Manganese porphyrins are a potent class of catalytic antioxidants whose activity was recently shown to be partially dependent upon flavin-containing enzymes (R. Kachadourian et al., 2004, Biochem. Pharmacol. 67, 77-85). We tested whether manganese porphyrins could redox cycle with the flavin-containing enzyme, cytochrome P450 reductase, and whether this results in the inhibition of xenobiotic metabolism. The effect of manganese porphyrins on xenobiotic metabolism in rat and human liver microsomes was assessed spectrofluorometrically by following the O-dealkylation of benzyloxy- (BROD) and methoxyresorufin (MROD). Redox cycling of manganese porphyrins with human cytochrome P450 reductase was assessed both spectrophotometrically and polarographically by following the consumption of NADPH and oxygen, respectively. The tetrakis(N-pyridinium-2-yl) meso-substituted manganoporphyrin, MnTEPyP5+, and the tetrakis(1,3-dimethyl imidazolium-2-yl) meso-substituted manganoporphyrin, MnTDMIP5+, were 40 to 100 times more potent inhibitors of rat and human liver microsomal BROD metabolism than the tetrakis(1,3-diethyl imidazolium-2-yl) meso-substituted manganoporphyrin, MnTDEIP5+, or the tetrakis(4-benzoic acid) meso-substituted manganoporphyrin, MnTBAP. A similar pattern of inhibition was also seen in beta-naphthoflavone-induced rat liver microsomal MROD metabolism. This pattern of xenobiotic metabolism inhibition correlated with the compound's ability to redox cycle with cytochrome P450 reductase where MnTEPyP5+ was more potent than MnTDEIP5+. Furthermore, redox cycling of MnTEPyP5+ with cytochrome P450 reductase was inhibited by the flavin domain inhibitor diphenyleneiodonium. Small changes to the carbon chain length on the imidazolium side groups had a large effect on this activity. It is possible that interactions between manganoporphyrins and flavin-dependent oxidoreductases can account for both adverse and beneficial effects of the compounds.