Metabolic activation of benzo[a]pyrene in vitro by hepatic cytochrome P450 contrasts with detoxification in vivo: experiments with hepatic cytochrome P450 reductase null mice.
Many studies using mammalian cellular and subcellular systems have demonstrated that polycyclic aromatic hydrocarbons, including benzo[a]pyrene (BaP), are metabolically activated by cytochrome P450s (CYPs). In order to evaluate the role of hepatic versus extra-hepatic metabolism of BaP and its pharmacokinetics, we used the hepatic cytochrome P450 reductase null (HRN) mouse model, in which cytochrome P450 oxidoreductase, the unique electron donor to CYPs, is deleted specifically in hepatocytes, resulting in the loss of essentially all hepatic CYP function. HRN and wild-type (WT) mice were treated intraperitoneally (i.p.) with 125 mg/kg body wt BaP daily for up to 5 days. Clearance of BaP from blood was analysed by high-performance liquid chromatography with fluorescence detection. DNA adduct levels were measured by (32)P-post-labelling analysis with structural confirmation of the formation of 10-(deoxyguanosin-N(2)-yl)-7,8,9-trihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene by liquid chromatography-tandem mass spectrometry analysis. Hepatic microsomes isolated from BaP-treated and untreated mice were also incubated with BaP and DNA in vitro. BaP-DNA adduct formation was up to 7-fold lower with the microsomes from HRN mice than with that from WT mice. Most of the hepatic microsomal activation of BaP in vitro was attributable to CYP1A. Pharmacokinetic analysis of BaP in blood revealed no significant differences between HRN and WT mice. BaP-DNA adduct levels were higher in the livers (up to 13-fold) and elevated in several extra-hepatic tissues of HRN mice (by 1.7- to 2.6-fold) relative to WT mice. These data reveal an apparent paradox, whereby hepatic CYP enzymes appear to be more important for detoxification of BaP in vivo, despite being involved in its metabolic activation in vitro.
Arlt
Volker M
VM
Section of Molecular Carcinogenesis, Institute of Cancer Research, Sutton, Surrey SM2 5NG, UK.
Stiborová
Marie
M
Henderson
Colin J
CJ
Thiemann
Markus
M
Frei
Eva
E
Aimová
Dagmar
D
Singh
Rajinder
R
Gamboa da Costa
Gonçalo
G
Schmitz
Oliver J
OJ
Farmer
Peter B
PB
Wolf
C Roland
CR
Phillips
David H
DH
D000818:Animals; D019540:Area Under Curve; D001564:Benzo(a)pyrene / Q000493:pharmacokinetics*; D001711:Biotransformation; D002273:Carcinogens / Q000493:pharmacokinetics*; D002851:Chromatography, High Pressure Liquid; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D008862:Microsomes, Liver / Q000201:enzymology*; D009153:Mutagens / Q000493:pharmacokinetics*; D009251:NADPH-Ferrihemoprotein Reductase / Q000235:genetics / Q000378:metabolism*; D016133:Polymerase Chain Reaction
D002273:Carcinogens; D009153:Mutagens; D001564:Benzo(a)pyrene; D003577:Cytochrome P-450 Enzyme System; D009251:NADPH-Ferrihemoprotein Reductase