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 (Gonzalez et al. (1985), J. Biol. Chem. 260, 7345-7441), but differed in 41 nucleotides, resulting in 11 changes and 2 deletions of amino acid residues. The P450/6βB spanned 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 from P450/6βB, 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 whether P450/6βB encodes 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 P450(6β-4) previously identified as CYP3A1. The recombinant 6β-B protein showed similar turnover rate and regioselectivity for testosterone with purified P450(6β-4) by the simultaneous addition of NADPH-cytochrome P450 reductase and cytochrome b5. These data suggest that P450/6βB encodes an active P450 form corresponding to CYP3A1 and P450PCN1 reported previously does not exist in rats.
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