Antioxidative effect of melatonin on human spermatozoa.

The ability of melatonin to suppress experimentally induced lipid peroxidation (LPO) in sperm membrane was investigated in 41 samples of infertile men. Iron/ascorbate (0.04/0.2 mmol)-induced LPO was measured by the formation of malondialdehyde (MDA) using the thiobarbituric acid method. Sperm incubated in the presence of melatonin (2-6 mmol) exhibited a concentration-dependent decrease of MDA generated from hydroperoxide of the sperm plasma membrane in the presence of promoter system. Addition of 6 mmol of melatonin significantly reduced the rate of lipid peroxidation in sperm of unselected donors (mean +/- SE in control samples = 26.4 +/- 2.9 vs. 6.5 +/- 1.1 nmol MDA/10(8) sperm in melatonin-treated samples: n = 16, p < .005). Inhibitory effect of melatonin was also significant in the presence of 0.015 mmol of ferrous ions (20.5 +/- 1.7 vs. 7.9 +/- 1.6 nmol MDA/10(8) sperm in melatonin-treated samples: n = 7, p < .02) and (.005 mmol of ferrous ions (20.2 +/- 2.8 vs. 9.9 +/- 2.4 nmol MDA/ 10(8) sperm: n = 6, p < .05). Comparing the effect of melatonin with that of Trolox, an analog of vitamin E. a similar effect at concentration of 0.1-0.2 mmol of Trolox was found (25.2 +/- 2.9 vs. 11.8 +/- 1.2 nmol MDA/10(8) sperm in Trolox-treated samples: n = 7, p < .005). The obtained data of in vitro experiments show that melatonin is 40-fold less efficient than Trolox in achieving the 50% reduction in LPO (4 vs. 0.1 mmol). Since the physiological concentration of melatonin in human semen is at the nanomolar level, its antioxidative role in vivo is probably of minor importance.