Haptoglobin is a protein circulating in plasma, which has the task of binding the hemoglobin circulating in the serum and conveying it to the monocyte-macrophage system for its degradation.
L'haptoglobin it is a glycoprotein (or mucoprotein) synthesized by the liver, and belongs to that class of proteins called alpha2-globulins.
It binds stably to the molecule ofhemoglobin, which is a large molecule contained in red blood cells (erythrocytes) and partly circulating in the blood, which has the task of carrying the oxygen introduced into the body with respiration up to organs and tissues. The hemoglobin-haptoglobin complex is purified from the blood in a few minutes by the monocyte-macrophage system, ie the Reticular Endothelial System (SRE), allowing the reuse of the substances that made it up, and in particular of iron (Fe). Therefore, people with significant intravascular and extravascular hemolysis (i.e. rupture of red blood cells with release of hemoglobin in or out of blood vessels) will have haptoglobin consumption, with reduced or no haptoglobin on blood tests, while haptoglobin levels do not decrease. in the presence of rhabdomyolysis (breakdown of muscle fibers), even severe, or in the case of myoglobinuria (loss in the urine of myoglobin, a protein present in the muscles).
The synthesis of haptoglobin decreases in patients with hepatic insufficiency and increases in inflammatory processes, as all hepatic activity will be directed towards the production of inflammatory proteins at the expense of normally produced proteins.
The link between hemoglobin and haptoglobin has similarities with that between antigen: antibody. The type 1-1 haptoglobin molecule has two heavy and two light chains linked together by disulfide bonds. There is an allele (i.e. a variant of a gene) which, following a gene duplication, encodes a double light chain protein which, due to a free sulfidyl (i.e. sulphate-containing) group, can in turn bind an additional chain light until it leads to the formation of haptoglobin aggregates (polymers) of different sizes.
A subject homozygous (i.e. with both genes encoding the altered protein) for this light chain variant is said to be type 2-2. A subject heterozygous for the two types of light chain is called type 1-2; these people may have various types of haptoglobin polymers with different, often higher molecular weights than those present in type 2-2 individuals. The haptoglobin variants (1-1, 1-2 and 2-2) are easily distinguished by high resolution electrophoresis. In the general population, the frequency of alleles for the two types of light chain is sufficiently high to make it possible to use the haptoglobin phenotype for paternity tests.
Normal levels of haptoglobin
Normal haptoglobin values are between 50-300 mg / dl in the blood. The normal ability to bind hemoglobin (Hb) is between 38 and 230 mg / l.
Increased levels of haptoglobin
A'high haptoglobin can be found in various conditions. We have listed the main causes of elevated haptoglobin below:
- Metastasizing neoplasms
- Acute and chronic inflammations
- Tissue necrosis (such as in myocardial infarction)
- Nephrotic syndrome
- Lymphomas and other neoplasms of the hematopoietic system
- Chronic inflammatory bowel diseases
Decreased levels of haptoglobin
A'low haptoglobin can be found in some conditions that are characterized by its own consumption. Here are the main causes of reduced haptoglobin:
- Liver failure (due to decreased synthesis)
- State of inflammation (inflammation: as already mentioned, the liver produces inflammatory proteins instead of normal proteins)
- Intravascular hemolysis
- Estrogen, oral contraceptives
- After blood transfusion (blood transfusion)