Mass spectroscopy and molecular modeling predict endothelial nitric oxide synthase dimer collapse by hydrogen peroxide through zinc tetrathiolate metal-binding site disruption

Endothelial nitric oxide synthase (eNOS) is inhibited by hydrogen peroxide (H₂O₂), but the mechanism has not been determined. Thus, the purpose of this study was to delineate the mechanism by which H ₂O₂ inhibits eNOS activity. Using mass spectroscopy, we found that the tetrathiolate cysteine residues 94 and 99 were susceptible to oxidation by H₂O₂. Molecular modeling predicted that these cysteic acid modifications would disrupt the van der Waals interactions and the hydrogen bonding network mediated by the tetrathiolate cysteines 94 and 99 resulting in changes in quaternary structure, zinc release, and dimer collapse. Using recombinant human eNOS (heNOS) to test the predictions of the molecular modeling we found that H₂O₂ caused disruption of the heNOS dimer and this was accompanied by zinc release and decreased NO generation. We also found that H₂O₂ increased the oxidation of tetrahydrobiopterin (BH₄) to dihydrobiopterin (BH₂), whereas preincubation of heNOS with excess BH₄ prevented the destruction of zinc tetrathiolate and dimer collapse and preserved activity. Interestingly, we found that the dimmer-stabilizing effect of BH₄ is due to its ability to act as a catalase mimetic. Further, we confirmed that, in ovine aortic endothelial cells, H₂O₂ could also induce dimer collapse and that increasing cellular BH₄ levels could maintain eNOS in its dimeric form and NO signaling when cells were challenged with H₂O₂. This study links the inhibitory action of H ₂O₂ on heNOS through the destruction of zinc tetrathiolate metal-binding site and dimer collapse both in vitro and in vivo.