Non-enzymatic antioxidative systems consists of co-enzyme Q10 (radical scavenger), dietary vitamins such as vitamins A (P-carotene; singlet oxygen quencher), C (ascorbate; diverse antioxidant functions) and E (a-tocopherol; chain-breaking compound) or other substances such as urate (radical scavenger) or aliphatic polyamines such as spermidine and spermine. Vitamin E is the most efficient compound in the lipid phase [ 149] . All these substances have in common that they detoxify free radicals  . Hence, it is not surprising to find the most important natural antioxidants such as vitamin C and E [151, 152], uric acid , glutathione  in highest concentrations in the seminal fluid. Reportedly, seminal vitamin C and E concentrations are lower in infertile men than in normal patients [155, 156]. In addition, patients with lower seminal ascorbate concentrations showed a higher, though not significant, percentage of sperm DNA fragmentation .
Moreover, other substances such as the aliphatic polyamines spermidine and sper-mine are present in seminal plasma and act directly as a free radical scavenger to inhibit LPO . Spermine, for instance, is present at high concentrations of about 0.6 mg/mL suggesting an important influence in the protection of spermatozoa against free radicals. On the other hand, there is also evidence that the ROS scavenging capacity of polyamines is only of marginal relevance because the rate constants for their reaction are relatively low . The antioxidative functions of polyamines are thought to be mainly due to their metal-chelating properties. In contrast, Allen and Roberts  showed significant cytotoxic effects of seminal plasma and spermine on lymphocyte cultures. Thus, the physiological function of spermine is still unclear.
The ROS scavenging activity of the seminal fluid not only does protect the male germ cell against oxidative damage but also regulates sperm functions, namely, capacitation. In this regard, it has been shown by de Lamirande et al.  that semenogelin, the main protein in the human seminal coagulum that is deriving from the seminal vesicle, or one of its degradation products, is scavenging superoxide at concentrations much lower than those of semen. This is an important observation, since superoxide triggers sperm hyperactivation and capacitation . Apparently, semenogelin and the element zinc, which is derived in large concentrations from the prostate, are important modulators of sperm capacitation by fine tuning the sperm cells' own generation of superoxide and nitric oxide .
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