Purification and Characterization of the Primary Acrosomal Autoantigen of Guinea Pig Epididymal Spermatozoa1

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Abstract
Previous studies showed that sperm auto- and alloantigens participate in guinea pig (GP) fertilization. In an effort to determine how alloantibodies to GP sperm acrosomal contents (AC) inhibit fertilization, we identified acrosomal auto- and alloantigens using Western blots. The predominant autoantigens migrated with Mr = 25,000, Mr = 51,000, and Mr = 55,000 under nonreducing conditions. The primary (Mr = 25,000) acrosomal autoantigen, AA1, was purified to homogeneity from AC by gel filtration, cation-exchange chromatography, chromatofocusing, and a final gel filtration. We also purified AA1 from an acidic glycerol extract of spermatozoa by gel filtration, chromatofocusing, and high-performance liquid chromatography on hydroxylapatite. AA1 is a protein and shares at least one antigenic determinant with a 51,000 Mr acrosomal component. AA1 is acrosome-specific, as determined by immunoabsorption and by indirect immunofluorescence on testicular cells. By quantitative enzyme-linked immunosorbent assay, AA1 comprises 6.4% of acrosomal protein in GP spermatozoa. On the basis of its physiochemical properties and localization, we conclude that AA1 is a unique sperm autoantigen. Surprisingly, several antibody preparations, including allo- and heteroantibodies with high anti-AA1 titers, did not inhibit fertilization in vitro. Thus, the mechanism by which alloantibodies to AC inhibit GP fertilization in vitro is not by binding to AA1.