Discussion 15 



accustomed to the idea that the initiating step of glycolysis is the hexo- 

 kinase reaction or a reaction preceding it such as the penetration of 

 glucose into the cell. This is the initiating reaction; the rates of the 

 following reactions do not depend on the amounts of enzyme, but on the 

 amounts of substrate supplied by the preceding steps. 



Mcllwain: One has to be careful in the use of the term "excess", 

 particularly if one implies unutilized excess ; as has been pointed out, the 

 intermediates often do not accumulate. I feel that what may seem to be 

 unutilized excess in potential enzyme activity is very important in 

 maintaining an ordered sequence of reactions, such as the tricarboxylic 

 acid cycle. In the cycle in a given tissue one may have certain stages 

 with enzyme capacities 50 times those of others. This is probably con- 

 cerned with keeping the cycle functioning as a cycle ; for in comparing 

 the levels of such enzymes e.g. in nervous tissue, one concludes that it is 

 the particular stages of the cycle where there is the possibility of diverg- 

 ing from it, that certain enzymes appear at high activity in comparison 

 with the others. Thus, the high activity can be related to the functioning 

 of the tissue. 



I have also been impressed by the extent to which the total level of 

 glycolytic systems in tissues from the central nervous system can give 

 approximately the same yield of energy-rich phosphate as the oxidative 

 series. Sometimes the glycolytic series may have enzyme levels of the 

 order of 20-30 times those of steps of the oxidative series. It appears to 

 be a real alternative, though normally functioning only briefly at maxi- 

 mum rate, and the high level of enzyme activity is necessary to make 

 the anaerobic reaction a real alternative. 



Krebs : Does this apply also to hexokinase? That is the critical point. 

 When I said "in excess" I meant that more can be dealt with by these 

 enzymes than can be supplied by the initial step. 



Mcllwain : The rate of hexokinase in, say, cerebral tissues, is approxi- 

 mately ten times that of the recorded rates of the initial stages of the 

 tricarboxylic acid cycle, so that the rate at which hexokinase operates 

 is more akin to the level of glycolytic enzymes. 



Hinshelwood: That is under the condition of an in vitro test. Are they 

 in these large excesses under the conditions of formation in the tissue? 

 The proportion of enzymes would be determined by the optimum con- 

 ditions of the original growth and formation of these enzymes in the 

 tissues, not by the level of their amount of activity in an in vitro test, 

 which is an artificially established condition. 



Mcllwain : A relevant point is that the high activity of the enzymes 

 themselves keeps their intermediate substrates at such relatively low 

 levels ; and that, I feel, is part of the mechanism of integration of their 

 individual reactions to a cycle. 



Siekevitz : On the hexokinase point, LePage has done experiments in 

 which he compared normal and tumour liver, and he found that in the 

 normal liver hexose diphosphate was a much better substrate for 

 glycolysis than glucose, and he came to the conclusion that hexokinase 

 is limiting (LePage, G. A. (1950). Cancer Res., 10, 77). 



Krebs : Many experiences have shown that hexokinase is limiting. 



