Inactivation of NF-κB Components by Covalent Binding of (−)-Dehydroxymethylepoxyquinomicin to Specific Cysteine Residues

Previously, we designed and synthesized a potent NF-kappaB inhibitor, DHMEQ. Although DHMEQ showed potent anti-inflammatory and anticancer activities in animals, its molecular target has not been elucidated. In the present study, its target protein was found to be p65 and other Rel homology proteins. We found that (-)-DHMEQ bound to p65 covalently with a 1:1 stoichiometry by conducting SPR and MALDI-TOF MS analyses. MS analysis of the chymotrypsin-digested peptide suggested the binding of (-)-DHMEQ to a Cys residue. Formation of Cys/(-)-DHMEQ adduct in the protein was supported by chemical synthesis of the adduct. Substitution of specific Cys in p65 and other Rel homology proteins resulted in the loss of (-)-DHMEQ binding. (-)-DHMEQ is the first NF-kappaB inhibitor that was proven to bind to the specific Cys by chemical methodology. These findings may explain the highly selective inhibition of NF-kappaB and the low toxic effect of (-)-DHMEQ in cells and animals.