The
heavy metals include at least 40 elements but
cadmium, lead, and
mercury have been most extensively studied. The
biological properties of
heavy metals are discussed in terms of three important characteristics: the ability to form, irreversibly, complexes and chelates with organic
ligands; the properties to form organic-metallic bonds; and the potential to undergo oxidation-reduction reactions. The formation of complexes and chelates within the body is shown to influence greatly the dynamics of transport, distribution, and excretion of several important
metal cations. The excretion of
uranium is influenced by acid-base balance in the body because
uranium forms complexes with
bicarbonate anions that are filtered by the kidneys. The biliary excretion of methylmercury depends on the formation of small molecular weight complexes with
sulfur-containing amiono
acids and the
peptides in the liver. The degree of enterohepatic recirculation of a variety of
heavy metals appears to depend on the chemical nature of the bilary complexes. The oxidation of elemental to divalent ionic
mercury is the crurial step in the retention and tissue deposition of inhaled
mercury vapor. That the oxidation process is, at least in part, catalyzed by the
enzyme,
catalase, explains the effects of
ethanol,
aminotriazole and the state of
acatalasemia on the metabolism of inhaled vapor in man and animals. The formation of covalent bonds between
metal cations and the
carbon atom usually greatly modifies the
biological properties of the
metal. Methylarsenic and
methylmercury compounds both differ from the inorganic forms in accumulation in animals.