SP600125, a selective JNK inhibitor, is a potent inhibitor of NAD(P)H: quinone oxidoreductase 1 (NQO1).

The c-Jun N-terminal kinases (JNKs) has been identified as a critical modulator in multiple cellular processes, including stress stimulus, inflammation, cell proliferation, apoptosis, etc. SP600125 is a widely used ATP-competitive reversible JNKs inhibitor. NAD(P)H: quinone oxidoreductase 1 (NQO1) is a flavoprotein mediated two or four electron-reduction of quinones. Here, we showed that SP600125 bind to the active pocket of NQO1 and inhibit NQO1 activity. SP600125 exhibits comparable inhibitory effects on NQO1-mediated quinone bioactivation, H₂O₂ generation, and cell death, as the specific NQO1 inhibitor dicoumarol (DIC). Importantly, the inhibitory effects of SP600125 on NQO1 are independent of JNKs inhibition. These results suggested that SP600125 is a novel NQO1 inhibitor, which provides new insights into the mechanism of action of SP600125. Furthermore, SP600125 should be used more cautiously as a JNKs inhibitor, especially when NQO1 is highly expressed. SP600125 competed with β-Lap (NQO1-bioactivated drugs) for binding to NQO1, and inhibited NQO1-dependent cell death.