Ovothiol A (N1-methyl-4-mercaptohistidine) is a highly reducing antioxidant which accumulates to very high levels in the eggs of certain marine invertebrates, including sea urchins, scallops and starfish, where it acts to quench the hydrogen peroxide released during fertilization.
This thiol-type antioxidant is also found in some human pathogens such as Trypanosoma cruzi and Leishmania donovani.
It is synthesized by the addition and oxidation of cysteine to histidine by 5-histidylcysteine sulfoxide synthase, followed by methylation and further reduction.