Superoxide activates uncoupling proteins by generating carbon-centered radicals and initiating lipid peroxidation: studies using a mitochondria-targeted spin trap derived from alpha-phenyl-N-tert-butylnitrone.

Although the physiological role of uncoupling proteins (UCPs) 2 and 3 is uncertain, their activation by superoxide and by lipid peroxidation products suggest that UCPs are central to the mitochondrial response to reactive oxygen species. We examined whether superoxide and lipid peroxidation products such as 4-hydroxy-2-trans-nonenal act independently to activate UCPs, or if they share a common pathway, perhaps by superoxide exposure leading to the formation of lipid peroxidation products. This possibility can be tested by blocking the putative reactive oxygen species cascade with selective antioxidants and then reactivating UCPs with distal cascade components. We synthesized a mitochondria-targeted derivative of the spin trap alpha-phenyl-N-tert-butylnitrone, which reacts rapidly with carbon-centered radicals but is unreactive with superoxide and lipid peroxidation products. [4-[4-[[(1,1-Dimethylethyl)-oxidoimino]methyl]phenoxy]butyl]triphenylphosphonium bromide (MitoPBN) prevented the activation of UCPs by superoxide but did not block activation by hydroxynonenal. This was not due to MitoPBN reacting with superoxide or the hydroxyl radical or by acting as a chain-breaking antioxidant. MitoPBN did react with carbon-centered radicals and also prevented lipid peroxidation by the carbon-centered radical generator 2,2'-azobis(2-methyl propionamidine) dihydrochloride (AAPH). Furthermore, AAPH activated UCPs, and this was blocked by MitoPBN. These data suggest that superoxide and lipid peroxidation products share a common pathway for the activation of UCPs. Superoxide releases iron from iron-sulfur center proteins, which then generates carbon-centered radicals that initiate lipid peroxidation, yielding breakdown products that activate UCPs.

Interactions of mitochondrial thiols with nitric oxide.

The interaction of nitric oxide (NO) with mitochondria is of pathological significance and is also a potential mechanism for the regulation of mitochondrial function. Some of the ways in which NO may affect mitochondria are by reacting with low-molecular-weight thiols such as glutathione and with protein thiols. However, the detailed mechanisms and the consequences of these interactions for mitochondria are uncertain. Here we review mitochondrial thiol metabolism, outline how NO and its metabolites interact with thiols, and discuss the implications of these reactions for mitochondrial and cell function.

Reversible glutathionylation of complex I increases mitochondrial superoxide formation.

Increased production of reactive oxygen species (ROS) by mitochondria is involved in oxidative damage to the organelle and in committing cells to apoptosis or senescence, but the mechanisms of this increase are unknown. Here we show that ROS production by mitochondrial complex I increases in response to oxidation of the mitochondrial glutathione pool. This correlates with thiols on the 51- and 75-kDa subunits of complex I forming mixed disulfides with glutathione. Glutathionylation of complex I increases superoxide production by the complex, and when the mixed disulfides are reduced, superoxide production returns to basal levels. Within intact mitochondria oxidation of the glutathione pool to glutathione disulfide also leads to glutathionylation of complex I, which correlates with increased superoxide formation. In this case, most of this superoxide is converted to hydrogen peroxide, which can then diffuse into the cytoplasm. This mechanism of reversible mitochondrial ROS production suggests how mitochondria might regulate redox signaling and shows how oxidation of the mitochondrial glutathione pool could contribute to the pathological changes that occur to mitochondria during oxidative stress.

Effects of nitrite and nitrate on the growth and acidogenicity of Streptococcus mutans.

UNLABELLED: It is hypothesised that exogenous nitrite acidified by metabolic products of acidogenic bacteria in the mouth will be converted to products which inhibit growth of the bacteria in question which contribute to dental caries. OBJECTIVES: The aims of this study were (1) to test the activity of both sodium nitrate and sodium nitrite at differing concentrations on the ability of Streptococcus mutans to lower the pH of its surroundings and hence (2) to determine whether either nitrate or nitrite might be bactericidal or bacteriostatic against S. mutans. METHODS: S. mutans NCTC 10449(T) was cultured in a liquid medium to which either sodium nitrate or sodium nitrite was added to a final concentration of 0.0, 0.2, 2.0, 20 or 200 mM, of which the first acted as a test substance negative control. After 24 h, the cultures were streaked onto agar to test for growth and the remaining culture used for pH measurement. The Mann-Whitney U-Test was used for statistical comparison of pH values. RESULTS: Nitrite at concentrations of 20 and 200 mM had a highly significant inhibitory effect (p < 0.001) on the ability of S. mutans NCTC 10449(T) to lower pH. Moreover, bacteria that had been subjected to these levels of nitrite were unable to recover on solid medium. Nitrate had no such effect on either the growth of the bacteria or on their ability to lower pH. CONCLUSIONS: It is concluded that nitrite, at final concentrations of either 20 or 200 mM, is both bactericidal and anti-acidogenic with respect to S. mutans, while lower concentrations of nitrite and all concentrations of nitrate are ineffective. Nitrite might be worthy of consideration as a mouth-rinse constituent.