10 

 swine diets in 1955, poultry in 1958, cattle in 1960, and 

 humans in 1961 (McDowell, 1992) . 



Zinc is a bluish white metal with atomic number 30 and an 

 atomic weight of 65.37. Zinc is a divalent cation, with a 

 specific gravity of 7.13 g/cm at 20°C, and melting and boiling 

 points of 419.5 and 906°C, respectively. It is derived from 

 numerous compounds, but the principal mineral ore is the 

 sulfide sphalerite, which is the source of most metallic Zn 



(NRC, 1979) . 



The biochemical basis for the essentiality of Zn is not 

 completely understood. Zinc metalloenzymes can be found in 

 virtually every enzyme class (Vallee and Galdes, 1984) . 

 Several biological roles for Zn have been clarified, including 

 those related to cell replication and differentiation 



(Hambridge et al . , 1986). Zinc has also been postulated to 

 have other roles independent of Zn metalloenzyme activity such 

 as gene expression (Wu and Wu, 1987), membrane structure and 

 function (Bettger and O'Dell, 1981), second messenger and 

 protective agent in molecular storage systems (Grummt et al . , 

 1986), and improvement of stability of human growth hormone 



(Cunningham et al . , 1991) . A great portion of the current 

 research on Zn metabolism is aimed at its functions in 

 molecular biology and nucleic acids. 



