# Potential Metal Contamination in Foods of Animal Origin--Food Safety Aspects
## 2 Major Toxic Metals in Food
### Mercury (Hg)
Absorption of inorganic mercury compounds is higher (1-40%), but this is influenced by species, age, diet, intestinal pH, and solubility of the compound. The absorption rate of organic forms is much higher; for example, methylmercury is almost completely absorbed (\(>\)90%) in mammals and chickens [120,121]. However, in ruminants, rumen microorganisms convert methylmercury to the inorganic form by demethylation, significantly reducing its absorption [122]. In fish, the absorption of organic mercury compounds (e.g., methylmercury) is 100 times faster than that of inorganic ones and is more efficient through the food chain [119]. Inorganic mercury is distributed almost equally between plasma and red blood cells [123]. In plasma, it is bound to sulfhydryl groups of proteins (primarily albumin), and in red blood cells, it is bound to hemoglobin and glutathione. Organic compounds (e.g., methylmercury) account for 90% of the total red blood cell concentration. Methylmercury has a high affinity for thiol-containing amino acids, allowing it to cross membranes through mechanisms that regulate amino acid transport. Inorganic mercury is generally unable to penetrate cell membranes, but its ionic form becomes more lipophilic when bound to selenium, improving its membrane transport. Elemental, inorganic, and organic mercury compounds can be interconverted within various organs and tissues of the body [124]. Tissue distribution also depends largely on the chemical form of mercury. Inorganic mercuric chloride (HgCl\({}_{2}\)) is found in high concentrations in the liver and kidneys, and only to a small extent in the brain and muscle. Methylmercury is distributed to all tissues, with the highest concentrations in the liver, kidneys, and spleen [125,126]. In poultry and other domesticated species, methylmercury is detected in much higher amounts in muscle tissue than inorganic compounds. The amount of mercury in the feed that is safe for the animal can reach concentrations in the muscle that can be dangerous (toxic) to the consumer. In fish, methylmercury is concentrated mainly in the muscle, while its inorganic compounds are concentrated in the epithelial cells of the gastrointestinal tract. Following oral exposure, it can also accumulate in the fur of mammals and in the feathers of birds, where mercury concentrations approach tissue levels [118,123]. Methylmercury crosses the blood-brain barrier and the placenta, reaching high tissue levels in fetal and maternal brain tissue. The passage of inorganic compounds through specialized membranes is low. The direction and extent of excretion also depend on the compound. Inorganic and elemental mercury are excreted from the body in the urine and feces. Methylmercury is excreted in the bile, where it binds to the sulfhydryl groups of glutathione [127,128]. The intestinal flora can convert it to an inorganic form, but intact molecules are reabsorbed via the enterohepatic circulation and continue to burden the body. The total body half-lives of methylmercury and mercuric chloride are 70 and 40 days, respectively, in humans [118]. Methylmercury has a half-life of 700 days in fish and persists in the body for about 2-5 times longer than inorganic compounds [114,119]. Organic mercury compounds can also be excreted in milk and through eggs in birds [129,130]. Cellular mechanisms caused by damage of inorganic and organic mercury compounds are basically similar, but due to their different tissue distribution, they can cause damage to different organs and tissues. Mercury ions (Hg\({}^{2+}\)) bind with high affinity primarily to thiol or sulfhydryl groups, but they can also bind to hydroxyl, carboxyl, and phosphoryl groups [120]. Sulfhydryl groups play a significant role in the structure and function of proteins; therefore, after mercury binding, enzyme activity decreases, membrane damage and other structural changes occur, and transport processes also become insufficient [123].