Ovothiols, a family of redox-active mercaptohistidine compounds from marine invertebrate eggs

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Abstract
We have previously reported a novel thiol compound, 1-methyl-N.alpha., N.alpha.-dimethyl-4-mercaptohistidine, or ovothiol, present at high concentration in the eggs of the sea urchin Strongylocentrotus purpuratus [Turner, E., Klevit, R., Hopkins, P.B., and Shapiro, B.M. (1986) J. Biol. Chem. 261, 13056-13063]. Here we report two related compounds, 1-methyl-N.alpha.-methyl-4-mercaptohistidine, or ovothiol B, from the scallop Chlamys hastata, and 1-methyl-4-mercaptohistidine, or ovothiol A, from the starfish Evasterias troschelii. These two compounds, as well as the S. purpuratus compound now designated ovothiol C, were isolated from eggs or ovarian tissue by S-carboxymethylation with [3H]iodoacetic acid, ion-exchange chromatography and ion-pairing high-pressure liquid chromatography. The structures of S-(carboxymethyl)ovothiols A and B were determined by 1H NMR, and that of ovothiol A was confirmed by comparison with authentic methylhistidine samples after desulfuration with Raney nickel. In the ovary of each species, the predominant methylation form of ovothiol accounts for at least 80% of the total 4-mercaptohistidine. The ovothiol concentration of the ovary far exceeds that of the testis or somatic tissues. The ovothiol C content of unfertilized S. purpuratus eggs is 1.14 .mu.mol/106 eggs, equivalent to approximately 4.3 mM average concentration; the glutathione (GSH + GSSG) content is 0.9 .mu.mol/106 eggs. In this species, high ovothiol levels persisted for the first 2 weeks of embryonic development. Ovothiol and glutathione accound for virtually all of the trichloroacetic acid soluble-SH groups in the egg; these results are compared to several previous studies. Ovothiol A was also isolated without prior alkylation; as previously reported for ovothiol C, it readily oxidizes in air to a disulfide dimer, and the reduced form confers NAD(P)H-O2 oxidoreductase activity on ovoperoxidase, an enzyme exocytosed at fertilization. Several experimental observations make it unlikely that the ovoperoxidase/ovothiol oxidase activity accounts for the "respiratory burst" and H2O2 generation that follow fertilization in S. purpuratus; an alternate possibility is that ovothiol functions in the control of H2O2 toxicity.

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