Phase I and phase II xenobiotic reactions and metabolism of the food-borne carcinogen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline in aggregating liver cell cultures

Biochem Pharmacol. 1993 Mar 9;45(5):1087-96. doi: 10.1016/0006-2952(93)90253-s.


Aggregating fetal liver cell cultures were tested for their ability to metabolize xenobiotics using ethoxycoumarin-O-deethylase (ECOD), as marker of phase I metabolism, and glutathione S-transferase (GST), as marker for phase II reactions. Significant basal activities, stable over 14 days in culture were measured for both ECOD and GST activities. The prototype cytochrome P450 inducers, 3-methylcholanthrene (3-MC) and phenobarbital (PB), increased ECOD and GST activities reaching an optimum 7 days after culturing, followed by a decline in activity. This decline was partially prevented by 1% dimethyl sulfoxide (DMSO) added chronically to the culture medium. DMSO was also found to induce ECOD activity and to a lesser extent GST activity. Furthermore, it potentiated in a dose-dependent manner the induction of ECOD by PB. The food-borne carcinogen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) is metabolically transformed through a number of pathways in vivo. It was therefore used to examine the metabolic capacity in fetal and adult liver cell aggregates. Metabolism of MeIQx was mainly through N2-conjugation, resulting in formation of the N2-glucuronide and sulfamate conjugates for non-induced fetal liver cells. These metabolites were also found in large amounts in non-induced adult liver cells. Low levels of cytochrome P450-mediated ring-hydroxylated metabolites were detected in both non-induced fetal and adult liver cells. After induction with arochlor (PCB) or 3-MC, the major pathway was ring-hydroxylation (cytochrome P450 dependent), followed by conjugation to beta-glucuronic or sulfuric acid. The presence of the glucuronide conjugate of N-hydroxy-MeIQx, a mutagenic metabolite, suggested an induction of P450 CYP1A2. The metabolism of MeIQx by liver cell aggregates is very similar to that observed in vivo and suggests that aggregating liver cell cultures are a useful model for in vitro metabolic studies in toxicology.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • 7-Alkoxycoumarin O-Dealkylase / biosynthesis
  • 7-Alkoxycoumarin O-Dealkylase / metabolism
  • Aging / metabolism
  • Animals
  • Cells, Cultured
  • Dimethyl Sulfoxide / pharmacology
  • Drug Synergism
  • Enzyme Induction
  • Glutathione Transferase / biosynthesis
  • Glutathione Transferase / metabolism
  • Liver / cytology
  • Liver / embryology
  • Liver / metabolism*
  • Methylcholanthrene / pharmacology
  • Phenobarbital / pharmacology
  • Quinoxalines / metabolism*
  • Rats
  • Xenobiotics / metabolism*


  • Quinoxalines
  • Xenobiotics
  • Methylcholanthrene
  • 2-amino-3,8-dimethylimidazo(4,5-f)quinoxaline
  • 7-Alkoxycoumarin O-Dealkylase
  • Glutathione Transferase
  • Dimethyl Sulfoxide
  • Phenobarbital