Previous studies revealed that while rat CYP2B1/2 are highly inducible by phenoharhital (PB), guinea pig CYP2B18A is expressed constitutively and has low inducible nature following PB treatment. To solve the low inducibility in guinea pigs. we cloned and sequenced CYP2B18A gene. Since the intron structures of the CYP2B2 gene have not yet been determined, we sequenced this gene as well as the 2B18A gene. Comparison of gene upstream between the two genes revealed that guinea pigs lack in the phenoharhital-responsive unit (PBRU) which is demonstrated to play a crucial role in PB-mediated induction of the CYP2B2. This observation seems to support the low-inducihility of CYP2B18A. Further, NF-κB and RBP-Jκ binding elements, which have been reported to mediate suppression of CYP2B2 gene, were not found in the CYP2B18A gene, supporting highly constitutive expression of this gene. To clarify the mechanism underlying PB-mediated induction, we analyzed further the structure of the CYP2B2 gene in the Qdj:SD rats that lack in response to PB-mediated induction of this protein. As the results, neither large deletion nor insertion was detected in the CYP2B2 gene of this mutant rats. In addition, no mutation was detected in the PBRU region and all exons. These results suggested that impaired induction of CYP2B2 in Qdj:SD rats is due either to mutation at the region different from the PBRU or exons, or defect in traps-acting factor(s).
