Metabolism of polychlorinated dibenzo-p-dioxins by rat liver microsomes.
The in vitro metabolism of several chlorinated dibenzo-p-dioxin congeners (PCDDs) was studied using rat liver microsomes as a source of CYP 1 enzymes. The reactions were kinetically first order in both enzyme and substrate and showed a general trend toward decreasing reactivity with increasing chlorination. Michaelis-Menten kinetics were followed for 1-chlorodibenzo-p-dioxin (1-CDD); the reactivity of the enzyme preparation toward 1-CDD exactly paralleled its activity toward 7-ethoxyresorufin. The unreactive congeners 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD) and 2,2'-dichlorobiphenyl (2,2'-DCB) acted as competitive inhibitors toward 1-CDD, with inhibition constants in the micromolar range, similar to the value of the Michaelis constant of 1-CDD. The inhibitory potency of furafylline, a mechanism-based inhibitor that is selective for CYP 1A2, declined in the order acetanilide (standard) > 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) > 1-CDD. We conclude that 1-CDD and 1,2,3,4-TCDD are oxidized almost exclusively by CYP 1A1, whereas 2,3,7,8-TCDD and 1,2,4,7,8-PeCDD are oxidized mainly by CYP 1A2. 1,2,3,7,8-PeCDD was oxidized too slowly for us to reach any conclusion about the P450 isozyme responsible.
Hu
K
K
Department of Chemistry and Biochemistry, University of Guelph, Ontario, Canada.
Bunce
N J
NJ
D000818:Animals; D019363:Cytochrome P-450 CYP1A1 / Q000378:metabolism; D019388:Cytochrome P-450 CYP1A2 / Q000378:metabolism; D004147:Dioxins / Q000737:chemistry / Q000378:metabolism; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism*; D000072317:Polychlorinated Dibenzodioxins / Q000031:analogs & derivatives* / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D013379:Substrate Specificity
D004147:Dioxins; D000072317:Polychlorinated Dibenzodioxins; C051452:1,2,3,7,8-pentachlorodibenzo-p-dioxin; D019363:Cytochrome P-450 CYP1A1; D019388:Cytochrome P-450 CYP1A2