ABSTRACT
Many isothiocyanates (ITCs), both naturally occurring and synthetic, are potent and selective inhibitors of carcinogenesis in animal models and are now viewed as a class of promising chemopreventive agents. We have investigated the ability of 11 ITCs to inhibit and/or inactivate P450 2A6- and 2A13-mediated coumarin 7-hydroxylation. Two of these 11 ITCs, phenylpropyl isothiocyanate (PPITC) and phenylhexyl isothiocyanate (PHITC), were potent inhibitors of P450 2A13. The K I values for the inhibition of P450 2A13-mediated coumarin 7-hydroxylation by PPITC and PHITC were approximately 0.14 and 1.1 microM, respectively. P450 2A6 was also inhibited by these two ITCs; however, the K I values indicated they were approximately 10-20-fold less potent for P450 2A6 than for P450 2A13. Most of the ITCs tested, including PPITC and PHITC, showed some degree of inactivation of both P450s; however, only one compound, tert-butyl isothiocyanate (tBITC), showed significant inactivation of P450 2A13 at a concentration of 10 microM. None of the ITCs caused significant inactivation of P450 2A6 at this concentration. tBITC inactivated P450 2A13 with an apparent K I of 4.3 microM and a k inact of 0.94 min (-1). Inactivation of P450 2A6 by tBITC was observed only at high concentrations and long incubation times. The observed differences in inhibition and/or inactivation of P450 2A6 and 2A13 by a few of the isothiocyanates suggest that these compounds may be useful for structure-function studies.
Subject(s)
Aryl Hydrocarbon Hydroxylases/drug effects , Coumarins/chemistry , Imidazoles/chemistry , Isothiocyanates/pharmacology , Mixed Function Oxygenases/drug effects , Aryl Hydrocarbon Hydroxylases/chemistry , Aryl Hydrocarbon Hydroxylases/metabolism , Binding Sites , Catalysis , Computer Simulation , Crystallography, X-Ray , Cytochrome P-450 CYP2A6 , Enzyme Inhibitors/pharmacology , Humans , Hydroxylation , Isothiocyanates/chemistry , Mixed Function Oxygenases/chemistry , Mixed Function Oxygenases/metabolism , Molecular StructureABSTRACT
Isothiocyanates have been shown to be potent inhibitors of carcinogenesis in animals exposed to a number of chemical carcinogens including the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). In this study the effects of benzyl isothiocyanate (BITC) and phenethyl isothiocyanate (PEITC), two naturally occuring isothiocyanates, on P450 2A6 and 2A13 were investigated. P450s 2A6 and 2A13 are thought to be the primary human P450 enzymes responsible for the in vivo metabolism of nicotine and NNK, respectively. In vitro, BITC and PEITC efficiently inhibited P450 2A6- and 2A13-mediated coumarin 7-hydroxylation. The inhibition of P450 2A6 and 2A13 by BITC was non-competitive with KI's of 4.1 and 1.3 microM, respectively. PEITC was a more potent inhibitor of both enzymes than BITC, with a KI of 0.37 microM for P450 2A6 and 0.03 microM for P450 2A13. P450 2A6-mediated metabolism of nicotine and P450 2A13-mediated alpha-hydroxylation of NNK were also inhibited significantly by these two isothiocyanates. Both BITC and PEITC were able to inactivate P450 2A6 and 2A13 in an NADPH-dependent manner potentially through the formation of adducts to the apoprotein. The potent inhibition of P450 2A6- and 2A13-mediated metabolisms together with the ability of BITC and PEITC to inactivate the enzymes suggests the possibility that these isothiocyanates could be developed as chemopreventive agents to protect smokers who are unwilling or unable to quit smoking against lung cancer.
Subject(s)
Anticarcinogenic Agents/pharmacology , Aryl Hydrocarbon Hydroxylases/metabolism , Chemoprevention , Isothiocyanates/pharmacology , Lung Neoplasms/prevention & control , Mixed Function Oxygenases/metabolism , Smoking/adverse effects , Aryl Hydrocarbon Hydroxylases/antagonists & inhibitors , Aryl Hydrocarbon Hydroxylases/drug effects , Cytochrome P-450 CYP2A6 , Humans , Lung Neoplasms/etiology , Mixed Function Oxygenases/antagonists & inhibitors , Mixed Function Oxygenases/drug effectsABSTRACT
Oltipraz (OPZ) is a known inducer of glutathione S-transferases and a mechanism-based inhibitor of cytochrome P450 1A2. Given the detoxification characteristics of this compound, the transcriptional effects of OPZ, along with the related naturally occurring compounds 3H-1,2-dithiole-3-thione (D3T) and sulforaphane (SF), were examined by gene expression profiling in murine BV-2 microglial cells, a neuronal macrophage cell type that mediates inflammatory responses in the brain. We show that the three compounds generate largely overlapping transcriptional changes in genes that are associated with detoxification and antioxidant responses. In addition, induction of an antioxidant/detoxification response in the microglial cells by OPZ, D3T, or SF was also able to protect cells from H2O2 -induced toxicity and to attenuate the production of reactive oxygen species in response to lipopolysaccharide treatment of cells. These results show that OPZ, D3T, and SF activate overlapping changes in gene expression and that they can regulate detoxification/antioxidant responses in multiple cells types, including cell types known to have a role in the production of oxidative stress.
