HMP Shunt and G6PD Deficiency — Phases, NADPH and Haemolysis

The hexose monophosphate shunt is also called the pentose phosphate pathway. It branches from glucose metabolism at glucose phosphate and provides reducing power and pentose sugars. For revision, follow the products rather than treating it as an extra list of glycolytic enzymes: NADPH supports redox defence and biosynthetic reactions, while ribose phosphate supports nucleotide production.

In human cells, the pathway operates in the cytosol. Its oxidative branch converts glucose phosphate into a pentose while generating NADPH and releasing carbon dioxide. The non-oxidative branch exchanges carbon fragments between sugars, linking the pathway to glycolysis. These different jobs explain why the branches are regulated according to cellular demand rather than always running together at the same rate.

A red cell illustrates the clinical importance. It repeatedly encounters oxidative stress and needs a supply of reduced glutathione to protect haemoglobin and its membrane. G6PD deficiency therefore presents as a haemolytic disorder even though the underlying lesion is metabolic. An infection or oxidant exposure can reveal a defect that caused no symptoms between episodes.

The oxidative branch starts with glucose-6-phosphate dehydrogenase, abbreviated G6PD. This reaction generates NADPH and a lactone intermediate. Lactonase then opens the lactone to form phosphogluconate. Phosphogluconate dehydrogenase performs the next oxidative reaction, generating additional NADPH, carbon dioxide and ribulose phosphate. Keep the pentose product separate from the ribose used directly in nucleotides.

A passage through the oxidative sequence produces two NADPH per glucose phosphate. The branch is conventionally regarded as unidirectional. Do not confuse its carbon dioxide release with mitochondrial oxidation: the pathway is cytosolic, and its distinctive reducing equivalent is NADPH. The dehydrogenase reactions account for the reducing power; hydrolysis accounts for the intervening lactone-opening step.

The non-oxidative branch interconverts pentoses and glycolytic intermediates. Ribulose phosphate can become ribose phosphate or xylulose phosphate. Transketolase and transaldolase then transfer carbon fragments among phosphorylated sugars. Fructose phosphate and glyceraldehyde phosphate provide the connection to glycolysis. This branch can supply pentoses from glycolytic intermediates or return surplus pentose carbon to central metabolism.

The direction depends on the relative need for pentoses and reducing power. A cell needing nucleotide precursors can draw on non-oxidative reactions without requiring every sugar to pass through the oxidative branch. A cell needing substantial reducing power can generate NADPH and recycle pentose carbon. Think of the non-oxidative phase as a flexible carbon exchange, rather than a second NADPH-producing phase.

The important chain is G6PD → NADPH → reduced glutathione → protection from peroxides. NADPH supplies reducing power to glutathione reductase, allowing oxidised glutathione to be returned to its reduced form. Reduced glutathione is then used in peroxide detoxification. Distinguish the enzyme recycling glutathione from the enzyme using glutathione to remove peroxide; their substrates and jobs differ.

If NADPH availability falls, antioxidant protection becomes inadequate during oxidative stress. Haemoglobin can denature and the red-cell membrane can be damaged. The resulting inclusions and membrane changes make the erythrocyte liable to destruction. This sequence explains why a metabolic enzyme deficiency produces characteristic blood-film findings rather than simply a low glucose concentration.

NADPH also contributes to anabolic reactions outside red cells, including lipid synthesis. The pathway therefore has a broader role than haemolysis prevention. For a clinical vignette, however, identify the affected cell and its immediate problem: an erythrocyte with insufficient antioxidant reserve. For a biochemical question, identify the product and branch before choosing an enzyme or cofactor.

G6PD deficiency has X-linked inheritance. Males are affected more often, but females are not automatically protected. X-inactivation can produce a wide range of enzyme activities in heterozygous females, including clinically significant deficiency. An exam option stating that a woman cannot develop oxidant haemolysis because the condition is X-linked is therefore too absolute.

Many affected people are well between episodes. The presentation depends on enzyme activity, the variant and the intensity of the oxidative challenge. Some variants are associated with chronic haemolysis, but episodic attacks are the familiar exam pattern. Avoid assuming that every individual with the diagnosis has the same baseline haemoglobin or reacts identically to an exposure.

Use family history as supporting evidence rather than as a requirement for diagnosis. A male patient with sudden jaundice and dark urine after an infection or an oxidant drug can fit the diagnosis even without a known affected relative. The clinical pattern, haemolysis investigations and enzyme assay need to be interpreted together.

The major trigger groups are infection, fava beans and oxidant medications. Infection itself can cause oxidative stress; haemolysis occurring during antibiotic treatment is not automatically attributable to the antibiotic. Ask what was taken, why it was taken and whether a concurrent illness could be contributing. This avoids turning a useful trigger list into a blanket prohibition on unrelated medicines.

Current drug guidance is more nuanced than older mnemonics. CPIC classifies medications by evidence and risk, and does not place every sulfonamide in the same category. Its assessment places sulfamethoxazole in a low-to-no-risk group at standard use. For prescribing questions, use the named medicine and relevant guidance; do not infer that all drugs in a broad family are either safe or dangerous.

An acute attack may cause pallor, fatigue, jaundice and dark urine. Laboratory evidence supports haemolysis: falling haemoglobin, increased indirect bilirubin and lactate dehydrogenase, reduced haptoglobin and a marrow reticulocyte response. The pattern is interpreted alongside the exposure history. A haemolysis panel identifies red-cell destruction but does not by itself establish the specific enzyme defect.

Heinz bodies are inclusions of denatured haemoglobin. Bite cells form when splenic processing removes damaged material from a red cell, leaving a notch. Blister cells may also occur. These are clues to oxidative red-cell injury; they are not exclusive proof of G6PD deficiency. The enzyme assay establishes the defect when obtained and interpreted appropriately.

A direct antiglobulin test is generally negative in this non-immune haemolysis. A positive result should prompt assessment for immune haemolysis or a coexisting process rather than forcing every finding into the G6PD diagnosis. When the stem asks what causes the inclusion, choose denatured haemoglobin. When it asks why the cell has a bite, choose splenic removal of damaged material.

A quantitative G6PD enzyme assay measures red-cell enzyme activity. During acute haemolysis, the most vulnerable cells may have been destroyed, leaving a population enriched in younger cells with greater activity. A result can therefore appear reassuring even when the history strongly suggests deficiency. The problem is the red-cell population being sampled, not spontaneous correction of the inherited disorder.

Repeat enzyme testing after recovery when the initial result conflicts with the clinical picture. Laboratory results should be interpreted alongside the severity and course of the current episode. In females, average enzyme activity and mosaic expression complicate screening further. A clinician should use the assay, testing history and clinical context rather than treating a single normal result as an absolute exclusion.

Separate screening from diagnosis of an acute episode. A drug-safety screen asks whether enzyme status permits a proposed regimen. A haemolysis work-up asks what caused current red-cell destruction. Both may involve G6PD measurement, but the timing, red-cell composition and consequences of a false-normal result differ.

1. Stop the suspected oxidant medicine and document the exposure.
2. Assess the severity of anaemia, renal function and ongoing haemolysis.
3. Treat an underlying infection and provide supportive care.
4. Use transfusion when the severity of the episode requires it.
5. Arrange repeat testing when needed and provide a clear avoidance plan.

Prevention is a substantial part of care. Patients need a usable list of relevant exposures and a record of their G6PD status that can inform future prescribing. Advice should remain specific enough to avoid harmful triggers without unnecessarily denying useful treatment. Severe neonatal hyperbilirubinaemia is another important presentation, so an adult trigger vignette should not be treated as the only clinical form.

For rapid revision, connect the biochemical and clinical halves in order: cytosolic pathway, oxidative NADPH production, glutathione recycling, oxidant exposure, damaged haemoglobin, bite cells and assay timing. This chain allows you to answer mechanism, morphology and drug questions from the same explanation. Revisit haemolytic anaemias to compare enzyme defects with immune and membrane causes.