158 GENERAL COMMENTS 



bond with ion transport is not known (for some suggestions see 

 Davies and Krebs, 1952) it is likely to involve a variety of factors, 

 amongst which the relative speeds of the reactions concerned are of 

 some importance. It has been calculated (Sloane-Stanley, 1952) 

 that for a tissue consisting of closely packed nerve fibres, similar to 

 those of squid axon but all between 1 and 10 micron diameter a 

 quantity of 10^ /xmoles of sodium/g wet wt. hr-^ would need to be 

 transported. This rate is very considerably in excess of any reaction 

 involving lipid metabolism which has so far been studied in brain 

 and at the moment would appear to be a serious obstacle to 

 interpretation of ion transport in terms of a lipid carrier molecule. 

 Another discrepancy occurs between events such as the rise and 

 fall of the action potential which may occupy 1 msec or less and 

 the rates of change in the levels of energy-rich phosphates which 

 have been measured in seconds only. That the energy derived 

 from the metabolism of, say, phosphocreatine, may be more than 

 sufficient for the above processes to occur can be inferred from 

 experiments upon Electrophorus electricus (Nachmansohn et al., 

 1943) in which it was found that the energy released during a 

 discharge was only a quarter of that made available by the 

 accompanying breakdown of phosphocreatine. It is unlikely that 

 experiments of this type can be performed with cerebral tissues 

 i?i vivo, for whereas the electrical energy derived from the electric 

 fish is a summation from a series of cells discharging in a similar 

 direction, the electrical energy of cerebral tissue measured across 

 any large section is the summation of discharges many of which 

 cancel one another out. In considering the above discrepancies, it 

 is to be remembered that the speeds of the chemical reactions 

 measured have been determined in arbitrary units, i.e. /xmoles/g 

 wet wt. tissue hr"^, which is not a true measure of enzymic 

 activity. Estimated in this way, phosphate metabolizing systems of 

 brain such as phosphocreatine phosphokinase and adenosine 

 triphosphatase can split off or transfer the phosphorus of their 

 substrates at 3600 and 2000 /xmoles/g hr" ^ respectively. In terms of 

 enzymic protein or of the weight of tissue occupied by the enzyme 

 these rates are probably very much higher and further work on 

 their localization, isolation and purification is needed. In this 

 connexion, work aimed at analysing, and localizing enzyme action 

 in microquantities of tissue or in individual nerve cells (see 

 Lowry, 1957) is of major importance. 



