186 BIOCHEMICALLY IMPORTANT MINERAL ELEMENTS 



anesthesia and paralysis of animals caused by excess Mg++ ions in the 

 body fluids. 



The magnesium requirement of human beings is not known with 

 certainty, but it has been estimated to lie in the range of 0.2 to 0.4 g. 

 per day for a normal adult. Studies of American dietaries indicate that 

 the daily magnesium intake per 3000 calories varies from 0.17 to 0.53 g. 

 This amount evidently meets the normal needs since cases of human 

 magnesium deficiency are almost never encountered. 



Iron and Copper. The iron contained in the animal and human body 

 is mostly present as a component of certain conjugated proteins, of 

 which the best known is hemoglobin. The iron is contained in the 

 prosthetic group of hemoglobin, which is called heme. Like chlorophyll, 

 heme belongs to the porphyrin class of substances and carries its iron 

 atom in the center of the porphyrin ring. 



Iron is also an essential component of several physiologically important 

 enzymes such as catalase, peroxidase, the cytochromes, and cytochrome 

 oxidase. Like hemoglobin, these enzymes are conjugated proteins with 

 an iron-porphyrin type of prosthetic group. The iron content of hemo- 

 globin and of the above enzymes lies in the range of 0.1 to 0.4 per cent. 

 There is also present in the animal body another iron containing protein, 

 ferritin, which, in contrast to the above materials, contains as much as 

 23 per cent of iron. Ferritin is present in the spleen and in the intestinal 

 wall, where it is probably involved in the metabolism of iron in the body, 

 particularly in absorption and storage. 



Copper also is known to be associated with certain proteins in living 

 tissues. It is an essential component of several enzymes and of a re- 

 spiratory pigment, hemocyanin. This substance is present in the blood 

 of certain lower animals, for example, the lobster, snail, and other in- 

 vertebrates, and acts as an oxygen carrier just as hemoglobin does in 

 higher forms. Hemocyanins from various species contain about 0.2 to 0.4 

 per cent of copper and range in molecular weight from 350,000 to several 

 milhon. The copper is easily removed on acidification, being fully 

 utilized as a source of food copper by animals. It has not been estab- 

 lished whether the copper is attached to a prosthetic group of the 

 porphyrin type. Although this might be expected by analogy with 

 hemoglobin, the easy removal of the metal argues against this possibility, 

 and no porphyrin derivative has been obtained from hemocyanin. How- 

 ever, turacin, a feather pigment of the turaco bird (South Africa), is a 

 copper-porphyrin derivative. 



The copper-containing enzymes include ascorbic acid oxidase, poly- 

 phenol oxidase, laccase, and several other oxidases. Each of these en- 

 zymes is a protein which contains a small amount of copper, ranging 

 from 0.15 to 0.34 per cent, as an integral part of the molecule. The 

 blood of the ox, sheep, and horse has been found to contain another 



