Definition
Haemoglobin is a protein found in the red blood cells, the erythrocytes. It gives the cells their red colour. Part of this protein is an iron ion.
This iron atom is present in the bivalent form, it is doubly positively charged (Fe2+). In the case of methaemoglobin, the iron ion is present in the trivalent form (Fe3+). This form of haemoglobin is unable to bind oxygen and thus perform its physiological function of supplying oxygen to the cells of the body.
The term “methaemoglobinaemia” describes the presence of methaemoglobin in the blood. This is physiological to a small extent (approx. 1.5% of the haemoglobin content in the blood). Only larger amounts of methaemoglobin in the blood can become dangerous.
Causes
Methemoglobin is formed by a chemical reaction: the oxidation of the divalent iron of hemoglobin to trivalent iron. The iron atom gives off a so-called electron, which gives it a positive charge. This chemical reaction occurs naturally in the body constantly and spontaneously.
The resulting methaemoglobin can no longer bind oxygen. Mechanisms then take place in the body to reduce the iron again, i.e. to give it a negative charge again. This can be done by proteins that can release an electron, i.e. negative charge, or by a special enzyme, methaemoglobin reductase, which catalyses a reaction in which the iron is converted back into its divalent form.
If there is a deficiency of this enzyme, methaemoglobinaemia can occur. If the blood contains 60-70% methemoglobin, this can be life-threatening due to the reduced ability of the blood to transport oxygen. There are other causes that can cause methaemoglobinaemia.
These include the absorption of substances that form methaemoglobin, e.g. nitrate, which is found, for example, in meat or cheese preservatives. Since newborns still show a reduced activity of the enzyme polyhemoglobin reductase, they are particularly at risk of high uptake of nitrate, which can also be found in drinking water, for example. Some drugs or poisoning with solvents can also lead to the formation of methaemoglobin.
Diagnosis
Above a certain level of methemoglobin in the blood, it takes on a brown colour. This can serve as a diagnostic criterion. For this purpose, a drop of blood is applied to filter paper, for example, and compared with a normal drop of blood.
Blood that is in a vein and has already released the oxygen it had bound can be confused with the blood of a methaemoglobinaemia because of its dark blue colour. The difference is that deoxygenated blood (blood that has already released the oxygen) turns bright red again due to the oxygen in the air. Blood that contains too much methaemoglobin is not able to do this and retains its brownish colour.
A further diagnostic possibility is the microscopic examination of the blood. In methaemoglobinaemia, so-called Heinz inner corpuscles are found. This is a special morphology of the red blood cells, which is present when the haemoglobin has lost its physiological form.