Heavy metal poisoning, amino acids

Mercury forms strong covalent bonds with sulfur. If the sulfur happens to be in an amino acid (cystine, cysteine, methionine), in a protein, or an enzyme then poisoning can occur. One result of this interaction is that Hg accumulates in toe and finger nails as well as in hair. Almost everyone has some Hg in their nails and hair, but an excessive amount can indicate chronic heavy metal poisoning. A survey of Hg in the hair of 17 general population people indicated 6.0 2.9 ppm. Dentists had 9.8 4.7 ppm. 

Crosslinking of protein monolayers by mercuric ion (MacRitchie, 1970) and silicic acid (Minones et al., 1973) has been reported. These studies are relevant to poisoning by heavy-metal ions and to silicosis, effects that seem likely to result from attack on the cell membrane proteins. Crosslinking by mercuric ion was detected by a spectacular increase in surface viscosity and a decrease in compressibility when a number of proteins (BSA, insulin, ovalbumin, and hemoglobin) were spread on 0.001 M mercuric chloride solution. Poly-DL-alanine was unaffected whereas poly-L-lysine and poly-L-glutamic acid were affected in a similar manner to the proteins, indicating that mercuric ion interacts with the ionizable carboxyl and amino groups on the protein side—chains. Silicic acid similarly caused protein monolayers... 

Many chemicals can bind to enzymes and either eliminate or drastically reduce their catalytic ability. These chemicals, called enzyme inhibitors, have been used for hundreds of years. When she poisoned her victims with arsenic, Lucretia Borgia was unaware that it was binding to the thiol groups of cysteine amino acids in the proteins of her victims and thus interfering with the formation of disulfide bonds needed to stabilize the tertiary structure of enzymes. However, she was well aware of the deadly toxicity of heavy metal salts like arsenic and mercury. When you take penicillin for a bacterial infection, you are taking another enzyme inhibitor. Penicillin inhibits several enzymes that are involved in the synthesis of bacterial cell walls. 

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