The metabolism of xenobiotics and endogenous compounds by the constitutive dog liver cytochrome P450 PBD-2.

We have investigated the metabolism of polychlorinated biphenyls and endogenous steroids by the major phenobarbital (PB)-inducible hepatic cytochromes P450 in dogs and rats, PBD-2 and PB-B, respectively. Previous results from our laboratory indicate that dog PBD-2 purified from microsomes of PB-treated animals is similar to rat PB-B with respect to structure and the regioselective metabolism of warfarin and androstenedione. The results also strongly suggest that PBD-2 is the P450 form responsible for metabolizing 2,2',4,4',5,5'-hexachlorobiphenyl (245-HCB) in liver microsomes from untreated dogs. In the present study, a cytochrome P450 with similar chromatographic behavior to that of PBD-2 has been purified from liver microsomes of untreated dogs. This protein is identical to PBD-2 based on (i) mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, (ii) reactivity with anti-PBD-2 IgG, (iii) amino-terminal sequence, and (iv) 245-HCB metabolite profile. Induction and antibody-inhibition data suggest that PBD-2 is responsible for the metabolism of 2,2',3,3',6,6'-hexachlorobiphenyl (236-HCB) in microsomes obtained from both untreated and PB-treated dogs. In contrast, metabolism of 4,4'-dichlorobiphenyl (4-DCB) by dog microsomes is poor, and does not appear to be catalyzed to a significant extent by PBD-2. Antibody-inhibition studies with intact microsomes corroborate previous results that androstenedione is metabolized by purified PBD-2 to the same major metabolite (16 beta-OH androstenedione) produced by rat PB-B. Dog PBD-2 metabolizes progesterone primarily to the 21-OH metabolite, while metabolism by rat PB-B leads to the formation of the 16 alpha-OH product. On the other hand, upon Ouchterlony double-immunodiffusion analysis, anti-PBD-2 IgG reacts strongly with PB-B but not PB-C, the major rat liver progesterone 21-hydroxylase. The data suggest that dog PBD-2 is a constitutive P450 important in the metabolism of various PCBs and endogenous steroids. Dog PBD-2 and rat PB-B appear to be similar enzymes, yet they differ in their regioselective metabolism of progesterone.