The importance of nickel enzymes, where nickel serves as an essential cofactor, in Archaea, bacteria, plants, and primitive eukaryotes, is well documented.~1 Despite the fact that no enzyme utilizing Ni has been found in mammalian species, the impact of Ni biochemistry on human health is also significant.~2 Indeed, nickel is known to cause cancer by an epigenetic mechanism,~3 which appears to involve substitution of Ni(II) for Fe(II) in nonheme iron dioxygenases that are involved in DNA and histone demethylation.4 Furthermore, exposure to nickel compounds can also elicit an immune reaction: nickel contact dermatitis is one of the most common allergies in the modern world,~5 and the molecular basis for the immune reaction is now beginning to emerge.~6 Exposure to high levels of nickel has also been shown to impair the normal homeostasis of essential metal ions.~7 Nonetheless, nickel is among the metals included in a group of "possibly essential elements" for animals and humans.~8
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