RBC membrane, cytoskeleton and physiology

Question 1

Describe the defects in sickle cell anemia

Answer 1

Sickle Cell Anemia

1. ATP is needed in Ca-ATP pump that pumps Ca2+ out of the RBC. If pump activation decreases→Increased Ca in cell→increased efflux of K+ = Gardos effect
2. Increased Ca2+ causes RBC membrane to become rigid→hemolysis of RBC in transit through small capillaries

Question 2

Describe how oxidant drugs or infection cause hemolysis of RBC in patients with deficiency of glucose –6-phosphate dehydrogenase.

Answer 2

1. G6PD catalyzes G6P→ produce NADPH
2. NADPH is needed to generate GSH, GSSG→GSH while NADPH→NADP+
3. During infection, macrophages generate H2O2, which cause cross linking of membrane proteins and lipid peroxidation. GSH destroys H2O2.
4. G6PD deficiency don’t have NADPH to convert GSSG→GSH
5. Results: hemolytic anemia due to rigidity from cross linking
6. Oxidant drugs/infection lead to formation of oxidants which cause:
   - Oxidation of Hb (Fe2+) to met-HB (Fe3+)
   - Oxidation and precipitation of denatured HB resulting in Heinz bodies, which affect permeability of ions and fragility
   - Cross linking of proteins in RBC, reducing flexibility

Question 3

Discuss the role of flipase (aminophospholipid translocase) in asymmetric distribution of phospholipids in the bilayer of RBC membrane.

Answer 3

1. During biosynthesis, phospholipids randomly distributed in lipid bilayer of reticulocytes. Flipase moves PE (phosphatidyl ethanolamine) and PS (phosphatidylserine) to internal leaflet of bilayer
2. PS in external leaflet is harmful, may initiate coagulation cascade
   - Promotes clot formation
   - Flipase requires ATP
   - In sickle cell, small amount of PS on outer leaflet→clot formation and vasoocllusion

Question 4

Discuss how genetic defect in cytoskeletal proteins (e.g. spectrin) leads to hemolytic anemia.

Answer 4

1. Cytoskeletal proteins interact with RBC integral proteins to maintain shape and deformability
2. Hereditary spherocytosis-reduced spectrin
3. Hereditary elliptocytosis-spectrin fails to form tetramer→weakened skeleton

Question 5

7. Discuss how abnormal accumulation of cholesterol in RBC membrane, due to alcoholism, leads to hemolytic anemia.

Answer 5

1. Acanthocytosis –rounded RBCs with thorny projections, increase in cholesterol and sphingomyelin. RBC take up excess cholesterol in membrane from circulating lipoproteins, due to chronic liver disease from alcoholism

Question 6

8. Discuss how single point mutation in the β-chain of hemoglobin in sickle red blood cells leads to hemolytic anemia, vaso-occlusion and disease pathophysiology.

Answer 6

1. Mutation in beta-globin chain, 6th AA gluamic acid→valine
2. With deoxygenation, polymerization occurs, forms insoluble long fibers with 14 double strands
3. Causes change in shape and decreased deformability→small capillary occlusion and adhesion to vascular endothelium→vasoocclusive crises and cell lysis

Question 7

What is the function of NADH in RBCs? What does a deficiency in NADH cytochrome b5 reductase result in and why?

Answer 7

-NADH converts metHB (fe3+) to Hb via NADH cy b5 reductase
-metHB can’t bind oxygen
-if heterozygous for enzyme defect, results in acute methemoglobinemia in response to oxidant drugs like chloroquine because
Hg–>metHb in presence of oxidants

Question 8

What is the mechanism of pyruvate kinase deficiency anemia?

Answer 8

• lower ATP production

- inability to maintain structural integrity of membrane–>hemolysis