DIGESTION. 137 



silver, or gold be changed to the colloidal state, 1 a state in which the particles of ultra- 

 microscopic size are held in solution or perhaps suspension they become extremely 

 active catalyzers even in exceedingly small quantities. 



.From the foregoing facts and from many others it may be assumed 

 that the unorganized enzymes exist in the colloidal state. 



The Rate and Completeness of Enzymic Action. The rate and 

 completeness of enzymic action are influenced by a variety of conditions, 

 among the more important of which are temperature and the rapidity of 

 the removal of the products of their action. 



Temperature. All enzymes are sensitive to changes in temperature. 

 At o C. they appear to be incapable of inducing changes in organic matter. 

 As the temperature is raised their reaction properties develop and increase 

 in velocity, until a temperature of 40 C. to 50 C. is reached, when they are 

 at their maximum. For this reason this degree of temperature is spoken of as 

 the optimum temperature. Beyond 50 C. the velocity of their action begins 

 to decrease and at 60 C. it comes to an end for the majority of enzymes. 

 At 100 C. all reaction ceases for the reason that the enzymes are destroyed, 

 especially if they are moist. 



The Removal of the Products of Enzymic Action. The completeness 

 of enzymic action will depend on the rapidity with which the products of 

 enzyme activity are removed. This is illustrated in the following: If a 

 substrate such as fat and the enzyme lipase be mixed with water in a dialyzing 

 test-tube, the fat will combine with water, after which the fat will undergo 

 a cleavage into a fat acid and glycerin. If the products of the reaction are 

 removed practically as rapidly as they are formed the reaction will continue 

 until all the fat is so transformed. Under such circumstances the action 

 will be complete. If, however, the reaction takes place in a receptacle the 

 character of which prevents the removal of the fat acid and glycerin, the 

 reaction will in time come to an end, leaving apparently a percentage of 

 fat unchanged. The explanation at one time given for the cessation of the 

 reaction was that the accumulation of the products interfered with the 

 further action of the enzyme. It is, however, now generally admitted that 

 under the circumstances the ferment, shortly after the appearance of the 

 cleavage products, initiates a reverse action, i.e., recombines the fat acid and 

 glycerin with the re- formation of the fat until a condition of chemic 

 equilibrium is established between the opposing tendencies. The dis- 

 covery that many ferments are thus capable of secondarily reversing their 

 primary action has assisted in the interpretation of a number of obscure 

 physiologic processes. It must not be overlooked that in this instance the 

 enzyme does not initiate the reverse action, but merely hastens what would 

 take place by reason of a want of chemic equilibrium between the substances 

 present. 2 



1 The colloidal state may be developed by passing an electric current through electrodes of 

 these metals placed in distilled water. With the passage of the current particles of the metals 

 are discharged from one of the electrodes into the water in the form of a cloud. 



2 Reversibility of a chemic reaction may be denned as a recombination of the products of the 

 reaction of the original compound until a condition of equilibrium is established between the 

 analytic and the synthetic tendencies. A classic illustration of the two phases of a chemic reaction 

 is the following: If chemically equivalent amounts of acetic acid and ethyl alcohol are mixed at a 

 definite temperature a reaction occurs which eventuates in the formation of ethyl acetate and 

 water. In this instance after a certain* percentage of these substances has thus united the prod- 



