MINERAL METABOLISM 395 



However, animals consuming these plants are not alTectcd, apparently 

 because the actual zinc contents of the plants are not much below 

 normal even though the plant cover is markedly reduced in quantity. 

 Thus a requirement has been rather difficiUt to prove in animals. 



First, it \vas demonstrated that zinc was an essential component o( 

 carbonic anhydrase. Since this enzyme catalyzes the exchange of 

 carbon dioxide in the red cell a nil occurs in all higher animals, zinc 

 must be essential. Other zinc proteins have been isolatetl, but proof 

 that the zinc is not accidental has yet to be produced. The hormone 

 insulin seems to be associated with zinc in the body. Some workers 

 feel that the metal is involved in the storage and utilization of the 

 hormone, but the problem is unsolved. Zinc is especially plentiful in 

 marine species where it may play special roles, as in the snail 

 Sycotypus, where zinc is the meial reported for the blood pigment 

 corresponding to hemoglobin and hemocyanin. 



Experimental zinc deficiencies have been produced in rats, pigs, and 

 chickens. In the first two, growth is retarded, diarrhea and vomiting 

 occur, and lesions of the skin, esophagus, and cornea appear. Chicks 

 require zinc for growth and for bone and feather development. Cur- 

 rent studies indicate that the required levels may be higher than pre- 

 viously suspected, perhaps above 0.01 per cent of the ration. The 

 known foods and feeds contain enough zinc for all domestic and 

 laboratory species, and zinc deficiency is probably not a practical 

 problem. 



Although Zn++ is not very poisonous, toxic levels can be reached 

 with massive residues from crop sprays or industrial dusts. Human 

 cases of zinc poisoning have resulted from prolonged storage and 

 treatment of highly acid foods in galvanized containers. In general 

 the symptoms are those of copper deficiency and may be alleviated by 

 administration of Cu + + . Apparently a large excess of Zn++ impedes 

 the utilization of copper. On the other hand, an experimental zinc 

 deficiency in pigs was produced on diets high in Ca++, so there must 

 also be an antagonism between these two elements. 



Incidental Elements 



A number of elements are widely distributed in animals but are not 

 known to play essential roles. As our knowledge increases, we may 

 find that some of these actually are essential. However, an almost uni- 

 versal distribution in soils and plants may asstne the presence of 

 such materials in animals without any need for or utilization of them. 

 Some elements play special roles in a few members of the animal 

 kingdom without any known general function. Still others are 



