BIOCHEMICALLY IMPORTANT MINERAL ELEMENTS 



189 



demonstrate a need for this element in rats or rabbits but showed that 

 the requirement, if any, of the rat for cobalt is less than 0.6 /xg. per day. 



However, vitamin B12 has recently been found to contain cobalt, and 

 since it plays a vital role in hemoglobin formation in many species, in- 

 cluding man, the need for cobalt is evident. Since vitamin B12 contains 

 only slightly more than 4 per cent of cobalt, and only a few micrograms 

 of the vitamin are needed daily, it is not surprising that requirement for 

 this element was difficult to demonstrate. 



Manganese. This element seems to be essential for both plant and 

 animal life, although as yet no specific manganese-containing substance 

 of biological importance is known. Since plants deprived of manganese 

 become chlorotic (contain too little chlorophyll), it is probable that the 

 element plays some role in chlorophyll synthesis (Fig. 8-1). Lack of 

 iron has a similar result (Fig. 8-2). In animals, manganese deficiency 

 leads to poor growth, failure of reproduction, and- abnormal bone de- 

 velopment. Chickens suffer from a condition known as perosis (slipped 

 tendon), which results from poorly shaped leg bones and is cured by 

 manganese plus choline. Eggs produced by hens on a low-manganese 

 ration hatch poorly. 



Although the detailed manner in which lack of manganese brings about 

 these difficulties is not known, it seems most probable that the explanation 

 will be found to lie in the effect of Mn++ ions on various enzymes. 

 Arginase is activated by a number of metal ions, of which Mn++ is 

 probably the most important under natural conditions. Carboxylase 

 requires either Mg++ or Mn++ for activity. Certain peptidases and 

 phosphatases also are known to be activated by Mn+ + . 



The human requirement for manganese is not definitely known, but it 

 is estimated to be in the neighborhood of a few mg. per day for the adult. 

 Whole cereals, and especially cereal brans, are high in manganese. Tea 

 is an outstanding source (150-900 parts per million in the dry leaves), 

 contributing several milligrams daily to those who use this beverage. 

 The daily intake on ordinary American diets has been calculated to be 

 at least 2.5 mg., which evidently meets all bodily needs, since human 

 manganese deficiency does not occur, as far as is known. 



Zinc. The essential nature of zinc for animal life has been demon- 

 strated by feeding young rats a ration very low in this element (Hove, 

 Elvehjem, and Hart). Poor growth and various abnormalities resulted. 

 It has also been found that about 0.3 per cent of zinc is present in the 

 enzyme carbonic anhydrase, which occurs in red blood cells, as well as 

 in the pancreas and stomach lining of animals. This enzyme plays a 

 vital role in the elimination of carbon dioxide from the body, as ex- 

 plained above (p. 187) . It has been suggested that carbonic anhydrase 

 also functions in the secretion of hydrochloric acid by the stomach lining. 

 Zinc likewise is a constituent of crystalline insulin, although zinc in- 



