CH. VIII.] ENZYMES. 185 



of the reaction on the nature of the dissociation of the 

 enzyme, which in many cases functions as an ampholyte 

 (see pages u and 31). At the isoelectric point of an 

 ampholyte it exists mainly in the undissociated state. If 

 the hydrogen-ion concentration be greater than at the 

 iso-electric point, the enzyme is positively charged, that is, 

 it exists mainly as kations : if the solution be alkaline 

 to the iso-electric point, the enzyme exists mainly as anions, 

 Michaelis has determined the iso-electric points of certain 

 of the enzymes by various methods, and concludes that 

 maltase, trypsin and erepsin are only active as anions ; 

 pepsin as kations ; whilst invertase is only active as un- 

 dissociated molecules. He makes the interesting sug- 

 gestion that though undissociated pepsin has no action on 

 ordinary proteins, yet it has the property of clotting milk, 

 that is a rennetic action (see p. 208). 



The action of most enzymes is retarded by the accumu- 

 lation of the products of the reaction, and in certain cases 

 the reaction is reversible. 



This is well seen in the case of lipase, which induces 

 the following reaction : 



Ethyl butyrate + water ~ > ethyl alcohol + butyric acid. 



The velocity of reaction is proportional to the amount 

 of the enzyme present, provided that the amount of the 

 enzyme is very small compared with that of the substrate. 

 If the amounts of enzyme and substrate are at all com- 

 parable, the laws of mass action are followed. But com- 

 plications are introduced by the fact that some of the 

 enzyme is thrown out of action by being absorbed by the 

 products of the action. 



In certain cases enzyme action is dependent on the 

 simultaneous presence of two substances. These are 

 sometimes called co-ferments. It has been shewn that 

 the zymase that is responsible for the alcoholic fermenta- 

 tion of sugar by yeast can only act in co-operation with 

 phosphates and some substance that is diffusible and not 



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