Updated: 5/1/2017

HMP Shunt

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Overview

  • Function 
    • generate NADPH
      • required for FA synthesis, steroid synthesis, reduction of oxidizing agents (H2O2 see figure)
    • provide ribose 5-phosphate
      • required for nucleotide synthesis
  • Pathway
    • occurs in cytoplasm of all cells
    • no ATP consumed or generated
    • 2 phases
      • oxidative
        • produces NADPH
        • glucose 6-phosphate (G6P) → 6-phosphogluconate
          • catalyzed by glucose 6-phosphate dehydrogenase (G6PDH)
            • rate limiting step
            • activated by NADP+, insulin
            • inhibited by NADPH
        • irreversible
      • nonoxidative
        • exchanging intermediate substrates between glycolysis and HMP shunt
          • catalyzed by transketolase
            • requires thiamine
        • reversible
  • Clinical relevance
    • glucose-6-phosphate dehydrogenase (G6PDH) deficiency   
      • pathophysiology
        • ↓ NADPH production
          • cells (specifically RBCs) lose protection against oxidizing agents
            • cannot regenerate glutathione
        • XR
        • most common human enzyme deficiency
        • ↑ prevalence among blacks
          • ↑ malarial resistance
            • by shortening the circulation life of RBCs
              • Plasmodium does not have enough time for life span
            • plasmodium does not have defense against free radicals
              • ↑ in free radicals kills parasite
      • presentation
        • episodic hemolytic anemia
          • intravascular hemolysis
          • normocytic
          • 2-3 days post precipitating stress
            • foods
              • fava beans
                • common in Mediterranean foods
                • presentation
                  • pallor, hemoglobinuria 24-48 post ingestion
            • drugs
              • sulfonamides, primaquine, antituberculosis drugs
            • infection
              • free radicals generated by the immune system
        • Heinz bodies  
          • oxidized hemoglobin that precipitates within RBCs
        • bite cells  
          • result from the phagocytic removal of Heinz bodies by macrophages 
        • back pain
      • test
        • active hemolysis screen
          • Heinz body prep 
 

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Questions (4)

(M1.BC.17.4707) A 12-year-old boy presents to the emergency department with fatigue and back pain. He has also had tea-colored urine over the last 24 hours. Upon being asked about changes to his routine over the last week, he remembers running a 5K, trying alcohol with a friend, and having a large meal at a friend's house with ingredients that he did not recognize. On physical exam he appears to be pale with conjunctival pallor. Labs are as follows:

Hemoglobin: 11 g/dL
Hematocrit: 32%
Mean Corpuscular Volume: 88 fL

Peripheral blood smear findings are shown in Figure A. An enzyme defect in which of the following pathways is likely responsible for this patient's symptoms?
Tested Concept

QID: 108445
FIGURES:
1

Amino acid synthesis

3%

(7/211)

2

Glycolysis

15%

(32/211)

3

Hemoglobin assembly

8%

(16/211)

4

Hexose mono-phosphate (HMP) shunt

58%

(123/211)

5

Porphyrin synthesis

13%

(27/211)

M 1 C

Select Answer to see Preferred Response

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(M1.BC.15.74) A 32-year-old male with HIV presents to his physician with complaints of shortness of breath and fatigue. He has a history of being non-compliant with his HAART regimen, and his CD4 count is found to be 149. The physician obtains a chest radiograph, which is shown in Figure A. Further workup demonstrates that the patient is infected with the organism demonstrated in Figure B. The physician prescribes the first-line treatment for this infection. One day later, the patient returns complaining of severe weakness and dark colored urine. On exam, the physician notes that the patient is jaundiced, and a peripheral blood smear demonstrates the findings shown in Figure C. Which of the following would most mimic this patient's condition? Tested Concept

QID: 106610
FIGURES:
1

Glutathione reductase deficiency

68%

(13/19)

2

Galactokinase deficinecy

5%

(1/19)

3

Ornithine transcarbamolyase deficiency

11%

(2/19)

4

Carnitine deficiency

5%

(1/19)

5

Cystathionine synthase deficiency

5%

(1/19)

M 1 B

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(M1.BC.13.22) A 55-year-old African American lawyer presents to your office with complaints of fatigue, back pain and dark urine that he notices in the morning when he wakes up. The patient is well known to you. He frequently travels to Bangkok where he has admitted to having unprotected sex. You have treated him in the past for chlamydia, gonorrhea and syphilis. This patient has a family history of sickle cell disease in 4 of his brothers. He has a past medical history of HIV and his most recent CD4 count was 147 cells/µL. His current medications include lisinopril, glyburide, metformin, ibuprofen, and buspirone. Appropriate prophylactic measures were taken one week ago account for his low CD4 count and his traveling. The patient's CBC demonstrates:

Hemoglobin: 9 g/dL
Hematocrit: 32%
White blood cell count: 3,600 cells/µL
Platelets: 155,000/µL

A peripheral blood smear is shown in Figure A. A chest radiograph is shown in Figure B.

Which of the following is the best explanation for this patient's symptoms?
Tested Concept

QID: 100087
FIGURES:
1

Decreased levels of CD55 and CD59

15%

(22/143)

2

Osteomyelitis of the vertebral column

10%

(15/143)

3

Defective conversion of glucose-6-phosphate to 6-phosphogluconate

59%

(85/143)

4

Side-effect of rifampin treatment

11%

(16/143)

5

Splenic insufficiency

3%

(4/143)

M 1 E

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