Chemoprotective properties of naturally occurring methylenedioxybenzenes and catecholic metabolites

grantor: University of TorontoNaturally occurring methylenedioxybenzenes (MDBs) are found in a wide variety of human food, essential oils and flavors. MDBs have been widely accepted as cytochrome P450 inhibitors. The chemoprotective effects of this group of dietary compounds are, however, still unknown. In the present work, the dietary MDBs, in particular isosafrole, have been found to: (1) prevent CCl4 -induced liver necrosis 'in vivo' and BrCCl3-induced hepatocyte injury 'in vitro'; (2) protect against carcinogens N-dimethylnitrosamine and chloroform hepatotoxicity 'in vivo'; (3) prevent phenetidine (a deleterious metabolite of phenacetin)-induced methemoglobinemia in mice. Isosafrole was highly protective even at dosages as low as 10 mg/kg, and was found to be a potent inhibitor of P450 isozymes CYP2E1, CYP2B1/2 and CYP1A2 that are involved in the bioactivation of the toxicants both ' in vivo' and 'in vitro'. The protective effects of MDBs were therefore predominantly attributed to the formation of inhibitory MDB metabolic intermediate-ferrocytochrome P450 complexes, which resulted in inactivation of P450 isozymes and thus prevented the metabolic activation of the xenobiotics. It was also discovered for the first time that these MDBs and their catecholic metabolites markedly inhibited reactive oxygen species (ROS)-mediated ferric nitrilotriacetate-induced renal necrosis in mice as well as lipid peroxidation with rat hepatocytes and liver microsomes. The antioxidative effect of isosafrole was however decreased if the hepatocytes were pre-incubated with other P450 inhibitors. This suggested that MDBs are potential antioxidants that could prevent ROS-mediated cytotoxicity mainly through their catecholic metabolites, which are direct antioxidants, and metal chelators. Catechol and its derivatives were also found to form stable catecholic ferric complexes at physiological pH, which possess greater superoxide dismutase mimic activity than uncomplexed catechols. These catecholic iron complexes were also much more effective than uncomplexed catechols at protecting hepatocytes against hypoxic cell injury. The chemoprotective properties of this group of compounds suggested that MDBs, particularly the non-genotoxic isosafrole, could protect against ' in vivo' chemical carcinogenesis catalyzed by cytochrome P450-dependent monooxygenases as well as ROS-mediated oxidative injury catalyzed by the presence of iron.Ph.D