What is G6PD (Glucose-6-Phosphate Dehydrogenase)?
G6PD is the first and rate-limiting enzyme of the pentose phosphate pathway, serving as the sole source of NADPH production in red blood cells, which is essential for maintaining cellular redox balance and protecting against oxidative stress. 1
Biochemical Function
G6PD catalyzes the conversion of glucose-6-phosphate to 6-phosphogluconate, generating NADPH in the process. 2 This NADPH is critical for:
- Maintaining reduced glutathione (GSH) levels, which protects cells from oxidative damage 2, 3
- Supporting multiple cellular systems including antioxidant pathways, nitric oxide synthase, NADPH oxidase, and cytochrome p450 systems 2
- Protecting red blood cells from hemolysis by maintaining membrane integrity against oxidative stress 4
Clinical Significance of G6PD Deficiency
G6PD deficiency is one of the most common human enzymopathies, affecting approximately 400-500 million people worldwide, caused by X-linked inherited mutations. 5, 4
Common Variants and Geographic Distribution
- GdA⁻ variant: Found in 10-15% of Black men and women, causes milder, self-limited hemolysis 6, 7
- Gdmed (Mediterranean) variant: Found predominantly in men from the Mediterranean, India, and Southeast Asia, can cause life-threatening hemolysis 6, 7
- Over 200 G6PD mutations are known, with approximately half being polymorphic and common in various populations 4
Clinical Manifestations
Patients with G6PD deficiency are typically asymptomatic but develop acute hemolytic anemia when exposed to oxidant stressors. 4
Triggers for hemolysis include:
- Oxidant drugs: dapsone, primaquine, sulfonamides, and methylene blue 1, 6, 7
- Fava bean ingestion (favism) 4
- Infections 4
- Neonatal jaundice and hyperbilirubinemia without overt hemolysis 8
Critical Clinical Implications
Methylene Blue Contraindication
Methylene blue should be avoided in patients with G6PD deficiency because it can induce severe hemolysis and paradoxically worsen methemoglobinemia. 1 This occurs because:
- G6PD-deficient patients cannot produce sufficient NADPH to reduce methylene blue to its active form (leukomethylene blue) 1
- Large doses of methylene blue act as an oxidant rather than a reducing agent in G6PD deficiency 1
- Alternative treatments (ascorbic acid or exchange transfusion) should be used instead 1
Screening Recommendations
Screening for G6PD deficiency is strongly recommended upon entry into care or before starting oxidant drug therapy in patients with predisposing racial or ethnic backgrounds. 7 This includes:
- All patients before methylene blue administration (ideally), or at minimum obtaining family history in emergencies 1
- HIV-infected patients before initiating dapsone, primaquine, or sulfonamides 6, 7
- Patients in malaria-endemic areas before primaquine or tafenoquine therapy 9, 4
Essential Cellular Role
G6PD is essential for cell survival under oxidative stress conditions. 10 Studies demonstrate that:
- Complete G6PD knockout cells are extremely sensitive to hydrogen peroxide and oxidizing agents 10
- G6PD is dispensable for pentose synthesis but absolutely essential for defense against oxidative stress 10
- The enzyme maintains cellular redox homeostasis across multiple cell types, not just erythrocytes 2, 3