COMPOSITION OF FOOD AND ACTION OF ENZYMES. 755 



be carried down mechanically by flocculent precipitates produced in 

 their solutions. If protein present in the solution is precipitated, 

 for instance, the enzymes may be carried down with it in part. 

 The mode of union of the enzyme with the precipitate in these 

 cases comes under the general head of mechanical adsorption. It 

 consists in a concentration of the enzyme at the limiting surface 

 between the particles of the precipitate and the solution. 



Incompleteness of their Action. In any given mixture of a sub- 

 stance and its enzyme the action of the latter is usually not com- 

 plete, that is, all of the substance does not disappear. One ex- 

 planation for this fact has been found in the reversibility of the 

 action of the enzyme. If the reaction proceeds in both directions, 

 then evidently under fixed conditions a final equilibrium will be 

 reached in which no further apparent change takes place, although 

 in reality the condition is not one of rest, but of balance between 

 opposing processes proceeding at a definite rate. In addition to 

 this factor it may be shown in some cases that the products of the 

 reaction serve to retard further action, possibly by forming a 

 compound of some kind with the enzyme. Within the body itself, 

 on the contrary, the action of an enzyme may be complete, since 

 the products are removed by absorption. 



Active and Inactive Form. In many cases it can be shown 

 that the enzyme exists within the cell producing it in an inactive 

 form or even when secreted it may still be inactive. This 

 antecedent or inactive stage is usually designated as zymogen 

 or preferment. The zymogen may be stored in the cell in the 

 form of granules which are converted into active enzyme at the 

 moment of secretion, or it may be secreted in inactive form and 

 require the co-operation of some other substance before it is 

 capable of effecting its normal reaction. In such cases the 

 second substance is said to activate the enzyme. In connection 

 with the process of activation various terms have been employed 

 to designate the substance responsible for the activation. Accord- 

 ing to a recent classification* it has been suggested that inorganic 

 substances causing activation shall be designated simply as acti- 

 vators, while organic substances playing a similar role shall be named 

 kinases. An example of the latter is found in the case of the entero- 

 kinase which activates the trypsin of the pancreatic secretion. 



Coenzymes or Coferments. In addition to the process of ac- 

 tivation it would seem that in some cases the action of an enzyme 

 is facilitated by, or perhaps is even dependent upon the presence 

 of some other substance. Perhaps the best example of this com- 

 bined activity is furnished by the influence of bile salts upon 

 lipase (p. 804) . These cases of coactivity are to be distinguished 

 from activation by the fact that the combination may be easily 

 * Samuely, in "Handbuch der Biochemie," i., 1908. 



