184 COMPOSITION OF THE DIGESTIVE JUICES. [cH. VIII. 



called the optimum temperature, which is generally about 

 45 C. Their action is suspended by cooling, but is com- 

 pletely destroyed by raising the temperature to 100 C. 



The enzymes are remarkably specific in their action, 

 that is, they act only on a particular substance or on a 

 group of substances having some similarity in chemical 

 composition and configuration. A striking example of 

 this is seen in the case of the glucosides (see page 103). 

 The enzyme maltase (a-glucase) hydro lyses a-methyl- and 

 a-ethyW-glucosides, but has no action on /3-methyl- or 

 /?-ethyl-d-glucosides, or on any /-glucoside or on d- or 

 /-galactosides. The enzyme emulsin (/3-glucase) acts only 

 on /3-ethyl, methyl or phenyW-glucosides. Lactase acts 

 only on the /5-galactosides. It is probable that the enzyme 

 first unites with the substrate, and to do this it must have 

 a configuration in space corresponding with that of the 

 substrate. According to Bayliss the preliminary union of 

 enzyme with substrate is a process of adsorption. Though 

 adsorption phenomena are probably of great importance 

 in enzyme action, this does not give any simple explana- 

 tion of the remarkable specificity which is characteristic 

 of enzyme action, but not of adsorption. 



An important factor in the action of an enzyme is the 

 concentration of hydrogen ions in the medium in which it 

 acts. For each enzyme there is a particular hydrogen-ion 

 concentration or PH at which the velocity of reaction is 

 greatest. This is the optimum reaction of that particular 

 enzyme. The optimum PH of certain enzymes is given on 

 page 23. It is interesting to note that in the case of 

 trypsin the optimum reaction seems to vary with the 

 nature of the substrate, being P H = 8 *o when acting on 

 trypsin and PH = 6*7 when acting on casein.* Michaelis 

 has made a special study of the significance of the optimum 

 reaction, and claims that it is partly dependent on the effect 



* The author has not been able to confirm this, finding p H = 8-1 the 

 optimum reaction for the hydrolysis of casein by trypsin. 



