Lysozyme in the ovary of tilapia (Oreochromis mossambicus): its purification and some biological properties.

Lysozyme was purified from the ovary of tilapia, Oreochromis mossambicus, with two steps, chitin coated-cellulose and Sephadex G-100, and its biological properties were investigated. Purified lysozyme had a molecular mass of 15kDa on SDS-PAGE under reducing condition. Analyses with antibody (a-EL) against the purified lysozyme revealed that serum and egg extract reacted with a-EL and the precipitin lines fused completely. The enzyme activities in serum and egg extract were inhibited by adding serially diluted a-EL. Therefore, egg extract and serum lysozyme was immunologically identical. Immunohistochemically, lysozyme was observed in the ooplasm of the oocytes laden with yolk but not in the follicle layers, egg envelope or immature oocytes (the peri-nucleolus stage). In addition, the enzyme activity in the large oocytes was higher than that in the small ones. These results suggest that lysozyme detected in the oocytes is derived from extra-ovarian tissue and transfers from the maternal circulation. Lysozyme activity in the serum of female tilapia increased with oocyte development, suggesting that the change in the enzyme level may be partially related to the reproductive events (especially vitellogenesis) of the female fish.