Heavy Metal Poisoning in Clinical Biochemistry

Critical vocabulary and biochemical mechanisms related to heavy metal poisoning (Lead, Mercury, Arsenic, Cadmium, Aluminium, and Iron) based on clinical biochemistry notes.

Sulphydryl (–SH) Binding

A mechanism by which heavy metals damage cells by binding to –SH groups in proteins and enzymes, leading to enzyme inhibition and neurotoxicity.

Metal Displacement

A cellular damage mechanism where heavy metals displace essential ions like $Ca^{2+}$ or $Zn^{2+}$, leading to enzyme dysfunction, anaemia, and bone disease.

Oxidative Stress

The generation of reactive oxygen species (ROS) by heavy metals, which causes membrane damage and organ failure.

Plumbism

A clinical term for Lead poisoning.

ALAD (delta-Aminolaevulinic Acid Dehydratase)

An enzyme inhibited by $Pb^{2+}$ during haem synthesis, leading to increased ALA in blood and urine.

Ferrochelatase

An enzyme inhibited by $Pb^{2+}$ that normally inserts $Fe^{2+}$ into Protoporphyrin IX; its inhibition leads to increased Zinc Protoporphyrin (ZPP).

Zinc Protoporphyrin (ZPP)

A compound formed when Zinc replaces $Fe$ in protoporphyrin due to ferrochelatase inhibition by lead.

Basophilic stippling

A haematological feature seen in the red blood cells of patients with Lead poisoning.

Burton's lines

Characteristic lead lines found on the gums as a clinical sign of chronic lead poisoning.

Lead Diagnosis Threshold

A Blood Lead Level $> 2.4\,\mu mol/L$ is considered significantly elevated and is the best diagnostic test.

Elemental Mercury

A form of mercury that exists as a toxic vapour at room temperature, often associated with dental amalgams and gold mining; causes respiratory distress on inhalation.

Methylmercury

An organic, lipophilic form of mercury found in large predatory fish that easily crosses the Blood-Brain Barrier and placenta.

Erethism

Irritability, which is a chronic CNS effect resulting from Mercury poisoning.

Lipoic acid

A cofactor for pyruvate dehydrogenase (PDH) that Arsenic binds to, thereby inhibiting PDH and decreasing ATP production.

Rice-water diarrhoea

A clinical feature of acute Arsenic poisoning, often accompanied by a garlic odour and severe gastroenteritis.

Mees' lines

White lines on the nails characteristic of chronic Arsenic poisoning.

Cadmium Biological Half-life

A very long period of $10-30\,\text{years}$ due to poor excretion, leading to accumulation in the kidney.

Itai-Itai (Ouch-Ouch) Disease

A bone disease caused by Cadmium replacing Calcium in bones, leading to osteomalacia and multiple painful fractures.

Beta2-microglobulin

A protein that increases in urine due to proximal tubular damage caused by Cadmium poisoning.

Dialysis Dementia

A progressive encephalopathy seen in chronic renal failure patients due to Aluminium toxicity from dialysis fluid or phosphate binders.

Desferrioxamine (DFO) Challenge

A diagnostic test used to help confirm Aluminium toxicity when serum levels are $> 2.2\,\mu mol/L$.

Fenton Reaction

$Fe^{2+} + H_2O_2 \rightarrow Fe^{3+} + HO^{\bullet} + OH^-$; this reaction produces hydroxyl radicals that cause lipid peroxidation and GI mucosal necrosis.

Iron Poisoning Latent Phase

Stage 2 of acute iron overdose ($6-24\,h$) characterized by apparent clinical recovery before later shock and acidosis.

Iron Toxicity Threshold

A serum iron level $> 90\,\mu mol/L$, often accompanied by metabolic acidosis and radio-opaque tablets on abdominal X-ray.