Zeina Al-Assaf. Mustafa Khader. Nayef Karadsheh

Transcription

1 6 Zeina Al-Assaf Mustafa Khader Nayef Karadsheh 1 P a g e

2 Metabolism in mature erythrocytes: During the maturation of RBCs most of its intracellular organelles are lost such as the nucleus and the mitochondria, however some metabolic enzymes that were synthesized in the erythrocyte precursors remain in the mature RBCs; these enzymes are required for the survival of the RBC in order to carry out its functions properly and maintain its membrane stability. As RBCs age the activity of their metabolic enzymes decreases leading to impaired function and membrane integrity which are recognized by the phagocytic cells of the RE system leading to their destruction. Main functions of those metabolic enzymes: 1. Prevention and repair of damage caused by ROS. 2. Generation of energy which is required for: a. Ion transport b. Phosphorylation of membrane proteins c. Priming (Activating) glycolytic reactions (As 2 ATP molecules are required in the first half of the glycolytic pathway) RBC s metabolic pathways: 1. Glycolysis: a. Provides the RBCs with ATP and the NADH that is required for the activity of Cytochrome B5 Reductase that converts MetHb to Hb. b. Is modified by the inclusion of the Rapoport-Luberin shunt which generates 2,3-Bisphosphoglycerate (an important regulator of the Hemoglobin s binding affinity to oxygen). This 2,3-BPG, is found in only trace amounts in most cells, however, is found at high concentrations in RBCs (4.5 mm) and serves to increase O2 delivery. 2 P a g e

3 2. Pentose phosphate pathway (Hexose monophosphate shunt): 5-10% of the glucose in the RBCs in the form of glucose-6- phosphate is diverted into this pathway that provides a major portion of NADPH which functions as a biochemical reductant that can keep G-SH reduced. Reduced glutathione (G-SH), the most important/efficient antioxidant present in the RBCs that can detoxify H2O2. This reaction forms oxidized glutathione (G-S-S-G) by the enzyme glutathione peroxidase. The cell regenerates G-SH in a reaction catalyzed by the enzyme Glutathione Reductase using NADPH. RBCs are totally dependent on the PPP for NADPH supply because unlike other cells, RBCs don t have an alternative source such as NADP+-dependent malate dehydrogenase (malic enzyme). Therefore, a deficiency in any of the PPP enzymes especially G6PD will make the RBCs more prone to oxidative damage and hemolysis. Genetic deficiencies in RBCs metabolic enzymes: 1. PPP enzyme deficiency: G6PD deficiency 2. Glycolytic enzyme deficiency: Pyruvate kinase (accounts for the majority of inherited defects in glycolytic enzymes). 3. Glucose 6 phosphate dehydrogenase deficiency: a hereditary disease characterized by Hemolytic Anemia caused by the inability of RBCs to detoxify ROS. It is the most common disease producing enzyme abnormality in humans, affecting million individuals worldwide having the highest prevalence in Middle East, tropical Africa, Asia and parts of the Mediterranean (Greece,South Italy and Spain). 3 P a g e

4 4 P a g e G6PD deficiency is X-linked deficiency which means that males are more prone than females G6PD deficiency provides resistance to Plasmodium Falciparum this is due to the short life span of RBCs as well as the high amount of ROS. Primaquine (an anti-malarial drug) acts by increasing ROS and thus killing the parasite. If a patient with G6PD deficiency and has malaria was given Primaquine as a treatment severe hemolysis will occur. G6PD has around 150 variants, the most common ones are: o Wild type B o Mediterranean B - (class II): There is a point mutation that changed 563 Serine to Phenylalanine, which doesn t change its migration in gel electrophoresis, but it has less than 10% of the residual activity. This variant is the most common in Jordan (60-70%). o African variant A - (Class III): Consists of two pointmutations 376 Asparagine to Aspartate and 202 Valine to Methionine. This type has 10-20% of the residual activity. o African variant A: A point mutation in only 376 Asparagine to Aspartate, this variant migrates ahead of the wild type towards the anode due to the negative charge of Aspartate. It has 80% of the residual activity. o Class I: A very severe variant that causes chronic hemolysis which has less than 2% of the residual activity

5 Precipitating factors in G6PD deficiency Some patients develop Hemolytic Anemia if they were exposed to oxidative stress which may be elicited by the following: 1. Oxidant drugs: (AAA) a. Antibiotics (Sulfamethoxazole and Chloramphenicol) b. Antimalarial (Primaquine) c. Antipyretics (Acetanilide but not Acetaminophen) 2. Favism: The hemolytic effect of ingesting fava beans which contain oxidants such as, Devicine and Isouramil which cause rapid decline in G-SH. It is worthy to note that not all G6PD deficiency patients suffer from Favism, but all patients with Favism have G6PD deficiency. 3. Infection: the most common precipitating factor of hemolysis in G6PD deficiency. The inflammatory response produces ROS in macrophages which can diffuse into RBCs and cause oxidative damage. 4. Neonatal Jaundice: G6PD deficient infants are more prone to develop neonatal jaundice. Molecular biology of G6PD: Most mutations that lead to a G6PD deficiency are Missense mutations and more specifically Point mutation. For example, G6PD A - and G6PD B - differ from their respective normal variants by a single amino acid. Note: Large deletions have not been identified suggesting that the complete absence of G6PD is lethal since it is a housekeeping enzyme. Classification of G6PD variants G6PD Mediterranean is a prototype of class II deficiency G6PD A - is a prototype of class III Class I is associated with Chronic Non- Spherocytic Hemolytic Anemia which occurs even in the absence of oxidative stress. 5 P a g e

6 Decline of G6PD activity with cell age: The activity of the normal enzyme declines as the RBC ages. However, this activity is sufficient to provide protection against oxidative damage and hemolysis. Nevertheless, only young G6PD A - RBCs can provide protection against oxidative damage. While the young Mediterranean variant G6PD B - RBCs cannot provide sufficient protection leading to hemolysis. That being said, if a patient presents with a hemolytic episode, a G6PD deficiency test needs to be performed. If the patient carries the G6PD A- variant, he might be misdiagnosed since young RBCs that are present in his blood after hemolysis have nearly normal activity. Therefore, this patient needs to be retested a few weeks after his previous episode. Pyruvate kinase deficiency: It is rare however it is the most common among the glycolytic enzymes (95% of glycolytic enzymes deficiency) while Phosphoglucose Isomerase is responsible for 4% Testing pyruvate kinase deficiency is difficult in the lab since we provide optimum conditions while the enzyme in physiologic conditions has limited substrates cofactors etc. Alteration in pyruvate kinase enzyme includes: 1. Abnormal response to the activator Fructose 1,6- bisphosphate 2. Abnormal K m or V max for substrates or coenzymes 3. Alteration of enzyme stability, activity, or quantity PK deficiency leads to decreased ATP production thus altering RBCs membrane and causing changes in cell shape and ultimately leading to its phagocytosis by RE system. The premature death and lysis results in Hemolytic Anemia. This causes an increase in 2,3- BPG synthesis which minimally helps in oxygen delivery. 6 P a g e