Abstract
LCS-1, a putative selective inhibitor of SOD1, is a substituted pyridazinone with rudimentary similarity to quinones and naphthoquinones. As quinones catalytically oxidize H(2)S to biologically active reactive sulfur species (RSS), we hypothesized LCS-1 might have similar attributes. Here, we examine LCS-1 reactions with H(2)S and SOD1 using thiol-specific fluorophores, liquid chromatography-mass spectrometry, electron paramagnetic resonance (EPR), UV-vis spectrometry, and oxygen consumption. We show that LCS-1 catalytically oxidizes H(2)S in buffer solutions to form RSS, namely per- and polyhydrosulfides (H(2)S(n), n = 2-6). These reactions consume oxygen and produce hydrogen peroxide, but they do not have an EPR signature, nor do they affect the UV-vis spectrum. Surprisingly, LCS-1 synergizes with SOD1, but not SOD2, to oxidize H(2)S to H(2)S(3-6). LCS-1 forms monothiol adducts with H(2)S, glutathione (GSH), and cysteine (Cys), but not with oxidized glutathione or cystine; both thiol adducts inhibit LCS-1-SOD1 synergism. We propose that LCS-1 forms an adduct with SOD1 that disrupts the intramolecular Cys(57)-Cys(146) disulfide bond and transforms SOD1 from a dismutase to an oxidase. This would increase cellular ROS and polysulfides, the latter potentially affecting cellular signaling and/or cytoprotection.