A silvery, slightly soft metal that burns very brightly in air, calcium is an alkaline earth element.
Its atomic number is 20 and its atomic symbol is Ca. It is the fifth most abundant metal in the Earth’s crust.
Biologically, calcium plays a role in cell signaling; it regulates gene expression and cell proliferation. It is a key cofactor in cellular enzymes involved in oxidative stress and is an essential nutrient.
Toxicity varies with oxidation state and solubility, and its biological effects are mediated by the intracellular production of reactive intermediates that include Cr(VI) compounds, thiylradicals and hydroxyl radicals, and ultimately Cr(III). At low concentrations (0.1-100 mM), cadmium binds to proteins, inhibits DNA repair, increases cytosolic free calcium levels, activates protein degradation and upregulates a wide range of cellular inflammatory cytokines.
Cadmium is a weak mutagen that is more soluble than most other carcinogenic metals. It enters cells through a variety of pathways and, under certain conditions, can be reduced to Cr(VI) by hydrogen peroxide, glutathione, ascorbic acid and GSH.
In vivo, cadmium has been shown to promote tumor growth in mouse models of malignant melanoma and prostate cancer at doses up to 1 mg/kg body weight. It also promotes apoptosis and suppresses cellular defense mechanisms such as heat shock, oxidative stress, stringent response, cold shock, and SOS [120-122].
Although it is difficult to identify a single chromium compound that causes a specific type of toxicity, the toxicity of chromium appears to be correlated with its ability to cross membranes and enter cells. Specifically, the oxidation state of the Cr(VI) form of the metal is responsible for its toxicity.