Oxidative Stress Apoptosis and DNA Damage 2011 Reactive Oxygen Species and Oxidative Stress

A free radical is defined as any species capable of independent existence that contains one or more unpaired electrons [1, 2]. Reactive oxygen species (ROS) include free radicals, as well as other oxygen-related reactive compounds. ROS are also formed during reduction of molecular oxygen to water in cellular respiration in the mitochondrial electron transport chain, via the cyclooxygenase pathway, as well as cellular enzymes such as cytochrome P450 oxidase and xanthine oxidase [3]. The most common ROS that have potential implication in reproductive biology include superoxide (O2-) anion, hydrogen peroxide (H2O2), peroxyl (ROO-), and hydroxyl (OH-) radicals [4, 5], ROS are produced by some physiological processes such as phagocytosis, arachidonic acid pathway, mitochondrial oxidative phosphorylation, and other processes. Moreover, ROS are produced from the spermatozoa themselves or from the leukocytes [6-8].

In the human ejaculate, ROS are mainly produced by leukocytes, while marginal amounts are produced by spermatozoa [9]. It is to be noted that leukocytes are more powerful generators of ROS than spermatozoa [10]. ROS were detected in seminal plasma of infertile men [11-13]. ROS are also regarded as essential participants in cell signaling and gene regulation [14-16]. However, low levels of ROS are necessary for spermatozoa to develop normally and be capable of fertilization [17, 18]. In addition, H2O2 and O2- were found to promote sperm capacitation, acrosome reaction, hyperactivation, and oocyte fusion [19] , High rates of ROS production that exceed the antioxidant capacity of the seminal plasma result in oxidative stress (OS) which is harmful to spermatozoa [20-22]. Oxidative stress also attacks the fluidity of the sperm plasma membrane and the integrity of DNA in the sperm nucleus [23], ROS may accelerate the process of germ cell apoptosis, leading to decline in sperm counts, DNA damage, and poor ART fertility outcomes [24]. All cellular components including lipids, proteins, nucleic acids, and sugars are potential targets of OS [25],

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