384 ANIMAL BIOCHEMISTRY 



magnesium occurs in complex salts with calciimi and phosphorus with 

 about a third of it available for other purposes when the amount in- 

 gested is insufficient. In addition to its structural role, Mg++ serves 

 as an activator for a variety of animal enzymes. Well-known examples 

 include all the kinases, transketolase, enolase, gluconolactonase, 6-phos- 

 phogluconic dehydrogenase, and at least some of the phosphatases. Al- 

 though the details are unknown, Mg++ probably participates in the 

 enzyme-substrate complex, perhaps as a linking agent. 



Calves and chicks require enough magnesium to make approxi- 

 mately 0.05 per cent (dry basis) of the ration. Since feeds normally 

 contain at least twice and usually three to four times this much, there 

 is not likely to be a deficiency problem. The abundance of mag- 

 nesium in both plants and animals almost automatically insures that 

 herbivores as well as carnivores will be adequately supplied. Allow- 

 ances have not been set except in the two cases mentioned. Green 

 plants and bone meal are rich sources. 



Naturally occurring deficiencies are imknown. The grass tetany 

 (page 372) once thought to be a result of magnesium deficiency is now 

 felt to arise from other causes. Experimental cases have been pro- 

 duced in rats, dogs, chicks, and calves. The characteristic symptoms 

 are reduced levels of blood Mg++, loss of the element from the 

 skeleton, dilation of the blood vessels giving a flushed appearance, 

 great excitability, convulsions, and death. 



As in the case of calcium, the efficiency of absorption of magnesium 

 depends upon the quantity of precipitating anion in the diet. Thus 

 ingested magnesium is excreted to a variable extent. Otherwise Mg+ + 

 is lost through the kidneys. 



Magnesium requirements, utilization, and excretion depend upon 

 the levels of both calcium and phosphate. An excess of any one of the 

 three can be antagonistic toward the others. A large dose of Mg+ + 

 tends to precipitate phosphate and lower its level below the normal 

 physiological range. This efl^ect thereupon interferes with the calcium 

 balance and with calcification. Thus magnesium ion can be adminis- 

 tered in toxic amounts, and the use of dolomitic limestone high in 

 MgCO.-j as a Ca++ supplement is potentially hazardous unless ade- 

 quate supplies of available phosphate are assured. Fortunately there 

 is no problem of Mg++ toxicity with ordinary feeds and foods. 



Manganese 



This element is required for proper bone formation and for the 

 development and function of the reproductive system. As is so gen- 

 erally true of the essential elements, manganese is necessary for the 



