Which Is the Most Significant Cause of Aging?

It becomes clearer and clearer that aging is a result of a significant number of causes and it would seem that counteracting one or several of them should not make a significant difference. Taken at face value, this suggests, for example, that free radicals and reactive oxygen species do not play a significant role in aging and that the lifespan of organisms cannot be significantly extended. In this review, I point to the fact that the causes of aging synergize with each other and discuss the implications involved.

Reflections on the Theories of Aging, of Oxidative Stress, and of Science in General. Is It Time to Abandon the Free Radical (Oxidative Stress) Theory of Aging?

Significance: Aging and oxidative stress are complex phenomena, and their understanding is of enormous theoretical and practical significance.

Recent Advances: Numerous hypotheses and theories that attempt to explain these phenomena have been developed. These hypotheses and theories compete with each other, with each claiming to be the correct one, while significantly contradicting each other.

Free radicals: how do we stand them? Anaerobic and aerobic free radical (chain) reactions involved in the use of fluorogenic probes and in biological systems.

Biologically significant conclusions have been based on the use of fluorogenic and luminogenic probes for the detection of reactive species. The basic mechanisms of the processes involved have not been satisfactorily elucidated. In the present work, the mechanism of the enzyme and photosensitized oxidation of NAD(P)H by resorufin is analyzed and appears to involve both aerobic and anaerobic free radical chain reactions.

Superoxide dismutase mimics, other mimics, antioxidants, prooxidants, and related matters.

A significant number of low molecular weight metal complexes as well as metal-free compounds that are capable of scavenging superoxide and/or other radicals and reactive species in simple systems have been proposed to be used as potential drugs in the case of various diseases and/or as antiaging agents. Some have been used or suggested to be used as diagnostic tools for the involvement of such species in biological processes. In the present work, analysis of such claims indicates that their use as specific detectors of superoxide or of other reactive oxygen species is unsupported and might be confusing.

Free radical paradoxes.

Unlike bigger and more advanced animals, Caenorhabditis elegans does not generate NO, yet it was recently found that NO produced by chemical or biological sources exerts profound effects in that animal, leading to increased life span and thermotolerance. The biological source was Bacillus subtilis, a natural food for C. elegans.

Reactive oxygen species and the free radical theory of aging.

The traditional view in the field of free radical biology is that free radicals and reactive oxygen species (ROS) are toxic, mostly owing to direct damage of sensitive and biologically significant targets, and are thus a major cause of oxidative stress; that complex enzymatic and nonenzymatic systems act in concert to counteract this toxicity; and that a major protective role is played by the phenomenon of adaptation. Another part of the traditional view is that the process of aging is at least partly due to accumulated damage done by these harmful species. However, recent workers in this and in related fields are exploring the view that superoxide radical and reactive oxygen species exert beneficial effects.

Mechanism of the peroxidase activity of Cu, Zn superoxide dismutase.

In addition to its very efficient catalysis of the dismutation of superoxide ( O(2)(-) ) into O(2) plus H(2)O(2), Cu, Zn SOD acts less efficiently as a non-specific peroxidase. This peroxidase activity is CO(2) dependent although very slow peroxidation of some substrates occurs in the absence of CO2. The mechanism of that CO(2) dependence is explained by the generation of a strong oxidant at the copper site by two sequential reactions with H(2)O(2), followed by the oxidation of CO(2) to the carbonate radical that then diffuses into the bulk solution.

The effects of superoxide dismutase on H2O2 formation.

Numerous reports of the effects of overproduction of SODs have been explained on the basis of increased H2O2 production by the catalyzed dismutation of O2-. In this review we consider the effects of increasing [SOD] on H2O2 formation and question this explanation.

Inactivation of copper, zinc superoxide dismutase by H2O2 : mechanism of protection.

Cu,Zn SOD is known to be inactivated by HO(2)(-) and to be protected against that inactivation by a number of small molecules including formate, imidazole, and urate. This inactivation has been shown to be due to oxidation of a ligand field histidine residue by a bound oxidant formed by reaction of the active site Cu(II) with HO(2)(-). We now report that protective actions of both formate and NADH increase as the pH was raised in the range 8.

The role of CO2 in metal-catalyzed peroxidations.

Transition metals, such as Cu(+2), Mn(+2), and Co(+2), have been seen to catalyze the bicarbonate enhanced oxidation of a variety of substrates by H(2)O(2). In several of these cases it has been demonstrated that CO(2), rather than bicarbonate, is the enhancing species. Mechanisms that are in accord with the data involve a hypervalent state that may be written (MO)(+n), or (MOH)(+n+1), or (M)(+n+2).

The role of CO2 in cobalt-catalyzed peroxidations.

Augmentation, by CO(2)/HCO(3)(-), of Co(II)-catalyzed peroxidations was explored to clarify whether the rate enhancement was due to CO(2) or to HCO(3)(-). The rate of oxidation of NADH by Co(II) plus H(2)O(2), in Tris or phosphate, was markedly enhanced by CO(2)/HCO(3)(-). Phosphate was seen to inhibit the Co(II)-catalyzed peroxidation, probably due to its sequestration of the Co(II).

Cross-compartment protection by SOD1.

The absence of SOD1 in yeast has been found to result in inactivation of Lys4p. This [4Fe-4S]-containing dehydratase is in the pathway of biosynthesis of lysine, hence the oxygen-dependent lysine auxotrophy seen in this case. O(2)(-) is known to oxidize and thus destabilize the [Fe-4S] clusters of dehydratases; hence, this would make perfect sense were it not for the fact that SOD1 localizes to the cytosol and the intermembrane space of mitochondria, whereas Lys4p localizes to the mitochondrial matrix.

Carbon dioxide mediates Mn(II)-catalyzed decomposition of hydrogen peroxide and peroxidation reactions.

Mn(II) can catalyze the decomposition of H(2)O(2) and, in the presence of H(2)O(2), can catalyze the oxidation of NADH. Strikingly, these processes depend on the simultaneous presence of both CO(2) and HCO(3)(-). This explains the exponential dependence of the rates on [HCO(3)(-)], previously noted by other workers.

CO2 enhanced peroxidase activity of SOD1: the effects of pH.

At pH 7.4, CO2, rather than HCO3-, markedly enhances the oxidation of diverse substrates by SOD1 plus H2O2. Since the concentration of CO2 would fall with rising pH in HCO3- buffers, it was of interest to explore the effects of pH on the peroxidase activity of SOD1 in the presence and in the absence of HCO3-.

Bicarbonate-enhanced peroxidase activity of Cu,Zn SOD: is the distal oxidant bound or diffusible?

The Cu,Zn SOD catalyzes the bicarbonate-dependent oxidation of a wide range of substrates by H2O2. A mechanism in accord with this activity has been described. It involves the generation of a strong oxidant (Cu(I)O, Cu(II)OH, or Cu(III)) by reaction of the active site Cu with H2O2, followed by oxidation of bicarbonate to CO3-* that in turn diffuses from the active site to oxidize the various substrates in free solution.

CO2, not HCO3-, facilitates oxidations by Cu,Zn superoxide dismutase plus H2O2.

The Cu,Zn superoxide dismutase catalyzes HCO(3)(-) -dependent oxidations by H(2)O(2). This activity has been shown to depend on the creation of a bound oxidant at the Cu(II) by interactions with H(2)O(2). The bound oxidant was then thought to oxidize HCO(3)(-) to CO(3)(.

The mode of decomposition of Angeli's salt (Na2N2O3) and the effects thereon of oxygen, nitrite, superoxide dismutase, and glutathione.

The classical view of the aerobic decomposition of Angeli's salt is that it releases NO(2)(-) + NO(-)/HNO the latter then reacting with O(2) to yield ONOO(-). An alternative that has recently been proposed envisions electron transfer to O(2) followed by decomposition to NO(2)(-) + NO. The classical view is now strongly supported by the observation that the rates of decomposition of Angeli's salt under 20% O(2) or 100% O(2) were equal.

Mutant Cu,Zn superoxide dismutases and familial amyotrophic lateral sclerosis: evaluation of oxidative hypotheses.

FALS-associated missense mutations of SOD1 exhibit a toxic gain of function that leads to the death of motor neurons. The explanations for this toxicity fall into two broad categories. One involves a gain of some oxidative activity, while the second involves a gain of protein: protein interactions.

Reversal of the superoxide dismutase reaction revisited.

Reversal of the superoxide dismutase (SOD) reaction was measured in terms of the reduction of tetranitromethane (TNM) by O2-. Cu,ZnSOD caused a biphasic reduction of TNM by H2O2. The rapid initial phase was stoichiometric with the enzyme and was followed by a slower catalytic phase that was oxygen dependent and was augmented by HCO3-.