Fig. 1. Inhibition of the tyrosine hydroxylase activity of mouse and human melanoma cells by H₂O₂. B16 mouse melanoma and SCL and BEU human melanoma cells were exposed to 1 mM H₂O₂ for 20 minutes, and then allowed to recover in H₂O₂-free medium for 8 or 16 hours. Cells were harvested, solubilized in 1% Igepal CA-630 and the tyrosine hydroxylase and protein content of the samples was analyzed. Results are expressed as mean±s.d. of three independent experiments. The tyrosine hydroxylase-specific activities (in µU/mg protein) of controls, treated as the 8 hours time point, were: 79±27, 170±22 and 303±27, for BEU, SCL and B16 melanoma cells, respectively.