Structure and Expression of the Rat CYP3A1 Gene: Isolation of the Gene (P450/6βB)and Characterization of the Recombinant Protein

A P450 gene (P450/6βB) of the CYP3A subfamily was isolated from a rat genomic library. Nucleotide sequencing of the exons revealed a high similarity with P450PCN1 cDNA (Gonzalezet al.(1985),J. Biol. Chem.260, 7345–7441), but differed in 41 nucleotides, resulting in 11 changes and 2 deletions of amino acid residues. TheP450/6βBspanned about 30 kbp and consisted of 13 exons, and was in exon number and size identical with CYP3A2 gene except in the 6th exon, which was shorter than that of CYP3A2. 6β-B mRNA, which may be transcribed fromP450/6βB,was detected on Northern blotting and by reverse transcription-polymerase chain reaction (RT-PCR). Profiles of the developmental change and induction by a treatment with several chemicals were very similar to those of P450PCN1 mRNA reported previously. P450PCN1 mRNA and gene, however, were not detected by PCR in rats. To determine whetherP450/6βBencodes an active protein, a cDNA was isolated and expressed. Expression of 6β-B cDNA in COS-1 cells was carried out and revealed that the recombinant protein comigrated with purified P4506β-4previously identified as CYP3A1. The recombinant 6β-B protein showed similar turnover rate and regioselectivity for testosterone with purified P4506β-4by the simultaneous addition of NADPH-cytochrome P450 reductase and cytochromeb5.These data suggest thatP450/6βBencodes an active P450 form corresponding to CYP3A1 and P450PCN1 reported previously does not exist in rats.