46 BACTERIOLOGICAL CHEMISTRY 



from 0-25 saturated ammonium sulphate ; chymotrypsin 

 crystallises from 0-01 N sulphuric acid, whilst trypsin 

 crystallises from solution in 0-5 saturated magnesium 

 sulphate solution in borate buffer at ^jH 9. 



The course of purification procedures can only be 

 followed by measurement of the activity of samples 

 of the enzyme at various stages, since no chemical methods 

 of estimating the enzyme as such are available. The 

 enzyme activity is expressed in terms of enzyme units, 

 which define the amount of change which the enzyme 

 can bring about under standardised conditions of con- 

 centration of substance acted on, ^^H, temperature, con- 

 centration of activators, salt concentration and amount 

 of change. As an example may be quoted Willstatter's 

 invertase unit, which is the amount of enzyme which will 

 reduce the rotation (after addition of alkali to bring 

 about mutarotation of a-glucose) of 4-0 g. of sucrose in 

 25 ml. of a 1 per cent, solution of NaH2P04 at 15-5° C. 

 to 0° in one minute. The activity of an enzyme prepara- 

 tion is usually quoted as the number of units per gram 

 of dry weight. It is clear that the unit and the necessary 

 set of conditions will differ for each enzyme, but must 

 be rigidly adhered to for any one enzyme. 



The Effect of Conditions on Enzyme Action. — (a) 



Concentration of Enzyme. — In general the rate of reaction 

 is proportional to the enzyme concentration as long as 

 other conditions such as piL are maintained constant by 

 the use of buffer solutions. In cases where deviations 

 from this rule have been observed they have usually been 

 traced to the gradual destruction of the enzyme, or the 

 production of inhibitors or other substances which com- 

 bine irreversibly with the enzyme and so alter its effective 

 concentration. It is to be emphasized that the equilibrium 

 condition of the reaction is not altered by the presence 

 or by the amount of the enzyme unless the products of 

 the reaction are involved in side reactions resulting in a 



