Aconitase, mitochondrial

When induced in macrophages, iNOS produces large amounts of NO which represents a major cytotoxic principle of those cells. Due to its affinity to protein-bound iron, NO can inhibit a number of key enzymes that contain iron in their catalytic centers. These include ribonucleotide reductase (rate-limiting in DNA replication), iron-sulfur cluster-dependent enzymes (complex I and II) involved in mitochondrial electron transport and cis-aconitase in the citric acid cycle. In addition, higher concentrations of NO,... 

The rate constant for Reaction (3) is in the range of 108 to 1091 mol-1s-1 [20]. Therefore, Reactions (3) and (4) may significantly enhance the concentration of ferrous ions and make Fenton reaction a better competitor with the peroxynitrite-inducible damage [21]. The formation of hydroxyl radicals in the reaction of superoxide with mitochondrial aconitase has... 

Gardner et al. [165] have shown that the redox-cycling agent phenazine methosulfate (PMS), mitochondrial ubiquinol-cytochrome c oxidoreductase, or hypoxia inactivated aco-nitase in mammalian cells. It has been proposed that the inactivation of aconitase is mediated by superoxide produced by prooxidants because the overproduction of mitochondrial MnSOD protected aconitase from inactivation by the prooxidants mentioned above except hyperoxia. Later on, the reaction of superoxide with aconitases began to be considered as one of the most important ways to NTBI generation in vivo. 

Additional ISC proteins are required later in the process for the insertion of ISC into mitochondrial Fe-S proteins. Isal and Isa2 are required specifically and in addition for the maturation of aconitase-type Fe-S proteins. 

Aconitase, an unstable enzyme,4 is concerned with the reversible conversion of cis-aconitate to either citric acid or isocitric acid. It may be noted that the entire system of tricarboxylic cycle enzymes are present in the mitochondria separated from cells, and, furthermore, it has been found that the mitochondrial enzymes differ from the isolated enzymes in that the former require no addition of D.P.N. (co-enzyme I) or T.P.N. (co-enzyme II) for activity. Peters suggests that the citrate accumulation is caused by the competitive reaction of the fluorocitrate with aconitase required for the conversion of citrate to isocitrate. This interference with the tricarboxylic acid... 

Nevertheless, the toxicity of fluoroacetate seems to be only partially due to the inhibition of aconitase. The competitive nature of the inhibition, its Xj value (Xj = 20-60 pM)," and the time-dependent nature (but reversible) of the inhibition of aconitase seem to be poorly compatible with the sharp and irreversible toxicity of fluorocitrate. Thus, it has been suggested that fluorocitrate can covalently bind with the proteins that are involved in citrate transport through the mitochondrial membrane. ... 

It is noteworthy that except for the Rieske center in Complex III, Complexes I and 11 are home to all the iron-sulfur clusters in the mitochondrial electron transfer chain and consequently most of the iron-containing carriers in the entire sequence. Hibbs subsequently showed that CAM-injured cells lose a substantial portion of their total intracellular iron (Hibbs et al., 1984) [later studies specifically identified loss of mitochondrial iron (Wharton et al., 1988)] and Drapier and Hibbs (1986) showed that the activity of another iron-sulfur-containing enzyme, aconitase, is also lost. In early 1987 Hibbs reported that the cytostatic actions of CAMs requires the presence of only one component in culture medium, L-arginine (Hibbs et al., 1987b). Thus, the stage was set for the discovery of a unique reactive species that targets intracellular iron, produced by CAMs. 

Treatment of isolated hepatocytes with authentic nitric oxide inhibits the electron transport chain at complexes I and II, and mitochondrial aconitase activity (Stadler et al., 1991). 

Effects of lL 1/3 and NMMA on islet-cell mitochondrial aconitase activity. Islets were treated for 18 hr with 5 U/ml lL-1, 0.5 mM NMMA, or lL-1/3 and NMMA. The islets were isolated and aconitase activity and the simultaneous release of nitrite were determined as described previously (Corbett et al., 1992b). Treatment of islets with lL-1/3 results in an 80% inhibition of mitochondrial aconitase activity, which is completely prevented by NMMA. lL-1 also stimulates a twofold increase in the level of nitrite released by islets, and NMMA prevents this nitrite formation. Reproduced with permission from J. Clin, Invest. (Corbett et al., 1992b), by the American Society for Clinical Investigation. 

Nitric oxide may be the active moiety of STZ that induces diabetes in this animal model. STZ contains a nitroso moiety and may release nitric oxide by a process analogous to the nitric oxide donor compounds SIN-1 and nitroprusside. Turk et al. (1993) have shown that incubation of rat islets with STZ at concentrations that impair insulin secretion results in the generation of nitrite and the accumulation of cGMP. STZ also inhibits mitochondrial aconitase activity of islets to a degree similar to that achieved by IL-1. These findings provide the first evidence that STZ impairs islet function by liberating nittic oxide. 

Our studies with mitochondrial respiration also ruled out the hypothesis that NO-induced impairment of mitochondrial function accounted for the decrease in protein synthesis. The reasons for this conclusion include first, a minimal decrease in aconitase activity and essentially no decrease in complex 1 or complex II activity was seen in HC exposed to KC supernatant or cytokines -I- LPS, despite a marked reduction in protein synthesis and second, exposure of HC to NO resulted in a profound and prolonged decrease in HC protein synthesis (up to 18 hr), but a very short-lived decrease in mitochondrial respiration (90 min)... 

Rodent KC and HC, as well as human HC, express an inducible NO synthase under septic or inflammatory conditions. In vivo in endotoxemia, this expression is transient. Our in vivo data indicate that this induced -NO serves a protective role in the liver and reduces hepatic injury in endotoxemia. This protective action may be mediated by the capacity of NO to neutralize oxygen radicals and prevent platelet adherence and aggregation. Our in vitro studies show that HC-derived -NO can activate soluble guanylate cyclase. Other in vitro effects include the nonspecific suppression of protein synthesis and a small reduction in mitochondrial aconitase activity. The relevance of these in vitro actions to hepatic function in vivo remains to be determined. 

Fig. 1 Enzymes localized to B. hominis mitochondrial-like organelle. The enzymes are 1 malic enzyme, 2 pyruvate NADP oxidoreductase, 3 acetate succinate CoA transferase, 4 succinate thiokinase, 5 a-ketoglutarate dehydrogenase, 6 isocitrate dehydrogenase, and 7 aconitase...

---