Quantitative Assessment of the Post-translational Modifications of Human Serum Albumin by Dimethyl Trisulfide.

Some bacteria, such as Fusobacterium nucleatum, act as dimethyl trisulfide (DMTS) producers in the host in vivo. DMTS acts as a sulfane sulfur donor and chemically modifies the sulfhydryl groups. This study explored the post-translational modifications of human serum albumin using DMTS. Quantitative assessments were conducted on mixed disulfides of mercaptoalbumin with mercaptomethane (Alb-SS-CH3) and albumin hydropersulfide (Alb-SSH) as post-translationally modified species. The hydropersulfide group was alkylated with iodoacetamide, resulting in the formation of an albumin-mercaptoacetamide mixed disulfide. The mixed disulfides were subsequently reduced with tris(2-carboxyethyl)phosphine, and the liberated mercaptomethane and mercaptoacetamide were fluorescently labeled with 4-fluoro-7-sulfamoylbenzofurazan (ABD-F). Quantification was performed using HPLC with fluorescence detection. Using this methodology, we examined the formation of Alb-SS-CH3 and Alb-SSH via the reaction between 4% human serum albumin and DMTS at 10-100 µM concentrations. Approximately two molecules of Alb-SS-CH3 and one molecule of Alb-SSH were generated from one DMTS molecule. Moreover, hydrogen sulfide was identified as an intermediate, suggesting its generation and subsequent reaction with intraprotein disulfide bonds, leading to the production of Alb-SSH. These results suggest the production of DMTS in humans in vivo should be involved in the elevation of Alb-SS-CH3 and Alb-SSH contents in plasma samples.

Isomerization of Oxidized Linoleate Metabolites with trans/cis Conjugated Diene Moiety to Those with trans/trans One in the Lipoxygenase/Linoleate/Hydrogen Polysulfide System.

Endogenous hydrogen polysulfides are radical scavengers, and the resulting thiyl radical may catalyze isomerization of the cis-double bond to a trans-double bond. This study examined whether oxidized linoleate species with trans/trans-conjugated diene moieties were generated in the 15-lipoxygenase/linoleate/hydrogen polysulfide system at a lower oxygen content. When 40 µL of 0.1 M phosphate buffer (pH 7.4) containing 1.0 mM linoleate, 1.0 µM soybean 15-lipoxygenase, and 100 µM sodium trisulfide was placed in a 0.6 mL polypropylene microtube for 1 h at 25 °C, the proportion of (E/E)-oxo-octadecadienoic acids (OxoODEs) content to the total OxoODEs content was estimated to be more than 80% (mol/mol). OxoODEs are generated through the pseudoperoxidase reaction of ferrous 15-lipoxygenase with hydroperoxy octadecadienoic acids (HpODEs), which are produced by the lipoxygenase reaction of ferric 15-lipoxygenase. The content of OxoODEs was positively correlated with the content of 9-HpODEs, indicating that 9-HpODEs production is involved in converting ferric 15-lipoxygenase to ferrous 15-lipoxygenase. Furthermore, when 40 µL of 0.1 M phosphate buffer (pH 7.4) containing 1.0 mM linoleate, 1.0 µM soybean 15-lipoxygenase, 100 µM sodium trisulfide, and nitroxyl radical (carbon-centered radical-trapping agent, 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-N-oxyl (CmΔP)) was incubated in a 0.6 mL polypropylene microtube at room temperature, CmΔP-(E/Z)-ODEs were isomerized to CmΔP-(E/E)-ODEs in a time-dependent manner and this isomerization was inhibited by a radical scavenger, Trolox. The results indicate that thiyl radicals derived from hydrogen polysulfides isomerize trans/cis conjugated diene moiety to the trans/trans moiety.

Catalytic Production of Oxo-fatty Acids by Lipoxygenases Is Mediated by the Radical-Radical Dismutation between Fatty Acid Alkoxyl Radicals and Fatty Acid Peroxyl Radicals in Fatty Acid Assembly.

Oxo-octadecadienoic acids (OxoODEs) act as peroxisome proliferator-activated receptor (PPAR) agonists biologically, and are known to be produced in the lipoxygenase/linoleate system. OxoODEs seem to originate from the linoleate alkoxyl radicals that are generated from (E/Z)-hydroperoxy octadecadienoic acids ((E/Z)-HpODEs) by a pseudoperoxidase reaction that is catalyzed by ferrous lipoxygenase. However, the mechanism underlying the conversion of alkoxyl radical into OxoODE remains obscure. In the present study, we confirmed that OxoODEs are produced in the lipoxygenase/linoleate system in an oxygen-dependent manner. Interestingly, we revealed a correlation between the (E/Z)-OxoODEs content and the (E/E)-HpODEs content in the system. (E/E)-HpODEs could have been derived from (E/E)-linoleate peroxyl radicals, which are generated by the reaction between a free linoleate allyl radical and an oxygen molecule. Notably, the ferrous lipoxygenase-linoleate allyl radical (LOx(Fe2+)-L·) complex, which is an intermediate in the lipoxygenase/linoleate system, tends to dissociate into LOx(Fe2+) and a linoleate allyl radical. Subsequently, LOx(Fe2+) converts (E/Z)-HpODEs to an (E/Z)-linoleate alkoxyl radical through one-electron reduction. Taken together, we propose that (E/Z)-OxoODEs and (E/E)-HpODEs are produced through radical-radical dismutation between (E/Z)-linoleate alkoxyl radical and (E/E)-linoleate peroxyl radical. Furthermore, the production of (E/Z)-OxoODEs and (E/E)-HpODEs was remarkably inhibited by a hydrophobic radical scavenger, 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO). On the contrary, water-miscible radical scavengers, 4-hydroxyl-2,2,6,6-tetramethylpiperidine 1-oxyl (OH-TEMPO) and 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrroline-N-oxyl (CmΔP) only modestly or sparingly inhibited the production of (E/Z)-OxoODEs and (E/E)-HpODEs. These facts indicate that the radical-radical dismutation between linoleate alkoxyl radical and linoleate peroxyl radical proceeds in the interior of micelles.

Lipoxygenase-mediated generation of lipid peroxides enhances ferroptosis induced by erastin and RSL3.

In cancer cells the small compounds erastin and RSL3 promote a novel type of cell death called ferroptosis, which requires iron-dependent accumulation of lipid reactive oxygen species. Here we assessed the contribution of lipid peroxidation activity of lipoxygenases (LOX) to ferroptosis in oncogenic Ras-expressing cancer cells. Several 12/15-LOX inhibitors prevented cell death induced by erastin and RSL3. Furthermore, siRNA-mediated silencing of ALOX15 significantly decreased both erastin-induced and RSL3-induced ferroptotic cell death, whereas exogenous overexpression of ALOX15 enhanced the effect of these compounds. Immunofluorescence analyses revealed that the ALOX15 protein consistently localizes to cell membrane during the course of ferroptosis. Importantly, treatments of cells with ALOX15-activating compounds accelerated cell death at low, but not high doses of erastin and RSL3. These observations suggest that tumor ferroptosis is promoted by LOX-catalyzed lipid hydroperoxide generation in cellular membranes.

2,2,6,6-Tetramethylpiperidine-1-oxyl acts as a volatile inhibitor of ferroptosis and neurological injury.

Ferroptosis, a type of oxidative stress cell death, has been implicated in cell injury in several diseases, and treatments with specific inhibitors have been shown to protect cells and tissues. Here we demonstrated that a treatment with the nitroxide radical, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), prevented the ferroptotic cell death in an airborne manner. Other TEMPO derivatives and lipophilic antioxidants, such as Trolox and ferrostatin-1, also prevented cell death induced by erastin and RSL3; however, only TEMPO exhibited inhibitory activity from a physically distant location. TEMPO vaporized without decomposing and then dissolved again into a nearby water solution. Volatilized TEMPO inhibited glutamate-induced cell death in mouse hippocampal cell lines and also reduced neuronal cell death in a mouse ischemia model. These results suggest that TEMPO is a unique cell protective agent that acts in a volatility-mediated manner.

Botanical sulfane sulfur donors inhibit ferroptotic cell death caused by the depletion of cysteine.

Inhibitors against cystine-glutamate antiporter, including erastin, elicit ferroptotic cell death. The erastin-induced ferroptotic cell death appears to be caused by cysteine as well as glutathione depletion. Cysteine is an essential substrate for sulfane sulfur producing systems in cells, generating persulfides that function as intracellular antioxidants and intermediates in iron-sulfur cluster production. Therefore, we examined whether botanical sulfane sulfur donors such as diallyl trisulfide (DATS) and dimethyl trisulfide (DMTS) prevent ferroptotic cell death in HT1080 cells treated with erastin. As a result, DMTS (20 µM) and DATS (10 µM) rescued the erastin-treated HT1080 cells by 69.6% and 91.6%, respectively. Furthermore, DMTS-containing squeeze of cabbage (2.0 g/L) and DATS-containing squeeze of garlic (0.07 g/L) rescued the erastin-treated HT1080 cells by 76.5% and almost 100%, respectively. In conclusion, the ingestion of trisulfides may bring about increased resistance to ferroptotic cell death in vivo.