314 A. C. R. Dean and Sir Cyril Hinshelwood 



of the parts played by adaptive processes and by the mechan- 

 isms of mutation and selection. 



2. Variations in Enzyme Activity during 

 the Growth Cycle 



In any normal growth cycle changes of pH occur and these 

 cause variations in the amounts of certain enzymes according 

 to a mechanism which is clear from the work of Gale and Epps 

 (1942). The activity of unit mass of an enzyme is a function 

 of the pH. If the pH is altered so as to lower the specific 

 activity, more enzyme is required to effect the same amount 

 of chemical change. Gale and Epps found that in certain 

 cases as the pH became adverse the amount of enzyme per cell 

 did in fact increase, the tendency being thus towards the 

 preservation of a constant total intensity of chemical action. 

 This result can be generally understood in the light of the 

 kinetic principles, outlined in the introduction, which lead to 

 the expectation of an optimum growth rate under the pre- 

 vailing conditions. 



The mechanism which operates in the experiments of Gale 

 and Epps we may call simple pH response. That it is not the 

 only mechanism causing variations in the amounts of enzymes 

 in a cell during the growth cycle is clear from the work of 

 McCarthy (1959), of which a brief summary will now be given. 



McCarthy investigated the variations during a growth cycle 

 of Bacterium lactis aerogenes of the amounts per cell of the 

 following enzymes: catalase, glucose and succinic dehydro- 

 genases, asparagine deaminase, and phosphatase. 



The effect observed by Gale and Epps was found in a 

 number of examples, especially with the deaminase. The 

 amount rose steeply towards the end of the growth cycle as 

 the pH fell. When an inoculum from the acid medium was 

 transferred to fresh medium at pH 7 the amount of enzyme 

 per cell rapidly fell, by a process of simple dilution (cf. 

 Pollock, 1958), the new material synthesized at the more 

 favourable pH containing a smaller content of the deaminase. 



