CYP2E1-mediated mechanism of anti-genotoxicity of the broccoli constituent sulforaphane

The broccoli constituent sulforaphane (1-isothiocyanate-4-methylsulfinylbutane) has previously been shown to protect rats against 9,10-dimethyl-1,2-benz[a]anthracene tumorigenesis, thought to be due, at least in part, to induction of phase II detoxification, We investigated the ability of sulforaphane to also inhibit the phase I enzyme cytochrome P450 isoenzyme 2E1 (CYP2E1), which is responsible for activation of several carcinogens, including dialkylnitrosamines., Using the p-nitrophenol hydroxylation assay in microsomes from livers of acetone-treated Sprague-Dawley rats, sulforaphane was shown to be a potent competitive inhibitor of CYP2E1 with a K-i of 37.0 +/- 4.5 mu M. In view of this result, we studied the capacity of sulforaphane to inhibit the genotoxicity of N-nitrosodimethylamine (NDMA), Sulforaphane at concentrations of >0.8 mu M inhibited the mutagenicity of NDMA (4.4 mg/plate) in Salmonella typhimurium strain TA100 after pre-incubation for 45 min with cytosol extract from livers of Balb/c mice pre-treated with acetone, Unscheduled DNA synthesis induced by NDMA (33.5 mu M) in mouse hepatocytes was inhibited in a dose-dependent manner by sulforaphane at 0.064-20 mu M, Sulforaphane was unable to inhibit mutagenicity of sodium azide (5 mu g/plate), a direct acting mutagen, in the Salmonella assay, It was not itself genotoxic in hepatocytes, as measured by unscheduled DNA synthesis, or mutagenic in the strain of Salmonella employed and cytotoxic only at high concentrations (greater than or equal to 0.5 mM), These findings suggest that inhibition of CYP2E1 by sulforaphane may offer chemoprotection against carcinogenic substrates of this enzyme.