Methoxyresorufin and benzyloxyresorufin: substrates preferentially metabolized by cytochromes P4501A2 and 2B, respectively, in the rat and mouse.

The cytochrome P450 isozyme specificity for the O-dealkylation of methoxyresorufin (MTR) and benzyloxyresorufin (BZR) in the rat and mouse was investigated. The induction of various alkoxyresorufin O-dealkylation activities was measured in male F344/NCr rats exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin or 3,4,5,3',4',5'-hexachlorobiphenyl. MTR and ethoxyresorufin (ETR) O-dealkylation activities were induced 30- and 80-fold, respectively, in the liver. ETR O-dealkylation activity was induced > 250-fold in the kidney, whereas the metabolism of MTR was induced only 30-fold in this extrahepatic tissue. Phenacetin, a fairly specific CYP1A2 inhibitor, caused concentration-dependent competitive inhibition of MTR O-dealkylation (ki approximately 20 microM at 0.5 microM substrate) in hepatic microsomes from 3,4,5,3',4',5'-hexachlorobiphenyl-treated rats. The corresponding ki for inhibition of ETR O-dealkylation by phenacetin was > or = 333 microM at a 0.5 microM substrate concentration. A monoclonal antibody displaying inhibitory activity against rat CYP1A1 inhibited ETR O-dealkylation activity, whereas it failed to inhibit MTR O-dealkylation activity. In contrast, a monoclonal antibody reactive with both CYP1A1 and CYP1A2 inhibited both O-dealkylation activities to an equal extent. Similar experiments, employing phenacetin or specific monoclonal antibodies, yielded comparable results when performed with mouse microsomes. The maximal induction of MTR O-dealkylation activity in mice was > 100-fold. The P450 isozyme specificity of BZR O-dealkylation was also examined in both rats and mice. Pregnenolone-alpha-carbonitrile, a strong inducer of CYP3A, only weakly induced BZR O-dealkylation activity. In addition, a monoclonal antibody that specifically inhibits CYP2B caused inhibition of BZR metabolism in microsomes from phenobarbital- or dexamethasone-pretreated rats. In B6C3F1 mice exposed to dietary Aroclor 1254, significant induction of hepatic MTR O-dealkylation activity was observed at concentrations lower than those required for the induction of ETR or BZR O-dealkylation. In summary, it would appear that MTR is a relatively specific substrate for CYP1A2 activity in rodents, while BZR appears to be relatively specific for CYP2B.