Multiple forms of cytochrome P-450: catalytic differences exhibited by two homogeneous forms of rabbit cytochrome P-450
Johnson, E.F.; Schwab, G.E.; Muller-Eberhard, U.
Molecular Pharmacology 15(3): 708-718
1979
ISSN/ISBN: 0026-895X PMID: 114747 Document Number: 149708
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 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 per mol cytochrome P-450, respectively, although with dissimilar apparent Km values 31 .mu.M and 9.6 .mu.M, respectively. Form 4 was 60 times more active than form 2 in the deethylation of 7-ethoxyresorufin and was 5 times more sensitive in the biphenyl reaction to the inhibitory effects of the differential inhibitor .alpha.-naphthoflavone. Densitometric scans of sodium dodecyl sulfate polyacrylamide gel electrophoretograms revealed that microsomes from rabbits treated with phenobarbital exhibit a relative predominance of form 2. 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. The distinct catalytic capacities of multiple forms of cytochrome P-450 represent significant determinants of xenobiotic metabolism by the cytochrome P-450 system.