*1.3.2 DNA damage*

*Free Radical Medicine and Biology*

may explain why increased spermatic cytoplasm could be linked to infertility [4]. In addition to leucocytes, infection in semen has also been implicated as a source of ROS. Exposure to heavy metals (e.g., cadmium, lead, iron and copper), pesticides, phthalate and pollution can lead to spermatozoa damage by excessive ROS [5]. Smoking has also been associated with decreased spermatic function. But industrial exposure not only induces oxidative stress but also disrupts the hypothalamic– pituitary-gonadal axis to inhibit the release of GnRH, LH and FSH in human and

Only the balance of ROS and antioxidants can keep the optimal spermatozoa function. Low level of ROS has been shown to be essential for fertilization, acrosome reaction, hyperactivation, motility and capacitation [8, 9]. ROS induces cyclic adenosine monophosphate (cAMP) in spermatozoa that inhibits tyrosine phosphatase, leading to tyrosine phosphorylation [10]. In particular, capacitation not only requires ROS, but also it can be inhibited by catalase (CAT) [11]. It has been described that high level of ROS can promote the acrosome reaction with the

Lipids are present in spermatozoa plasma membrane in the form of polyunsaturated fatty acids (PUFA), most susceptible to oxidative damage [13, 14]. Once there is generation of lipid peroxide radical, it will react with the neighboring lipid molecule, triggering a chain reaction that can lead to >50% oxidation of the spermatozoa plasma membrane [15]. Byproducts of lipid oxidization include mutagenic and genotoxic molecules malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE), leading indirectly to DNA damage [16]. Buffalo spermatozoa are more prone to oxidative damage than that of cattle, since it is rich in polyunsaturated fatty acids

mechanism of ROS-modulated tyrosine phosphorylation [12].

like arachidonic acids and docosahexaenoic acids [17].

**74**

animal [6, 7].

**Figure 1.**

**1.3 Pathological effects on spermatozoa**

*Factors contributing to oxidative stress-induced male infertility.*

*1.3.1 Lipid peroxidation of plasma membrane*

Free radicals have the capability to directly damage spermatozoa DNA via single- and double-strand DNA breaks, cross-links and chromosomal rearrangements [18, 19]. ROS also can cause various types of gene mutations such as point mutations and polymorphism, resulting in decreased semen quality [20]. Other mechanisms such as denaturation and DNA base-pair oxidation also may be involved. Although most of the spermatozoa genome (85%) is bound to central nucleoprotamines that protect it from free radical attack [21], infertile men often have deficient protamination, which may make their sperm DNA more vulnerable to ROS damage [22]. A common byproduct of DNA oxidation, 8-hydroxy-2-deoxyguanosine (8-OH-2- deoxyguianosine), has been considered a key biomarker of this oxidative DNA damage [23].
