Lecture 45

Question 1

RBC function

Answer 1

transport of oxygen from lungs to tissues

pg 1176

Question 2

RBC structure

Answer 2

• lack nucleus and membrane-bound cellular organelles
• shaped as a biconcave disc to maximize the cell surface for gas exchange
• extremely flexible to pass through narrow capillaries

pg 1176

Question 3

RBC life

Answer 3

• short life span -> average of 120 days
• fast turnover -> 10¹² RBC produced daily

pg 1176

Question 4

RBC metabolism overview

Answer 4

• no mitochdondria, no ER, etc -> no fatty acids used for energy
• glycolysis: synthesis of ATP (2 per 1 glucose), production of 2,3 BPG (bisphosphoglycerate), and reduction of Fe³⁺ to Fe²⁺ (2+ can reversibly bind to O₂
• pentose phosphate pathway (PPP) or hexose monophosphate shunt (HMP shunt): role of NADPH in RBC

pg 1177

Question 5

anaerobic glycolysis

Answer 5

• for 1 glucose: 2 ATP net energy yield, 2 NADH, 2 pyruvate
• NAD+ is regenerated through lactate production (to keep glycolysis going by converting pyruvate to lactic acid)
• lactate is sent to the liver (part of Cori cycle) for production of glucose via gluconeogenesis

pg 1179

Question 6

GLUT 1

Answer 6

• distribution: human erthyrocyte, blood-brain barrier, blood-retinal barrier, blood-placental barrier, blood-testis barrier
• expressed in cell types with barrier functions
• high-affinity glucose transport system
• RBCs rely exclusively on glucose for energy and require this transporter

pg 1180

Question 7

pyruvate kinase deficiency

Answer 7

• RBCs have their own type of pyruvate kinase for use in glycolysis
• results in hemolytic anemia (nonspherocytic)
• symptoms: fatigue, unusually pale skin, SOB, jaundice, increased risk of gallstones

pg 1182

Question 8

pyruvate kinase deficiency vs G6PD deficiency

Answer 8

• G6PD deficiency most common cause of hemolytic anemia -> pyruvate kinase deficiency is 2nd most common
• distinguish: pyruvate kinase deficiency has lack of Heinz bodies (precipitated hemoglobin)

pg 1182

Question 9

2,3-bisphosphoglycerate (2,3-BPG)

Answer 9

• formed from 1,3-BPG via a mutase
• allosteric regulator of O₂ binding to Hb
• most abundant organophosphate in RBC
• rapidly degraded in blood stored for transfusion
• increased in adapting to high altitudes

pg 1183

Question 10

hemoproteins

Answer 10

• in hemoglobin (transport) and myoglobin (storage in muscle), iron must be in Fe²⁺ form to reversibly bind oxygen
• cytochrome can be in either form and has function in ETC
• cytochrome P450 can be in either form and has function in hydroxylation
• catalase can be in either form and functions in degradation of H₂O₂

pg 1185

Question 11

heme iron oxidation

Answer 11

• hemoglobin (Fe²⁺) oxidized by drugs and endogenous oxidants to methemoglobin (Fe³⁺)
• metHb unable to bind O₂ and leads to methemoglobinemia
• metHb reduced to Hb by NADH-cytochrome b5 reductase

pg 1186

Question 12

methemoglobinemia

Answer 12

• “chocolate cyanosis” -> blue coloration of the skin and mucous membranes, brown-colored blood as a result of the dark-colored metHb
• babies have half the capacity to reduce metHb

pg 1186

Question 13

methemoglobinemia symptoms and treatment

Answer 13

Symptoms

• related to degree of tissue hypoxia
• anxiety, headache, and dyspnea
• rarely -> coma and death when metHb is more than 70%

Treatments

• methylene blue (a reducing agent) -> reduces metHb back to Hb and shows results within minutes

pg 1186

Question 14

acquired methemoglobinemia

Answer 14

• oxidative stress
• certain drugs and/or their metabolites causing inability to maintain iron in its Fe²⁺ state

pg 1187

Question 15

congenital methemoglobinemia

Answer 15

• deficiency of NADH-cytochrome b5 reductase
• mutations in the α- or β- globin chain producing abnormal HbM resistant to the reductase (rare)

pg 1187

Question 16

deficiency of NADH-cytochrome b5 reductase

Answer 16

• autosomal recessive disorder
• close relative mating
• makes skin very blue, but no other clinical presentation
• “Kentucky Blue People”

pg 1188

Question 17

HMP shunt

Answer 17

hexose monophosphate shunt

pg 1190

Question 18

HMP shunt oxidative reactions

Answer 18

• 3 irreversible steps -> produces NADPH
• rate-limiting committed step: glucose 6-phosphate dehydrogenase (G6PD)
• inhibitors: NADPH (competitively)
• activators: insulin stimulates G6PD expression (signal of high nutrient state)

pg 1190

Question 19

HMP shunt nonoxidative reactions

Answer 19

• results in ribose 5-phosphate
• reversible steps
• interconvert sugars with 3 to 7 carbons
• enzymes: transaldolase and transketolase

pg 1191

Question 20

transketolase

Answer 20

• requires TPP (from thiamine, vit. B1)
• important in diagnosis of thiamine deficiency
• done by measurement of its activity in RBCs

pg 1191

Question 21

HMP shunt direction

Answer 21

• determined based on the cellular need
• oxidative and non-oxidative can occur independently

pg 1193

Question 22

need for NADPH only

Answer 22

• oxidative: NADPH
• nonoxidative: convert ribulose 5-P to glucose 6-P to produce more NADPH

pg 1193

Question 23

need for NADPH + ribose 5-P

Answer 23

• oxidative: NADPH and ribulose 5-P
• isomerase converts ribulose 5-P to ribose 5-P

pg 1193

Question 24

need for ribose 5-P only

Answer 24

• only nonoxidative reactions
• high NADPH inhibits G6PD
• transketolase and transaldolase convert fructose 6-P and glyceraldehyde 3-P to ribose 5-P

pg 1193

Question 25

need for NADPH and pyruvate

Answer 25

• oxidative: NADPH and ribulose 5-P
• nonoxidative: convert ribulose 5-P to fructose 6-P and glyceraldehyde 3-P, glycolysis converts intermediates to pyruvate

pg 1193

Question 26

NADPH biological roles

Answer 26

• electron donor for synthesis of: FAs, cholesterol, steroids
• elecron donor for neutralization of ROS: hydrogen peroxide, superoxide, hydroxyl radical
• reducing equivalents for cyt P450 monooxygenase system: biosynthesis of steroids, detoxification of xenobiotics and drugs
• play role in phagocytosis -> destruction of pathogens by macrophages and neutrophils
• substrate for synthesis of NO

pg 1195

Question 27

NADPH role in RBC

Answer 27

44:20 from lecture
* pentose phosphate pathway is the only source of NADPH in RBC

pg 1196

Question 28

G6PD deficiency

Answer 28

• glucose 6-phosphate dehydrogenase
• episodic hemolytic anemia induced by oxidative stress
• RBC contain Heinz bodies (precipitated hemoglobin)
• one of most common single gene disorders
• X-linked (males predominantly affected)

pg 1197

Question 29

G6PD variants

Answer 29

• Class I: very severe (< 10% of residual enzyme activity)
• Class II: severe (acute hemolytic anemia)
• Class III: moderate (10%-60% residual enzyme activity)
• Class IV: no clinical symptoms (>60% residual enzyme activity)

pg 1198

Question 30

G6PD genetics

Answer 30

• high degree of population specific polymorphism
• > 400 putatively distinct G6PD variants identified
• > 200 mutations -> most point missense
• mutations result in altered enzyme kinetics -> E stability (majority of mutations), E active site, E allosteric sites

pg 1198

Question 31

G6PD precipitating factors

Answer 31

• infections
• foods containing fava beans substances: divicine and isouramil, suspected to bind and decrease GSH levels in RBC
• certain drugs (ex: sulfonamides, nitrofurans, aspirin, etc)

pg 1199