166 PHYSIOLOGY 



there will be a definite amount of all four substances present in the mixture, namely, 

 water, alcohol, ester, and acid. This equilibrium point can be shifted by altering 

 the amount of any of the four substances. Thus the interaction of methyl acetate and 

 water can be diminished to any desired extent by adding to the mixture the products 

 of the interaction, namely, methyl alcohol and acetic acid. 



There is evidence that some of the ferment actions are reversible. Thus 

 maltase acts on maltose with the formation of two molecules of glucose. 

 If the maltase be added to a concentrated solution of glucose, we get a 

 reverse effect, with the production of a disaccharide which has been desig- 

 nated as isomaltose or revertose. To this reverse action may be due a 

 certain amount of the retardation observed in the action of trypsin on 

 coagulable protein. A more important factor is probably the combination 

 of the ferment itself with the end-products and the consequent removal 

 of the ferment from the sphere of action. Several facts speak for such a 

 mode of explanation. Thus the action of lactase on milk sugar is not 

 retarded by both its end-products, namely, glucose and galactose, but only 

 by galactose. In the same way the action of invertase on cane sugar is 

 retarded by the end-product fructose, but not at all by the other end- 

 product, glucose. 



So far therefore a study of the velocity of ferment actions would lead 

 us to suspect that the ferment combines in the first place with the substrate, 

 and that this combination is a necessary step in the alteration of the sub- 

 strate. In the second place, the ferment is taken up to a certain extent 

 by some or all of the end-products, and this combination acts in opposition 

 to the first combination, tending to remove the ferment from the sphere 

 of action, and therefore to retard the whole reaction. Other facts can be 

 adduced in favour of these conclusions. Thus it has been shown that 

 invertase ferment, which is destroyed when heated in watery solution at a 

 temperature of 60 C., can, if a large excess of its substrate, cane sugar, 

 be present, be heated 25 higher without undergoing destruction. The 

 same protective effect is observed in the case of trypsin. Trypsin in watery 

 or weakly alkaline solutions undergoes rapid decomposition. At 37 C. it 

 may lose 50 per cent, of its proteolytic power within half an hour. If, on 

 the other hand, trypsin be mixed with a protein such as egg albumin or 

 caseinogen, or with the products of its own action, namely, albumoses and 

 peptones, it can be kept many hours without undergoing any considerable 

 loss of power. 



It has been found that, whereas maltase splits up all the a-glucosides, it has no 

 power on the /?-glucosides ; that is to say, maltase will fit into a molecule of a certain 

 configuration, but is powerless to affect a molecule which differs from the first only 

 in its stereochemical structure. On the other hand, emulsin, which breaks up /S-gluco- 

 sides, has no influence on a-glucosides. This specific affinity of the ferments for optically 

 active groups of bodies suggests that the ferment itaelf may be optically active. We 

 cannot of course isolate the ferment and determine its optical behaviour ; but that 

 it is optically active is rendered probable both by these results and certain results 

 obtained by Dakin on lipase, the fat-splitting ferment. Dakin carried out his experi- 

 ments on the esters of mandelic acid. Mandelic acid is optically inactive, but this 

 optically inactive modification consists of a mixture of equal parts of dextro-rotatory 



