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A phenobarbital-inducible hepatic mitochondrial cytochrome P-450 immunochemically related to microsomal P-450b
1Department of Animal Biology, School of Veterinary Medicine, University of Pennsylvania, Philadelphia 19104-6048.
Abstract:
We have purified and characterized a phenobarbital (PB)-inducible hepatic mitochondrial cytochrome P-450 (P-450), termed P-450mt4, which is distinctly different from the previously characterized mitochondrial isoforms. The level of induction of P-450mt4 by PB in the male livers is nearly 20-fold, as against a marginal induction in the female livers, suggesting that it may be a male predominant isoform. P-450mt4 shows a close resemblance to microsomal P-450b (the major PB-inducible form) with respect to electrophoretic migration (apparent molecular mass of 50 kDa) and immunological cross-reactivity, although it exhibits a distinct isoelectric pH (pI 6.9 vs 6.5 for P-450b), peptide fingerprint pattern, and amino acid composition. Further, the N-terminal sequence analysis shows over 90% positional identity (39 out of 42) between P-450mt4 and P-450b, suggesting that it is a close relative of the P-450 IIB gene family. In vitro reconstitution experiments show that P-450mt4 can metabolize a wide range of substrates such as benzphetamine, (dimethylamino)antipyrine, aflatoxin B1, and vitamin D3, exclusively in the presence of mitochondrial-specific ferredoxin and ferredoxin reductase as electron carriers. P-450mt4 is translated as a 53-kDa precursor, which is transported into mitochondria under in vitro conditions and processed into a mature 50-kDa protein. These results provide conclusive evidence for the occurrence of a male-specific P-450 belonging to the IIB gene family in rat liver mitochondria.
Insights
Researchers identified a new rat liver mitochondrial enzyme, phenobarbital-inducible cytochrome P-450mt4. This male-predominant enzyme is related to the P-450 IIB gene family and metabolizes various substrates.
Area of Science:
- Biochemistry
- Molecular Biology
- Pharmacology
Background:
- Cytochrome P-450 enzymes are crucial for drug metabolism and detoxification.
- Mitochondrial P-450 isoforms are less characterized than their microsomal counterparts.
- Phenobarbital induces specific P-450 enzymes in the liver.
Purpose of the Study:
- To purify and characterize a novel phenobarbital-inducible hepatic mitochondrial cytochrome P-450.
- To determine the substrate specificity and electron transport requirements of the new enzyme.
- To elucidate the gene family relationship and tissue distribution of this P-450 isoform.
Main Methods:
- Enzyme purification and characterization (electrophoresis, isoelectric focusing, peptide mapping).
- N-terminal amino acid sequencing.
- In vitro reconstitution assays with specific substrates and electron carriers.
- In vitro translation and mitochondrial import studies.
Main Results:
- A phenobarbital-inducible mitochondrial P-450 (P-450mt4) was purified from male rat liver.
- P-450mt4 showed significant induction in males but not females, suggesting male predominance.
- It shares similarities with microsomal P-450b but has distinct biochemical properties and belongs to the P-450 IIB gene family.
- P-450mt4 metabolizes diverse substrates using mitochondrial electron carriers.
- It is synthesized as a precursor and imported into mitochondria.
Conclusions:
- A male-specific, phenobarbital-inducible mitochondrial P-450 (P-450mt4) belonging to the P-450 IIB gene family exists in rat liver.
- This finding expands the known diversity of mitochondrial P-450 enzymes and their roles in hepatic metabolism.
- P-450mt4 represents a novel player in xenobiotic and endogenous compound metabolism within rat liver mitochondria.