Role of cytochrome P-450 in hydrogen peroxide-induced cytotoxicity to LLC-PK1 cells

The current study was designed to test the role of cytochrome P-450 in hydrogen peroxide-induced cytotoxicity, and to determine whether it may serve as a source of catalytic iron. Hydrogen peroxide led to iron release (as measured by iron/bathophenanthroline complex) from the microsomes prepared from LLC-PK1 cells. Cimetidine, which inhibits cytochrome P-450 by interacting with the heme iron, significantly blocked iron release, whereas ranitidine, which has a similar structure as cimetidine but is a weak inhibitor of P-450, did not have an effect (H2O2, 0.42 +/- 0.04; H2O2 + cimetidine, 0.23 +/- 0.02 nmol/mg protein; N = 4, P < 0.01). Exposure of LLC-PK1 cells to hydrogen peroxide (2.5 mM) resulted in a significant increase in the bleomycin-detectable iron (iron capable of catalyzing free radical reactions) content that was prevented by cimetidine, but not ranitidine. We then examined the effect of the inhibitors of cytochrome P-450 on cell death (as measured by Trypan blue exclusion) after exposure of LLC-PK1 cells to 2.5 mM hydrogen peroxide for 120 minutes. Inhibition of cytochrome P-450 by cimetidine significantly reduced the cell death; the effect was observed with 0.05 mM and was concentration dependent with 1 mM affording almost complete protection (H2O2, 59 +/- 1.3% vs. H2O2 + cimetidine, 11 +/- 0.7%; N = 5, P < 0.01). In contrast, ranitidine did not show any protection. We confirmed that the protective effect of cimetidine was not related to scavenging hydrogen peroxide or hydroxyl radicals or chelating iron. A second inhibitor of cytochrome P-450, piperonyl butoxide, had a similar dose-dependent beneficial effect against hydrogen peroxide-induced cell injury. Our data thus indicate an important role of cytochrome P-450 in hydrogen peroxide-induced cytotoxicity to LLC-PK1 cells and suggest that cytochrome P-450 may serve as a source of catalytic iron.