370 ANIMAL BIOCHEMISTRY 



found in muscle and thought to play roles in making the chemical 

 energy ol ATP available in the form of muscular work. Both enzymes 

 catalyze: 



ATP + H2O -^ ADP + H0P03= + energy 



one being activated by calcium, the other inhibited by this ion but 

 activated by magnesium. 



Since higher animals possess endoskeletons, their complex circula- 

 tory systems are relatively exposed to mechanical damage. The evolu- 

 tionary development preventing loss of the circulatory fluid is termed 

 blood clotting. This complex process (see pages 296 and 297) requires 

 Ca++ in at least one step. Thrombin thus formed catalyzes the con- 



Prothrombin -> thrombin 



thromboplastin 



version of fibrinogen to fibrin, which polymerizes to form the clot. 

 Therefore, otherwise trivial injiuies become serious when the Ca + + 

 concentration of the blood is low. For experimental uses, blood clot- 

 ting is readily prevented by addition of any material precipitating or 

 complexing with the calcium ions. 



The permeability of cells is affected by the presence of ions, partic- 

 ularly by divalent cations and most especially by Ca+ + , which precipi- 

 tates or forms complexes with so many anions. Association of the 

 Ca++ with the cellular membranes controls the charge on the mem- 

 brane, which in turn controls the diffusion of soluble electrolytes by 

 way of electrostatic interactions. Even the penetration of non-elec- 

 trolytes into cells is affected by the presence of Ca+ + , which forms 

 organic salts with the carboxyl groups of the membrane materials. 

 These calcium salts are much more readily wet by lipides than are 

 most polar compounds occurring biologically. Passage of relatively 

 non-polar molecules into cells may be dependent upon this property 

 of calcium compounds. 



Since the proper functioning of these several physiological systems 

 depends on the maintenance of a rather definite concentration of cal- 

 cium ion, some control mechanism must be operating. Inasmuch as 

 the concentrations in all other tissues depend on that in blood, regula- 

 tion of the level in the blood suffices for the animal as a whole. The 

 necessary control is exercised in some unknown way by a hormone 

 secreted from the parathyroid system into the blood. Removal of these 

 little glands located near the thyroid leads to reduction in the Ca+ + 

 level of the blood and increased excretion to the point of deficiency 



