2,4-dinitrophenol

**Mechanism:** DNP is a lipophilic weak acid. It picks up protons in the intermembrane space, diffuses across the inner mitochondrial membrane, and releases them in the matrix. This bypasses ATP synthase (complex V) and collapses the proton motive force. The electron transport chain (ETC) keeps running and may speed up, so O2 consumption rises, but ATP synthesis falls. The energy is released as heat.

**Effects on the ETC and metabolism:** - Increased O2 consumption and increased substrate oxidation (glucose, fatty acids), with a high metabolic rate. - Decreased ATP:O2 ratio (low P/O ratio), and decreased ATP production. - Increased NADH oxidation, so the NADH/NAD+ ratio falls. - Increased glycolysis and TCA cycle flux, with compensatory lactate production from anaerobic glycolysis, so lactic acidosis can occur. - Heat production causes hyperthermia.

**Contrast with other ETC agents (high-yield):** - ETC inhibitors (rotenone, antimycin A, cyanide, CO, azide) block electron flow, so O2 consumption falls. - ATP synthase inhibitors (oligomycin) block proton flow through complex V. The gradient builds up and ETC activity falls. - Uncouplers (2,4-DNP, FCCP, aspirin at toxic doses, thermogenin/UCP-1 physiologically) increase O2 consumption and heat while ATP falls.

**Related uncouplers:** - Salicylate (aspirin) overdose causes uncoupling, which produces hyperthermia, along with mixed respiratory alkalosis and anion gap metabolic acidosis. - UCP-1 (thermogenin) in brown fat is the physiologic uncoupler. It is important in neonates and in cold adaptation.

**Clinical significance:** - DNP was used in the 1930s as a weight-loss agent. It was withdrawn from use because of cataracts, neuropathy, agranulocytosis, skin rash, and deaths from hyperthermia. - It is banned for human consumption but still sold illicitly online and used by bodybuilders and for weight loss. It is also used industrially (dyes, explosives, pesticides, wood preservatives). - Toxicity presentation: profuse sweating, flushing, tachycardia, tachypnea, fever or severe hyperthermia, agitation, and a yellow discoloration of skin and urine. It can progress to rhabdomyolysis, AKI, seizures, cardiovascular collapse, and death. Rapid rigor mortis may occur after death. - Management: supportive care. Use aggressive external cooling, IV fluids, and benzodiazepines for agitation and seizures. Consider activated charcoal if ingestion was recent. Treat rhabdomyolysis and electrolyte abnormalities. **Antipyretics (acetaminophen, NSAIDs) are ineffective** because the fever comes from uncoupling, not from a hypothalamic set-point change. **Dantrolene** has been tried but evidence for benefit is limited. It is not an established treatment. - Differential diagnosis for hyperthermia with high CO2 and muscle rigidity: malignant hyperthermia, neuroleptic malignant syndrome, serotonin syndrome, thyroid storm, salicylate toxicity, and anticholinergic toxicity. DNP and salicylate toxicity both involve uncoupling.

**Board pearl:** "Increased O2 consumption + decreased ATP + increased heat" points to an uncoupler. The ETC is still functioning, but the proton gradient is dissipated.