Primary structures of cytochrome P-450 isozyme 5 from rabbit and rat and regulation of species-dependent expression and induction in lung and liver: identification of cytochrome P-450 gene subfamily IVB.

The primary structure of rabbit cytochrome P-450 isozyme 5 has been derived from the nucleotide sequence of cloned cDNA. Identical sequences were obtained for cDNAs constructed with mRNA from four different sources, lung and liver of untreated rabbits and liver from rabbits treated once or four times with phenobarbital. Isozyme 5 shows significant sequence identity only with rabbit P-450p2 (54%) and rat P-450LA omega (53%), which places it in a previously unrecognized cytochrome P-450 gene subfamily (IVB). A cDNA library was also constructed from rat pulmonary mRNA and screened with cDNA encoding rabbit isozyme 5. The amino acid sequence derived from a positive clone was compared with that of rabbit isozyme 5 and found to be 87% identical, significantly greater than observed between other similar forms of cytochrome P-450 from rabbit and rat. Alignment of the primary structures of rabbit isozyme 5 (506 residues), rat isozyme 5 (511 residues), rabbit P-450p2, and rat P-450LA omega shows 43% structural identity and a common 16-residue peptide near position 300 that is unique to these forms of cytochrome P-450. Analysis of mRNA from lung and liver of rabbit, rat, guinea pig, and hamster indicates that species and tissue differences in the expression and induction of isozyme 5 are likely regulated at the level of transcription. These differences fall into one of the following three groups: first, expression in lung and liver and induction in liver by phenobarbital (rabbit); second, expression in lung and liver but no hepatic induction (hamster); and third, expression in lung and little or no expression in liver regardless of treatment (rat and guinea pig).