Mercury is a heavy metal that has been used for centuries as a medicine and a poison. Common exposures come from contaminated seafood, dental amalgams, and vaccines for infants. Mercury can exist in 11 different chemical states or compounds. At the molecular level, it forms bonds with sulfhydryl groups on an enzyme, which are parts of the enzyme that contain a sulfur atom that is attached to a hydrogen atom (SH). Binding of mercury can change the shape of the enzyme and block its activity. Enzymes inhibited by mercury include acetylcholinesterase, catalase, dipeptyl peptidase (CD26), amylase, lipase, lactase and glucose-6-phosphatase.
Acetylcholine is one of the main neurotransmitters that nerves use to control muscle movement. After release, acetylcholine must be degraded in order to stop the “go” signal from continuing to stimulate the receiving cell. Acetylcholine is degraded by an enzyme called acetylcholinesterase. This enzyme is found in the synaptic cleft, which is the space between the "fingertips" of a nerve cell and the neighboring cell that the nerve activates. Mercury inhibits this enzyme differently in different species, depending on whether it can easily find a sulfhydryl group to latch onto. For human acetylcholinesterase, it takes millimolar amounts of mercuric chloride (HgCl2) to inhibit the enzyme.
Catalase is an enzyme that converts hydrogen peroxide into water and oxygen. Hydrogen peroxide is regularly produced by cells as they make energy in a process called cellular respiration. Hydrogen peroxide is toxic at high levels, so cells get rid of it via the enzyme catalase. Though it is widely known that mercury inhibits catalase, it may do so by binding to sites other than sulfhydryl groups. It is interesting to note that when a person absorbs elemental mercury, which causes brain damage, catalase is the enzyme in the red blood cells that converts elemental mercury into an ionic form (mercuric salt).
Mercury also inhibits the enzyme found in skeletal muscle called creatine kinase. Muscle cells contract by using an energy molecule called adenosine triphosphate (ATP), a molecule with three -- thus the “tri” prefix -- phosphates. Energy is released for an enzyme when the enzyme grabs ATP and breaks off one phosphate, resulting in adenosine diphosphate (ADP) -- “di” means two. A quick way of making ATP is to take a phosphate from a sugar molecule called phosphocreatine and add it to ADP. Creatine kinase is the enzyme that recharges ADP into ATP in this way. Mercury inhibits creatine kinase in several ways. Mercury blocks creatine kinase’s ability to bind ADP or the magnesium ion that the enzyme needs in order to function properly.
Mercury binds to sulfhydryl groups, which is found on the amino acid cysteine. Since cysteine is a common amino acid in many enzymes, mercury inhibits a whole host of enzymes. The "Journal of Applied Toxicology" reported the effects of inorganic mercury in the liver tissue of freshwater fish. Mercury inhibited many enzymes involved in digestion of food molecules, such as protein, carbohydrate and fat: amylase, lipase, lactase and maltase. Mercury also inhibited glucose-6-phosphatase, an enzyme involved in the production and export of glucose in cells.