Characterization of Cytochrome P2‐450 (20‐S) mRNA

Abstract

Mouse liver cytochrome P₂‐450 is defined as the major isosafrole‐inducible form of P‐450 which is most specific for isosafrole metabolism. λAhP‐1 represents a 15.5 × 10³‐base‐pair segment of mouse genomic DNA having the cytochrome P₁‐450 gene (∼ 4600 base pairs) located in the middle portion. Using various subclones as probes, we investigated the differential expression of P₁‐450 mRNA and P₂‐450 mRNA induction as a function of association with the Ah locus, 3‐methylcholanthrene or isosafrole dosage, tissue specificity, and developmental age. Both P₁‐450 (23‐S) mRNA and P₂‐450 (20‐S) mRNA induction processes are regulated by the Ah receptor. P₂‐450 mRNA is about 10‐fold more sensitive than P₁‐450 mRNA to induction by either 3‐methylcholanthrene or isosafrole. Phenobarbital pretreatment has no effect at all on either P₁‐450 mRNA or P₂‐450 mRNA. Whereas both P₁‐450 mRNA and P₂‐450 mRNA are induced by 3‐methycholanthrene in C57BL/6N liver, P₁‐450 (23‐S) mRNA but not P₂‐450 (20‐S) mRNA is induced by 3‐methylcholanthrene in C57BL/6N kidney. P₁‐450 mRNA induction by 3‐methylcholanthrene is measurable in C57BL/6N liver at day 15 of gestation, and the expression becomes enhanced with increasing age. P₂‐450 mRNA induction by 3‐methylcolanthrene in C57BL/6N liver appears about 7 days later during development than 3‐methylcholanthrene‐inducible P₁‐450 mRNA. Both 3‐methylcholanthrene‐induced P₁‐450 mRNA and P₂‐450 mRNA are detectable in DBA/2N liver; their appearance is later in development, however, and at lower concentrations than that seen with C57BL/6N liver. P₁‐450 (23‐S) mRNA and P₂‐450 (20‐S) mRNA appear to hybridize to a common 5′ fragment of the P₁‐450 gene.