118 ENZYMES 



(d) Catalytic acceleration (positive or negative) is proportional 

 to the concentration of the catalyst. This is true only within 

 limits. At very high concentrations of catalyst the acceleration 

 is not quite proportional to concentration. 



(e) The catalyst is not destroyed during the reaction, but may 

 suffer a change in physical state or be altered chemically by some 

 subsidiary reaction. 



(/) Some catalysts are specific in their action. They act best 

 in certain reactions. For example, hydriodic acid is slowlj^ 

 oxidised by hydrogen peroxide and by persulphates. The former 

 reaction is activated by tungstic acid, but not the latter. 



The great majority of vital catalytic reactions have, as catalyst, 

 an enzyme. Enzymes themselves cannot be detected or estimated. 

 Their presence is made apparent by their action. By estimating 

 the amount of the products of enzyme activity an idea of the rate 

 of reaction may be gained. Many attempts have been made to 

 isolate and purify certain enzymes and, though complete success 

 has not been granted to any investigator, much has been learned 

 of their nature and of the conditions necessary for enzyme action. 



(a) Enzymes are colloidal. They can readily be separated from 

 crystalloids by dialysis or ultra-filtration. Chemically, they 

 resemble their substrate or are so closely associated with their 

 sulistrate that existence apart is impossible. It may be that the 

 colloidal character of enzymes is the secret of their action. At 

 any rate, an artificial oxidising enzyme has been prepared by 

 mixing a suspensoid — finely divided manganese, with an emulsoid 

 — ^gum acacia. The adsorption complex so formed, if suitable 

 crystalloids were present, reacted as an artificial " laccase." 



(b) Enzymes retain their activity only over a very well-defined 

 range of temperature. It is common knowledge that physio- 

 logical processes take place most rapidly at body temperature. 

 Every biological laboratory is equipped with devices for keeping 

 incubators at a constant temperature — say, 37°-40° C. Before 

 these appliances had been perfected, investigators in this realm 

 had to keep their experimental material on their person. The 

 Abbe Spallanzani (1729-1799), in his classical work on digestion, 

 carried his digest-tubes in small pockets in his armpits for several 

 days. During the Great War, when scientific work had to be 

 carried out in all sorts of places, at least one physiologist, bereft 

 of gas regulators, had to resort to this simple but efficient method 

 of maintaining a fairly uniform temperature. In this way, 

 reactions in which they play a part differ from those usually 

 styled chemical. The rate of most chemical processes is doubled 

 or trebled when the temperature is raised 10° C. The enzymes 



