Reactive Oxygen Species: A Double-Edged Sword in Reproduction

Abstract

Antioxidants are compounds that protect cells against the damaging effects of reactive oxygen species (ROS) such as singlet oxygen, superoxide, peroxyl radicals, hydroxyl radicals and peroxynitrite. Oxidative damage occurs to cells in vivo and in vitro from exposure to free radicals generated by exogenous agents (e.g., radiation, chemicals, hyperoxia) and endogenous processes such as normal cellular metabolism. An imbalance between antioxidants and ROS results in oxidative stress, leading to cellular damage. Oxidative stress (OS) has been linked to cancer, ageing, atherosclerosis, ischemic injury, inflammation, and neurodegenerative diseases. Under extreme oxidative conditions, or if the antioxidant protective mechanisms of cells are compromised, cellular injury and death may occur. Early mammalian embryos are susceptible to damage from reactive oxygen species, and they increase the production of oxygen free radicals when cultured in vitro. ROS generation results from mitochondrias oxidative phosphorylation. The electrons will leak from the inner mitochondrial membranes, being transferred by the oxygen molecule, resulting in an unpaired electron in the orbit. This leaking results in the generation of the superoxide molecules. ROS can also be generated by the cytoplasmic NADPH-oxidase, cytochrome p450 enzymes and the xanthine oxidoreductase enzymes. The excess of OS can have negative effects on the cellular environment and can result in impaired cellular growth in the embryo or apoptosis resulting in embryo fragmentation. Similarly to what happens in females, oxidative energy production in males is inevitably associated with the generation of ROS excessive concentrations of which can lead to cellular pathology. It has been established that ROS can function as signaling molecules and evidence is emerging that sperm may generate low and controlled concentrations of ROS, specifically O2-H2O2, as well as other species such as nitric oxide (NO), which, in his turn, act to mediate the processes of capacitation, hyperactivation and acrosome reaction crucial to the acquisition of fertilizing ability. In the present review the most important antioxidants and their mechanisms of action related to animal reproduction, are discussed.