Which heavy metal binds to sulfhydryl groups of enzymes and causes a garlic odor on the breath?

Explanation

## Arsenic Poisoning: Mechanism and Clinical Signature **Key Point:** Arsenic binds to sulfhydryl (–SH) groups of enzymes, particularly pyruvate dehydrogenase and other mitochondrial enzymes, causing cellular dysfunction. The characteristic garlic odor on breath and in body fluids is pathognomonic for arsenic poisoning. ### Mechanism of Arsenic Toxicity 1. **Sulfhydryl Binding:** Arsenic (especially trivalent As³⁺) irreversibly binds to –SH groups on cysteine residues in enzymes 2. **Enzyme Inhibition:** Blocks pyruvate dehydrogenase, α-ketoglutarate dehydrogenase, and other TCA cycle enzymes 3. **Energy Depletion:** Impairs ATP production, leading to cellular necrosis 4. **Metabolic Acidosis:** Results from impaired metabolism and lactate accumulation ### Garlic Odor: Forensic Signature **High-Yield:** The garlic odor (dimethyl arsine gas) is produced when arsenic is metabolized and is detectable on: - Breath - Sweat - Urine - Gastric contents (in autopsy) This odor is so characteristic that it is a key forensic clue in suspected arsenic poisoning cases. ### Comparison with Other Heavy Metals | Metal | Enzyme Target | Odor | Mechanism | |-------|---------------|------|----------| | Arsenic | Pyruvate dehydrogenase (–SH groups) | Garlic | Sulfhydryl binding | | Mercury | Sulfhydryl groups (different enzymes) | None | Protein denaturation | | Lead | ALA dehydratase, ferrochelatase | None | Heme synthesis inhibition | | Cadmium | Metallothionein binding | None | Renal tubular damage | **Clinical Pearl:** The garlic odor may fade within hours to days as the volatile dimethyl arsine is exhaled, so its absence does not rule out arsenic poisoning. However, its presence is highly suggestive. **Mnemonic:** **GARLIC = Gaseous Arsenic Release, Lethal Inhibition of Citric acid cycle, Irreversible sulfhydryl binding, Cellular necrosis**