Table 2.
Chemical Attributes of Reactive Oxygen Species, Reactive Nitrogen Species, and Reactive Sulfur Species (Interactions with Metals and Metalloproteins Not Included in This Overview)
| Species | Occurrence/formation | Chemistry |
|---|---|---|
| Dioxygen, O2 | Aerobic life | Has unpaired electrons—reacts readily with other radicals, poor 1e− oxidant, but otherwise easily reduced by 2, 3, and 4e−. |
| Superoxide, O2− | 1e− reduction of O2 | Has one unpaired electron. Good reductant. Under acidic conditions, it can be an oxidant. Reacts with other radicals. |
| Hydrogen peroxide, H2O2 | 1e− reduction of O2− | Two-electron oxidant. Electrophilic. Not a radical species. Can modify RSH (below). |
| Hydroxyl radical, HO• | 1e− reduction of H2O2 | Potent 1e− oxidant. A short-lived radical species capable of abstracting an e− or hydrogen atom from most biological molecules. |
| Nitric oxide, NO | Enzymatic, NO2− reduction | Has unpaired e−. Poor oxidant. Reacts with other radicals (O2 and O2− and other organic radicals [R•]). Can quench radical chemistry. |
| Nitrogen dioxide, NO2 | Oxidation of NO | Good 1e− oxidant. Radical species. Reacts with other radicals (can make R-NO2 when reacted with R•). |
| Dinitrogen trioxide, N2O3 | Oxidation of NO by O2 | Not a radical. Electrophilic and can nitrosate nucleophiles (add equivalent of NO+). Synthesis only relevant at high concentrations of NO. |
| Peroxynitrite, ONOO− | Reaction of NO and O2− | Not a radical, but can generate both HO• and NO2. Rearranges to give nitrate (NO3−). Can oxidize by 2e− (via peroxide-like chemistry). |
| Nitroxyl, HNO | S-nitrosothiol reduction | Reacts readily with thiols. A good hydrogen atom donor. Can act as an antioxidant by quenching radical reactions. |
| Nitrite, NO2− | Oxidation of NO, dietary | Unreactive at neutral pH. Not a radical. Nitrosating agent under acidic conditions. Can be reduced to NO (under acidic conditions). |
| Hydrogen sulfide, H2S | Geochemical, enzymatic | Good metal ligand. Not a radical. Can react with other biological electrophilic sulfur species (e.g., RSSR, RSOH). |
| Thiol, RSH | Endogenous (e.g., cysteine) | Good metal ligand. Not a radical. Can be oxidized to give other biologically relevant sulfur species. |
| Thiyl radical, RS· | 1e− oxidation of RSH | Radical species. Good 1e− oxidant. Will react with other radical species such as NO. |
| Disulfide, RSSR | Oxidation of RSH | Not a radical. Electrophilic. Can be reduced back to RSH under biological conditions. |
| S-Nitrosothiols, RSNO | Nitrosation of RSH | Not a radical. Can be reduced to RSH and HNO. Can transfer NO+ to another thiol (transnitrosation). |
| Sulfenic acid, RSOH | 2e− oxidation of RSH | Not a radical. Electrophilic (reacts with other thiols to give disulfides, RSSR). |
| Hydropersulfide, RSSH | Enzymatic | Not a radical. Superior nucleophile and can be electrophilic (akin to RSSR). Good 1e− reductant. Readily reduced back to RSH. |
| Hydropolysulfide, RSSnH | Oxidation of RSH | Properties similar to RSSH (although enhanced in all aspects). |
| Dialkyl polysulfide, RSSnR | Oxidation of RSH | Properties similar to RSSR (although enhanced in all aspects). Can also be nucleophilic. |
| SSNO− | Unknown, to be determined | Fairly stable. Appears to be a source of NO. Chemical properties pending. |
| SNO− | Unknown | Fleeting. Chemical properties pending. |