Calpain-calpastatin: a novel, complete calcium-dependent protease system in human spermatozoa

Calpain, a calcium (Ca2+)-activated cysteine protease presents in several somatic mammalian cells, has been demonstrated to mediate specific Ca2+-dependent reactions including cell fusion. Because spermatozoa cells have an absolute Ca2+ requirement for penetration of oocytes, we have postulated that calpain would also be found in mammalian spermatozoa. Here we show that whole sperm homogenate and cell fractions prepared from ejaculated human spermatozoa contain calpain activity. Specific calpain inhibitors impaired this proteolytic activity. Unlike the enzyme described in somatic cells, sperm calpain was mostly particulate in nature and its activity was maximal at pH 9.0. Presence of sperm calpain was confirmed by immunoblot analysis using specific anti-calpain I and anti-calpain II antibodies. A 67 kDa calpain II protein and a 75 kDa calpain I protein were detected. Also spermatozoa contain the endogenous calpain inhibitor, calpastatin. We detected 158.8 +/- 24.5 (mean +/- SD) fmol calpastatin/mg sperm protein. Immunoblot analysis using specific antibodies showed a 68 kDa calpastatin protein located in the cytosolic fraction. This is the first demonstration that a complete calpain-calpastatin system exists in mammalian spermatozoa. Because calpain is a unique effector system for calcium-dependent processes, our data reveals a novel mechanism by which calcium exerts its regulatory functions in spermatozoa.