Effect of haem on cytochrome P-450 synthesis.

1. The expression of the phenobarbital-inducible cytochrome P-450 mRNA species (P-450 IIB1 and IIB2) were investigated in different tissues of rats following treatment with 2-allyl-2-isopropylacetamide. 2. The mRNAs were detected as a single 2.1 kb mRNA species by Northern blot analysis. These mRNAs were readily detected in liver, lung and kidney but were not detected in testis, brain or erythroid tissue. 3. When rats were administered 2-allyl-2-isopropylacetamide, cytochrome P-450 levels were elevated specifically in the liver and kidney but remained undetectable in testis, brain and erythroid spleen. Thus these cytochrome P-450 mRNAs are expressed and induced by drug in a tissue-specific fashion. Levels of mRNA for 5-aminolaevulinate synthase, the rate controlling enzyme of haem biosynthesis, were also induced by drug in a similar tissue-specific fashion. 4. The proposal that haem is required for the transcription of cytochrome P-450 IIB1/IIB2 and other cytochrome P-450 genes was investigated in rat liver using succinylacetone, a specific inhibitor of the haem biosynthetic pathway. 5. While 2-allyl-2-isopropylacetamide induced levels of cytochrome P-450 IIB1/IIB2 mRNAs, succinylacetone administration did not affect this induction. However, succinylacetone substantially elevated both basal and drug-induced levels of mRNA for 5-aminolaevulinate synthase. 6. Since 5-aminolaevulinate synthase mRNA synthesis is inhibited by the end-product haem, the results show that lowered haem levels do not affect cytochrome P-450 gene transcription. The work does not provide evidence for the suggestion that haem is required for cytochrome P-450 gene transcription.