| Summary: | 博士 === 國立臺灣大學 === 生化科學研究所 === 90 === A sperm-agglutinating factor was purified from carp eggs and the conditioned medium of cortical reacted eggs (CM). It was identified to be the carp ovarian cystatin. Three cystatin isoforms were found. The cystatin isolated from the CM had a higher sperm-agglutinating activity than that isolated from eggs although they have identical N-terminal amino acid sequence, mass, and positive charges. Differences in sperm-agglutinating activity between the cystatins of the CM and eggs may be caused by the different conformations because they differ in circular dichroism spectrum and tryptic map.
Cystatin is discharged from cortical granules to perivitelline space after fertilization and abundant in the perivitelline fluid (PVF) of the early stages of embryos. Cystatin rapidly agglutinates spermatozoa via electrostatic interaction. Other basic proteins also agglutinate carp spermatozoa. Their activities are inhibited by salt and high pH. Cystatin binds to the entire surface of carp spermatozoa. The PVF of early embryos agglutinate carp spermatozoa. The activity is related to the cystatin content and influenced by ionic strength and pH. Therefore, cystatin is the major sperm-agglutinating factor of PVF. Owing to the rapid action of cystatin on spermatozoa agglutination and the presence of high content of cystatin in PVF, cystatin is considered important for preventing polyspermy in carp eggs.
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