The cytochrome P450 enzyme CYP2E1 catalyzes the oxidative metabolism of many solvents and other small organic molecules. CYP2E1 is expressed in adult and fetal human liver in addition to extrahepatic tissues such as lung and placenta. Treatment of primary cultures of human hepatocytes with ethanol induces CYP2E1 protein, and this is consistent with the finding that hepatic CYP2E1 protein and mRNA levels are increased in individuals with alcoholism. Although only a few drugs (e.g., acetaminophen have been identified as substrates for CYP2E1, many low molecular weight procarcinogens are activated by this cytochrome P450 (P450). Chlorzoxazone 6-hydroxylation, N-nitrosodimethylamine N-demethylation and p-nitrophenol hydroxylation can be used to measure the catalytic activity of CYP2E1. Thus, the recombinant human CYP2E1 activity was measured by its ability to hydroxylate p-nitrophenol to p-nitrocatechol. The reaction was performed in 50 mM potassium phosphate, pH 7.4 ( Assay Buffer), initiated by addition 20 μL of 500 ug/ml CYP2E1 to 10 μL of 5 mM substrate p-nitrophenol and 30 ul of 26 mM NADPH in a total volume of 500 ul. Incubated at 37 ℃ for 30min, then read at a wavelength of 535 nm after acidification of the reaction mixture with trichloroacetic acid followed by neutralization using 2 M NaOH.