A 35-year-old female with a history of recurrent infections and hemolytic episodes is suspected to have a defect in the non-oxidative phase of the pentose phosphate pathway. Which investigation would best confirm a deficiency in transketolase activity?

Explanation

## Transketolase Deficiency and the Non-Oxidative PPP ### The Non-Oxidative Phase of the Pentose Phosphate Pathway ```mermaid flowchart LR A["Ribulose-5-phosphate<br/>(from oxidative phase)"]:::outcome --> B["Transketolase<br/>cofactor: TPP"]:::action B --> C["Glyceraldehyde-3-phosphate<br/>+ Sedoheptulose-7-phosphate"]:::outcome C --> D["Gluconeogenesis<br/>& Glycolysis"]:::action style B fill:#fff3cd ``` **Key Point:** Transketolase is the rate-limiting enzyme of the non-oxidative PPP. It requires thiamine pyrophosphate (TPP) as an essential cofactor and catalyzes the transfer of 2-carbon units between sugars. ### Why TPP Supplementation Test is Diagnostic Transketolase deficiency can result from: 1. **Genetic deficiency** of the enzyme protein → activity does NOT improve with TPP 2. **Thiamine deficiency** → activity IMPROVES dramatically with TPP supplementation The **TPP stimulation test** differentiates between these two mechanisms: - **Genetic transketolase deficiency:** Basal activity low; minimal improvement with TPP - **Thiamine deficiency:** Basal activity low; >25% increase in activity after TPP addition **High-Yield:** The TPP stimulation test is the gold standard for confirming transketolase deficiency and distinguishing enzyme deficiency from cofactor deficiency. ### Clinical Correlation **Clinical Pearl:** Transketolase deficiency is rare. More commonly, reduced transketolase activity reflects thiamine (vitamin B₁) deficiency, which causes Wernicke-Korsakoff syndrome. The TPP stimulation test guides whether supplementation will help. **Mnemonic:** **TPP Test** — Transketolase needs Thiamine Pyrophosphate. If activity rises with TPP, the problem is vitamin deficiency, not enzyme deficiency.