What is intracellular methemoglobin?
Methemoglobin (MetHb) is a dysfunctional form of hemoglobin that is incapable of transporting oxygen, thus reducing blood oxygenation and potentially inducing tissue hypoxemia.
What causes methemoglobinemia?
Methemoglobinemia can be due to certain medications, chemicals, or food or it can be inherited from a person’s parents. Substances involved may include benzocaine, nitrates, or dapsone. The underlying mechanism involves some of the iron in hemoglobin being converted from the ferrous [Fe2+] to the ferric [Fe3+] form.
When the iron in the hemoglobin molecule is in the ferric Fe3+) state hemoglobin is termed?
Methemoglobin forms when hemoglobin is oxidized to contain iron in the ferric [Fe3+] rather than the normal ferrous [Fe2+] state. Any of the four iron species within a hemoglobin molecule that are in the ferric form are unable to bind oxygen.
Why is SpO2 low in methemoglobinemia?
Low SpO2 readings occur because pulse oximeters utilize light absorption at 660 and 940 nm to calculate the ratio of oxy-hemoglobin to deoxy-hemoglobin in blood. Methemoglobin absorbs light at both of those wavelengths, thus the presence of these additional hemoglobin species makes SpO2 calculation inaccurate.
What is the difference between Hematin and methemoglobin?
MetHb differs from Hb only in that the iron moiety of the heme groups is in the ferric rather than the ferrous state. MetHb forms in vivo at low levels normally and at much higher levels in the presence of oxidative compounds (Bodansky, 1951). MetHb is unable to bind oxygen and must be reduced to Hb to be functional.
Why does oxygen not bind to Fe3+?
This property is used for oxygen transport by hemoglobin. But iron is buried in a pocket guarded by histidine residues. This prevents oxidation of ferrous by oxygen but the affinity helps carrying oxygen. In ferric form, affinity is lost and cannot carry oxygen.
How does methylene blue help with methemoglobinemia?
Methylene blue is an effective antidote for methemoglobinemia due to its own oxidizing properties. It oxidizes NADPH, forming the reduced product leukometh- ylene blue. Leukomethylene blue in turn acts as a reducing agent converting me- themoglobin to hemoglobin and thus restoring oxygen carrying capacity.
What prevents Fe2+ oxidizing to Fe3+?
The distal histidine, which is not bound to the heme, helps prevent oxidation of Fe2+ to Fe3+. Oxygen does not bind to Fe3+.
Does ferritin bind Fe2+ or Fe3+?
The core of ferritin isolated from tissues contains Fe3+, but Fe2+ is required for experimental core formation in protein coats; reduction of Fe3+ to Fe2+ facilitates iron removal from protein coats.