Isoniazid(INH), a widely used antituberculosis drug, has beenassociated with serious drug-induced liver injury (DILI). INH-modifiedproteins have been proposed to play important roles in INH DILI; however,it remains to be determined whether INH or reactive metabolites bindirreversibly to proteins. In this study, mass spectrometry was usedto define protein modifications by INH in vitroandin patients taking INH therapy. When INH was incubated with N-acetyl lysine (NAL), the same isonicotinic-NAL (IN-NAL)adducts were detected irrespective of the presence or absence of anyoxidative enzymes, indicating auto-oxidation may have been involved.In addition, we found that INH could also bind to human serum albumin(HSA) via an auto-oxidation pathway, forming isonicotinic amide adductswith lysine residues in HSA. Similar adducts were detected in plasmasamples isolated from patients taking INH therapy. Our results showthat INH forms protein adducts in the absence of metabolism. [ABSTRACT FROM AUTHOR]