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.