Antioxidant enzyme response to hypericin in EMT6 mouse mammary carcinoma cells

Antioxidant enzyme activities were measured following exposure to hypericin +/- irradiation in EMT6 cells. CuZnSOD and catalase activities peaked within 0.5 h following irradiation for nontoxic 0.5 microM hypericin and toxic 1.0 microM hypericin. Catalase remained elevated up to 3 h for 1.0 microM hypericin + light. MnSOD activity was elevated immediately following irradiation for both doses. These levels returned to control by 1 h for 0.5 microM hypericin, but were depressed after 1 h for 1.0 microM hypericin. This suggests that mitochondria impairment may be a critical factor in hypericin phototoxicity. Glutathione reductase was inhibited immediately following irradiation with 1.0 microM hypericin, suggesting that an altered status of the glutathione pool contributed to cytotoxicity. Glutathione peroxidase activities were elevated following irradiation but returned to control levels within 0.5 h for both doses, implicating hydroperoxide formation as an early event in hypericin phototoxicity. Inhibition by hypericin in the dark was demonstrated for purified CuZnSOD, Se-dependent glutathione peroxidase, glutathione S-transferase, and glutathione reductase activities in vitro. Irradiation did not potentiate hypericin-mediated glutathione reductase inhibition and decrease inhibition for the other enzymes. Collectively, these data demonstrate an antioxidant enzyme response to hypericin photoactivation and confirm a role for oxygen in hypericin phototoxicity.