|[Frontiers in Bioscience 1, e78-86, August 1,1996]|
OXIDATIVE STRESS AND ROLE OF ANTIOXIDANTS IN NORMAL AND ABNORMAL
Department of Urology, Tulane University School of Medicine, New Orleans, Louisiana, USA
Received 05/30/96; Accepted 07/02/96; On-line 08/01/96
Mammalian spermatozoa are rich in polyunsaturated fatty acids and, thus, are very susceptible to ROS attack which results in a decreased sperm motility, presumably by a rapid loss of intracellular ATP leading to axonemal damage, decreased sperm viability, and increased midpiece morphology defects with deleterious effects on sperm capacitation and acrosome reaction (25). Lipid peroxidation of sperm membrane is considered to be the key mechanism of this ROS-induced sperm damage leading to infertility (Fig 1) (18).
Lipid peroxidation (LPO) is the most extensively studied manifestation of oxygen activation in biology. The most common types of LPO are: (a) non-enzymatic membrane LPO, and (b) enzymatic (NADPH and ADP dependent) LPO. The enzymatic reaction involves NADPH-cytochrome P-450 reductase and proceeds via an ADP-Fe3+ O2.- (perferryl) complex (26). In spermatozoa, production of malondialdehyde (MDA), an end product of LPO induced by ferrous ion promoters, has been reported (20). Formation of MDA can be assayed by the thiobarbituric acid (TBA) reaction which is a simple and useful diagnostic tool for the measurement of LPO for in vitro and in vivo systems (27).
Spermatozoa, unlike other cells, are unique in structure, function, and susceptibility to damage by LPO (18). In order to understand the biological mechanisms of LPO in infertility, three important questions need to be addressed: (a) What are the mechanisms of LPO of sperm in vivo? (b) What are the consequences of damage to sperm membrane, proteins, and nucleic acids? (c) What regulates the antioxidant defense mechanisms in seminal plasma?
In general, the most significant effect of LPO in all cells is the perturbation of membrane (cellular and organellar) structure and function (transport processes, maintenance of ion and metabolite gradients, receptor-mediated signal transduction, etc.). Low levels of NADH and glutathione, as a result of the increased activity of glutathione peroxidase to remove metabolites of LPO, will further affect cellular Ca2+ homeostasis. Minor alterations in sperm membranes in selected cases of dyspermia can be reversed by GSH therapy (28). Studies on how these cellular changes caused by LPO affect seminal parameters and sperm function and reversal of these effects are open to further investigations.
Besides membrane effects, LPO can damage DNA and proteins, either through oxidation of DNA bases (primarily guanine via lipid peroxyl or alkoxyl radicals) or through covalent binding to MDA resulting in strand breaks and cross-linking (26). ROS can also induce oxidation of critical -SH groups in proteins and DNA, which will alter structure and function of spermatozoa with an increased susceptibility to attack by macrophages (15). The oxidative damage to mitochondrial DNA is well known to occur in all aerobic cells which are rich in mitochondria and this may include spermatozoa. In addition, the redox status of human spermatozoa is likely to affect phosphorylation and ATP generation with a profound influence on its fertilizing potential (29). Aitken et al. recently showed that stimulation of endogenous NADPH-dependent ROS generation in human sperm appears to regulate acrosome reaction via tyrosine phosphorylation (30). In general, the oxidizing conditions increase tyrosine phosphorylation with enhanced sperm function while reducing conditions have the opposite effect. However, this has been debated for a long time, and it is still not clear whether sperm have a NADPH-dependent oxygenase system. Nonetheless, how these mitochondrial DNA or membrane changes regulate specific sperm functions in association with altered tyrosine phosphorylation is an interesting area for further investigation. These studies may open a new series of diagnostic tool in clinical infertility to assess sperm function and damage.