Determination of the CYP2B6 Component of 7-Ethoxy-4-Trifluoromethylcoumarin O-Deethylation Activity

CYP2B6 protein has been isolated and purified from human liver (1 , 2 ). In a panel of 60 individual human liver microsome samples, this P450 accounts for <l% of total P450 content in human liver (2 ). However, a large mtermdlvldual variability has been reported for the hepatic levels of CYP2B6 mRNA (3 ) and protein (1 , 2 , 4 ). Experiments with primary cultures of human hepatocytes have shown that CYP2B6 is inducible by treatment of the cells with phenobarbital (5 ), dexamethasone (5 , 6 ), or rifampm (rifampicm) ($6). By contrast, the level of this P450 in hepatocyte cultures does not appear to be altered by other P450 mducers such as /3-naphthoflavone (5 ), 2,3,7,8-tetrachlorodibenzo-p-dloxm (5 ) or pregnenolone 16a-carbonitnle (6 ). Several substrates have been ldentlfied for cDNA-expressed CYP2B6, including I-l-ethoxycoumarin (3 , 7 , 8 ), benzo[a]pyrene (9 ), phenanthrene (10 ), and methoxychlor (11 ). However, little is known regardmg the role of this P450 in drug metabolism, although cDNA-expressed CYP2B6 is an active catalyst of lidocaine N -deethylatlon (12 ) and human hver microsomal CYP2B6 appears to be a high-Km catalyst of cyclophosphamlde 4-hydroxylation (8 ). Detailed mvestlgations of hepatic mlcrosomal CYP2B6 have been limited because: 1 Many of the heterologous anti-CYP2B antibody preparations are not useful owing to their slgmficant cross-reactivity with other human P450 enzymes; 2 A CYP2B6-specific chemical inhibitor has yet to be found; and 3 A dlagnostlc catalytic activity for human hepatlc mlcrosomal CYP2B6 has not been identified.