## Dimercaprol (BAL) and Heavy Metal Chelation **Key Point:** Mercury forms the strongest metal-sulfhydryl (thiol) complexes and is the MOST effectively chelated by dimercaprol (BAL). BAL works by binding to the sulfhydryl groups of cysteine residues, displacing the metal and forming a water-soluble complex for urinary excretion. ### Chelation Efficacy by Heavy Metal | Heavy Metal | BAL Efficacy | Mechanism | Clinical Use | | --- | --- | --- | --- | | **Mercury** | **Excellent** | **Strong SH-complex formation** | **First-line for acute Hg poisoning** | | Arsenic | Excellent | Strong SH-complex formation | First-line for acute As poisoning | | Lead | Moderate | Weak SH-complex; EDTA preferred | Adjunct only; not first-line | | Cadmium | Poor | Weak SH-complex; redistributes to kidney | Contraindicated; worsens renal damage | | Iron | None | Not a thiol-binding metal | Deferoxamine used instead | **High-Yield:** **BAL (dimercaprol) is the chelator of choice for mercury and arsenic poisoning** because both metals form exceptionally stable sulfhydryl complexes. For lead, EDTA or succimer is preferred because BAL-lead complexes are weaker and may redistribute toxin to the brain. ### Why Mercury? Mercury has an extremely high affinity for sulfhydryl (thiol, -SH) groups: - Binds to cysteine residues in proteins and enzymes - Inhibits critical enzymes (e.g., thioredoxin reductase, glutathione peroxidase) - BAL displaces Hg and forms a stable Hg-BAL complex that is water-soluble and renally excreted **Clinical Pearl:** BAL must be given **early** (within 24 hours of acute mercury exposure) for maximum efficacy. It does NOT cross the blood-brain barrier well, so it is less effective for organic mercury (methylmercury) that has already accumulated in the CNS. **Mnemonic:** **BAL = Best for Arsenic & (Liquid) mercury** — the two heavy metals with the strongest thiol affinity.
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