title: Purification and characterization of the dog hepatic cytochrome P-450 isozyme responsible for the metabolism of 2,2',4,4',5,5'-hexachlorobiphenyl.
abstract: The biochemical basis for the marked difference in the rate of the hepatic metabolism of 2,2',4,4',5,5'-hexachlorobiphenyl (245-HCB) by Beagle dogs and Sprague-Dawley rats has been investigated. Control dog liver microsomes metabolize this substrate 15 times faster than control rat liver microsomes. Upon treatment with phenobarbital (PB), at least two cytochrome P-450 isozymes are induced in the dog, and the hepatic microsomal metabolism of 245-HCB is increased on both a per nanomole P-450 basis (twofold) and a per milligram protein basis (fivefold). One of the PB-induced isozymes, PBD-2, has been purified to a specific content of 17-19 nmol/mg protein and to less than 95% homogeneity, as evidenced by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In a reconstituted system containing cytochrome b5, this isozyme shows an activity toward 245-HCB which is greater than threefold that seen in intact liver microsomes from PB-induced dogs. A reconstituted system containing the major isozyme induced by PB in the rat (PB-B) metabolizes 245-HCB at 1/10 the rate observed with purified PBD-2. Antibody inhibition studies have shown that PBD-2 accounts for greater than 90% of the hepatic microsomal metabolism of 245-HCB in control and PB-induced dogs, while PB-B only accounts for about half of the metabolism of this compound by microsomes obtained from PB-treated rats. Immunoblot analysis has revealed that the level of PBD-2 in dog liver microsomes increases nearly sixfold with PB treatment, and this increase correlates well with the fivefold increase in the rate of hepatic microsomal metabolism of 245-HCB by dogs. Together these data support a primary role for isozyme PBD-2 in the hepatic metabolism of 245-HCB in control and PB-induced dogs. In addition, these results suggest that, in contrast to rats, dogs can readily metabolize 245-HCB as a result of the presence of a cytochrome P-450 isozyme with efficient 245-HCB metabolizing activity.
mesh_terms: D000595:Amino Acid Sequence; D000596:Amino Acids / Q000032:analysis; D000818:Animals; D003577:Cytochrome P-450 Enzyme System / Q000302:isolation & purification* / Q000378:metabolism; D004285:Dogs; D007527:Isoenzymes / Q000302:isolation & purification* / Q000378:metabolism; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism*; D006899:Mixed Function Oxygenases / Q000302:isolation & purification / Q000378:metabolism; D011078:Polychlorinated Biphenyls / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D013045:Species Specificity; D013379:Substrate Specificity
chemical_list: D000596:Amino Acids; D007527:Isoenzymes; D003577:Cytochrome P-450 Enzyme System; D011078:Polychlorinated Biphenyls; D006899:Mixed Function Oxygenases; C014024:2,4,5,2',4',5'-hexachlorobiphenyl