PT  - JOURNAL ARTICLE
AU  - JOHNSON, ERIC F.
AU  - SCHWAB, GEORGE E.
AU  - MULLER-EBERHARD, URSULA
TI  - Multiple Forms of Cytochrome P-450: Catalytic Differences Exhibited by Two Homogeneous Forms of Rabbit Cytochrome P-450
DP  - 1979 May 01
TA  - Molecular Pharmacology
PG  - 708--718
VI  - 15
IP  - 3
4099  - http://molpharm.aspetjournals.org/content/15/3/708.short
4100  - http://molpharm.aspetjournals.org/content/15/3/708.full
SO  - Mol Pharmacol1979 May 01; 15
AB  - Two highly purified forms of hepatic cytochrome P-450 were isolated from rabbits treated with the inducers phenobarbital and 2,3,7,8-tetrachlorodibenzo-p-dioxin. Both forms were shown to be metabolically active when reconstituted with lipid and homogeneous NADPH-cytochrome P-450 reductase. Form 2, obtained from rabbits treated with phenobarbital, and form 4, purified from rabbits treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin, hydroxylated biphenyl at comparable rates (Vmax) 4.2 and 3.4 mol min-1 mol-1 cytochrome P-450, respectively, although with dissimilar apparent Km values, 31 µM and 9.6 µM, respectively. Form 4 was 60 times more active than form 2 in the deethylation of 7-ethoxyresorufin. Moreover, form 4 was five orders of magnitude more sensitive in the biphenyl reaction to the inhibitory effects of the differential inhibitor α-naphthoflavone than was form 2. Densitometric scans of sodium dodecyl sulfate polyacrylamide gel electrophoretograms revealed that microsomes from rabbits treated with phenobarbital exhibit a relative predominance of form 2. In similar fashion, microsomes from 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated animals show a dominance of form 4. The metabolic activities of the induced microsomes reflect the influence of the major form of cytochrome P-450. Similar trends are seen in the activities of phenobarbital-induced microsomes and reconstituted form 2 just as 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced microsomes and reconstituted form 4 share like metabolic properties. Thus, the distinct catalytic capacities of multiple forms of cytochrome P-450 represent significant determinants of xenobiotic metabolism by the cytochrome P-450 system. ACKNOWLEDGMENTS We thank Ms. Maryann C. Zounes for her technical assistance in providing the SDS PAGE, and Dr. Richard L. Norman for providing the densitometric scans for the SDS PAGE.