Isolation and comparison of four cytochrome P-450 enzymes from phenobarbital-induced rat liver: three forms possessing identical NH2-terminal sequences.

Phenobarbital pretreatment of male rats induced four microsomal cytochrome P-450 enzymes, at least one of which has previously not been reported. Three of the induced forms of the cytochrome possess identical NH2-terminal amino acid sequence for the first 32 residues (PBRLM5, 6 and 7). This sequence is identical to that shown earlier for PB-4 and PB-5. Apparent values for minimum molecular weight on SDS-PAGE were 52,000 (PBRLM4), 53,000 (PBRLM5), 53,500 (PBRLM6) and 54,000 (PBRLM7). Isomeric metabolite patterns from testosterone and progesterone differed for each enzyme further indicating their unique natures. Studies reveal similarity of PBRLM4 to PB-1, of PBRLM5 to P-450b and PB-4, and PBRLM6 to P-450e and PB-5. PBRLM7, which does not correspond to any reported forms, metabolizes steroids poorly. It preferentially hydroxylates testosterone at the 16 beta-position. It is the largest and least active of the enzymes shown for all of the substrates tested. This study further provided a cautionary note against assuming that chromatographic pools, like a P-450 PB-B fraction, are homogeneous.