Fig. 2. Dose dependence of H₂O₂ inhibition of tyrosine hydroxylase activity and effect of the length of the oxidative challenge. (A) Semiconfluent B16 melanoma cells were challenged for 20 minutes with varying concentrations of H₂O₂, and allowed to recover in fresh medium for 16 hours before enzyme activity and protein concentration determinations. Results shown are the mean±s.d. for three independent experiments. (B) Cells were exposed to H₂O₂ (0.5 mM) for the times shown and allowed to recover for 8 hours before determination of their tyrosine hydroxylase- () and dopa oxidase- () specific activities. Results are the mean±s.d. of at least three independent experiments.