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Received for publication September 27, 2004.
Revised November 25, 2004.
Accepted for publication December 20, 2004.
Modulation of UDP-glucuronosyltransferase 2B7 (UGT2B7)-catalyzed morphine glucuronidation by cytochrome P450 (P450, CYP) was studied. The effects of CYP isozymes on the kinetic parameters of UGT2B7-catalyzed glucuronidation of the morphine 3-hydroxyl group were examined by simultaneous expression of UGT2B7 and either CYP3A4, 1A2 or 2C9 in COS-1 cells. While co-expression of CYP3A4 with UGT2B7 caused little effect on Vmax, the Km was increased by about 9.8-fold in comparison with the UGT2B7 single expression system. The other P450 isozymes (CYP1A2 and CYP2C9) had some effects on Km and Vmax values. Immunoprecipitation of UGT from solubilized human liver microsomes resulted in co-precipitation of CYP3A4 with UGT2B7. The protein-protein interaction between CYP3A4 and UGT2B7 was further confirmed by overlay assay using glutathione S-transferase - CYP3A4 fusion protein. Addition of CYP3A4 to untreated COS microsomes expressing UGT2B7 had no or minor effects on morphine glucuronidation. In contrast, the formation of morphine-3-glucuronide by detergent-treated microsomes from COS-1 cells expressing UGT2B7 was reduced by CYP3A4, whereas the formation of the 6-glucuronide was enhanced. These results strongly suggest that 1) the glucuronidation activity of UGT2B7 towards morphine is specifically modulated by interaction with CYP3A4 in microsomal membranes; and 2) CYP3A4 alters UGT2B7 regioselectivity so that the ratio of morphine activation/detoxication is increased. This study provides the first evidence that P450 is not only involved in oxidation of drugs but also modulates the function of UGTs.
Key words:
Cytochrome P450, Structure/function/mechanism, UDP-glucuronyltransferases
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