The assay varied the concentration of L-DOPA (0.5, 1.0, 1.5 and
2.5 mM) and inhibitor (0, 20, 40, 60 and 80 mg/ml). The inhibition
constants were obtained by the second plots of the apparent
Michaelis-Menten constant (Kmapp) versus the concentration of the
inhibitor as described by Chen et al. (2005) and calculated follow: