (green), 5 μM (blue), 10 μM (orange), 33 μM catechol (brown). ( C) Lineweaver-Burk plot comparing the reciprocal velocities of activity against the reciprocal concentration of catechol in the absence (black) and presence (red) of 100 μM serotonin. The higher slope observed for serotonin indicates some level of inhibition. ( D) Michaelis-Menten determination of equilibrium inhibition constant. Activity assays in the absence (black) and presence (red) of 100 μM serotonin were fit to a two-site Michaelis-Menten model. While the K
M for catechol remains relatively the same, the V
max shows some noticeable differences thereby suggesting a non-competitive mechanism of inhibition between serotonin and catechol.