Ovothiol A (N1-methyl-4-mercaptohistidine) is a highly reducing antioxidant mercaptohistidine, which accumulates to very high levels in the eggs of certain marine invertebrates, including sea urchins, scallops and starfish,[1] where it acts to scavenge hydrogen peroxide released during fertilization.

[2] This thiol is also found in some human pathogens including trypanosomes and members of the genus Leishmania.

[3] It is synthesized by the addition and oxidation of cysteine to histidine by 5-histidylcysteine sulfoxide synthase, followed by methylation and further reduction.