Hemopexin is a glycoprotein that binds free heme to itself, thereby counteracting oxidative damage in tissues. The liver takes up the combined heme-hemopexin complex and renders it harmless. Abnormal hemopexin levels may be seen, for example, in malignant melanoma and hemolytic anemia.
What is hemopexin?
The protein hemopexin has a strong binding ability to heme, which is found in hemoglobin, enzymes, and myoglobin. Unbound heme can lead to oxidative stress, so the body needs to regulate it. Hemopexin is also known as beta-18-glycoprotein. Glycoproteins are not only composed of protein, but also have a carbohydrate component. Hemopexin also belongs to the beta-globulins, which are a subgroup of globulins. These proteins are found in blood serum and are not soluble in water. Their functions are related to the immune system, among other things. In addition, they have numerous specific functions as enzymes, biological transport molecules or regulators of blood properties, for example pH. In addition to beta-globulins, three other groups exist in the human body, which biology calls alpha-1-, alpha-2-, and gamma-globulins.
Function, effects, and roles in body and health
When hemopexin meets a free heme molecule in the blood, the two substances form a bond with each other. In blood, heme occurs as part of the red blood pigment hemoglobin, which contains iron and represents a component of red blood cells (erythrocytes). Its main function is to transport oxygen. In muscles, hemoglobin is equivalent to myoglobin, which, however, can bind oxygen much more strongly. By forming a heme-hemopexin complex, hemopexin protects the organism from damage by free heme, which can cause harmful oxidation of tissue. So-called reactive oxygen species mediate the process. These include radicals such as alkoxyl radical, hydroxyl radical and peroxyl radical, but also hydroperoxide, hypochlorite anion, ozone and hydrogen peroxide. Under controlled conditions, the human body uses such reactive oxygen species to fight parasites, bacteria and viruses. Energy conversion in mitochondria also releases small amounts of reactive oxygen species. However, especially in higher concentrations, they lead to oxidative stress, which not only affects proteins and enzymes, but can also affect cytomembranes and genes. If the oxidation is due to free heme, hemopexin can help limit the damage or preventively stop the process before major impairments occur. According to some studies, hemopexin also plays a role in inflammatory processes. However, researchers have found both increased and decreased hemopexin levels to be correlates. The exact rules followed by the underlying processes have not yet been definitively determined.
Formation, occurrence, properties, and optimal values
In its primary structure, hemopexin consists of 462 amino acids that are building blocks joined together in a long chain with the help of peptide bonds. The HPX gene, which in humans is located on the eleventh chromosome, is responsible for the synthesis of the protein. Like a blueprint, the genetic code determines the sequence of amino acids within such a chain. Ribosomes use a copy of the DNA (the messenger RNA or mRNA) to translate the genetic information into a polypeptide. After translation is complete, the manufactured amino acid chain folds and eventually assumes the spatial structure of hemopexin. Only in this three-dimensional form is the bioprotein fully functional. Hemopexin is produced in the liver, which also synthesizes most other globulins. In addition, the liver is responsible for the production of heme and absorbs the hemopexin when it has bound heme to it. This process is part of the natural blood purification of the human body. In blood serum, the level of hemopexin in healthy people ranges from 50 to 115 mg per deciliter.
Diseases and disorders
Abnormal hemopexin levels may occur in the context of various diseases. In the presence of malignant melanoma, the measured concentration may increase. Malignant melanomas are malignant tumors that grow from melanocytes. Melanocytes are skin cells that contain the pigment melanin.This substance is not only responsible for the color tone of the skin, but also absorbs UV light. Although the absorption is not complete, this mechanism is an important protection against potentially harmful radiation. UV radiation is a component of natural sunlight. Therefore, excessive sunbathing and sunburn are among the risk factors associated with the development of melanoma. Malignant melanoma is also known as black skin cancer because the disease shows up as a dark tumor of brown to black coloration in the skin. Statistically, however, the externally visible melanoma disappears in about 20% of those affected. However, this type of cancer often spreads at an early stage and leads to further ulcers in other regions. Treatment options include surgical removal of the tumor and, if necessary, radiation therapy or chemotherapy. If the malignant melanoma has already metastasized, therapy also considers these. In hemolytic anemia, the hemopexin content in the blood typically decreases, as this form of anemia is characterized by the dissolution of heme-containing red blood cells (erythrocytes). The hemopexin binds the released heme, giving it a different overall structure with altered properties than unloaded hemopexin. When analyzed, laboratory tests can therefore detect a reduced level of hemopexin in the blood serum – in some cases the protein is no longer detectable at all. Pathologic hemolysis occurs in the context of various diseases, including sickle cell and spherocytic cell anemia, rhesus incompatibility, and malaria.
