Heavy Metals

An umbrella term for metallic elements with relatively high density (conventionally >5 g/cm3), many of which are toxic to biological systems at low concentrations. This wiki uses "heavy metals" broadly to encompass both strictly toxic metals and essential trace metals that become toxic in excess.

Classification

Toxic (No Known Biological Requirement)

- lead (Pb): Disrupts heme synthesis (inhibits ALAD and ferrochelatase), neurotoxic, crosses the blood brain barrier, immunosuppressive. No safe threshold.
- mercury (Hg): Binds sulfhydryl groups, inhibits glutathione peroxidase, crosses BBB as methylmercury. CNS, renal, and GI toxicity.
- cadmium (Cd): 17-30 year biological half-life; renal primary target; disrupts Ca/Zn/Fe homeostasis; carcinogenic. Induces metallothionein.
- arsenic (As): Inhibits pyruvate dehydrogenase (blocks Krebs cycle), proinflammatory, carcinogenic. Technically a metalloid.
- aluminium (Al): Enters brain via transferrin receptor; implicated in neurodegeneration.

Essential but Toxic in Excess

- iron (Fe): Required for heme, electron transport; excess drives ferroptosis via Fenton chemistry. Both deficiency and overload cause dysbiosis.
- zinc (Zn): Cofactor for 300+ enzymes; excess causes neuronal death and amyloid-beta aggregation.
- copper (Cu): Required for cytochrome c oxidase, SOD; excess generates ROS. Elevated in atherosclerotic plaques.
- nickel (Ni): Cofactor for bacterial urease and hydrogenase; toxic to host via metalloestrogen activity, nickel allergy, and nickel neurotoxicity.
- manganese (Mn): Required for SOD2, arginase; excess causes parkinsonism. Crosses BBB.
- chromium (Cr): Cr(III) may assist insulin signaling; Cr(VI) is a Group 1 carcinogen causing DNA strand breaks.

Shared Toxicity Mechanisms

Despite differing target organs and binding preferences, toxic heavy metals converge on common pathways [balali mood 2021 toxic mechanisms five heavy metals]:

1. ROS generation and oxidative stress: Depletion of GSH, SOD, CAT, GPx; lipid peroxidation.
2. Sulfhydryl group binding: Inactivation of enzymes dependent on -SH groups.
3. DNA damage: Strand breaks, 8-OHdG adducts, genomic instability.
4. dysbiosis: Selective killing of SCFA-producing commensals; enrichment of metal-tolerant pathobionts; barrier breakdown increasing further metal absorption [anchidin norocel 2025 heavy metal gut probiotics biosensors].
5. inflammation: Proinflammatory cytokine induction (TNF-alpha, IL-6, IL-8); nf kappa b activation.
6. mis metallation: Displacement of essential metals from enzyme active sites (e.g., Cd replacing Zn, Pb replacing Ca).

The Gut as Primary Exposure Site

Approximately 60% of ingested heavy metals are absorbed in the intestine, making the gut the first point of contact for dietary metal exposure [anchidin norocel 2025 heavy metal gut probiotics biosensors]. The gut metal microbiome axis describes the bidirectional relationship: metals reshape microbial communities, and those communities in turn modulate metal absorption, speciation, and toxicity.