The only reason your red blood cells haven't oxidized into rust.
Every RBC in your body is protected by one enzyme. One. No mitochondria, no TCA cycle, no other backup plan. Just G6PD, a handful of NADPH molecules, and a protein called glutathione standing between your blood cells and complete oxidative collapse. What happens when it's gone? You'll know it the hard way after a bowl of fava beans.
The oxidative phase is irreversible. The non-oxidative phase is reversible. clinical medicine love to flip this. The clue is NADPH production: anything that makes NADPH is going forward and can't come back.
Red blood cells have no mitochondria, so they cannot run the TCA cycle or oxidative phosphorylation. Their only source of NADPH is the pentose phosphate pathway. And NADPH's most critical job in RBCs is keeping glutathione in its reduced form.
The cycle: Oxidative stress generates hydrogen peroxide (H2O2). Glutathione peroxidaseUses reduced glutathione (GSH) as the electron donor to neutralize H2O2 into water. Found in all cells but especially critical in RBCs. converts H2O2 to water using 2 GSH → GSSG. Then glutathione reductaseUses NADPH to reduce oxidized glutathione (GSSG) back to 2 GSH. This is the step that depends on the PPP. uses NADPH to regenerate 2 GSH from GSSG. Cycle complete.
Without G6PD: No NADPH → no glutathione regeneration → H2O2 oxidizes hemoglobin → hemoglobin denatures and precipitates into Heinz bodies → macrophages in the spleen "bite" them out, creating bite cells → the structurally compromised RBC cannot survive intact.
Important: G6PD deficiency often drives both extravascular and intravascular hemolysis at the same time, not one clean label.
Extravascular (the sequence above): splenic macrophages target Heinz-body damage and create bite cells; those weakened cells are then trapped and destroyed in the spleen.
Intravascular: severe oxidative stress from unchecked H2O2 also injures the RBC lipid bilayer directly, so red cells can rupture inside the circulation. That intravascular component is what produces free hemoglobin and the classic hemoglobinuria (dark/tea-colored urine).
Both fatty acid synthase and HMG-CoA reductase (the cholesterol synthesis rate-limiting enzyme) require NADPH as the electron donor for their reductive reactions. Liver and adrenal cortex cells run the PPP heavily for this reason.
Board cross-link: Statins block HMG-CoA reductase (cholesterol synthesis). Fatty acid synthesis happens in the cytosol and requires NADPH. The PPP is the primary NADPH supplier in the cytosol. Mitochondrial NADPH (from ISOCITRATE DEHYDROGENASE in the TCA cycle) stays in the mitochondria and can't cross the inner membrane.
Hepatic cytochrome P450 enzymes use NADPH to oxidize drugs, steroids, and xenobiotics. Each cycle consumes one NADPH. This is a very high-demand process in the liver and is why the liver has one of the highest PPP activity rates in the body.
Connection: Drugs metabolized by CYP450 (like primaquine, dapsone) generate reactive oxygen species as byproducts. In a normal person, NADPH handles this. In a G6PD-deficient person, NADPH is already insufficient under baseline conditions. Add a CYP450-activating drug and the RBC is overwhelmed.
Neutrophils intentionally use NADPH to generate the oxidative burst · a controlled explosion of reactive oxygen species to kill bacteria. The enzyme NADPH-oxidase converts NADPH + O2 → superoxide (O2•−). Superoxide is then converted to other killing agents (H2O2, hypochlorous acid via MPO).
Disease: Chronic Granulomatous Disease (CGD) · NADPH-oxidase is non-functional (X-linked recessive, same inheritance as G6PD). Neutrophils engulf bacteria but cannot kill catalase-positive organisms. Classic presentation: recurrent Staphylococcus, Aspergillus, Serratia, Nocardia infections. Diagnosis: negative dihydrorhodamine (DHR) flow cytometry or nitroblue tetrazolium (NBT) test.
CGD and G6PD both affect NADPH but in opposite directions. G6PD = not enough NADPH (no defense for RBCs). CGD = NADPH-oxidase broken (neutrophils can't weaponize NADPH). Know which enzyme is affected in each.
| Feature | Detail |
|---|---|
| Inheritance | X-linked recessive · males affected, females carriers (can have mild disease if lyonization is skewed) |
| Population | 400 million worldwide; highest in sub-Saharan Africa, Mediterranean, Middle East, Southeast Asia |
| Malaria connection | G6PD-deficient RBCs are resistant to Plasmodium falciparum. This is why the allele is maintained at high frequency (heterozygote advantage, like sickle cell) |
| Baseline | Most patients are completely asymptomatic at baseline. Enzymopathy requires a trigger to become clinically apparent |
| Type of hemolysis | Both · typically occurs simultaneously. Extravascular (dominant): Heinz bodies form → splenic macrophages "bite" them out → bite cells are trapped and destroyed in the spleen. Raises unconjugated bilirubin. Intravascular: severe oxidative stress ruptures the RBC lipid bilayer directly inside the circulation → free Hb in plasma → hemoglobinuria (dark/tea-colored urine), low haptoglobin. |
| Category | Examples | Trace It |
|---|---|---|
| Antimalarials | Primaquine, chloroquine | Generate reactive oxygen species via CYP450 metabolism; overwhelm depleted NADPH |
| Antibiotics/Antiparasitics | Dapsone, nitrofurantoin, rasburicase | Direct oxidant stress; dapsone especially potent trigger |
| Food | Fava beans (favism) | Vicine and convicine in fava beans generate H2O2 directly after gut absorption |
| Infection | Any (bacterial or viral) | Most common trigger overall. Fever + inflammatory cytokines + respiratory burst activation all generate oxidative stress |
| Other oxidants | Naphthalene (mothballs), methylene blue | Direct oxidant; naphthalene via skin contact, inhalation, or ingestion in children |
| Test | Finding | Why |
|---|---|---|
| Hemoglobin | Decreased | RBC destruction |
| Reticulocyte count | Elevated | Bone marrow compensating with new RBC production |
| LDH | Elevated | Released from lysed RBCs |
| Indirect bilirubin | Elevated | Heme breakdown product; splenic clearance of damaged cells raises unconjugated bilirubin even when intravascular lysis also contributes |
| Haptoglobin | Decreased | Binds free plasma hemoglobin; drops fastest when intravascular lysis dumps Hb into plasma (often alongside splenic clearance in G6PD crises) |
| Peripheral smear | Heinz bodies, bite cells | Denatured Hb precipitates; macrophage removal creates the "bite" appearance |
| G6PD enzyme activity | Falsely normal during crisis | See board trap below |
Do NOT test G6PD enzyme activity during or immediately after a hemolytic crisis. During active hemolysis, the most G6PD-deficient RBCs are destroyed first. The surviving RBCs are the newest reticulocytes (which have more G6PD activity). Result: the blood sample looks like it has adequate G6PD. Test will be falsely normal. Wait 3 months after the crisis resolves.
The non-oxidative phase requires transketolase, which is a thiamine-dependent enzymeThiamine (B1) pyrophosphate is the cofactor for transketolase. It also serves as a cofactor for pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase in the TCA cycle.. In a thiamine-deficient alcoholic, transketolase fails. 4-carbon and 7-carbon intermediates accumulate. These sugars draw water osmotically into cells, causing neuronal swelling, especially in the mammillary bodies and periaqueductal gray → Wernicke's encephalopathy (confusion, ataxia, ophthalmoplegia).
Critical protocol: Alcoholic patient presenting to the ED → give 100 mg thiamine BEFORE glucose. Giving glucose first accelerates thiamine consumption and can precipitate acute Wernicke's.
Do not force G6PD crises into a single bucket. Extravascular clearance happens when splenic macrophages attack Heinz-body: damaged RBCs (bite cells, splenic trapping). Intravascular lysis happens when oxidative injury breaches the membrane in the bloodstream → free hemoglobin drives hemoglobinuria. Real episodes commonly involve both mechanisms together.
Testing G6PD during crisis gives a falsely NORMAL result. The most deficient cells are already gone. The reticulocytes that survive have more enzyme. Wait 3 months after crisis to test accurately.
clinical medicine love: patient with P. vivax malaria who is being started on primaquine (the only drug that clears liver hypnozoites). You MUST check G6PD status first. If deficient, primaquine triggers hemolysis. Alternative: tafenoquine or forgo hypnozoite clearance.
NADH = generated by glycolysis and TCA cycle = used in oxidative phosphorylation = energy. NADPH = generated by PPP = used for reductive biosynthesis and protection. These are NOT interchangeable. NADPH cannot enter the electron transport chain.
Oxidative phase = IRREVERSIBLE. Non-oxidative phase = REVERSIBLE. If you see a question asking which phase the cell can run backward to feed glycolysis, it's the non-oxidative phase.
If a question asks which PPP enzyme is affected by thiamine deficiency, the answer is TRANSKETOLASE (non-oxidative phase). G6PD does not need thiamine. Pyruvate dehydrogenase also needs thiamine but that's the TCA entry point, not PPP.
X-linked recessive. Glucose-6-phosphate + NADP+ yields 6-phosphogluconate + NADPH. That NADPH feeds glutathione reductase to keep GSH reduced. No G6PD = no NADPH = hemoglobin oxidizes = Heinz bodies = bite cells = splenic extravascular clearance plus direct membrane injury that can lyse cells intravascularly (free Hb → hemoglobinuria). Most common inherited enzymopathy worldwide (400 million). Mnemonic: the enzyme's job is in the name: it dehydrogenates G6P.
Requires thiamine (B1) pyrophosphate as a cofactor. Without B1, transketolase fails, sugar intermediates (sedoheptulose-7-P, erythrose-4-P) accumulate, cause osmotic neuronal swelling. This is Wernicke's encephalopathy: confusion, ataxia, ophthalmoplegia. Always give IV thiamine BEFORE IV glucose in an alcoholic. Transaldolase does NOT need thiamine.
End product of the oxidative phase. Can also be generated by the non-oxidative phase from glycolytic intermediates (reversible). High-demand tissues: bone marrow, skin, intestinal epithelium, tumors. Board setup: rapidly dividing cells run the PPP at high rates to make ribose-5-P for DNA replication. Key board distinction: ribose-5-P = nucleotides. NADPH = antioxidant and reductive biosynthesis. Same pathway, different products used for different jobs.
NADH: produced by glycolysis and TCA cycle. Donates electrons to Complex I of the ETC. Job = energy. NADPH: produced by PPP (and ISOCITRATE DH in mitochondria). Used for glutathione reductase, fatty acid synthesis, cholesterol synthesis, cytochrome P450 reactions, NADPH-oxidase in neutrophils. Job = protection and biosynthesis. NADPH cannot enter the ETC. They are NOT interchangeable. Same letters; entirely different jobs.
OXIDATIVE phase = IRREVERSIBLE. Produces 2 NADPH + CO2 + ribose-5-P. G6PD is the rate-limiting, irreversible step. NON-OXIDATIVE phase = REVERSIBLE. Transketolase + transaldolase. Can run forward (ribose-5-P to glycolytic intermediates) or backward (glycolytic intermediates to ribose-5-P). The bidirectionality is what makes it clinically useful: cells can switch which product they need based on demand.
Triggers: (1) Primaquine/chloroquine (CYP450 generates ROS). (2) Dapsone (potent oxidant). (3) Fava beans (vicine + convicine generate H2O2 in gut). (4) Infection: THE MOST COMMON trigger overall. (5) Naphthalene/mothballs. (6) Rasburicase (generates H2O2 from uric acid). SAFE: penicillin, hydroxychloroquine. Always check G6PD before prescribing primaquine. Do NOT test G6PD during crisis (falsely normal). Wait 3 months.
Run oxidative phase forward: G6P yields NADPH + ribose-5-P. If excess ribose-5-P is generated, run non-oxidative phase to recycle it into glycolytic intermediates. The cell gets NADPH for protection and ribose for nucleotide synthesis in the same cycle. Classic in rapidly dividing cells under oxidant stress.
Oxidative phase generates NADPH (the goal) and excess ribose-5-P. The non-oxidative phase runs in reverse, converting that ribose-5-P back into F6P and G3P for glycolysis. Net effect: extra NADPH, no ribose accumulation, no wasted carbon. This is the RBC protection mode under acute oxidant stress (infection, drugs).
F6P + G3P (glycolytic intermediates) enter the non-oxidative phase and are rearranged into ribose-5-P for nucleotide synthesis. No NADPH is generated. No CO2. Fully reversible. This is how rapidly dividing cells (bone marrow, intestinal epithelium) make massive amounts of nucleotides without necessarily running the full oxidative phase.
G6P enters the oxidative phase. Produces 2 NADPH + CO2 + ribose-5-P. The ribose-5-P is the goal here; the NADPH is a bonus the cell will use for reductive biosynthesis. This is the normal daytime liver mode when hepatocytes are generating ribose for ATP synthesis and NADPH for fatty acid biosynthesis simultaneously.
G6PDThink: "G6PD is the bodyguard of the RBC." Silent until a threat appears. The moment an oxidant drug, fava bean, or infection shows up, the missing enzyme gets exposed. Board setup: patient is well, then TRIGGER, then crisis. is the rate-limiting enzymeRate-limiting = irreversible = statin-equivalent in this pathway. The first committed step that can't turn back. G6PD commits glucose-6-phosphate to the PPP. Once you go this way, you don't come back. of the pentose phosphate pathway. It converts glucose-6-phosphateG6P is the entry molecule for the PPP. It is also the substrate for glycolysis (hexokinase). The cell decides at this branch point: energy pathway or protection/biosynthesis pathway. High NADP+ (low NADPH) signals the PPP to run faster. into 6-phosphogluconate while reducing NADP+ to NADPHThis is the key product. NADPH (not NADH) is the RBC's entire antioxidant currency. Think of NADPH as the rechargeable battery for glutathione. NADH is the battery for ATP. Same name, different plug, different device..
The oxidative phaseIRREVERSIBLE. One-way street. G6P goes in; NADPH + CO2 + ribose-5-P come out. Can never run backward. Key board test: "which phase is irreversible?" Answer: oxidative. Why? G6PD's step is exergonic and can't be reversed under physiological conditions. produces 2 NADPH and is irreversible. The non-oxidative phaseREVERSIBLE. Bidirectional traffic. Transketolase and transaldolase shuffle carbons back and forth between the PPP and glycolysis. The cell can make ribose-5-P from glycolytic intermediates OR convert excess ribose-5-P back into glycolytic intermediates. The direction depends on demand. is reversible and relies on transketolase (B1 dependent)Wernicke's connection: no thiamine = no transketolase function = sugars pile up = osmotic neuronal swelling. Classic alcoholic with confusion, ataxia, and lateral gaze palsy. Fix: IV thiamine BEFORE IV glucose. Glucose without thiamine accelerates B1 consumption and triggers acute Wernicke's. "Thiamine before sugar in every alcoholic." and transaldolaseThe non-thiamine enzyme of the non-oxidative phase. Transfers 3-carbon dihydroxyacetone groups between sugars. No vitamin cofactor. Board favorite: "which PPP enzyme needs thiamine? Transketolase." "Which one does NOT? Transaldolase." Two enzymes, one vitamin requirement, only one of them..
In a G6PD-deficient patient, an oxidant triggerThe list: primaquine, dapsone, fava beans, infection (most common), naphthalene/mothballs, rasburicase, nitrofurantoin, sulfa drugs. Safe list: penicillin, hydroxychloroquine. Board setup always includes either a drug name or fava beans in the social history. That's the clue. causes hemoglobin to denature into Heinz bodiesHeinz bodies = denatured hemoglobin precipitated against the RBC membrane. Visible on supravital stain (crystal violet, brilliant cresyl blue). NOT visible on routine Giemsa/Wright stain. This is why you need to specifically order a supravital stain to confirm G6PD-related hemolysis. Regular smear shows bite cells; supravital adds Heinz bodies., and splenic macrophages bite them out to create bite cellsAlso called "degmacytes." The macrophage in the spleen grabs the Heinz body stuck to the membrane and pinches it off, taking a chunk of the RBC with it. The remaining cell looks like someone took a bite out of it. This is the classic peripheral smear finding in G6PD hemolytic crisis. Compare: echinocytes = PK deficiency (chronic, no bite), bite cells = G6PD (episodic, oxidant-triggered)..
Testing G6PD during a crisis gives a falsely normal resultThe most G6PD-deficient cells are destroyed first. What's left are the youngest reticulocytes, which have more G6PD. So the blood sample that survived the crisis looks almost normal. Always retest 2-3 months after the event when the RBC population has returned to steady state. This is one of the most-tested timing traps in hematology clinical medicine.. G6PD-deficient RBCs are also malaria-resistantHeterozygote advantage: Plasmodium generates oxidant stress inside the RBC as it replicates. In a G6PD-deficient cell, this oxidative burden overwhelms the already-depleted NADPH, and the cell collapses around the parasite before it can complete its cycle. Same principle as sickle cell trait and malaria: the variant survives in a malaria-endemic environment because it costs less than it benefits. (heterozygote advantage), explaining the high allele frequency in malaria-endemic regions. The CGD connectionChronic Granulomatous Disease (CGD) affects NADPH oxidase, the enzyme in neutrophils that uses NADPH to generate superoxide (the oxidative burst). NBT test is negative (no superoxide produced = no color change). Classic bugs: Staphylococcus, Aspergillus, Serratia, Nocardia, Burkholderia. G6PD = insufficient NADPH (RBC protection fails). CGD = NADPH oxidase broken (can't weaponize NADPH). Same currency, different spender.: NADPH oxidase uses the same NADPH that G6PD makes, but in neutrophils to kill bacteria. NADPH oxidase deficiency = CGD.
