The oxidatively induced DNA lesions 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyG) and 4,6-diamino-5-formamidopyrimidine (FapyA) are formed abundantly in DNA of cultured cells or tissues exposed to ionizing radiation or to other free radical-generating systems. In vitro studies indicate that these lesions are miscoding, can block the progression of DNA polymerases, and are substrates for base excision repair. However, no study has yet addressed how these lesions are metabolized in cellular extracts. The synthesis of oligonucleotides containing FapyG and FapyA at defined positions was recently reported. These constructs allowed us to investigate the repair of Fapy lesions in nuclear and mitochondrial extracts from wild type and knock-out mice lacking the two major DNA glycosylases for repair of oxidative DNA damage, OGG1 and NTH1. The background level of FapyG/FapyA in DNA from these mice was also determined. Endogenous FapyG levels in liver DNA from wild type mice were significantly higher than 8-hydroxyguanine levels. FapyG and FapyA were efficiently repaired in nuclear and mitochondrial extracts from wild type animals but not in the glycosylasedeficient mice. Our results indicated that OGG1 and NTH1 are the major DNA glycosylases for the removal of FapyG and FapyA, respectively. Tissue-specific analysis suggested that other DNA glycosylases may contribute to FapyA repair when NTH1 is poorly expressed. We identified NEIL1 in liver mitochondria, which could account for the residual incision activity in the absence of OGG1 and NTH1. FapyG and FapyA levels were significantly elevated in DNA from the knock-out mice, underscoring the biological role of OGG1 and NTH1 in the repair of these lesions.A large number of DNA base modifications are formed by oxidative damage to DNA (for review, see Ref. 1). Some of these lesions are generated at high rates, even in the absence of exogenous DNA-damaging agents. For instance, it was estimated that 100 -500 8-hydroxyguanines 3 are formed per day in a human cell (2). 8-oxoG is one of the most studied DNA lesions, and it is often used as a biomarker of oxidative DNA damage. However, ring-opened formamidopyrimidine lesions, 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyG) and 4,6-diamino-5-formamidopyrimidine (FapyA), are formed at equal or higher levels than 8-oxoG after oxidative stress (3-5). These lesions result from hydroxyl radical attack on guanine and adenine, respectively, followed by one-electron reduction of the hydroxyl adduct radicals (1), which are also intermediates in the formation of 8-oxoG and 8-oxoA (6). Formation of Fapy lesions in DNA upon UV radiation has also been reported (7). FapyA (8) and FapyG (9) are miscoding in vitro, both directing the preferential misincorporation of adenine opposite the lesions by a bacterial DNA polymerase (Klenow exo Ϫ ). Experiments using the methylated analogue of FapyG, i.e. 2,6-diamino-4-hydroxy-5-Nmethylformamidopyrimidine (Me-FapyG), have suggested that formamidopyrimidines might also constitute blocks to DNA poly...

No abstract

There is an age-associated decline in the mitochondrial function of the Wistar rat heart. Previous reports from this lab have shown a decrease in mitochondrial cytochrome c oxidase (COX) activity associated with a reduction in COX gene and protein expression and a similar decrease in the rate of mitochondrial protein synthesis. Damage to mitochondrial DNA may contribute to this decline. Using the HPLC-Coularray system (ESA, USA), we measured levels of nuclear and mitochondrial 8-oxo-2'-deoxyguanosine (8-oxodG) from 6-month (young) and 23-month-old (senescent) rat liver DNA. We measured the sensitivity of the technique by damaging calf thymus DNA with photoactivated methylene blue for 30s up to 2h. The levels of damage were linear over the entire time course including the shorter times which showed levels comparable to those expected in liver. For the liver data, 8-oxodG was reported as a fraction of 2-deoxyguanosine (2-dG). There was no change in the levels of 8-oxodG levels in the nuclear DNA from 6 to 23-months of age. However, the levels of 8-oxodG increased 2.5-fold in the mitochondrial DNA with age. At 6 months, the level of 8-oxodG in mtDNA was 5-fold higher than nuclear and increased to approximately 12-fold higher by 23 months of age. These findings agree with other reports showing an age-associated increase in levels of mtDNA damage; however, the degree to which it increases is smaller. Such damage to the mitochondrial DNA may contribute to the age-associated decline in mitochondrial function.

We have examined the substrate specificity and inhibitor sensitivity of H2O2 formation by rat heart mitochondria. Active H2O2 production requires both a high fractional reduction of Complex I (indexed by NADH/NAD+ + NADH ratio) and a high membrane potential, delta psi. These conditions are achieved with supraphysiological concentrations of succinate. With physiological concentrations of NAD-linked substrates, rates of H2O2 formation are much lower (less than 0.1% of respiratory chain electron flux) but may be stimulated by the Complex III inhibitor antimycin A, but not by myxothiazol. Addition of Mn2+ to give 10 nmol/mg of mitochondrial protein enhances H2O2 production with all substrate combinations, possibly by repleting mitochondrial superoxide dismutase with this cation. Contrary to previously published work, no increased activity of H2O2 production was found with heart mitochondria from senescent (24 month) rats, relative to young adults (6 month).

No abstract

Base excision repair is the main pathway for repair of oxidative base lesions in DNA. Mammalian cells must maintain genomic stability in their nuclear and mitochondrial genomes, which have different degrees of vulnerability to DNA damage. This study quantifies DNA glycosylase activity in mitochondria and nucleus from C57/BL 6 mouse tissues including brain, liver, heart, muscle, kidney, and testis. The activities of oxoguanine DNA glycosylase (OGG1), uracil DNA glycosylase, and endonuclease III homologue 1 (NTH1) were measured using oligonucleotide substrates with DNA lesions specific for each glycosylase. Mitochondrial content was normalized to citrate synthase activity and mitochondrial function was assessed by measuring cytochrome c oxidase (COX) activity. In nuclear and mitochondrial extracts, the highest DNA glycosylase activities were in testis. Brain and heart, tissues with the highest oxidative load, did not have higher levels of OGG1 or NTH1 activity than muscle or kidney, which are more glycolytic tissues. In general, mitochondrial extracts have lower DNA glycosylase activity than nuclear extracts. There was no correlation between glycosylase activities in the mitochondrial extracts and COX activity, suggesting that DNA repair enzymes may be regulated by a mechanism different from this mitochondrial enzyme.

1. We have examined systematically the relationship between the percentage reduction of cardiac mitochondrial NAD and the flux through oxidative phosphorylation, as measured by O2 uptake. Reduction of NAD was varied by varying the concentration of palmitoyl-L-carnitine, pyruvate, 2-oxoglutarate or glutamate in the presence of malate as the oxidizable substrate. 2. In the presence of ADP (State 3 respiration) there was a substantially linear positive relationship between O2 uptake and the percentage reduction of NAD. Coupled respiration in the absence of ADP also showed an increase with increasing NADH, with the exact shape of the relationship being variable. 3. When pyruvate and 2-oxoglutarate dehydrogenase activity were increased by increasing medium Ca2+ concentration within the range 5 nM to 1.23 microM, at non-saturating substrate concentrations, there was again a positive relationship between O2 uptake and the reduction of NAD; however, rates of O2 uptake tended to be higher at given values of NAD reduction when the incubation medium contained Ca2+. This is taken to indicate an activation by Ca2+ of the enzymes of phosphorylation or of the respiratory chain, in addition to the dehydrogenase activation. 4. When carboxyatractyloside plus ADP were used to generate 50% State 3 rates of O2 uptake with pyruvate or 2-oxoglutarate, sensitivity to Ca2+ was retained. However, when oligomycin plus 1 mM-ADP and 1 mM-ATP were used to generate 50% State 3, no such dependence was seen. 5. The results are interpreted to indicate a substantial role for substrate dehydrogenation in the overall regulation of oxidative phosphorylation when substrates are available at near-physiological concentrations.

Aging is strongly correlated with the accumulation of oxidative damage in DNA, particularly in mitochondria. Oxidative damage to both mitochondrial and nuclear DNA is repaired by the base excision repair (BER) pathway. The "mitochondrial theory of aging" suggests that aging results from declining mitochondrial function, due to high loads of damage and mutation in mitochondrial DNA (mtDNA). Restriction of caloric intake is the only intervention so far proven to slow the aging rate. However, the molecular mechanisms underlying such effects are still unclear. We used caloric-restricted (CR) mice to investigate whether lifespan extension is associated with changes in mitochondrial BER activities. Mice were divided into two groups, receiving 100% (PF) or 60% (CR) of normal caloric intake, a regime that extends mean lifespan by approximately 40% in CR mice. Mitochondria isolated from CR mice had slightly higher uracil (UDG) and oxoguanine DNA glycosylase (OGG1) activities but marginally lower abasic endonuclease and polymerase gamma gap-filling activities, although these differences were tissue-specific. Uracil-initiated BER synthesis incorporation activities were significantly lower in brain and kidney from CR mice but marginally enhanced in liver. However, nuclear repair synthesis activities were increased by CR, indicating differential regulation of BER in the two compartments. The results indicate that a general up-regulation of mitochondrial BER does not occur in CR.