MINERAL METABOLISM 369 



thai any one oC ihc elements listed is more important than another 

 because more is known about it. Nor is it safe to say that aiiv element 

 is non-essential, for it is philosophically impossible to prove that an 

 element is not required. 



On the contrary, as methods are devised and improved new members 

 appear on the list of required elements. Therefore, the group clas- 

 sified as incidental elements includes those found in animals, though 

 for several of these there is no known role or requirement in animals. 

 A few have the special functions mentioned in particular species. 



Where the available information warrants, the special biological 

 roles, dietary requirements, deficiency states, toxicities, excretions, and 

 physiological interactions are discussed for each ion listed. Certain 

 more generalized roles are considered in four sections at the end of the 

 chapter. 



Cal 



cium 



This most abundant inorganic element of the higher animals is 

 located predominantly in the skeletal tissues, where it plays a primary 

 role. Outside the bones and teeth calcium is distributed in small 

 amoimts throughout all the tissues where it has special roles in con- 

 trolling nerve and muscle action, blood clotting, and cell permeabil- 

 ity. The intercellular fluid averages about 20 mg. of Ca++ per 100 g. 

 of tissue. Of this quantity some is in free form and some is complexed 

 with proteins. 



Part of the soluble calcium participates in the transmission of nerve 

 impulses. The overall process was mentioned on page 306 and de- 

 pends in part on the formation of acetylcholine from choline and 

 acetyl CoA in the presence of choline acetylase. The catalytic activity 

 of this enzyme requires both calcium and potassium ions. Thus with- 

 out Ca++ acetylcholine is not formed and transmission of impulses 

 along the nerves does not occur. 



Stimulation of a muscle fiber by a nerve impulse leads to contrac- 

 tion of the fiber when the biochemical environment is suitable (see 

 page 303). Although the detailed mechanism is in doubt, Na+, K + , 

 and Ca + + are required. Sodium apparently is necessary for the con- 

 tractility of animal muscle in contrast to potassium, which has a para- 

 lyzing effect. Since K+ is always present, the concentration might often 

 rise to levels interfering with muscular function. However, Ca+ + 

 antagonizes this property of K+ and maintains a state of balance. 

 Hence, though indirect, this role of calcium is of prime importance in 

 muscle. More directly, Ca + + affects the activities of two enzymes 



