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Dissociation of cytochrome P-450 inactivation and induction
Abstract:
Polycyclic aromatic hydrocarbons bind to a cytosolic receptor that translocates into the nucleus and induces the synthesis of mRNA for a subset of cytochrome P-450 isozymes. The failure to detect a "phenobarbital" receptor, however, suggests that the induction of P-450b/e, the isozymes induced by phenobarbital, is mediated by a less direct mechanism. One attractive alternative is that a receptor exists for an endogenous substance that is specifically turned over by the trace amounts of P-450b/e present in uninduced liver. Changes in the concentration of this substance caused by agents that inhibit P-450b/e would then trigger the induction response. We report here that suppressing the catalytic activities of P-450b/e for prolonged periods with 1-aminobenzotriazole, a mechanism-based irreversible inhibitor, neither induces P-450b/e nor interferes with induction of these isozymes at the level of transcription by phenobarbital. The results argue against mechanisms for the induction of P-450b/e keyed to the effective catalytic availability of these isozymes.
Insights
Phenobarbital induces cytochrome P-450 isozymes through indirect mechanisms. Suppressing P-450b/e activity did not alter phenobarbital-induced P-450b/e levels, refuting catalytic availability as the induction trigger.
Area of Science:
- Biochemistry
- Molecular Biology
- Pharmacology
Background:
- Polycyclic aromatic hydrocarbons induce cytochrome P-450 isozymes via a known receptor mechanism.
- The induction of cytochrome P-450b/e by phenobarbital lacks a detected receptor, suggesting an indirect pathway.
Purpose of the Study:
- To investigate the mechanism of phenobarbital-induced cytochrome P-450b/e (P-450b/e) expression.
- To test the hypothesis that induction is mediated by changes in the turnover of an endogenous substance by P-450b/e.
Main Methods:
- Utilized 1-aminobenzotriazole, a mechanism-based irreversible inhibitor, to suppress P-450b/e catalytic activity.
- Administered phenobarbital to induce P-450b/e and assessed transcriptional levels.
Main Results:
- Prolonged suppression of P-450b/e activity with 1-aminobenzotriazole did not induce P-450b/e.
- Inhibition of P-450b/e catalytic activity did not interfere with phenobarbital-mediated induction of P-450b/e at the transcriptional level.
Conclusions:
- The results challenge mechanisms for P-450b/e induction that rely on the enzyme's catalytic availability.
- Phenobarbital induction of P-450b/e likely involves signaling pathways independent of direct substrate turnover by the isozymes.