Hemichrome

A hemichrome (FeIII) is a form of low-spin methemoglobin (metHb).Hemichromes, which precede the denaturation processes of haemoglobin (Hb), are mainly produced by partially denaturated haemoglobins and form histidine complexes.Hemichromes are usually associated with blood disorders.Hemichromes can be classified in two main categories: reversible and irreversible.Upon blood exiting the body, haemoglobin in blood transits from bright red to dark brown, which is attributed to oxidation of oxy-hemoglobin (HbO2) to methemoglobin (met-Hb) and ending up in hemichrome (HC). For forensic purposes, the fractions of HbO2, met-Hb and HC in a bloodstain can be used for age determination of bloodstains when measured with Reflectance Spectroscopy .Hemichromes form an insoluble macromolecule (macromolecular aggregate) by copolymerization with the cytoplasm of band 3. Covalent bonds reinforce the aggregate interactions of the hemichromes which are accumulated on the surface of the membrane. However, hemichromes are less stable than their native form.Hemoglobin A in humans can form hemichromes even under physiological conditions as a result of pH and temperature alterations, and the autoxidation of oxyhaemoglobin. Hemichrome formation, followed by a band 3 clustering and the formation of Heinz bodies, can take place during the physiological clearance of damaged red blood cells. The difference between a normal red blood cell (RBC) and a red blood cell with unstable haemoglobin (such as in the case of haemolytic anaemia) is that, in a normal RBC, the formation of Heinz bodies is significantly delayed. In cells with unstable haemoglobin, hemichromes are formed soon after the cell has been released into the bloodstream and they precipitate on the membrane’s surface.When haemoglobin is exposed to certain conditions, reversible or irreversible hemichromes are formed.