Noun
methemoglobin (countable and uncountable, plural methemoglobins)
(biochemistry) An oxidized form of hemoglobin, containing ferric rather than ferrous iron, that cannot transport oxygen.
In such cases, the enzyme methemoglobin reductase will be able to eventually reactivate methemoglobin by reducing the iron center. Source: Internet
Nitric oxide is capable of converting a small fraction of hemoglobin to methemoglobin in red blood cells. Source: Internet
This method has the advantage of avoiding the formation of methemoglobin (see below). Source: Internet