78 GENERAL METABOLISM in vitrO 



General Metabolism 



One of the major aims of studies of metabolism in vitro is the 

 integration of the resuhs obtained with the events occurring in 

 vivo. This aim is most hkely to be assisted if the preparations used 

 in vitro correspond, in the metabohc activity being studied, to the 

 tissue ill vivo. Though this ideal is as yet only partly achieved, 

 much information has been provided relevant to the major theme. 

 The following account relates to the general metabolism, in tissue 

 preparations in vitro^ of phosphorylated compounds found to be 

 affected by differing conditions and stimuli in vivo. 



Acid-soluble Phosphates 



Quantities and syntheses. — Removal of the brain results in a 

 rapid breakdown of phosphocreatine and adenosine triphosphate 

 (Table 10). When slices of cerebral tissue are incubated in a 

 suitable oxygenated saline containing glucose these phosphates are 

 resynthesized (Table 12), phosphocreatine reaching levels of 

 about 1-3-1-5 /xmoles/g wet wt. tissue. Although the levels of 

 phosphocreatine and adenosine triphosphate finally reached appear 

 to be lower than levels found in vivo (Table 1), differences can be 

 partly ascribed to the higher water content of the slices. As 

 normally prepared, slices imbibe some 20% of their weight of 

 water before being weighed making the results in Table 12 too low 

 by about one-fifth. Even so the levels of phosphocreatine and 

 adenosine triphosphate finally reached do not normally exceed 

 60% of the quantities found in vivo. Other causes include loss of 

 up to 50% of adenine from nucleotides to the medium (Acs et al., 

 1953) and 70-80% of the free creatine of the tissue (Thomas, 

 1956). Addition of these to the incubation medium (Thomas, 

 1956, 1957) results in an increased resynthesis to quantities of 

 2-0 jLtmoles phosphocreatine and 1-8-1-9 /xmoles adenosine triphos- 

 phate/g wet wt. tissue corrected for swelling. Resynthesis to 

 these higher levels is extremely slow, requiring some 2-3 hr, in 

 contrast to the initial rapid increase in phosphocreatine which takes 

 place within a few minutes. This situation is analogous to the 

 slow synthesis of glycogen in cerebral slices in vitro (Le Baron, 1955) 

 but may also be due to a slow penetration of creatine and adenine 

 into the tissue (Thomas, 1956). 



The exchange of inorganic phosphate between a cerebral slice 



