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Ovoperoxidase, the enzyme implicated in hardening the extracellular coat of the fertilized sea urchin egg, is inserted into the assembling uncrosslinked (soft) fertilization membrane via specific interactions with a protein, proteoliaisin (P. Weidman, E. Kay, and B. M. Shapiro (1985). J. Cell. Biol. 100, 938-946), and the vitelline scaffold. Dityrosine crosslinks introduced by ovoperoxidase have been postulated to harden the assembled structure from such indirect data as the discovery of dityrosine in hard fertilization membranes (Foerder and B. M. Shapiro (1977). Proc. Natl. Acad. Sci. USA 74, 4214-4128; H. G. Hall (1978). Cell 15, 343-355). In this report, we show directly that soft fertilization membranes (SFM) contain no dityrosine residues but acquire these crosslinks in vitro only during hardening. In vitro hardening alters the susceptibility of the fertilization membrane to disruption in cation-depleted solutions and in detergent; the kinetics of these phenomena are all similar to those of hardening in vivo. Ovoperoxidase substrates were identified as a class of high-molecular-weight proteins of SFM by polyacrylamide gel electrophoresis after in vitro hardening or after an ovoperoxidase-catalyzed radioiodination reaction. The specificity of ovoperoxidase for particular substrates decreased once it was no longer associated with these polypeptides within the SFM. Moreover, after disruption of the SFM, ovoperoxidase had an increased capacity to iodinate an exogenous protein, myoglobin. These data suggest that assembly of ovoperoxidase into a specific locus within the soft fertilization membrane provides a regulatory mechanism to guarantee the crosslinking of only certain appropriately juxtaposed tyrosyl residues in the assembled structure.

The ovoperoxidase from the egg of the sea urchin, Strongylocentrotus purpuratus, has been purified to apparent homogeneity. Ovoperoxidase is secreted from the egg at fertilization and is responsible, in vivo, for hardening of the fertilization membrane by forming cross-links between protein tyrosyl residues. Purification was accomplished by activation of cortical granule exocytosis with acetic acid, followed by NH4SO4 precipitation, DEAE-Sephacel chromatography in the absence of divalent cations, and CM-Sephadex chromatography. The purified enzyme is a glycoprotein of Mr 70,000, based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The enzyme exhibits a UV-visible spectrum typical of heme peroxidases (epsilon 412 = 1.19 X 10(5) M-1 cm-1). Ovoperoxidase catalyzes the oxidation of tyrosine, guaiacol, iodide, and bromide, but not chloride, and can employ either H2O2 or, with 8% relative efficiency, ethyl peroxide as an oxidative substrate. Phenylhydrazine, 3-amino-1,2,4-triazole, azide, and sulfite all inhibit purified ovoperoxidase at concentrations similar to those that inhibit hardening in vivo. Inhibition by 3-amino-1,2,4-triazole is reversible, requires H2O2, and is slow relative to substrate turnover. The purified enzyme is sensitive to protease cleavage in the native state, yielding an active product of Mr approximately 50,000 which varies slightly depending upon the protease employed. Ovoperoxidase should provide a useful tool for the study of fertilization membrane formation as a paradigm of macromolecular assembly and modification.