1 Department of Obstetics and Gynecology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA, USA 2 Instituto de Infertilidad Masculina, Unidad de la Mujer, USP-Hospital Santa Teresa, La Coruña, Spain
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Introduction |
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As we showed in a previous study (Ollero et al., 2000), when docosahexaenoic acid (DHA) levels in human sperm are expressed as nmol/cell, instead of as moles percentage of total fatty acid, there is an actual decrease in DHA content in mature compared with immature sperm. These results were confirmed in an in-vivo model using sperm obtained from the seminiferous tubules and epididymis of mice. Mature epididymal sperm showed a 3-fold decrease in DHA and a 6-fold decrease in total saturated fatty acid content compared with immature sperm obtained from the seminiferous tubules (when expressed as nmol/cell). However, when DHA levels were expressed as percentage of total fatty acid, an apparent increase in DHA levels was observed in mature epididymal sperm. This could be explained by the relative higher loss of saturated fatty acid (which comprises ~75% of total sperm fatty acid) compared with DHA. Therefore, the `well established' increase in DHA levels during the process of sperm maturation appears to be more the result of how the data was expressed than the actual DHA content per cell. This also explains, at least in part, why the molar content of malondialdehyde produced by immature sperm with proximal cytoplasmic retention (which is enriched in DHA compared with other fatty acids), under conditions leading to lipid peroxidation, is significantly higher than in mature sperm (Aitken et al., 1994
; Huszar and Vigue 1994; Gomez et al., 1996
). As we showed in a previous study, malondialdehyde production in human sperm originates, for the most part, from the peroxidation of DHA, which is the most abundant polyunsaturated fatty acid in human sperm (Alvarez and Storey, 1995
)
Concerning the changes in membrane fluidity, although there is a net decrease in DHA content during the process of sperm maturation, there is also a concomitant loss in total saturated fatty acid, cholesterol and desmosterol, which would make the sperm membranes less rigid. This could result in a net increase in membrane fluidity. In conclusion, during the process of sperm maturation, a critical level of DHA may be retained by sperm resulting in optimal membrane fluidity required to support sperm motility and the early steps of fertilization, and minimal risk of oxidative damage to the sperm membranes and DNA.
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Notes |
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References |
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Alvarez, J.G. and Storey, B.T. (1995) Differential incorporation of fatty acids into and peroxidative loss of fatty acids from phospholipids of human spermatozoa. Mol. Reprod. Dev., 42, 334346.[ISI][Medline]
Gomez, E., Buckinham, D.W., Brindle, J., Lanzafame, F., Irvine, S. and Aitken, R.J. (1996) Development of an image system to monitor the retention of residual cytoplasm by human spermatozoa: correlation with biochemical markers of the cytoplasm space, oxidative stress, and sperm function. J. Androl., 17, 276287.
Huszar, G. and Vigue, L. (1993) Correlation between the rate of lipid peroxidation and cellular maturity as mesaured by creatine kinase activity in human spermatozoa. J. Androl., 15, 7177
Ollero, M., Powers, D.P. and Alvarez, J.G. (2000) Variation of docosahexaenoic acid content in subsets of human spermatozoa at different stages of maturation: implications for sperm lipoperoxidative damage. Mol. Reprod. Dev., 55, 326334.[ISI][Medline]
Ollero, M., Guzman-Gil, E., Lopez, M.C., Sharma, R.K., Agarwal, A., Larson, K., Evenson, D.K., Thomas, A. Jr and Alvarez, J.G. (2001) Characterization of subsets of human spermatozoa at different stages of maturation: implications in the diagnosis and treatment of male infertility. Hum. Reprod., 16, 19121921