Benzene metabolism by reconstituted cytochromes P450 2B1 and 2E1 and its modulation by cytochrome b5, microsomal epoxide hydrolase, and glutathione transferases: evidence for an important role of microsomal epoxide hydrolase in the formation of hydroquinone.
Benzene metabolism was investigated using two purified rat hepatic MFO systems containing either cytochrome P450 2B1 or cytochrome P450 2E1. Studies performed over a wide substrate concentration range indicate that cytochrome P450 2B1 represents a relatively low-affinity form of cytochrome P450 with respect to benzene metabolism while cytochrome P450 2E1 is substantially more efficient at low benzene concentrations (apparent Km value 0.17 mM). Cytochrome b5 stimulated benzene metabolism by both cytochromes P450 2B1 and P450 2E1. With cytochrome P450 2E1 the stimulation of benzene metabolism by cytochrome b5 was very pronounced (up to 6-fold) at low concentrations of benzene and was most effective (up to 15-fold) with respect to formation of hydroquinone. The metabolites observed in these studies were phenol and hydroquinone. Cytochrome P450 2E1 metabolized phenol with an affinity and capacity comparable to those of benzene. Hydroquinone was the major product formed at all substrate concentrations, while some catechol was formed at all substrate concentrations, while some catechol was formed at higher concentrations of phenol. Phenol metabolism was also stimulated by cytochrome b5. The metabolism of benzene by cytochrome P450 2E1 in the presence of the major microsomal epoxide hydrolase, mEHb, yielded phenol, hydroquinone, and benzene dihydrodiol. Interestingly, the addition of mEHb did not lead to a decrease of the toxicologically important metabolite hydroquinone as might be expected from sequestration of the intermediate benzene oxide to the vicinal dihydrodiol pathway but rather led to a marked (more than 4-fold) increase in the formation of hydroquinone, suggesting catalysis by mEHb of a predominant attack at the homoallylic position rather than at a carbon atom which forms the epoxide ring of benzene oxide. The addition of glutathione transferases plus glutathione did not yield GSH conjugates during benzene metabolism. However, metabolism of phenol by cytochrome P450 2E1 in the presence of glutathione yielded a nonenzymatically formed glutathione conjugate derived from hydroquinone or from an oxidative product of hydroquinone.
Snyder
R
R
Joint Graduate Program in Toxicology, Environmental and Occupational Health Sciences Institute, Piscataway, New Jersey 08855-1179.
Chepiga
T
T
Yang
C S
CS
Thomas
H
H
Platt
K
K
Oesch
F
F
D000818:Animals; D001554:Benzene / Q000378:metabolism*; D019392:Cytochrome P-450 CYP2E1; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D015786:Cytochromes b5 / Q000378:metabolism*; D004851:Epoxide Hydrolases / Q000378:metabolism*; D005978:Glutathione / Q000378:metabolism; D005982:Glutathione Transferase / Q000378:metabolism*; D006873:Hydroquinones / Q000378:metabolism; D066298:In Vitro Techniques; D008099:Liver / Q000201:enzymology*; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology; D010089:Oxidoreductases, N-Demethylating / Q000378:metabolism; D019800:Phenol; D010636:Phenols / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley
D006873:Hydroquinones; D010636:Phenols; D019800:Phenol; D015786:Cytochromes b5; D003577:Cytochrome P-450 Enzyme System; D019392:Cytochrome P-450 CYP2E1; D010089:Oxidoreductases, N-Demethylating; D005982:Glutathione Transferase; D004851:Epoxide Hydrolases; D005978:Glutathione; D001554:Benzene; C031927:hydroquinone