ABSTRACT
Zinc is an essential trace element on which many biochemical functions depend (1), because it is a constituent and cofactor in over 200 human enzymes that regulate lipid, protein, and nucleic acid synthesis and degradation (2). Zinc is one of the most abundant elements in the human body, present mainly in skeletal muscle (57%), bone (29%), teeth, hair, skin (6%), and testes. Only 0.1% is bound to plasma proteins (3). In the plasma, one-third is loosely attached to albumin and two-thirds are firmly bound to globulins. The metal protects against the formation of free radicals, by displacing iron ions from membrane proteins, and by inducing significant metallothionein (MT) levels which, with their high content of sulfhydryl residues, scavenge hydroxyl and superoxide radicals (4,5). ZnMT also appears to protect against cadmium-induced nephrotoxicity (6). A similar protective role is exhibited by preexposure to zinc sulfate (7). Parallel protective mechanisms are apparent with respect to the hepatotoxicity of cadmium, as pretreatment with zinc induces elevated MT levels in the liver of experimental animals (8-12).
