Special issue on “recent topics of redox chemistry and biology”

Redox reactions mediated by various reactive species play an important role in the birth, health, disease, and death of humans. Some redox reactions proceed by regulated manner, while others randomly. The underlying mechanisms and physiological and pathophysiological effects have been the subjects of extensive studies and arguments. A large number of excellent reviews have been published over the years to summarize the progress owing to the contributions of many researchers in this field but many issues still remain to be addressed. In this special issue of Free Radical Research, we aimed at providing recent topics of several issues in redox chemistry and biology, special attention being paid to include topics which have not been covered before and to avoid overlap from existing popular topics.

Coronavirus disease 2019, COVID-19, is a pandemic affecting billions of people worldwide now. Chiu and his colleagues reviewed the role of glucose-6-phosphate dehydrogenase (G6PD) against SARS-CoV-2, the pathogenic agent which caused hyperinflammation and a cytokine storm, leading to cell death by imbalanced redox status [1]. They showed that G6PD deficiency was a predisposing factor of COVID-19 and that individuals with G6PD deficiency were more susceptible to coronavirus infection than individuals with normally functioning G6PD.

Hyodo introduced their work on noninvasive visualization of free radicals derived from biological molecules using an in vivo dynamic nuclear polarization (DNP) magnetic resonance imaging (MRI) system [2]. They discussed on the free radicals of flavins, ubiquinone, vitamin E, vitamin C, and melanin. The DNP-MRI method may help to shed more light on the action and role of these radicals in vivo. Yamada and his colleagues summarized recent advances in the detection of lipid-derived radicals in vitro and in cultured cells [3]. They also discussed the possibility of visualization and structural analysis of lipid-derived radicals generated not only in cells but also in animal tissue samples using fluorescence probe that combines with piperidine-based nitroxide.

Oxidative modification of DNA bases induced by multiple oxidizing species is one of the most important reactions in redox chemistry. Chatgilialoglu focused on the mechanistic aspects for the various lesion formation and their interconnections in purine DNA damage by hydroxyl radical [4]. It was shown that, although majority of the purine DNA lesions are generated by various ROS, the formation of 5′,8-cyclopurine (cPu) lesions in vitro and in vivo relies exclusively on the hydroxyl radical attack. Methodologies generally utilized for the purine lesions quantification in biological samples are critically discussed. Carbon ion radiotherapy is more effective than conventional photon beam radiotherapy in treating cancer because of its excellent dose localization and high biological effect on tumors. Matsumoto reviewed the generation of reactive oxygen species by local ionization in water and subsequent estimation by simulations and pulse radiolysis experiments using EPR pain trap and spin probe [5].

Akagawa reviewed protein carbonylation at the amino acid side chains, a major hallmark of oxidative modification of proteins mediated by reactive oxygen species and lipid oxidation-derived aldehydes [6]. Analytical approaches for the measurements of carbonylated proteins were also reviewed. Cholesterol is oxidized by enzymes and non-enzymatic oxidants to give diverse products termed oxysterols including hydroxides, hydroperoxides, epoxides, and carbonyl compounds. Miyamoto et al. [7] reviewed the generation and measurement of reactive sterol oxidation products with emphasis on electrophilic oxysterols which modify a variety of proteins, changing their structures and eliciting a broad range of cellular effects. They also showed that secosterol aldehydes were formed as secondary products derived from cholesterol hydroperoxide decomposition.

Iron is an essential element for humans but iron promotes free radical generation and lipid peroxidation, causing oxidative damage and cell death. Shi and his colleagues reviewed rapid progress in recent years in understanding molecular mechanisms of iron homeostasis with a focus on epigenetic regulation of hepcidin, ferritin, and ferroptosis [8]. Recent topics including the interactions between methionine and iron and iron regulation in stroke were discussed. Ferroptosis is a form of regulated cell death characterized by the iron-dependent decomposition of lipid hydroperoxides, which exhibits distinct features from apoptosis and necrosis. Yin and colleagues summarized the recent progress on ferroptosis in connection with cardiovascular disease and atherosclerosis, highlighting the role of free radical mediated lipid peroxidation [9]. The evidence underlying the ferroptosis and challenges in the field was also critically discussed.

Nanomaterials defined as materials possessing, at minimum, one external dimension measuring 1-100nm have received much attention for unique properties and potential novel applications. Various types of nanoparticles are known to induce oxidative stress and may exert toxicity. Horie and Tabei reviewed potential toxicity of nanoparticles [10]. It was shown that intracellular ROS generation by nanoparticles depends on their physical and chemical properties, such as the crystalline phase, adsorption ability, and solubility and that the particle size was not a direct factor that induces oxidative stress.

Role of antioxidants against detrimental oxidation of biological molecules has received much attention. Aldini and his colleagues reviewed recent findings on the antioxidant and antiinflammation effects of carnosine, an endogenous dipeptide, and discussed underlying molecular mechanisms [11]. It was discussed also that carnosine acts indirectly by activating the Nrf2 transcription factor, which would explain many of the effects evoked by this peptide such as anti-inflammatory, antioxidant, antiglycation and anti-carbonyl effects and taken together would explain its therapeutic effect. Niki and Noguchi [12] reviewed the levels of oxidation products of vitamin E found in humans under normal and pathological conditions focusing on α-tocopherol quinone (α-TQ) and 5-nitro-γ-tocopherol (5-NγT). Both absolute concentrations of α-TQ and 5-NγT and the molar ratios to the parent tocopherols were elevated significantly in the plasma of patients with various diseases compared with healthy subjects. In general, the molar ratio 5-NγT/γ-tocopherol was higher than the ratio α-TQ/α-tocopherol. It was shown that α-TQ and 5-NγT were useful biomarker for oxidative stress and nitrative stress in humans.

Like lipoproteins, intracellular lipid droplets consist of a core formed by hydrophobic neutral lipids surrounded by a monolayer of amphiphilic phospholipids. Fedorova and her colleagues addressed the oxidizability of lipid droplets [13]. The effects of unsaturation of phospholipids, triacyclglycerols, and cholesterol esters on lipid droplet size and oxidation sensitivity were carefully examined. It was demonstrated that not only the total level of lipid unsaturation, but also the ratio between different lipid species played a significant role in determining lipid oxidation rate.

Acknowledgement

The authors thank all the authors for their excellent contributions and reviewers for critical and constructive comments. The authors hole this special issue will be interesting and inspiring to many readers.

Disclosure statement

No potential conflict of interest was reported by the author(s).