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

Factors Important in the Use of Fluorescent or Luminescent Probes and Other Chemical Reagents to Measure Oxidative and Radical Stress

Numerous chemical probes have been used to measure or image oxidative, nitrosative and related stress induced by free radicals in biology and biochemistry. In many instances, the chemical pathways involved are reasonably well understood. However, the rate constants for key reactions involved are often not yet characterized, and thus it is difficult to ensure the measurements reflect the flux of oxidant/radical species and are not influenced by competing factors. Key questions frequently unanswered are whether the reagents are used under 'saturating' conditions, how specific probes are for particular radicals or oxidants and the extent of the involvement of competing reactions (e.g., with thiols, ascorbate and other antioxidants). The commonest-used probe for 'reactive oxygen species' in biology actually generates superoxide radicals in producing the measured product in aerobic systems. This review emphasizes the need to understand reaction pathways and in particular to quantify the kinetic parameters of key reactions, as well as measure the intracellular levels and localization of probes, if such reagents are to be used with confidence.

Mechanisms and consequences of protein cysteine oxidation: the role of the initial short-lived intermediates

Thiol groups in protein cysteine (Cys) residues can undergo one- and two-electron oxidation reactions leading to the formation of thiyl radicals or sulfenic acids, respectively. In this mini-review we summarize the mechanisms and kinetics of the formation of these species by biologically relevant oxidants. Most of the latter react with the deprotonated form of the thiol. Since the pKa of the thiols in protein cysteines are usually close to physiological pH, the thermodynamics and the kinetics of their oxidation in vivo are affected by the acidity of the thiol. Moreover, the protein microenvironment has pronounced effects on cysteine residue reactivity, which in the case of the oxidation mediated by hydroperoxides, is known to confer specificity to particular protein cysteines. Despite their elusive nature, both thiyl radicals and sulfenic acids are involved in the catalytic mechanism of several enzymes and in the redox regulation of protein function and/or signaling pathways. They are usually short-lived species that undergo further reactions that converge in the formation of different stable products, resulting in several post-translational modifications of the protein. Some of these can be reversed through the action of specific cellular reduction systems. Others damage the proteins irreversibly, and can make them more prone to aggregation or degradation.

The amplex red/horseradish peroxidase assay requires superoxide dismutase to measure hydrogen peroxide in the presence of NAD(P)H

A sensitive fluorescence assay based on Amplex Red (AR) oxidation by horseradish peroxidase (AR/HRP) is described which continuously monitor rates of H2O2 production by microsomal enzymes in the presence of relatively high concentrations of NADPH. NADPH and NADH are known to interact with HRP and generate significant quantities of superoxide anion, a radical that spontaneously dismutates to form H2O2 which interferes with the AR/HRP assay. Microsomal enzymes generate H2O2 as a consequence of electron transfer from NADPH to cytochrome P450 hemoproteins with subsequent oxygen activation. We found that superoxide anion formation via the interaction of NADPH with HRP was inhibited by superoxide dismutase (SOD) without affecting H2O2 generation by microsomal enzymes. Using SOD in enzyme assays, we consistently detected rates of H2O2 production using microgram quantities of microsomal proteins (2.62 ± 0.20 picomol/min/µg protein for liver microsomes from naïve female rats, 12.27 ± 1.29 for liver microsomes from dexamethasone induced male rats, and 2.17 ± 0.25 picomol/min/µg protein for human liver microsomes). This method can also be applied to quantify rates of H2O2 production by oxidases where superoxide anion generation by NADH or NADPH and HRP can interfere with enzyme assays.

Oxygen-dependence of mitochondrial ROS production as detected by Amplex Red assay

The initial rates of superoxide plus hydrogen peroxide (ROS) generation by intact or permeabilized rat heart mitochondria and coupled inside-out bovine heart submitochondrial particles (SMP) oxidizing NAD-dependent substrates, NADH, and succinate were measured by detecting resorufin formation in the Amplex Red assay at various oxygen concentrations. Linear dependences of the initial rates on oxygen concentration within the range of ~125-750 μM were found for all significant mitochondrial generators, i.e. the respiratory complexes and ammonium-stimulated dihydrolipoamide dehydrogenase. At lower oxygen concentrations upon its decrease from air saturation level to zero, the time-course of resorufin formation by SMP catalyzing coupled oxidation of succinate (the total ROS production by respiratory complexes II and III and by the reverse electron transfer (RET)-mediated by complex I) also corresponds to the linear dependence on oxygen with the same first-order rate constant determined in the initial rate studies. Prolonged incubation of SMP generating succinate-supported complex I-mediated ROS affected neither their NADH oxidase nor ROS generating activity. In contrast to SMP significant deviation from the first-order oxygen dependence in the time-course kinetics during coupled oxidation of succinate by intact mitochondria was evident. Complex I catalyzes the NADH:resorufin oxidoreductase reaction resulting in formation of colorless reduced resorufin. Hydrogen peroxide oxidizes reduced resorufin in the presence of peroxidase, thus showing its dihydroresorufin peroxidase activity. Combined NADH:resorufin reductase and dihydroresorufin peroxidase activities result in underestimation of the amount of hydrogen peroxide generated by mitochondria. We conclude that only initial rates of the mitochondrial ROS production, not the amount of resorufin accumulated, should be taken as the reliable measure of the mitochondrial ROS-generating activity, because of the cycling of the oxidized and reduced resorufin during Amplex Red assays fed by NADH and other possible reductant(s) present in mitochondria.

Antioxidant Supplements and Gastrointestinal Diseases: A Critical Appraisal

The gastrointestinal tract digests and absorbs dietary nutrients, protects the body against physical and chemical damage from contents in its lumen, provides immunity against external antigens, and keeps an optimum environment for the gut microbiota. These functions cannot be performed normally in several diseases of which the following are discussed here: irritable bowel syndrome and inflammatory bowel disease, which includes Crohn's disease and ulcerative colitis. Because these diseases are associated with oxidative stress, a host of antioxidant supplements are used for maintenance and recovery of the gut functions. However, the benefits of these supplements have not been established. The available 80 human trials were rated for levels of confidence and for benefits of the antioxidant supplements. For Crohn's disease, the supplements for which clear benefits occurred in at least 2 studies were allopurinol, Boswellia serrata (frankincense or shallaki), Artemesia species (wormwood), Tripterygium wilfordii (léi gōng téng), and omega-3 fatty acids. Similar beneficial supplements for ulcerative colitis were allopurinol, Matricaria chamomilla (chamomile), Curcuma longa (curcumin in turmeric), and omega-3 fatty acids. There was also a clear benefit for ulcerative colitis in 2 studies where a multiherbal Chinese medicine preparation and an Ayurvedic medicine preparation were used. For irritable bowel syndrome, there was only a marginal benefit of some of the antioxidant supplements. Thus, some antioxidant supplements may be beneficial at certain stages of specific diseases. This is consistent with the current concept that antioxidants act by inhibiting oxidative stress pathways in a tissue- and environment-specific manner and not by simply acting as scavengers.

Benefits of antioxidant supplements for knee osteoarthritis: rationale and reality

Arthritis causes disability due to pain and inflammation in joints. There are many forms of arthritis, one of which is osteoarthritis whose prevalence increases with age. It occurs in various joints including hip, knee and hand with knee osteoarthritis being more prevalent. There is no cure for it. The management strategies include exercise, glucosamine plus chondroitin sulfate and NSAIDs. In vitro and animal studies provide a rationale for the use of antioxidant supplements for its management. This review assesses the reality of the benefits of antioxidant supplements in the management of knee osteoarthritis. Several difficulties were encountered in examining this issue: poorly conducted studies, a lack of uniformity in disease definition and diagnosis, and muddling of conclusions from attempts to isolate the efficacious molecules. The antioxidant supplements with most evidence for benefit for pain relief and function in knee osteoarthritis were based on curcumin and avocado-soya bean unsaponifiables. Boswellia and some herbs used in Ayurvedic and Chinese medicine may also be useful. The benefits of cuisines with the appropriate antioxidants should be assessed because they may be more economical and easier to incorporate into the lifestyle.

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.

CRITICAL ISSUES

It is important to develop a maximally correct theory that may then trigger significant practical breakthroughs.

FUTURE DIRECTIONS

None of these theories is entirely correct or close enough to the truth. However, most of them contain many correct elements (CE). Finding these CE is possible by analysis of these theories. Once the CE are found, they can be merged by synthesis in a better new theory. An analysis of some of the theories of aging followed by synthesis is attempted.

How effective are antioxidant supplements in obesity and diabetes?

Obesity is a central health issue due to its epidemic prevalence and its association with type 2 diabetes and other comorbidities. Obesity is not just being overweight. It is a metabolic disorder due to the accumulation of excess dietary calories into visceral fat and the release of high concentrations of free fatty acids into various organs. It represents a state of chronic oxidative stress and low-grade inflammation whose intermediary molecules may include leptin, adiponectin and cytokines. It may progress to hyperglycemia, leading to type 2 diabetes. Whether or not dietary antioxidant supplements are useful in the management of obesity and type 2 diabetes is discussed in this review. Only the benefits for obesity and diabetes are examined here. Other health benefits of antioxidants are not considered. There are difficulties in comparing studies in this field because they differ in the time frame, participants' ethnicity, administration of antioxidant supplements, and even in how obesity was measured. However, the literature presents reasonable evidence for marginal benefits of supplementation with zinc, lipoic acid, carnitine, cinnamon, green tea, and possibly vitamin C plus E, although the evidence is much weaker for omega-3 polyunsaturated fatty acids, coenzyme Q10, green coffee, resveratrol, or lycopene. Overall, antioxidant supplements are not a panacea to compensate for a fast-food and video-game way of living, but antioxidant-rich foods are recommended as part of the lifestyle. Such antioxidant foods are commonly available.