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A novel procedure for the preparation of drug-modified proteins is presented.Separation of target protein from bioactivation system by a semi-permeable membrane.Purified and membrane-bound cytochrome P450s can be used for drug bioactivation.Analysis of intact protein adducts by mass spectrometry without laborious workup.The described method is applicable to reactive metabolites of significant half-life.The modification of critical cellular proteins by reactive metabolites (RMs) resulting from P450-dependent drug bioactivation is considered essential to the onset of many idiosyncratic drug reactions. In this study, we report a novel method that can be used to prepare and study drug-protein adducts. Drug bioactivation by P450s was performed in a small container containing a mini-dialysis tube with the model target protein human glutathione-S-transferase P1-1 (hGST P1-1), allowing RMs to translocate from P450 to hGST P1-1 via a semi-permeable membrane (6–8 kDa). GST P1-1 modification was evaluated by LC–MS analysis of intact protein adducts and following digestion of protein with trypsin. As proof of principle, the described methodology was first applied to the direct electrophile monochlorobimane. A highly active P450 BM3 mutant (CYP102A1M11H) was subsequently used for bioactivation of acetaminophen, clozapine, diclofenac (DF) and mefenamic acid (MFA), but hGST P1-1 adducts were only observed for the latter two drugs. CYP2C9 and CYP3A4, which metabolize DF to p-benzoquinone imines, were tested to investigate the applicability of human P450s. Finally, it was evaluated whether bioactivation of MFA by human and rat liver microsomes resulted in modification of hGST P1-1. The results show that our adduct preparation method can also be used in combination with membrane-bound P450 bioactivation systems, as long as formed RMs have sufficient life-time to reach hGST P1-1 inside the dialysis tube.