Muscle and Electric Organs 615 



The development of our knowledge regarding the chemistry of energy- 

 yielding reactions in muscle is one of the most fascinating stories in modern 

 biochemistry and is discussed extensively in most physiology and biochem- 

 istry texts.--" It is probable that the underlying reactions by w-hich energy 

 is made available to the contractile proteins are similar in all muscles. Three 

 types of energy liberations are used: (1) breakdown of the high-energy 

 phosphate bonds in the specihc phosphate donors, adenosine triphosphate 

 and either phosphocreatine or arginine phosphate; (2) glycolysis of carbo- 

 hydrate; and (3) oxidation of carbohydrate. 



The breakdown of adenosine triphosphate (ATP) liberates about 11,000 

 calories per mol for each phosphate linkage broken. The reactions of the 

 organic phosphates set off the chain of glycolysis whereby glycogen is split, 

 and its component sugars are phosphorylated, going through many inter- 

 mediaries to form lactic acid.-"'- "^" Part of the lactic acid is oxidized and a 

 larger part is reconverted to glycogen, usually in the liver. Between the ATP 

 and the glycolytic systems is interposed a carrier of phosphate, phosphagen. 

 This phosphagen in vertebrate muscle is creatine, which is reversibly phos- 

 phorylated and dephosphorylated to phosphocreatine. 



Phosphocreatine was discovered as muscle phosphagen in 1927. In the 

 following year it was found that the phosphagen of crustacean muscle is 

 arginine phosphate (phosphoarginine). The structural formulas of these 

 two substances are given in Fig. 234. From 1928 to 1937 appeared a series 

 of papers concerning the distribution of phosphocreatine (PC) and phos- 

 phoarginine (PA) in the animal kingdom. The results have been sum- 

 marized several times.-^' -■^- ^''''*- ^'^ The distribution of phosphagens is in- 

 dicated in Table 74. 



TABLE 74. DISTRIBUTION OF PHOSPHAGENS (References in text) 



