Structural determinants for alcohol substrates to be oxidized to formaldehyde by rat liver microsomes

Specificity of substrate and inhibitor probes for cytochrome P450s: evaluation of in vitro metabolism using cDNA-expressed human P450s and human liver microsomes

1. We evaluated the specificity of 15 substrates and 14 inhibitors of the cytochrome P450s using nine human P450 forms expressed in HepG2 cells using a recombinant vaccinia virus and also in human liver microsomes. 2. Coumarin, 7-ethoxyresorufin, 7-benzyloxyresorufin, tolbutamide, aniline and diazepam were form-selective substrates towards CYP2A6, the CYP1A subfamily, CYP2B6, the CYP2C subfamily, CYP2E1 and the CYP3A subfamily respectively. However, a selective substrate for CYP2D6 was not found among the chemicals tested. 3. SKF-525A inhibited > 40% of the metabolic activity of all substrates tested, and the inhibitory effects differed among P450 forms. Sulphaphenazole, 7,8-benzoflavone, quinidine and troleandomycin were selective inhibitors of the CYP2C subfamily (except CYP2C19), the CYP1A subfamily, CYP2D6 and the CYP3A subfamily respectively. Methoxsalen (CYP2A6 inhibitor) inhibited the metabolic activity of CYP1A2 as well as that of CYP2A6. Diethyldithiocarbamate (CYP2E1 inhibitor) inhibited the metabolic activities of CYP2A6 and CYP2C19 in addition to that of CYP2E1. 4. Our results indicated that substrates and inhibitors reported as P450 selective probes are not necessarily specific for individual human P450 forms. These results may provide useful information regarding human P450 substrates and inhibitors in vitro using human liver microsomal samples.

Increased oxidation of ethylene glycol to formaldehyde by microsomes after ethanol treatment: role of oxygen radicals and cytochrome P450

The production of ferryl-type oxidants by microsomes from ethanol-fed rats and pair-fed controls was determined by assaying for the production of formaldehyde from ethylene glycol. Microsomes from the ethanol-fed rats were more reactive than controls in oxidizing ethylene glycol. Catalase was a powerful inhibitor for this reaction, superoxide dismutase was slightly inhibitory and hydroxyl radical scavengers had no effect. These results suggest an important role for H2O2, but not O2-. or .OH in the overall pathway for oxidizing ethylene glycol to formaldehyde. The production of H2O2 by microsomes was increased after ethanol treatment, the extent of increase corresponding to the increase in oxidation of ethylene glycol. A variety of inhibitors and ligands of cytochrome P450, including miconazole, diethyldithiocarbamate, tryptamine, and 4-methylpyrazole, inhibited formaldehyde production by both microsomal preparations. Anti-cytochrome P4502E1 IgG also inhibited the reaction with both microsomal preparations and prevented the increase caused by ethanol treatment. These results indicate that microsomes from ethanol-treated rats are more reactive than pair-fed controls in generating ferryl-type oxidants and that increased production of H2O2 by cytochrome P4502E1 plays a role in the elevated oxidation of ethylene glycol to formaldehyde.