Free radicals: what they are and what they do

Free radicals (FRs) have gained significant attention in the field of biology and medicine due to their “double-edged sword roles” in numerous physiological processes as well as their implications in a wide range of pathologies. FRs comprising reactive oxygen species (ROS) and reactive nitrogen species (RNS), are derived from endogenous and exogenous sources. Because of their unique characteristics, FRs have the ability to adversely affect vital biomolecules such as nucleic acids, lipids, and proteins, thereby altering the normal intracellular redox state leading to oxidative stress (OS) development. FR overproduction and the resulting OS have been implicated in a number of diseases such as diabetes mellitus, neurodegenerative and cardiovascular diseases, autoimmune disorders, and various cancers. This chapter focuses on the chemistry, properties, formation, and sources of FRs, followed by a discussion on their molecular targets. The chapter also provides a brief overview of the general roles of ROS and RNS in health and disease.