Glutathione S-transferase pi trapping method for generation and characterization of drug–protein adducts in human liver microsomes using liquid chromatography–tandem mass spectrometry

Covalent binding of reactive metabolites (RMs) to proteins is thought to play an important role in the processes leading to adverse drug reactions. Therefore, there is great interest in methodologies that enable the characterization of covalent binding of drugs to proteins. To facilitate the study of drug–protein adducts, we have developed a liquid chromatography–tandem mass spectrometry (LC–MS/MS) method for characterizing RM-modified proteins formed through drug bioactivation in human liver microsomes (HLMs), which are commonly used for the in vitro drug bioactivation studies. The technique was illustrated by the trapping of RMs of acetaminophen (APAP) and raloxifene with human glutathione S-transferase pi (hGSTP) as a model target protein. After hGSTP-supplemented HLM incubations, the modified/unmodified hGSTP fractions were collected by high-performance liquid chromatography. hGSTP fractions were digested with trypsin, and then analyzed by linear ion trap–orbitrap mass spectrometry followed by a SEQUEST database search. Characteristic MS/MS fragment ions of RM-modified peptides were identified by searching for possible adducted-mass shifts. The method successfully revealed that RMs of both drugs adducted to Cys-47 of hGSTP and the mass shifts corresponded to modification by the N-acetyl-p-benzoquinone imine form of APAP and diquinone methide form of raloxifene, respectively. The developed method would be a possible tool for widespread use for the generation and characterization of drug–protein adducts in HLMs and has the potential to assess the risk of covalent binding of drugs to proteins.