Related Experiment Videos
Theophylline N-demethylations as probes for P4501A1 and P4501A2
1School of Pharmacy, West Virginia University, 26506.
Abstract:
Polycyclic aromatic hydrocarbons present in cigarette smoke induce cytochromes P4501A1 and P4501A2. These isozymes are of toxicological importance because they convert several environmental pollutants to reactive intermediates that form covalent adducts with cellular DNA resulting in mutations and/or malignant transformations. The aim of our research was to investigate whether theophylline metabolites could be used as probes for P4501A1 and P4501A2. It is known that at least two human P450 isozymes are involved in theophylline metabolism. The N-demethylations of theophylline to 3-methylxanthine (3-MX) and 1-methylxanthine (1-MX) appear to be mediated by P4501A1 and/or P4501A2 and the 8-hydroxylation by different isozymes. Theophylline metabolism was measured in liver microsomes from control, benzo(a)pyrene (BP)-, and isosafrole (ISO)-induced rats. Theophylline was also incubated in microsomes prepared from cells expressing high levels of human P4501A1 and P4501A2. A plot of v vs. v/S was linear for 3-MX in the ISO-induced microsomes, but nonlinear for 1-MX, indicating that P4501A2 mainly forms 3-MX, whereas P4501A1 and 1A2 probably mediate 1-MX. A similar nonlinear relation was also obtained in the BP-induced microsomes. Incubation of theophylline in the microsomes from the cells indicated that only 1-MX could be measured in cells expressing P4501A1, and both 1-MX and 3-MX were formed in the P4501A2 cell microsomes. Therefore 1-MX seems to be mediated by P4501A1/P4501A2 and 3-MX specifically by P4501A2, and theophylline N-demethylations can be used as probes for P4501A1 and P4501A2.(ABSTRACT TRUNCATED AT 250 WORDS)
Insights
Theophylline metabolites, 1-methylxanthine (1-MX) and 3-methylxanthine (3-MX), can serve as probes for cytochromes P4501A1 and P4501A2. This research demonstrates that specific theophylline N-demethylations can effectively monitor the activity of these crucial enzymes.
Area of Science:
- Biochemistry
- Toxicology
- Pharmacology
Background:
- Polycyclic aromatic hydrocarbons in cigarette smoke induce cytochromes P4501A1 and P4501A2.
- These enzymes are critical in metabolizing environmental pollutants, potentially leading to DNA adducts, mutations, and cancer.
- Understanding and monitoring P4501A1 and P4501A2 activity is vital for toxicology and risk assessment.
Purpose of the Study:
- To determine if theophylline metabolites can be utilized as biomarkers for P4501A1 and P4501A2 activity.
- To elucidate the specific P450 isozymes responsible for the N-demethylation of theophylline.
- To establish a method for assessing P4501A1 and P4501A2 induction using theophylline metabolism.
Main Methods:
- Theophylline metabolism was analyzed in rat liver microsomes induced with benzo(a)pyrene (BP) and isosafrole (ISO).
- Theophylline was incubated with microsomes from cells engineered to express high levels of human P4501A1 and P4501A2.
- Metabolite formation (1-methylxanthine and 3-methylxanthine) was quantified to determine enzyme kinetics and specificity.
Main Results:
- Theophylline N-demethylation to 3-methylxanthine (3-MX) was primarily mediated by P4501A2.
- The N-demethylation to 1-methylxanthine (1-MX) was mediated by both P4501A1 and P4501A2.
- Experiments with human P450-expressing cells confirmed that 1-MX is formed by P4501A1/P4501A2, while 3-MX is specific to P4501A2.
Conclusions:
- Theophylline N-demethylations, specifically the formation of 1-MX and 3-MX, serve as reliable probes for P4501A1 and P4501A2 activity.
- This finding offers a novel approach for monitoring the induction and activity of these critical xenobiotic-metabolizing enzymes.
- Theophylline metabolism provides a valuable tool in toxicological studies and in assessing exposure to environmental pollutants that induce P4501A enzymes.