Boar Acrosin Digestion of the Porcine Egg Coat, Zona Pellucida, and Rearrangement of the Zona Proteins

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Abstract
Mechanically isolated, structurally intact porcine zonae pellucidae composed of three families of glycoproteins (PZP1–3) were digested with purified boar acrosin, and then the solubilized and unsolubilized fractions were separately analyzed by HPLC and/or SDS-PAGE. In isotonic solution, PZP1 was first degraded and then PZP2, whereas PZP3 was quite resistant to acrosin, reflecting the organization of these families in the zona structure. As the proteolytic hydrolysis proceeded, high-molecular-weight products appeared in the unsolubilized fraction. These products disintegrated on treatment with β-mercaptoethanol. The zona materials solubilized by acrosin were separated into seven fractions by reverse phase HPLC. The total yield of the latter was only about 5% by weight. Thus, limited sites of the porcine zona were cleaved by the homologous sperm acrosin. Since five fractions contained peptides that were more hydrophilic than the original proteins, these peptides seemed likely to be present on the outer surface of the zona structure. SDS-PAGE of the unsolubilized fraction showed that acrosin cleaved the zona at many more sites in hypotonic solution than in isotonic solution. Thus, structural relaxation of the inner region of the zona was indicated to be induced under hypotonic conditions. However, no high-molecular-weight products were formed in hypotonic solution, suggesting that the native architecture of the zona is a prerequisite for their formation.