A 28-year-old man from Delhi presents to the emergency department with acute onset confusion, vomiting, and rapid breathing. His wife reports he consumed a bottle of antifreeze (ethylene glycol) 6 hours ago. Laboratory findings: pH 7.15, HCO₃⁻ 12 mmol/L, serum lactate 9 mmol/L, anion gap 18 mEq/L, serum osmolality 380 mOsm/kg. Urine microscopy shows calcium oxalate crystals. Which TCA cycle enzyme is most severely inhibited by the toxic metabolite of ethylene glycol?

Explanation

## Ethylene Glycol Toxicity and TCA Cycle Inhibition ### Metabolism of Ethylene Glycol to Toxic Metabolites **Key Point:** Ethylene glycol is metabolized by alcohol dehydrogenase → glycolaldehyde → glycolic acid (glycolate) → glyoxylate → **oxalic acid (oxalate)**. The intermediate **glycolate** and the end-product **oxalate** are the primary toxic metabolites responsible for enzyme inhibition and metabolic acidosis. ``` Ethylene glycol → Glycolaldehyde → Glycolic acid (Glycolate) → Glyoxylate → Oxalate (ADH) (Aldehyde DH) (Glycolate oxidase) ``` ### Why Succinate Dehydrogenase is the Most Severely Inhibited TCA Enzyme **High-Yield:** **Glycolate (glycolic acid)**, the major accumulating metabolite of ethylene glycol poisoning, is a **structural analogue of succinate** and acts as a **competitive inhibitor of succinate dehydrogenase (Complex II)**. This is the well-established primary TCA cycle enzyme inhibited in ethylene glycol toxicity. | Feature | Succinate | Glycolate | |---------|-----------|-----------| | Structure | 4-carbon dicarboxylic acid | 2-carbon hydroxy acid (structural mimic) | | Enzyme target | Succinate dehydrogenase (SDH) | Competitive inhibitor of SDH | | Result of inhibition | Blocked TCA at succinate → fumarate step | Impaired oxidative phosphorylation, lactic acidosis | | Additional effect | — | Oxalate chelates Ca²⁺ → AKI, hypocalcemia | ### Mechanism of Lactic Acidosis in Ethylene Glycol Poisoning 1. **Glycolate accumulation** → Competitive inhibition of **succinate dehydrogenase** → TCA cycle blockade. 2. **Impaired NADH re-oxidation** → Pyruvate shunted to lactate by LDH → **Lactic acidosis**. 3. **Oxalate formation** → Chelates Ca²⁺ and Mg²⁺ → Calcium oxalate crystals in renal tubules → **AKI**. 4. **Glycolic acid** itself contributes to the **high anion gap metabolic acidosis**. 5. **Result**: Elevated anion gap, elevated osmolar gap (early), elevated lactate, calcium oxalate crystalluria. **Clinical Pearl:** The **osmolar gap** is elevated early (parent compound); as metabolism proceeds, the **anion gap** widens due to glycolate and oxalate accumulation. Calcium oxalate crystals in urine are pathognomonic. ### Why Other Enzymes Are Not Primarily Affected | Enzyme | Why Not the Answer | |--------|--------------------| | Pyruvate dehydrogenase (A) | Not directly inhibited by glycolate or oxalate; affected only secondarily by NADH buildup | | α-Ketoglutarate dehydrogenase (B) | Oxalate has some inhibitory effect but is NOT the primary/most severely inhibited TCA enzyme; glycolate's structural mimicry of succinate makes SDH the primary target | | Isocitrate dehydrogenase (D) | No structural similarity to glycolate or oxalate; not a recognized target | **Mnemonic:** **GLYcolate inhibits SDH** — glycolate mimics succinate and competitively inhibits succinate dehydrogenase, the primary TCA enzyme blocked in ethylene glycol poisoning. ### Clinical Correlation - **Calcium oxalate crystals in urine** → Oxalate precipitation in renal tubules → Acute tubular necrosis → AKI. - **Hypocalcemia** → Oxalate chelates serum Ca²⁺ → Risk of tetany, arrhythmias. - **Treatment**: **Fomepizole** (alcohol dehydrogenase inhibitor) to block formation of toxic metabolites; **hemodialysis** to remove glycolate and oxalate; supportive care. [cite: Lehninger Principles of Biochemistry 7e Ch 19; Casarett and Doull's Toxicology 9e; Harrison's Principles of Internal Medicine 21e Ch 471]