324 PRINCIPLES OF GENERAL PHYSIOLOGY 



either by chemical or physical change. Hydrolytic actions, and it is in these that 

 the question arises, are practically thermo-neutral, as pointed out by van't Hull' 

 (1909, p. 1075); the heat change is very small; in the conversion of one gram- 

 molecule of methyl acetate to one of alcohol and one of acid, only 0'9 large 

 calories, thus : 







Heat of combustion of methyl acetate - - 170-6 



,, acetic acid - 61'7 



232-3 

 methyl acetate - 233 -2 



Difference - 0'9 



In other words, only 0'38 per cent, of the heat of combustion of the ester. The 

 small amount of energy required to change the equilibrium position, in the case of 

 ethyl butyrate, in the experiments of Dietz from 85-5 per cent, of ester, when acid 

 was the catalyst, to that of 75 per cent, when lipase was used, might quite 

 conceivably, as Herzog (1910, p. 196) points out, be obtained from surface or 

 volume energy of some kind. In fact, the difference between catalysis by acid and 

 that by enzyme consists essentially in the circumstance that the former results in 

 an equilibrium in a homogeneous system, the latter in one in a heterogeneous 

 system. 



If this shifting of the equilibrium position is due to the supply of energy from some 

 component of the system, it seems difficult to suppose that it can be from the enzyme itself : 

 if so, the equilibrium should not be the same with different amounts of enzyme, whereas \\e 

 have seen that, in experiments in which it was certain that equilibrium \v;is really attained, 

 the concentration of enzyme played no part in the equilibrium. It may be, nevertheless, that 

 the amount of energy required is so small that it was supplied by the smallest coin-eiitiati.ni 

 used. 



A subsidiary point is worth mention here. We have seen that lipase is increased in its act i\ it y 

 by bile-salts, and the question arises, does this increased activity affect both tin- components 

 of the reversibler eaction? It has been shown by Hamsik (1910) that it does ; the position of 

 equilibrium is unaffected. The fact serves to confirm the view taken of the mode of action 

 of the co-enzyme in this case, namely, that it increases the active surface of the en/vine. 



Bourquelot et Bridel (1914) made the interesting observation that, if maltase and eimilsin 

 together act on glucose and alcohol, so that a mixture of the a- and /3-ghtooadee is formed, the 

 same composition of the sj'stem in equilibrium is attained, whatever the relative amount or 

 the order in which the enzymes are added. There must, therefore, be a conversion of the one 

 glucoside into the other, presumably after previous hydrolysis. 



MODE OF ACTION 



The manner in which catalysts act appears to be of more than one kiml. s.> 

 that no satisfactory general statement can he made. 



Formation of Intermediate Compounds. In one case, that of the acceleration 

 of the reaction between hydriodic acid and hydrogen peroxide by molylidic arid, 

 this stage of intermediate combination has been satisfactorily shown by Brode 

 (1901) to be passed through. A series of permolybdic acids is formed by tin- 

 action of hydrogen peroxide on the catalyst ; these are formed with great 

 rapidity and, when formed, they react also with great rapidity on hydriodic acid, 

 with separation of iodine and return of the catalyst to its original form of 

 molybdic acid. Both these reactions together occur at a greater rate- than the 

 original uncatalysed reaction between hydriodic acid and hydrogen peroxide, 

 so that the criterion of Ostwald (1899, p. 517) as to the conditions to be satisfied 

 for such an explanation to be admissible are'present. 



It must l>e admitted, however, that this case of catalysis with the formation of an inter- 

 mediate comjMmnd of a chemical nature appears to be an exceptional one. In some cases, 

 indeed, as in thfe catalysis of methyl acetate by hydrochloric acid, it is found that the 

 reaction by way of methyl chloride takes loiujrr than the spontaneous actual one, so that this 

 obvious intermediate compound is excluded. 



Adsorption. It was suggested above that the increased concentration 

 produced by adsorption might perhaps be sufficient to account for the greater 

 rate of reaction. But it scarcely seems possible to explain the existence of such 



