Reactive Species

The metabolism of oxygen by cells generates potentially deleterious and highly reactive species (RS). Due to their high reactive nature, these species can be difficult to observe. Under physiological conditions, the rate of oxidant generation is balanced by the amount of oxidant elimination. However, an imbalance between pro-oxidant and antioxidant compounds results in oxidative/reductive stress. Oxidative stress is a process characterized by the reduced capacity of a biological system to overcome an overproduction of RS and free radicals (FR). These species are involved in cell regulation in which the redox status plays an essential role in a plethora of signaling processes that act to control cell physiology. They are increasingly understood to play a key role in the regulation of vital cellular processes such as cell proliferation and differentiation, immune responses, cell motility, and programmed cell death. It is now clear that cells need to achieve a fine-tuned balance between oxidants and antioxidants in order to function properly. Disruptions in cell redox homeostasis are associated with tumor development and are co`nsidered to be a critical component in several chronic diseases. Besides, FR, ignored as biochemical entities not worth of close scrutiny, are now recognized as contributing factors in many pathological conditions. In this chapter, we start with a description of the most typical RS, which are reactive oxygen species (ROS), reactive nitrogen species (RNS), carbon reactive species, reactive halide species, reactive pseudohalide species, and reactive sulfur species (RSS). Comprehensive discussion of their fundamental chemistry, generation, and roles is provided. Besides, the chapter describes their interactions with each other, which may occur under certain conditions (e.g., pH and the flux of O2) to generate sometimes more reactive RS and FR.