GENERAL METABOLISM in Vttro 85 



phosphorus in phosphocreatine derives from the y-phosphorus of 

 the adenosine triphosphate. However, resynthesis of phospho- 

 creatine from creatine and adenosine triphosphate in cerebral 

 homogenates, or with a purified preparation of creatine 

 phosphokinase, takes place slowly under conditions which can 

 only be regarded as highly artificial, such as pH 9-0 and with high 

 concentrations of substrate (Narayanaswami, 1952; Ennor and 

 Rosenberg, 1955). At present, little significance can be attached to 

 this process as a means of resynthesis of phosphocreatine in the 

 intact tissue. Attempts to replace adenosine triphosphate with 

 phosphorylated amino acids did not lead to any appreciable 

 synthesis of phosphocreatine (Tseitlin, 1953). Accepting that 

 adenosine triphosphate is the precursor of phosphocreatine the 

 turnover rate for phosphocreatine in cerebral slices can be calcu- 

 lated to be 160-170 mmoles phosphorus/g/hr. Under these condi- 

 tions the oxygen uptake of the tissue slice is 55 /xmoles Og/g/hr 

 from which it is calculated that for each atom of oxygen consumed 

 1-5 atoms of phosphate are converted to phosphocreatine. This 

 value for the rate of synthesis of phosphocreatine is in good agree- 

 ment with a similar value derived from studies under diflFerent 

 conditions and with a simpler technique (p. 124). It is of interest 

 that the rate is approximately half of the theoretical rate for 

 synthesis of adenosine triphosphate. 



Of other phosphates mentioned, incorporation of radioactive 

 phosphorus into phosphorylethanolamine was studied by Ansell 

 and Dawson (1951). As in vivo the rate of incorporation was 

 sufficiently high to show that this phosphate was synthesized 

 directly and was not formed as a result of a breakdown of phos- 

 pholipid. 



Generally the synthesis of the acid-soluble phosphates follows a 

 pattern similar to that described for brain i?i vivo. Thus inorganic 

 phosphate entering the tissue is incorporated most rapidly into 

 adenosine triphosphate and sliglitly less rapidly into phospho- 

 creatine. The establishment of the ability of the tissue slice to 

 resynthesize quantities of phosphocreatine and adenosine triphos- 

 phate approaching those existing in vivo together with the demon- 

 stration that the pattern of synthesis is also similar, provided a cell 

 containing preparation permitting the detailed examination of 

 several agents and conditions known to alter phosphate metabolism 

 in vivo. Details of these are given in Chapters 4 and 5. 



