**Abstract**

Oxidative stress occurs when the production of potentially destructive reactive oxygen species (ROS) exceeds the body's own natural antioxidant defences, resulting in cellular damage. Spermatozoa oxidative stress is intimately linked to several reproductive pathologies including the failure of spermatozoa cryopreservation and spermatozoa-egg recognition and fertilization. In this light, this review focuses on (i) the effects of oxidative stress on spermatozoa and application of antioxidants; (ii) production of ROS during cryopreservation; and (iii) oxidative stress in male infertility. This literature describes both a physiological and a pathological role of ROS in fertility. A delicate balance between ROS necessary for physiological activity and antioxidants to protect from cellular oxidative injury is essential for fertility.

**Keywords:** spermatozoa, oxidative stress, antioxidants, cryopreservation, infertility

### **1. Effects of oxidative stress on spermatozoa**

#### **1.1 What is oxidative stress?**

Oxidative stress occurs when a system has an imbalance between oxidation and reduction reactions, leading to generation of excess oxidants or molecules that accept an electron from another reactant [1]. A free radical is a molecule or element with an unpaired electron that is extremely reactive in an attempt to reach an electronically stable state. ROS are free radical derivatives of oxygen (O2) containing molecules. Some of the clinically important ROS identified include peroxyl (·ROO—) and hydroxyl (·OH—) radicals, superoxide (·O2 —) anion, and H2O2. Nitrogen compounds such as nitric oxide (NO) and peroxynitrite anion (ONOO) also appear to play a role in oxidation and reduction reactions. Common molecules that receive the unpaired electron are lipids in membranes and carbohydrates in nucleic acids [2]. This leads to potential cellular membrane and DNA damage when ROS are greater than the antioxidant-carrying capacity (**Figure 1**).

#### **1.2 Production of ROS**

The process of mitochondrial oxidative phosphorylation uses nicotinamide adenine dinucleotide (NADH) as an electron donor and O2 as an electron acceptor in the electron transport chain, coupling both reduction and oxidation reactions with the synthesis of adenosine triphosphate (ATP), and about 1–5% O2 transformed into ROS [3]. Another intrinsic source of spermatic ROS production is cytoplasmic glucose-6-phosphate dehydrogenase (G-6-PDH). This cytoplasmic source of ROS

### **Figure 1.**

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

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 animal [6, 7].

### **1.3 Pathological effects on spermatozoa**

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 mechanism of ROS-modulated tyrosine phosphorylation [12].
