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A protein of about 150 kDa affecting sperm kinetic motility and viability was purified from rabbit seminal plasma. The incubation of rabbit sperm with this purified seminal plasma protein caused significant changes in sperm viability and motility. Moreover, the seminal protein showed a noticeable reactivating effect on immotile spermatozoa. A 10-mg amount of purified protein, added to immotile rabbit spermatozoa suspended in Tris-citrate, pH 7.4, resulted in a 48% reactivation. It is known that circulating insulin-like growth factors are bound to specific high-affinity binding proteins and form complexes with relative molecular masses of about 150 kDa. Western blotting analyses proved the existence of insulin-like growth factor in the protein purified from rabbit seminal plasma and immunofluorescence staining showed the existence of IGF-1 receptor in rabbit spermatozoa. Therefore, we suggest that the purified rabbit seminal plasma protein may represent the protein complex delivering IGF to the sperm cells thus affecting their physiological functions.

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