Redox properties of novel antioxidant 5,8-Dihydroxycoumarin: implications for its prooxidant cytotoxicity

The aim of this work was to characterize the redox properties of the new antioxidant 5,8-dihydroxycoumarin (5,8-DHC), isolated from sweet grass (Hierochlo? odorata L.), and to determine its impact on its cytotoxic action. Reversible electrochemical oxidation of 5,8-DHC at pH 7.0 was characterized by the midpoint potential (E(p/2)) of 0.23 V vs. the normal hydrogen electrode. 5,8-DHC was slowly autoxidized at pH 7.0, and it was active as a substrate for peroxidase (POD, EC 1.11.1.7) and tyrosinase (TYR, EC 1.14.18.1). Oxidation of 5,8-DHC by POD/H202 yielded the product(s) which reacted with reduced glutathione and supported the oxidation of NADPH by ferredoxin:NADP+ reductase (FNR, EC 1.18.1.2) and NAD(P)H:quinone oxidoreductase (NQO1, DT-diaphorase, EC 1.6.99.2). The concentration of 5,8-DHC for 50% survival of bovine leukemia virus-transformed lamb kidney fibroblasts (line FLK) during a 24-h incubation was (60 +/- 5.5) microM. Cytotoxicity of 5,8-DHC was decreased by desferrioxamine, catalase, the antioxidant N,N'-diphenyl-p-phenylene diamine, and potentiated by 1,3-bis-(2-chloroethyl)-1-nitrosourea and dicumarol, an inhibitor of NQO1. This shows that 5,8-DHC possesses the oxidative stress-type cytotoxicity, evidently due to the action of quinodal oxidation product(s). The protective effect of isoniazide, an inhibitor of cytochrome P-450 2E1, points to hydroxylation of 5,8-DHC as additional toxification route, whereas the potentiating effect of 3,5-dinitrocatechol, an inhibitor of catechol-o-methyltransferase (COMT, EC 2.1.1.6), points to the o-methylation of hydroxylation products as the detoxification route.