FACTORS AFFECTING METABOLISM in vitVO 103 



and inability to support phosphate levels in the slice is related to 

 the balance between the energy requirements of the intact tissue 

 and the energy yield during the oxidation of the substrate. The 

 complete oxidation of glucose yields —680 kcal while the oxida- 

 tion of pyruvate via the tricarboxylic acid cycle yields a free energy 

 of about —270 kcal. The oxidation of citrate to malate yields 

 —200 kcal while the oxidation of oc-oxoglutarate to oxaloacetate 

 yields —160 kcal (Burton and Krebs, 1953). There thus appears 

 to be a correlation between the free energy of oxidation of a 

 substrate and its ability to maintain adequate levels of phospho- 

 creatine in the slice. It also seems relevant that of the inter- 

 mediates listed in Table 16 only pyruvate, by combining with 

 oxaloacetate via acetyl-CoA, is likely to be capable of maintaining 

 the tricarboxylic acid cycle operating at its maximum speed. 

 Glutamic acid occupies a curious position for when used as a sole 

 substrate or in the presence of glucose, fumarate or malate, the 

 levels of phosphocreatine or adenosine triphosphate are markedly 

 reduced. Parallel with this reduction another labile phos- 

 phate accumulates (Mcllwain, 1952^), the quantities reaching 

 0-3-0-4 /xmoles phosphorus/g wet wt. Such a phosphate has not 

 been found during the metabolism of other substrates. Although 

 an identity with y-glutamyl phosphate has been suggested its 

 nature is unknown. Certainly its extreme lability in acidified 

 molybdate solutions is similar to that of the N-phosphoryl 

 amino acids. The action of glutamate in decreasing the levels of 

 adenosine triphosphate has been connected with the increased 

 synthesis of glutamine which takes place during the oxidation of 

 glutamate (Acs et al., 1953), a process well known to require 

 energy in the form of adenosine triphosphate. However, glutamic 

 acid occurs free in cerebral tissues at concentrations of 8-10 mM 

 (Weil-Malherbe, 1952; Schwerin et al., 1950) and at 10-20 mM 

 appears to be essential for the maintenance of the potassium ion 

 concentration of cerebral slices (Terner et al., 1950). Its effect in 

 decreasing levels of energy-rich phosphates in vitro, particularly in 

 the presence of glucose, thus seems to merit further examination. 

 Maintenance of phosphocreatine levels depends upon factors 

 other than the substrate. Thus after incubation in the absence 

 of glucose and in the presence of oxygen for 30 min, depleted levels 

 of phosphocreatine were restored by adding glucose (Mcllwain 

 and Gore, 1953). However incubation under anaerobic conditions 



