Mercury(2+);selenium(2-)

Mercury mainly affects the nervous system. Exposure to high levels of metallic, inorganic, or organic mercury can permanently damage the brain, kidneys, and developing fetus. Effects on brain functioning may result in irritability, shyness, tremors, changes in vision or hearing, and memory problems. Acrodynia, a type of mercury poisoning in children, is characterized by pain and pink discoloration of the hands and feet. Mercury poisoning can also cause Hunter-Russell syndrome and Minamata disease. Chronic oral exposure to high concentrations of selenium compounds can produce a disease called selenosis. The major signs of selenosis are hair loss, nail brittleness, and neurological abnormalities (such as numbness and other odd sensations in the extremities). Animal studies have shown that selenium may also affect sperm production and the female reproductive cycle. (L619, L7)

High-affinity binding of the divalent mercuric ion to thiol or sulfhydryl groups of proteins is believed to be the major mechanism for the activity of mercury. Through alterations in intracellular thiol status, mercury can promote oxidative stress, lipid peroxidation, mitochondrial dysfunction, and changes in heme metabolism. Mercury is known to bind to microsomal and mitochondrial enzymes, resulting in cell injury and death. For example, mercury is known to inhibit aquaporins, halting water flow across the cell membrane. It also inhibits the protein LCK, which causes decreased T-cell signalling and immune system depression. Mercury is also believed to inhibit neuronal excitability by acting on the postsynaptic neuronal membrane. It also affects the nervous system by inhibiting protein kinase C and alkaline phosphatase, which impairs brain microvascular formation and function, as well as alters the blood-brain barrier. Mercury also produces an autoimmune response, likely by modification of major histocompatibility complex (MHC) class II molecules, self peptides, T-cell receptors, or cell-surface adhesion molecules. Selenium readily substitutes for sulfur in biomolecules and in many biochemical reactions, especially when the concentration of selenium is high and the concentration of sulfur is low. Inactivation of the sulfhydryl enzymes necessary for oxidative reactions in cellular respiration, through effects on mitochondrial and microsomal electron transport, might contribute to acute selenium toxicity. Selenomethionine (a common organic selenium compound) also appears to randomly substitute for methionine in protein synthesis. This substitution may affect the structure and functionability of the protein, for example, by altering disulfide bridges. Inorganic forms of selenium appear to react with tissue thiols by redox catalysis, resulting in formation of reactive oxygen species and causing damage by oxidative stress. (L619, L7, A8, A25, A26)

Mercury is absorbed mainly via ingestion and inhalation, then distributed throughout the body via the bloodstream, where a portion binds to sulfhydryl groups on haemoglobin. Mercury can undergo oxidation to mercuric mercury, which takes place via the catalase-hydrogen peroxide pathway. The mercury atom is able to diffuse down the cleft in the catalase enzyme to reach the active site where the heme ring is located. Oxidation most likely occurs in all tissue, as the catalase hydrogen peroxide pathway is ubiquitous. Following oxidation, mercury tends to accumulate in the kidneys. Mercury is excreted mainly by exhalation and in the faeces. Selenium may be absorbed through inhalation and ingestion, while some selenium compounds may also be absorbed dermally. Once in the body, selenium is distributed mainly to the liver and kidney. Selenium is an essential micronutrient and is a component of glutathione peroxidase, iodothyronine 5'-deiodinases, and thioredoxin reductase. Organic selenium is first metabolized into inorganic selenium. Inorganic selenium is reduced stepwise to the intermediate hydrogen selenide, which is either incorporated into selenoproteins after being transformed to selenophosphate and selenocysteinyl tRNA or excreted into the urine after being transformed into methylated metabolites of selenide. Elemental selenium is also methylated before excretion. Selenium is primarily eliminated in the urine and feces, but certain selenium compounds may also be exhaled. (L619, A6, L7)