27 - RBC Metabolism Flashcards
Describe the general process of maturation and senescence (aging) of RBCs
- Erythrocytes lose nuclei before they enter the circulation
- mRNA disappears 1-2 days after release
- There is no protein synthesis and no replacement of damaged molecules
- The cytoskeleton and membranes of the RBC degenerate and lose elasticity
Describe the end of life for RBCs
- Old, inelastic RBCs are trapped in the spleen and phagocytosed by macrophages
- This is considered to be EXTRAVASCULAR hemolysis because the spleen is outside of the vasculature
- NO RBCs are naturally lysed in the vasculature
What is hereditary spherocytosis?
- A defect of the cytoskeleton of the RBC which shortens its lifespan
- The mutation is in the spectrin gene
- The mutation results in rounded, short-lived cells
What is the difference between intravascular and extravascular lysis?
- Intravascular hemolysis occurs in the vasculature
- Extravascular hemolysis occurs in the spleen
Describe intravascular hemolysis ***
- Mechanical disruption (such as injury)
- There is a release of hemoglobin from RBCs
- This causes hemoglobinuria (hemoglobin in the urine)
Describe extravascular hemolysis
- Removal of “stiff” RBCs
- Release of bilirubin from RBCs
- Possible jaundice can occur
What is the purpose of RBC metabolism?
- Keeping iron reduced (Fe2+)
- Maintaining K+/Ca++ gradients
- Keeping protein SH-groups reduced (thiol groups)
- Maintaining cell shape
How does RBC metabolism keep iron reduced?
Using NADH
It needs NADH to keep iron reduced, especially in the presence of O2
How does RBC metabolism maintain the K+/Ca++ gradient?
Using ATP
ATP is needed to maintain this gradient
How does RBC metabolism keep protein SH-groups reduced (thiol groups)?
Using NADPH
It needs NADPH to keep the SH groups (thiol groups) on the proteins reduced
How does RBC metabolism maintain cell shape?
Using ATP
It needs ATP to maintain cell shape and keep the cytoskeleton functioning
If the cell is starved from ATP it will not “bounce back” after mechanical damage
Give a summary of what the cell needs to accomplish each purpose of RBC metabolism
- Reduce iron (NADH)
- Maintain K+/Ca++ (ATP)
- Keep SH groups reduced (NADPH)
- Maintain cell shape (ATP)
What will happen if RBC metabolism fails?
- Cell will fill with Ca++
- Cell will release K+
- Cell will lose its biconcave shape (it will assume a spherical shape)
What is the consequence of this?
The spleen is unable to clear spherical RBCs from the circulation
What happens the spleen becomes enlarged (splenomegaly)?
- The spleen is removed
- You give RBCs an “extra lease on life” because they do not get filtered out as quickly
Where do RBCs get most of their metabolic energy?
Glucose
NEED to realize that RBCs do NOT rely on insulin ***
What pathways contribute to the breakdown of glucose for RBC metabolic energy?
Cytoplasmic pathways
- Glycolysis
- Petose phosphate pathway
If RBCs don’t respond to insulin, what do they respond to?
pH ***
Need to know that acidosis improves the oxygen saturation of blood
What steps in RBC glycolysis are regulated?
- Hexokinase
- Phosphofructokinase 1
These are both regulated by pH –> Both hexokinase and phosphofructokinase 1 have a high pH optimum
This means that a falling blood pH INHIBITS glycolysis ***
What does glycolysis in RBCs produce?
ATP and NADH
What is the significance of RBCs not responding to insulin?
Unlike other cells, red blood cells do not adjust their metabolism to the availability of glucose
WHY are RBCs regulated by pH?
It is a mechanism that helps to normalize pH in acidic conditions
- Under hypoxic conditions, tissues synthesize lactate and become acidic
- Under these conditions, red blood cells will stop glycolysis in order to prevent further lactic acid buildup
- When glycolysis is inhibited, red blood cells begin oxidizing lactate to obtain NADH for the defense of hemoglobin
- In effect, lactate consumption by red blood cells helps bring the pH of acidic tissues back to normal
Under normal conditions what is the outcome of glycolysis?
You get 2 molecules of ATP from 1 molecule of glucose –> This is enough for RBCs ***
What do we call glycolysis without ATP gain?
An “energy clutch”
- Instead of ATP, the RBC produces NADH
How does the prodcution of NADH occur?
- The glycolytic intermediate 1,3 bisphosphoglycerate (1,3 BPG) can be converted into 2,3 bisphosphoglycerate (2,3 BPG)
- 2,3 BPG can be reintroduced into glycolysis by dephosphorylation to 3-phosphoglycerate
- There is no ATP gain if these reactions occur and glycolysis produces only NADH
What is 2,3 BPG regulated by?
pH
- Low pH (acidic) reduces 2,3 BPG
- 2,3 BPG decreases the affinity that hemoglobin has for oxygen
- So when pH is low, 2,3 BPG is low and the affinity of oxygen to Hb is high
Overall, this means that acidosis improves the oxygen saturation in the blood - high affinity of oxygen to Hb
What process directly counteracts low 2,3 BPG inducing high oxygen affinity in low pH?
The Bohr effect
- The Bohr Effect directly decreases oxygen affinity of hemoglobin acidic environments
What should you remember about an acidic environment?
- Less glycolysis
- Less 2,3 BPG production
- Improved oxygen saturation
Describe the process of the pentose phosphate pathway in RBCs
- Red blood cells contain the enzymes for the pentose phosphate pathway
- In the red blood cell, the function of the PPP is to provide reduction equivalents in the form of NADPH***
How is the PPP regulated?
The regulation of the PPP is very simple
- A low intracellular NADPH concentration will activate glucose 6-phosphate dehydrogenase and direct glucose 6-phosphate to the PPP
- Pentose products are then reintroduced into glycolysis
What is the importance of glutathione
Due to the abundance of oxygen, the red blood cell is at risk of sustaining damage by reactive oxygen species.
High concentrations of Glutathione (GSH) protect red blood cells from reactive oxygen
What are the two types of oxidative damage to RBCs?
- ROSs
- Superoxide radical
Describe what ROSs do and how glutathione (GSH) offers protection
- ROS oxidizes protein sulfhydryl groups
- The ROS can denature structural proteins and enzymes
- GSH functions to keep sulfhydryl groups reduced and non-reactive
Describe what surperoxide radicals (O2-) do and how glutathione (GSH) offers protection
- Superoxide radical (O2-) is converted to H2O2 by superoxide dismutase
- Highly reactive and damaging
- GSH provides electrons to convert H2O2 to H2O
What can cytoskeletal problems cause in RBCs?
Sphyerocytic anemia
What can enzymatic problems in RBCs cause?
Enzymopathies
This causes nonspherocytic anemias
- G6 PD deficiency
- Pyruvate kinase deficiency
What happens in a glucose 6 phosphate dehydrogenase (G6 PD) deficiency
Glucose 6 phosphate dehydrogenase (G6 PD) deficiency causes hemolytic anemia for lack of NADPH
It is the MOST COMMON pentose phosphate pathway defect ***
Describe the genetics and patient population of G6 PD deficiency
- X-linked recessive disorder
- Prevalent in African American and Mediterranean populations due to its protection against malaria
What is a hemolytic crisis triggered by?
- Infections
- H2O2 producing drugs
- Fava beans
All of these things will produce H2O2 which will cause problems
What is the result of a hemolytic crisis due to a PPP defect such as G6 PD deficiency?
Hemolysis
- Splenomegaly
- Jaundice
Remember: In G6PD deficiency and other PPP defects, hemolysis occurs because red blood cells are lacking NADPH for antioxidant reactions ***
You will see denaturation of proteins within RBCs such as hemoglobin
Hemoglobin will start precipitating granules
What happens in a pyruvate kinase deficiency?
Pyruvate kinase deficiency causes hemolytic anemia for lack of NADH and ATP
MOST COMMON glycolysis defect but it is still very rare –> far more rare than PPP defects
Describe the genetics of glycolysis defects
- Hereditary
- Results in a non-spherocytic hemolytic anemia
Are the crises of glycolysis defects (i.e. pyruvate kinase deficiency) triggered by ROS?
NO
- Crises are NOT triggered by ROS
What will we see in pyruvate kinase deficiency?
Hemolysis
- Splenomegaly
- Jaundice
On a microscopic image you will see BLEBBING OUT of the outer membrane ***
Describe the metabolism of cancer cells
- Similar to the metabolism of RBCs
- Tumor cells get energy from glycolysis
- Tumors produce large amounts of lactate
- Tumor cells can operate under hypoxic conditions
What is an enzyme that cancer cells express that allows them to function in hypoxic conditions?
Hypoxia-inducible factor 1 alpha
(HIF-1-alpha)
This is a good target for chemotherapy ***
How are cancer cells similar to RBCs?
Very similar to RBCs –> consume glucose, produce lactate, able to live in hypoxic environment
Describe the cytoskeleton of RBCs
- The elasticity of RBCs depends on the cytoskeleton
- On the inside of the plasma membrane, a meshwork of spectrin filaments provides a scaffold that is connected to membrane glycoproteins
- If the cytoskeleton degenerates, the red blood cells become inflexible, spherical or ellipsoid in shape and are removed from the circulation in the spleen
What is the RBC cytoskeletal defect we discussed?
Hereditary spherocytosis (sess common is elliptocytosis)
Describe hereditary spherocytosis
- Autosomal dominant
- Prevalence is 1/2000
- The most common mutation is an Ankyrin mutation***
- The less common mutations are in Band 3, spectrin and protein 4.2
