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.
Laboratory of Comparative Carcinogenesis, National Cancer Institute, Frederick Cancer Research and Development Center, MD 21702.
P V
PV
Nerurkar
S S
SS
Park
P E
PE
Thomas
R W
RW
Nims
R A
RA
Lubet
D000818:Animals; D019388:Cytochrome P-450 CYP1A2; D019362:Cytochrome P-450 CYP2B1; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis / Q000378:metabolism*; D004790:Enzyme Induction; D008297:Male; D051379:Mice; D008861:Microsomes / Q000201:enzymology; D010078:Oxazines / Q000378:metabolism*; D010088:Oxidoreductases / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000378:metabolism*; D051381:Rats; D011916:Rats, Inbred F344; D015262:Xenobiotics
D065607:Cytochrome P-450 Enzyme Inhibitors; D010078:Oxazines; D015262:Xenobiotics; C076580:7-methoxyresorufin; C007649:ethoxyresorufin; C057754:benzyloxyresorufin; C044943:pentoxyresorufin; D003577:Cytochrome P-450 Enzyme System; D010088:Oxidoreductases; D019388:Cytochrome P-450 CYP1A2; D019362:Cytochrome P-450 CYP2B1; C000617645:cytochrome P-450 1A2, mouse