Nitric oxide as a cellular antioxidant: a little goes a long way.

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 Fe2+ -induced cellular lipid peroxidation. Using oxygen consumption as a measure of LPO, we simultaneously measured nitric oxide and oxygen concentrations with NO* and O2 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 microM) 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 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 a continuous production of NO* that would yield a steady-state concentration of only 10-20 nM is capable of inhibiting Fe2+ -induced LPO.