744 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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 wall be 

 reached in which no further apparent change takes place, although 

 in reahty 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 in an inactive form, and 

 even when secreted it may still be inactive. This antecedent 

 or inactive stage is usually designated as zymogen or proferment. 

 The zymogen may be stored in the cell in the form of granules 

 which are converted into active enzyme at the moment of secre- 

 tion, 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 activa- 

 tion various terms have been employed to designate the sub- 

 stance responsible for the activation. According to a recent classi- 

 fication* it has been suggested that inorganic substances causing 

 activation shall be designated simply as activators, while organic 

 substances playing a similar role shall be named kinases. An ex- 

 ample of the latter is found in the case of the enterokinase which 

 activates the trypsin of the pancreatic secretion. 



Coenzijmes 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 

 hpase (p. 794). 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. 



