Methemoglobin (English: methaemoglobin) (pronounced "met-hemoglobin") is a form of the oxygen-carrying metalloprotein hemoglobin, in which the iron in the heme group is in the Fe3+ (ferric) state, not the Fe2+ (ferrous) of normal hemoglobin. Methemoglobin cannot bind oxygen, unlike oxyhemoglobin. It is bluish chocolate-brown in color. In human blood a trace amount of methemoglobin is normally produced spontaneously, but when present in excess the blood becomes abnormally dark bluish brown. The NADH-dependent enzyme methemoglobin reductase (diaphorase I) is responsible for converting methemoglobin back to hemoglobin.
Normally one to two percent of a person's hemoglobin is methemoglobin; a higher percentage than this can be genetic or caused by exposure to various chemicals and depending on the level can cause health problems known as methemoglobinemia. A higher level of methemoglobin will tend to cause a pulse oximeter to read closer to 85% regardless of the true level of oxygen saturation.
Common causes of elevated methemoglobin
- Reduced cellular defense mechanisms
- Various pharmaceutical compounds
- Environmental agents
- Inherited disorders
- Some family members of the Fughate family in Kentucky, due to a recessive gene, had blue skin from an excess of methemoglobin.
- In cats
Amyl nitrite is administered to treat cyanide poisoning. It works by converting hemoglobin to methemoglobin, which allows for the binding of cyanide and the formation of non-toxic cyanomethemoglobin.
Methemoglobin saturation is expressed as the percentage of hemoglobin in the methemoglobin state; That is MetHb as a proportion of Hb.
- 1-2% Normal
- Less than 10% metHb - No symptoms
- 10-20% metHb - Skin discoloration only (most notably on mucous membranes)
- 20-30% metHb - Anxiety, headache, dyspnea on exertion
- 30-50% metHb - Fatigue, confusion, dizziness, tachypnea, palpitations
- 50-70% metHb - Coma, seizures, arrhythmias, acidosis
- Greater than 70% metHb - Death
Increased levels of methemoglobin are found in blood stains. Upon exiting the body, bloodstains transit from bright red to dark brown, which is attributed to oxidation of oxy-hemoglobin (HbO2) to methemoglobin (met-Hb) and hemichrome (HC).
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- Bremmer et al PloS One 2011 http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021845