To investigate a possible new physiological role of carbon monoxide (CO), an endogenous gas involved in cell signaling and cytotoxicity, we tested the hypothesis that the mitochondrial generation of reactive oxygen species by CO activates mitochondrial biogenesis in the heart. In mice, transient elevations of cellular CO by five- to 20-fold increased the copy number of cardiac mitochondrial DNA, the content of respiratory complex I-V and interfibrillar mitochondrial density within 24 hours. Mitochondrial biogenesis is activated by gene and protein expression of the nuclear respiratory factor 1 (NRF1) and NRF2, of peroxisome proliferator-activated receptor gamma co-activator-1α, and of mitochondrial transcription factor A (TFAM), which augmented the copy number of mitochondrial DNA (mtDNA). This is independent of nitric oxide synthase (NOS), as demonstrated by the identical responses in wild-type and endothelial NOS (eNOS)-deficient mice, and by the inhibition of inducible NOS (iNOS). In the heart and in isolated cardiomyocytes, CO activation involved both guanylate cyclase and the pro-survival kinase Akt/PKB. Akt activation was facilitated by mitochondrial binding of CO and by production of hydrogen peroxide (H2O2). Interference with Akt activity by blocking PI 3-kinase and by mitochondrial targeting of catalase to scavenge H2O2 prevented binding of NRF1 to the Tfam promoter, thereby connecting mitochondrial H2O2 to the pathway leading to mtDNA replication. The findings disclose mitochondrial CO and H2O2 as new activating factors in cardiac mitochondrial biogenesis.

These novel findings disclose a duality of reactive oxygen species (ROS) effect in the heart's response to LPS in which oxidative mitochondrial damage is opposed by oxidant stimulation of biogenesis.

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Peroxynitrite anion is a powerful oxidant which can initiate nitration and hydroxylation of aromatic rings. Peroxynitrite can be formed in several ways, e.g. from the reaction of nitric oxide with superoxide or from hydrogen peroxide and nitrite at acidic pH. We investigated pH dependent nitration and hydroxylation resulting from the reaction of hydrogen peroxide and nitrite to determine if this reaction proceeds at pH values which are known to occur in vivo. Nitration and hydroxylation products of tyrosine and salicyclic acid were separated with an HPLC column and measured using ultraviolet and electrochemical detectors. These studies revealed that this reaction favored hydroxylation between pH 2 and pH 4, while nitration was predominant between pH 5 and pH 6. Peroxynitrite is presumed to be an intermediate in this reaction as the hydroxylation and nitration profiles of authentic peroxynitrite showed similar pH dependence. These findings indicate that hydrogen peroxide and nitrite interact at hydrogen ion concentrations present under some physiologic conditions. This interaction can initiate nitration and hydroxylation of aromatic molecules such as tyrosine residues and may thereby contribute to the biochemical and toxic effects of the molecules.