Preface

The term "reactive oxygen species" (ROS) refers to a group of reactive molecules and free radicals produced by molecular oxygen. The production of ROS is a bane to all aerobic species. ROS formed as a byproduct of the mitochondrial electron transport in aerobic respiration tends to cause several harmful events. Only phagocytic cells are responsible for ROS production as part of the host cell defense systems. Recent work has demonstrated that ROS has a role in cell signaling, including apoptosis, gene expression, and the activation of cell signaling cascades. ROS are oxygen-containing molecules with high chemical activity. The existence of an electron with only a single bond gives these molecules, which can take a variety of shapes, their reactivity. In this condition, electrons possess an affinity to make stronger bonds, resulting in chemical reactions. ROS might be as simple as superoxide (O2-.) molecules or as complex as hydrogen peroxide (H2O2).

The book begins with a chapter summarizing the history of the Fenton reaction. The chapter provides insights into the Fenton reaction and highlights its importance in creating three fields of peroxide oxidation chemistry: hydroxyl radicals, ferryl-oxo-ions, and perferryl-oxoions. Singlet oxygen, a highly reactive form of molecular oxygen, is essential in environmental and biomedical processes. The detection and quantification of singlet oxygen species provide critical information to understand their involvement and mechanism in many processes. Superoxide dismutases (SODs) of intracellular pathogens are the main determinants of their survival inside the host niche. Furthermore, they also play a vital role in the severity of disease and virulence of these pathogens by protecting them from extracellular host-derived ROS activity. The chapter on SOD in psychiatric disorders provides a significant account of modulation of SOD activity in schizophrenia, bipolar disorder, depression, and Alzheimer's disease. The discussion shows the significance of SOD in preventing oxidative stress and initiation of apoptosis. The authors of the chapter on ROS in autoimmune (AI) diseases elucidate the protective roles of ROS in various disorders despite its proven role in exacerbation of AI diseases. ROS plays a complex role in AI responses, and they have been linked to the initiation, generation, and amplification of novel epitopes. The chapter discusses the important role of ROS in the pathophysiology of rheumatoid arthritis (RA) involved in the initiation of various signaling pathways.

A chapter on aging summarizes the various mechanisms underlying skin aging in terms of UV radiation and ROS as well as the role of antioxidants in impeding these processes. Furthermore, it hints at potential future explorations and challenges in the field of skin aging. In the chapter on epigenetic modifications in the aging process, the authors investigate the role of histone methylation in the process of aging and oxidative stress with an emphasis on the role of the SET-18 gene in the aging process.

Excess ROS in the brain generates oxidative stress that causes damage to the CNS, ultimately leading to neurodegeneration and incompetence. One chapter examines the possible mechanisms and inhibition to check oxidative stress and its associated symptoms. Alzheimer's disease (AD) is a major neurodegenerative disease worldwide. A chapter on AD deals with the role of metal chelation therapy as a possible treatment for the disease. This chapter focuses on how metal ion imbalance causes oxidative stress and affects AD pathology.

The role of nuclear factor erythroid 2-related factor 2 (Nrf2) as a dual-edged sword has been a majorly researched topic over the last decade. The crosstalk between Nrf2 and nuclear factor kappa B (NF-B) is central to the construction of the redox response network. The chapter on Nrf2 gives an idea about the therapeutic potential of targeting Nrf2 under multiple clinical settings. ROS typically accumulate in plants during abiotic and biotic stress conditions, resulting in oxidative damage and, eventually, programmed cell death. One chapter provides an overview of ROS regulation in plants and the key enzymes involved in abiotic stress tolerance mechanisms. The chapter on reducing agents provides conclusive evidence on oxidative stress and discusses how variations in agents' levels could cause reversal.

Particulate matter (PM) is a kind of air pollutant that consists of a mixture of solid particles and liquid droplets. Thus, an organism's ability to respond to PM inhalation necessitates anti-oxidative, anti-inflammatory, and cellular stress defenses, which can be compromised in susceptible people with chronic diseases such as diabetes and obesity. A chapter on this topic elucidates the mechanisms through which PM affects health and gives an account of research models in particle inhalation studies. The mechanisms and applicability of advanced oxidation processes for hydroxyl radicals and singlet oxygen species for water treatment are discussed. According to research, free radicals have a significant impact on aging, the damage of which can be corrected by adequate antioxidant defense and optimal antioxidant nutrient intake. This chapter emphasizes the importance of alternative antioxidants in the body for eliminating free radicals and their harmful effects. It explains the importance of alternative antioxidants (from plant sources) in the body for quenching free radicals and their harmful effects.

In recent decades, there has been a surge of interest in the role of ROS in various diseases. From basic science research to clinical trials, the biomedical community has made rapid progress toward a better understanding of ROS-metabolizing systems and their role in health and diseases. The reduced antioxidant defense may mimic the effect of increased ROS generation, and the dominant mechanism may vary depending on an individual's genetic predisposition. This book provides insights into ROS on researchbased innovations and the mechanisms of antioxidants in combating them.

I am grateful to Prof. Mahdi Abumadni and Dr. Bassam Awary for their support and encouragement throughout the project. My friend Prof. Haseeb Ahsan needs special mention, as he is always available whenever needed for proofreading and corrections. My mother and family members are the pillars behind me and without their support, this work would have been next to impossible. I am very thankful to the staff at IntechOpen, particularly Author Service Manager Ms. Romina Rovan for her timely assistance throughout this project.

> **Rizwan Ahmad** College of Medicine, Imam Abdulrahman Bin Faisal University, Dammam, Saudi Arabia
