ABSTRACT
The hatching enzyme is a protease(s) that digests and solubilizes the egg envelope at the time of hatching. The presence of hatching enzymes has been confirmed in many animals, such as mammals, lower vertebrates, insects, and other invertebrates (Ishida, 1985; Yamagami, 1988; Perona and Wassarman, 1986; Sawada et al., 1990). However, their molecular properties and structures are not known due to the difficulty of their purification. As a result of recent progress in the technology for purification and in molecular biology, there is now information on the molecular characteristics of the hatching enzyme for several animals, such as fish, sea urchin, and frog. Among them, the patching enzymes of the medaka, Oryzias latipes, (Yasumasu et al., 1989a, b; 1992b; 1994) and the sea urchin, Hemicentrotus pu.lcherrimus (LePage and Gache, 1989, 1990; Ghiglione et al., 1994) were highly purified and their cDNAs and genomic DNAs were cloned. The clarification of their molecular characteristics and structures would enable us to further the studies on interesting problems of the hatching enzyme. One of these problems concerns the interesting role of this enzyme: "How does the hatching enzyme digest the egg envelope very efficiently?" It is a simple question, but its mechanism is elaborately established as a consequence of interaction between the structures of the egg envelope and the hatching enzyme. The egg envelope of medaka is a very tough and hard structure, and many kinds of proteases-except the hatching enzyme can hardly digest it efficiently. In medaka, the hatching enzyme is not a single enzyme but an enzyme system involving two proteases (Yasumasu et al., 1988; Yamagami et al., 1993), which were designated high choriolytic enzyme (HCE, or choriolysin H; EC 3.4.24.67) and low choriolytic enzyme (LCE, or choriolysin L; EC 3.4.24.66) . HCE and LCE digest the egg envelope cooperatively.
