**2. Free radicals: general characteristics**

A free radical (FR) is defined as any atom, molecule or a fragment of atoms and molecules with one or more unpaired electrons, capable of short independent existence. The abstraction or gain of one electron by a nonradical molecule may (or may not) convert it to a radical species [13]. Free radicals may have a positive, negative or a neutral charge [14]:

A → minus one electron → A+●.

B → plus one electron → B−●.

It is precisely the presence of an unpaired electron that results in certain common properties shared by most radicals. Free radicals are generally unstable and highly reactive. They can either donate an electron to or accept an electron from other molecules, thus behaving as oxidants or reductants [13].

**3. Sources of ROS in semen**

**Table 1.** Overview of reactive oxygen and nitrogen species.

**Radicals Non-radicals**

Hydroxyl radical OH● Hypochlorous acid HOCl Peroxyl radical ROO● Hypobromous acid HOBr Alkoxyl radical RO● Ozone O3

Lipid peroxyl radical LOO● Lipid peroxide LOOH

Nitric oxide NO● Nitrous acid HNO2

●− Hydrogen peroxide H2

Physiological and Pathological Roles of Free Radicals in Male Reproduction

http://dx.doi.org/10.5772/intechopen.70793

● Singlet oxygen <sup>1</sup>

● Nitrosyl cation NO+

Nitroxyl anion NO<sup>−</sup> Dinitrogen tetroxide N2

Dinitrogen trioxide N2

Peroxynitrite ONOO<sup>−</sup> Peroxynitrous acid ONOOH Nitronium (nitryl) cation NO2

Nitryl chloride NO2

Alkyl peroxynitrite ROONO

O2

119

Δg

O4

O3

+

Cl

**Reactive oxygen species (ROS)**

**Reactive nitrogen species (RNS)**

Superoxide O2

Hydroperoxyl radical HO2

Nitrogen dioxide NO2

pathogens [19].

kocytospermia is present [21].

Virtually every ejaculate may contain potential sources of ROS. Leukocytes activated by multiple factors, especially inflammation and infection, are among significant ROS producers in semen [17]. Subpopulations of leukocytes, which may be found in semen, mainly consist of polymorphonuclear (PMN) leukocytes (50–60%) and macrophages (20–30%) [18]. PMN leukocytes represent an important source of ROS due to their abundant presence in semen. Furthermore, external stimuli induce the activation of macrophages, leading to an oxidative burst and ROS overgeneration. Under normal circumstances, these monocytes are of paramount importance in defending male reproductive structures against nearby cells and

The Endz test based on myeloperoxidase staining is an efficient technique to quantify seminal leukocytes during semen quality assessment [20]. According to the World Health

/mL, leu-

Organization (WHO), if the leukocyte concentration in the ejaculate exceeds 1 × 106

In cells, one-electron modification of molecules can yield sulfur-, oxygen-, carbon- and nitrogen-derived free radicals [14]. Furthermore, ions of transition metals have a radical nature [13].

The most common and important free radicals related to biological systems are oxygenderived radicals called reactive oxygen species (ROS) and nitrogen-derived molecules, defined as reactive nitrogen species (RNS) [15]. ROS represent a broad category of molecules including radical and non-radical oxygen derivatives [16]. Reactive nitrogen species are nitrogen-free radicals and commonly accepted as a subclass of ROS [13, 15]. A summary of the most common oxygen- and nitrogen-derived free radicals is provided in **Table 1**.


**Table 1.** Overview of reactive oxygen and nitrogen species.
