86 GENERAL METABOLISM in vitro 



Degradatory systems. — When the cellular structure is disrupted 

 the balance between synthesis and metabolism is disturbed. The 

 activity of many enzyme systems becomes predominantly one of 

 degradation of substrates. Amongst such systems adenosine 

 triphosphatase, myokinase, and creatine phosphokinase and 

 inorganic pyrophosphatase predominate. Adenosine triphosphate 

 is converted to inorganic phosphate and adenylic acid presumably 

 by the combined action of an adenosine triphosphatase (Meyerhof 

 and Geliazkowa, 1947; Gore, 1951; Lowry et al., 1954) and 

 cerebral myokinase (Colowick and Kalkar, 1943; Oliver, 1954, 

 1955). The system, however, is not a simple one and probably 

 contains at least two enzymes acting on adenosine triphosphate, one 

 activated by magnesium ions and inhibited by calcium ions and 

 the other activated by calcium ions (Pappius and Elliot, 1954; 

 Naidoo and Pratt, 1956). The nature and extent of splitting by 

 these enzymes is not clear. The adenosine triphosphatase studied 

 by Lowry et al. (1954) removed only the terminal phosphate from 

 adenosine triphosphate and was inhibited by adenosine diphos- 

 phate. The enzyme studied by Gore (1951) split two phosphate 

 groups from adenosine triphosphate but it is not clear whether this 

 preparation contained myokinase. In both instances the rates of 

 splitting were extremely rapid being 800 and 2000 [xmoles/g v/et 

 wt. tissue hr~^ respectively (cf. Dubois and Potter, 1953). In contrast, 

 other reactions consuming adenine nucleotides appear to operate 

 at a low level of activity. These include the adenosine-5 -phosphate 

 de-aminase of dog brain (Muntz, 1953) and the transphosphoryla- 

 tions occurring between inosine triphosphate and adenosine mono- 

 and diphosphates in extracts of sheep brain (Krebs and Hems, 

 1955). The greatest rate calculable for any of these reactions is 

 about 50ju.moles/g wet wt. hr~i for adenosine-5-phosphate de- 

 aminase. The preparations of Krebs and Hems may have been 

 affected by preliminary treatment of the brain with acetone. 

 Adenosine-5-phosphate is split by a specific 5-nucleotidase at 

 rates of up to 200 /xmoles/g wet wt. hr~^ (Naidoo and Pratt, 1954). 

 The enzyme is also active against inosine-5 -phosphate (Reis, 

 1951). 



Although cerebral dispersions contain phosphatases such as 

 thiamine pyrophosphatase and the nonspecific acid and alkaline 

 phosphatases (see Pratt, 1953 ; Naidoo and Pratt, 1953 ; Feigen and 

 Wolf, 1955) simple dispersions made in sodium chloride do not 



