Free Radical Biomedicine: Principles, Clinical Correlations, and Methodologies

The term free radical refers to any chemical species capable of independent existence that contains one or more unpaired electrons. Free radicals and related reactive species are formed in biological systems, and can cause detrimental as well as beneficial biological effects. Free radical biomedicine deals with the biological effects of free radicals and related reactive species as well as antioxidants with an emphasis on the involvement of these reactive species and antioxidants in heath and disease. This chapter provides a brief historical overview of this rapidly evolving field and introduces common terms and concepts in free radical biomedicine. These include reactive oxygen and nitrogen species (ROS/RNS), antioxidants, oxidative stress, redox signaling, and free radical paradigm. The aims and layout of the book are also described in this chapter.

Reactive oxygen and nitrogen species (ROS/RNS) are collective terms that refer to a number of oxygen- or nitrogen-containing reactive species. They are produced from various cellular pathways as well as exogenous sources. ROS/RNS are able to cause damage to a variety of biomolecules, including lipids, proteins, and nucleic acids, leading to tissue injury and disease processes. Under certain circumstances, the well-controlled production of ROS/RNS also fulfils important physiological roles, including antimicrobial activity and participation in cell signaling. The biological effects of ROS/RNS, including both beneficial and detrimental effects are dependent on the types of ROS/RNS and their concentrations and duration of exposure, as well as the types of tissues/cells that the ROS/RNS are produced from or act on. The causal or contributing role of ROS/RNS in various diseases makes it necessary to devise strategies to mitigate the tissue injury and therefore protect against the disease process. Use of exogenous compounds with antioxidant properties and upregulation of endogenous cellular antioxidant enzymes by chemical inducers are among the potential approaches to the intervention of diseases with augmented ROS/RNS as underlying mechanisms.

Due to the detrimental nature of reactive oxygen and nitrogen species (ROS/RNS), a number of antioxidants have been evolved to control these reactive species in mammals, including humans. The endogenous antioxidative defenses include antioxidant enzymes/proteins and non-protein antioxidant compounds synthesized by cells. Mammalian cells and tissues also contain antioxidant compounds derived from the diet. In addition, a number of synthesized antioxidant compounds have become available. This chapter provides a survey of the various types of antioxidants encountered in free radical biomedicine. The chemical or biochemical properties, biological activities, and molecular regulation of antioxidants are discussed in the context of their implications for disease intervention.

There is substantial evidence supporting that oxidative stress plays a causal role in the pathophysiology of various cardiovascular diseases in animal models. These include hypertension, atherosclerosis, myocardial ischemia-reperfusion injury, heart failure, cardiac arrhythmias, and cardiotoxicity induced by certain drugs and environmental toxicants. A number of cellular sources of reactive oxygen and nitrogen species (ROS/RNS) are identified in cardiovascular tissues, including NAD(P)H oxidases, xanthine oxidoreductase, mitochondrial electron transport chain, uncoupled endothelial nitric oxide synthase, lipoxygenases, and cytochrome P450 system. Augmented formation of ROS/RNS from these sources results in cardiovascular injury via various mechanisms. Suppression of the augmented formation of ROS/RNS by overexpression of endogenous antioxidants or administration of exogenous antioxidants attenuates the severity of cardiovascular diseases in animal models. In contrast to animal studies, large scale clinical trials on using antioxidant vitamin supplements (mainly vitamin E) in general populations for the intervention of human cardiovascular diseases have been disappointing. Possible reasons for the negative results include the doses and forms of the vitamins used, the time-window of intervention, and the general populations included in the trials. In this context, multiple small clinical trials in selected patients with unusual oxidative stress show the benefits for antioxidant therapies in human cardiovascular diseases. Future studies should focus on the development of more effective antioxidants and testing their effectiveness in well-designed clinical trials.

Both diabetes mellitus (often simply referred to as diabetes) and metabolic syndrome are prevalent metabolic disorders that pose significant impact on public health. Development of these metabolic disorders involves the complex interactions between genes and environment, and the exact underlying pathophysiology remains to be fully elucidated. Extensive studies in animal models provide ample evidence for oxidative stress as an important pathophysiological component of both diabetes and metabolic syndrome. In line with experimental studies, observational epidemiological studies and interventional clinical trials suggest that oxidative stress may also play an important role in the pathophysiology of diabetes and metabolic syndrome in humans. The increasing recognition of the involvement of oxidative stress in diabetes and metabolic syndrome has prompted extensive research to develop antioxidant-based approaches to the preventive and therapeutic intervention of these prevalent metabolic disorders.

Due to its high rate of oxygen utilization and richness in lipids containing polyunsaturated fatty acids, the central nervous system is vulnerable to reactive oxygen and nitrogen species (ROS/RNS)- mediated injury. Indeed, increasing evidence shows oxidative stress as an important mechanism underlying various neurological disorders. This chapter summarizes both experimental and clinical data supporting oxidative stress as a critical pathophysiological component of such common neurological diseases as stroke, Parkinson’s disease, and Alzheimer’s disease. The role of oxidative stress in other neurological disorders, including amyotrophic lateral sclerosis, Huntington’s disease, traumatic brain injury, spinal cord injury, and drug/xenobiotic-induced neurotoxicity is also discussed in this chapter.

Pulmonary diseases are common clinical entities encountered by physicians and represent a major issue of public health worldwide. Understanding of the detailed pathophysiological mechanisms of pulmonary disorders is of critical importance for developing more effective therapeutic approaches to combating these diseases. This chapter discusses experimental and clinical studies showing oxidative stress as a crucial pathophysiological factor in diverse pulmonary diseases. These include chronic obstructive pulmonary disease, asthma, cystic fibrosis, acute respiratory distress syndrome, pulmonary ischemia-reperfusion injury, idiopathic pulmonary fibrosis, and drug/xenobiotic-induced pulmonary toxicity. The development of antioxidant-based modalities for the intervention of these pulmonary diseases is also covered in this chapter.

Due to their essential roles in diverse physiological processes, the liver and the gastrointestinal system are the major sites of many common diseases. Multiple pathophysiological mechanisms have been identified to underlie various hepatic and gastrointestinal disorders. This chapter discusses the oxidative stress mechanism in the disease processes of hepatic and gastrointestinal systems and introduces antioxidant-based modalities for the disease intervention. The disease entities covered in the chapter include alcoholic fatty liver disease, nonalcoholic fatty liver disease, Wilson’s disease, liver ischemia-reperfusion injury, drug/xenobiotic-induced hepatotoxcity, inflammatory bowel disease, peptic ulcer disease and Helicobacter pylori infection, and gastroesophageal reflux disease.

Kidney diseases are broadly classified into acute kidney injury and chronic kidney disease. They collectively affect approximately 10% of the adult population in the United States and worldwide, and represent a major issue of public health. As our understanding of the pathophysiology of kidney diseases becomes more advanced, various new mechanistically-based therapeutic strategies for treating these diseases have been developed. This chapter summarizes experimental and clinical evidence supporting a causative role of oxidative stress in the pathophysiology of both acute kidney injury and chronic kidney disease. It also discusses the recent development of antioxidant-based intervention of kidney disorders.

Cancer remains a leading cause of death in the human population worldwide. Both genetic and environmental factors are involved in the development of cancer. Recent work also demonstrates a critical role for reactive oxygen and nitrogen species (ROS/RNS) in multistage carcinogenesis. This chapter examines the oxidative stress mechanism of carcinogenesis as well as antioxidant-based modalities for cancer intervention. The chapter also discusses ROS as a potential tumor killing modality in cancer therapy.

Aging is a universal, complicated biological process characterized by the progressive accumulation in an organism of diverse, deleterious changes with time that increase the chance of disease and death. Although the molecular mechanisms of aging remain elusive, increasing evidence suggests a critical role for free radicals and related reactive species, especially reactive oxygen species in the process of aging. This chapter provides an overview of the evidence from both experimental models and human subjects that supports an oxidative stress mechanism of aging.

The causal involvement of reactive oxygen and nitrogen species (ROS/RNS) in the pathophysiology of common human diseases has been discussed in the preceding chapters. This chapter summaries experimental and clinical evidence that shows a causative involvement of ROS/RNS in other disease processes and related conditions. These include skin and eye disorders, diseases of reproductive system, arthritic disorders and sepsis, as well as exercise, regenerative medicine, and nanomedicine.

Detection and quantification of free radicals and related reactive species in biological systems are a critical step in understanding the pathophysiological role of these reactive species in disease processes. In this regard, a variety of methods and techniques have been developed over the past two to three decades to detect free radicals and related reactive species, including the commonly encountered reactive oxygen and nitrogen species (ROS/RNS) in mammalian cells or tissues. This chapter provides an overview of the major technical approaches to detecting biological ROS/RNS and describes some well-established ROS/RNS-detecting assays with an emphasis on their principles, advantages, and potential limitations. The commonly detected ROS/RNS include superoxide, hydrogen peroxide, hydroxyl radical, peroxynitrite, and nitric oxide. In addition, assays for detecting singlet oxygen, hypochlorous acid, and molecular oxygen are also discussed in this chapter.

Lipids, proteins, and nucleic acids are major cellular targets of reactive oxygen and nitrogen species (ROS/RNS). Oxidative damage to these cellular constituents has been increasingly recognized as a significant pathophysiological event leading to many disease processes. Thus, determination of ROS/RNSelicited damage to lipids, proteins, and nuclear acids is crucial for investigating the oxidative mechanisms of human diseases and assessing the efficacy of antioxidant-based therapies. This chapter first introduces the general experimental approaches to assessing the effects of ROS/RNS on cellular biomolecules and then describes the commonly used methods to determine ROS/RNS-mediated biological damage, with an emphasis on the assays that detect lipid peroxidation, protein oxidation, and oxidative DNA base modifications. The chapter ends with a discussion of biomarkers of oxidative stress and their value in assessing disease pathophysiology as well as the efficacy of antioxidant intervention.

The detrimental effects of reactive oxygen and nitrogen species (ROS/RNS) in mammalian systems are determined not only by the amounts and reactivity of the ROS/RNS formed, but also by the status of cellular and tissue antioxidant defenses. Thus, detection and measurement of endogenous antioxidants are an important part of investigation into the mechanism of oxidative stress in human disease development. This chapter provides an overview of the general experimental approaches to assessing antioxidant defenses and describes the basic methodologies for measuring the activities and levels of the major cellular and tissue antioxidants.