186 PHYSIOLOGY 



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 altera- 

 tion of the substrate. 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 o-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 mole- 

 cule which differs from the first only in its stereochemical structure. On the 

 other hand, emulsin, which breaks up /3-glucosides, has no influence on o-gluco- 

 sides. This specific affinity of the ferments for optically active groups of 

 bodies suggests that the ferment itself 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 experiments 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 and laevo-rotatory mandelic acid. The esters prepared 

 from the optically inactive acids are themselves optically inactive. Dakin 

 found that, when an optically inactive mandelic ester was acted upon by a 

 lipase prepared from the liver, the final results of the action were also inactive ; 

 but if the reaction were interrupted at the half-way point, the mandelic acid 

 which had been liberated was dextro-rotatory, while the remainder of the ester 

 was 1 SB vo -rotatory. Thus the rate of hydrolysis of the dextro-component of 

 the ester is greater than that of the Isevo-component, a result which can be 



