Singlet oxygen is a highly reactive electrophilic species that reacts rapidly with electron-rich moieties, such as the double bonds of lipids, thiols, and ascorbate (AscH-). The reaction of ascorbate with singlet oxygen is rapid (k = 3 x 10(8) M(-1) s(-1)). Here we have investigated the stoichiometry of this reaction. Using electrodes to make simultaneous, real-time measurements of oxygen and hydrogen peroxide concentrations, we have investigated the products of this reaction. We have demonstrated that hydrogen peroxide is a product of this reaction. The stoichiometry for the reactants of the reaction (1 1O2 + 1AscH--->1H2O2 + 1dehydroascorbic) is 1:1. The formation of H2O2 results in a very different oxidant that has a longer lifetime and much greater diffusion distance. Thus, locally produced singlet oxygen with a half-life of 1 ns to 1 micros in a biological setting is changed to an oxidant that has a much longer lifetime and thus can diffuse to distant targets to initiate biological oxidations.

Nitric oxide (NO • ) is an effective chain-breaking antioxidant in free radical-mediated lipid oxidation (LPO). It reacts rapidly with peroxyl radicals as a sacrificial chain-terminating antioxidant. The goal of this work was to determine the minimum threshold concentration of NO • required to inhibit Fe 2+ -induced cellular lipid peroxidation. Using oxygen consumption as a measure of LPO, we simultaneously measured nitric oxide and oxygen concentrations with NO • -and O 2 -electrodes. Ferrous iron and dioxygen were used to initiate LPO in docosahexaenoic acid-enriched HL-60 and U937 cells. Bolus addition of NO • (1.5 μM) inhibited LPO when the NO • concentration was greater than 50 nM. Similarly, using (Z)-1-[N-(3-ammoniopropyl)-N-(n-propyl)amino]diazen-1-ium-1,2-diolate (PAPA/NO) as a NO • donor we found that an average steady-state NO • concentration of at least 72 ± 9 nM was required to blunt LPO. As long as the concentration of NO • was above 13 ± 8 nM the inhibition was sustained. Once the concentration of NO • fell below this value, the rate of lipid oxidation accelerated as measured by the rate of oxygen consumption. Our model suggests that the continuous production of NO • that would yield a steady-state concentration of only 10 -20 nM is capable of inhibiting Fe 2+ -induced LPO.

Singlet oxygen is a highly reactive electrophilic species that reacts rapidly with electron-rich moieties, such as the double bonds of lipids, thiols, and ascorbate (AscH-). The reaction of ascorbate with singlet oxygen is rapid (k = 3 × 10 8 M -1 s -1 ). Here we have investigated the stoichiometry of this reaction. Using electrodes to make simultaneous, real-time measurements of oxygen and hydrogen peroxide concentrations, we have investigated the products of this reaction. We have demonstrated that hydrogen peroxide is a product of this reaction. The stoichiometry for the reactants of the reaction (1 1 O 2 + 1AscH-→ 1H 2 O 2 + 1dehydroacorbic) is 1:1. The formation of H 2 O 2 results in a very different oxidant that has a longer lifetime and much greater diffusion distance. Thus, locally produced singlet oxygen with a half-life of 1 ns -1 μs in a biological setting is changed to an oxidant that has a much longer lifetime and thus can diffuse to distant targets to initiate biological oxidations.