Extracellular Antioxidants Protect Against Lipid Peroxidation

In order to prevent intracellular oxygen radical-induced damage, spermatozoa mostly rely on superoxide dismutase (SOD) and glutathione peroxidase (GPX) as the primary antioxidant enzymes, since mammalian spermatozoa have been shown to lack catalase activity. Spermatozoa are largely bereft of cytoplasm and cytoplasmic space, and therefore, these enzymes must exert their antioxidant activity while bound to the cytoskeleton of the axoneme, as it has been suggested for glycolytic enzymes [26], The role of intracellular GPX and SOD in preventing lipid peroxidation and motility loss has been previously reported [4, 27]. Inhibition of intracellular SOD results in the release of very high levels of superoxide anion to the extracellular medium thus providing support for the important role of SOD in preventing intracellular oxygen radical-induced damage. On the other hand, in order to prevent extracellular oxygen radical-induced damage, these cells rely heavily, but not exclusively, on the presence of extracellular antioxidants in reproductive tract fluids that accompany the spermatozoa on their journey through the male and into the female reproductive tracts.

The presence of such antioxidants in seminal plasma is important because it has an impact on the methods that are used to prepare spermatozoa for assisted conception purposes. Thus, if spermatozoa are washed free of seminal plasma and then pelleted in a simple defined culture medium in preparation for swim-up, then ROS-generating germ cells and leukocytes can attack normal spermatozoa in the same cell suspension in the absence of antioxidant interference. Such attack can result in extensive DNA damage [28 ] loss of motility and impairment of sperm-oocyte fusion [29]. However, if the culture medium is supplemented with antioxidants such as vitamin E, then this kind of collateral damage can be significantly reduced [29].

The biochemical composition of the antioxidants present in epididymal and seminal plasma is known to include a variety of antioxidant enzymes (a secreted form of SOD, specific isoforms of GPX, and a variety of peroxiredoxins) as well as a wide range of small molecular mass free radical scavengers including vitamin C, vitamin E, uric acid, acetylcarnitine, tryptophan, tyrosine, and taurine [30-32]. Another form of antioxidant, which is rarely discussed, is lactoferrin. The latter was identified as the sperm-coating antigen many years ago [33] and serves an extremely important role in limiting the ability of transition metals, particularly iron and copper, to access the fatty acids in the sperm plasma membrane. Limiting the local availability of free transition metals is significant as far as spermatozoa are concerned because these metals they catalyze both the first chain initiation and the propagation of lipid peroxidation chain reactions.

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