Zinc

Radiopaedia • CC-by-nc-sa 3.0 • de

Zinc (chemical symbol Zn) is a trace element with a key role as a constituent of enzymes, e.g. carbonic anhydrase, and as part of zinc finger proteins, vital for the correct folding of macromolecules, such as DNA. More recently zinc has been found to act as an important cellular messenger .

Chemistry

Basic chemistry

Zinc is a transition metal, with a bluish-white hue. It has the atomic number 30, and atomic weight 65.37. Its valence states are -1, +1, and +2 .

Radiochemistry

There are five stable isotopes of zinc, yet almost 50% of the naturally occurring zinc is zinc-64. Zinc-66 and zinc-68 account for 28% and 18% of the total respectively. The remaining 4% is mainly zinc-67. Zinc-70 only forms 0.6% of the zinc on earth. Thirty-two isotopes of zinc have been discovered in total .

Nutrition

Zinc concentrations tend to be highest in red meat, liver, milk and cheese.

Absorption, transport and storage

Zinc is absorbed via several gut transporters, e.g. divalent cation transporter 1 (DCT 1). However zinc is also secreted into the gut, partially as a constituent of biliary and GI secretions. This secretion is thought to be important for the homeostasis of zinc . Approximately 70% of the zinc in the circulation is bound to albumin. Storage of zinc in the cells is under investigation, but the protein metallothionein is thought to be important.

Biological function

Zinc has key roles:

  • zinc ions act as intra- and intercellular messengers (cf. calcium)
  • as the metallic component of the active sites of zinc finger moieties, central to zinc finger proteins, ensuring the correct folding of macromolecules, e.g. nucleic acids and proteins
  • catalytic ion in enzymes, e.g. carbonic anhydrases

Deficiency

Zinc deficiency is surprisingly common in the developing world, especially amongst infants and the elderly. Up to two billion people are estimated to be zinc deficient globally. Lack of body zinc may result in a constellation of sequelae:

Toxicity

True zinc toxicity is rare. Overdose of zinc is generally fairly well tolerated, and most importantly interferes with the metabolism of copper, which may result in copper deficiency.

If oral overdose, then emesis is the commonest outcome. Conversely if there is inhalational exposure to zinc dusts then metal fume fever, a self-limiting chest condition, may result.

Radiological importance

  • Zn-63 zinc citrate is a niche PET tracer used in neuroimaging research
  • historically zinc-cadmium sulfide (ZnCdS) was employed as a phosphor for image intensifier screens

History and etymology

The word zinc is derived from the German word Zinke, and likely ultimately from the Old High German zinko meaning prong, possibly because zinc takes on this morphology as it cools after smelting .