BACKGROUND: Male infertility is the major cause of conception failure in about 25% of all infertile couples. Understanding the causes of male infertility depends to a certain extent on the proteins present on the spermatozoa. The study aim was to investigate ®rst, whether there is any difference in the expression of sperm membrane proteins between fertile and infertile males; and second, whether there is any functional signi®cance of these proteins in the spermatozoa. METHODS: Six different

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... ocols were employed to extract sperm membrane proteins. A 57 kDa protein was identi®ed and puri®ed using different chromatographic techniques. The homogeneity and isoelectric point of the protein was con®rmed by 2D-electrophoresis. The protein was characterized by immuno¯uorescence, ELISA,¯ow cytometry, SDS±PAGE and Western blot analysis. The role of 57 kDa protein in sperm±oocyte binding was studied in vitro. RESULTS: All six sperm extracts of normozoospermic and infertile subjects showed 16±18 major and 12±15 minor protein bands. However, in one of the methods, the lysis buffer containing N-octyl-b-D-glycopyranoside (NOG) resulted in an additional protein band at the 57 kDa region in 95% of normozoospermic samples. The protein was either absent (~80%) or negligible (~20%) in infertile subjects. The protein was localized to the head of non-acrosome-reacted spermatozoa (NAR), and shifted to the equatorial segment in acrosome-reacted (AR) spermatozoa. The antibody directed against puri®ed 57 kDa protein inhibited binding of human sperm to zona-free hamster oocytes in a dose-dependent manner. CONCLUSIONS: The results suggest that lack and/or low expression of 57 kDa protein may be one of the reasons for infertility in men. Therefore, the protein could be used as a marker for sperm quality in men.