A MAJOR GLUCOCORTICOID-INDUCIBLE P450 IN RAT-LIVER IS NOT P450 3A1

A new P450 3A cDNA (RL33) has been cloned from a liver cDNA library of untreated male rat. RL33 is 2032 nucleotides in length and has an ope n reading frame of 502 amino acid residues. The nucleotide sequence of its 5'-noncoding region is completely identical with that of a genomi c clone of P450 3A1 isolated by Burger et ah. [Proc. Natl. Acad, Sci. USA 89, 2145-2149 (1992)]. Compared with rat P450 3A1, P450 RL33 showe d 98 and 97% identities in the nucleotide and deduced amino acid seque nces, respectively, with the deletion of 2 amino acids and substitutio n of 12 amino acids. These residues were localized around amino acids 107-230. Recently Kirita and Matsubara have isolated the same P450 3A cDNA (cDEX) from dexamethasone (DEX)-treated rat liver [Arch. Biochem. Biophys. 307, 253258 (1993)]. Northern blot analysis using an oligonu cleotide probe specific for P450 RL33/cDEX revealed that P450 RL33/cDE X mRNA was induced strongly by pregnenolone 16 alpha-carbopitrile and DEX and weakly by phenobarbital (PB) and triacetyloleandomycin. We con structed a P450 3A cDNA library by the reverse transcriptase-polymeras e chain reaction using common primers to P450 RL33/cDEX, 3A1, and 3A2, and subcloned the cDNAs into pUC119. The expression level of P450 RL3 3/cDEX mRNA was investigated by identifying each clone with the above oligonucleotide probe. P450 RL33/cDEX mRNA represented over 70% of the total P450 3A mRNA from untreated, PB-, and DEX-treated rat liver. Th ese results indicated that the major DEX-inducible form of P450 3A is P450 RL33/cDEX and not P450 3A1.