Methemoglobinemia is the clinical state in which circulating hemoglobin is present with iron in the oxidized Fe3+ state instead of the usual Fe2+ state.
Fe3+ cannot carry oxygen. Fe 3+ also moves the oxyhemoglobin dissociation curve to the left, thus what little oxygen is present on other hemoglobin molecules has a harder time being released into the tissues. The lack of oxygen is obvious from the chocolate brown appearance (shown below) of methemoglobineminic blood.
In health, a small amount of oxidized Fe3+ iron is formed by auto-oxidation of the circulating red blood cells. Fe2+ is oxidized to Fe3+ at the rate of about 3% per day.
This iron is reduced back to Fe2+ by the enzyme NADH-cytochrome b5 reductase and reduced glutathione.
Jalapa La Paz
Las Piedras, Puerto Rico