Spermatozoa as ROS Producers

In spermatozoa, like in other living cell, energy is produced to a large extent by means of oxidative phosphorylation in the mitochondria as well as through oxidation of hydrogen in the form of nicotinamide adenine dinucleotide (NADH). During the process of energy production in the electron transfer chain (ETC), elementary oxygen (O2) takes up four electrons and is thus reduced to highly reactive free radicals as intermediate products with water (H2O) as end product. Nevertheless, this process is not efficient enough to convert all the consumed oxygen into energy. As a result, 1-5% of the consumed oxygen is converted into free radicals [39] . ROS that are produced via this mechanism are regarded as cytotoxic by-products involved in the aetiology of disease and ageing [40] . and spermatozoa are no exception. Therefore, male germ cells are competent producers of ROS such as superoxide and H2O2 (reviewed in [41]).

Particularly, morphologically abnormal spermatozoa exhibiting excess residual cytoplasm with its content of glucose-6-phosphate dehydrogenase [42], which fuels ROS production [43, 44], are deemed immature. As a result, such spermatozoa have impaired fertilizing capacity [45]. Although lower in the total amount as compared to the ROS production by activated leukocytes, the male germ cells' own ROS production appears to be of clinical importance. This is due to the so-called "intrinsic" ROS production in spermatozoa, which correlates considerably stronger with different sperm parameters such as DNA fragmentation than "extrinsic" ROS production by leukocytes [34] .

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