Structure of Hemoglobin
• The protein globin and the iron-containing pigment heme make up a hemoglobin molecule. Approximately 65–70% of hemoglobin is produced by normoblasts, while another 30-35 percent is produced in the reticulocyte stage. An iron (Fe++) atom is found in protoporphyrin, which makes up each heme molecule. Heme production mostly takes place in normoblast mitochondria. Two pairs of dissimilar polypeptide chains joined to a heme molecule make up each globin chain. Greek letters are used to identify each polypeptide chain. Various globin chains are synthesized at the embryonic, fetal, and postnatal (adult) phases of development.
– Four different forms of globin chains—alpha, beta, gamma, and delta—are formed in adults (α, β, γ, δ). Every typical hemoglobin has α chain.
- Two α-globin chains and two b-globin chains (α2 β2) make up hemoglobin A (HbA). In mature red blood cells, it typically makes up more than 95% of the hemoglobin. During the first year of life, adult hemoglobin replaces fetal hemoglobin (hemoglobin flipping).
– Adults typically have low levels of hemoglobin A2 (HbA2), which has two α and Fetal hemoglobin (Hb F), which has two α and two γ-globin chains (α2 γ2).
It is the primary hemoglobin throughout fetal development and is typically present in adults at low concentrations.
Two δ chains (α2 δ2) in hemoglobin molecules can be used to separate them from one another using liquid chromatography or electrophoresis.
Purchased Chemically Engineered Hemoglobin Substitutes
• Methemoglobin (Hi): This type of hemoglobin has iron in its ferric form (as opposed to the ferrous form seen in normal hemoglobin) and is unable to mix with oxygen. Methemoglobinemia is the term for an increased level of methemoglobin in red blood cells. It could be bought or inherited. The primary acquired causes are chemicals and medicines.
• Sulfhemoglobin (SHb): This protein is created when certain medications and substances (such as phenacetin and sulfonamides) irreversibly oxidize hemoglobin. For the transfer of oxygen, it is useless.
• Hemoglobin exposed to carbon monoxide gas becomes carboxyhemoglobin (HbCO). Due to its ability to cause hypoxia death in those exposed to it swiftly, carbon monoxide is regarded as the "silent killer."
Errors in the Hemoglobin
Hemoglobin deficiencies can manifest as either a decreased (quantitative) or abnormal (qualitative) synthesis of normal hemoglobin. If not otherwise indicated, the term hemoglobinopathy often denotes a qualitative hereditary condition.
• Defective quality in hemoglobins (abnormal): Mutations at the α- or β-globin gene can cause aberrant hemoglobin to be produced, which has an altered amino acid composition (e.g. sickle cell anemia). The molecule's heme component is in normal condition.
The aberrant hemoglobin variant may differ from a normal hemoglobin molecule in terms of its physiologic or physical characteristics even if it may function normally.
Quantitative hemoglobin defect: Thalassemia, for example, is a type of genetic mutation in which there is a quantitative reduction in the synthesis of α-or β-globin chains and a net reduction in the creation of hemoglobin.
ANALYTIC HEMOLYSIS
Definition
Anemias caused by an increase in the rate of red cell death are known as hemolytic anemias.
• Hemolysis is the term for the premature or accelerated rate at which circulating red blood cells are destroyed. Red blood cells have a normal lifespan of about 120 days. The ensuing anemias are hemolytic anemias, which can have a prominent characteristic of a reduced red cell life span of as little as 10–15 days.
• The patient does not exhibit anemia if the compensatory marrow regeneration (via enhanced erythropoiesis) is sufficient; this condition is known as compensated hemolytic illness.
• Nevertheless, compensatory mechanisms fall short in cases of moderate to severe hemolysis, leading to the development of hemolytic anemia-like symptoms in patients.
The Process and Outcomes of Hemolysis
A. Characteristics of Enhanced Red Cell Destroying
Senescent RBCs are older RBCs that have reached the end of their 120-day natural life cycle. The reticuloendothelial—RE) cells of the spleen, liver, and bone marrow are the primary extravascular site where 80–90% of the physiological elimination of senescent RBCs occurs.
Hemolysis, the excessive breakdown of red blood cells, is the cause of hemolytic anemia. Hemoglobin breakdown products that are produced during RBC lysis accumulate as a consequence of increased hemolysis. They lead to:
Elevated blood levels of unconjugated bilirubin: jaundice.
• Dark-colored stool due to an increase in stercobilinogen.
• Higher levels of urobilinogen in the urine, which results in darker urine.
• The bone marrow accumulates more iron as a result of heme being released.
• Typical signs of intravascular hemolysis-related anemia
- Anemia
- Lower serum levels of haptoglobin: Intravascular hemolysis is characterized by it.
Hemosiderinuria – Hemoglobinuria
Elevation of plasma lactate dehydrogenase (LDH)
Location of RBC destruction
Red cell destruction can occur intravascularly or extravascularly.
Destroying extravascular red blood cells
• It is the primary process via which healthy red blood cells are destroyed. Additionally, in several hemolytic anemias, it is the primary pathogenetic mechanism of hemolysis.
• The reticuloendothelial (RE) system's mononuclear phagocytes are responsible for the premature death of red blood cells. Extravascular hemolysis, sometimes known as macrophage-mediated hemolysis, is the process by which the contents of red blood cells are broken down inside phagocytic macrophages and are not visible in the plasma. Organs such as the spleen, liver, and bone marrow experience "work" hyperplasia as a result of extravascular hemolysis in the RE system. For red cells to pass through the splenic sinusoids successfully, they must undergo drastic morphological changes. Normal red blood cells are easier to do this because of their unique cell membrane. When extravascular hemolysis occurs, red blood cells are either phagocytosed because they are perceived as "foreign" or lose some of their deformability, which makes it harder for them to pass through the splenic sinusoids.
• Hemoglobin is released by lysed or destroyed senescent red blood cells in the RE system. Heme and globin are the byproducts of hemoglobin breakdown. Globin is broken down even more into amino acids and then added back to the pool of amino acids. Protoporphyrin and iron, which is recycled, are separated from the heme portion. Protoporphyrin is transformed into bilirubin, which circulates in the blood and binds to unconjugated serum albumin.
• The liver is where bilirubin is conjugated, and the gut receives the expelled conjugated bilirubin through bile.
• Urobilinogen is the term for the group of colorless, water-soluble chemicals that are produced when gut bacteria break down conjugated bilirubin. The majority of urobilinogen oxidizes into stercobilinogen, which is then expelled as stercobilin in feces.
• The ileum and colon of the gut reabsorb around 20% of the urobilinogens, which are then recycled back into the liver and the bile. A tiny portion of this reabsorbed urobilinogen is present in the bloodstream, where it is filtered by the renal glomerulus and eliminated as both urobilinogen and urobilin in the urine.
The following are the typical results of extravascular hemolysis:
- Anemia: Anemia is the result of excessive blood cell breakdown.
- Jaundice: Jaundice is brought on by an overabundance of bilirubin.
- Splenomegaly: This condition is frequently brought on by hyperplasia of the mononuclear phagocyte system. The majority of patients benefit from splenectomy since the spleen is where most extravascular hemolysis occurs. Hemoglobinemia and hemoglobinuria are not seen in extravascular hemolysis.
Intravascular hemolysis of red blood cells
The minor mechanism of natural red cell death is called intravascular hemolysis. In certain hemolytic anemias, it may represent a significant pathogenic process. Hemolysis is the process by which red blood cell components, including hemoglobin, are released into the circulation and end up in the plasma.
Intracellular parasites (e.g., malarial parasites), complement fixation (e.g., antibody binding against red cell antigens), mechanical injury (e.g., trauma by heart valves), or exogenous toxic injury (e.g., clostridial sepsis) can all result in intravascular hemolysis.
The following are the typical results of intravascular hemolysis:
1. Hemoglobinemia: Free hemoglobin is released during intravascular hemolysis and binds to plasma hemoglobin. The mononuclear phagocyte system quickly breaks down the hemoglobin and haptoglobin complex that has formed, preventing hemoglobin from being excreted into the urine.
2. Serum haptoglobin levels have decreased, which is indicative of intravascular hemolysis. A α globulin that is typically seen in serum is called hemoglobin. It produces a complex when it binds particularly to the hemoglobin-containing globin component. The cells of the mononuclear phagocytic system eliminate this compound. Because hemoglobin binds to free hemoglobin, its levels are lowered in all hemolytic anemias. Since it can also be decreased in cirrhosis, it is not a diagnostic marker for hemolysis.
3. Hemoglobinuria: Free increased hemoglobin is found in the urine when the plasma hemoglobin content surpasses the haptoglobin binding capacity (hemoglobinuria). Because hemoglobin oxidizes to brown methemoglobin, hemoglobinuria can give urine a dark red or brown (cola/root beer) color.
4. Hemosiderinuria (see Chapter 50, page 438): Iron liberated from hemoglobin may be taken up by the renal proximal tubular cells in hemoglobinuria of chronic intravascular hemolysis. By using the Prussian blue reaction, iron can be found in the urine sediment. Iron builds up as hemosiderin within tubular cells.
5. Anemia: Anemia is the result of excessive blood cell breakdown.
6. Jaundice: Because it results from the breakdown of red blood cells, excessive bilirubin synthesis causes jaundice, which is also referred to as hemolytic or prehepatic jaundice.
As opposed to extravascular hemolysis, there is no splenomegaly.
8. Elevation of plasma lactate dehydrogenase (LDH)
