CHEMICAL CHANGES IN LIVING MATTER. FERMENTS 187 



best explained by the assumptions (a) that the enzyme or a substance closely 

 associated with it is a powerfully optically active substance ; (6) that actual 

 combination takes place between the enzyme and the ester undergoing hydro- 

 lysis. Since the additive compounds thus formed in the case of the dextro- 

 and laevo-components of the ester would not be optical opposites, they would 

 be decomposed with unequal velocity, and thus account for the liberation of 

 the optically active mandelic acid. 



We may conclude that in the action of ferments on the food sub- 

 stances, whether carbohydrate or protein, an essential factor is the 

 combination of the ferment with the substrate. Only the part of the 

 substrate, which is thus combined with the ferment, can be regarded 

 as the active mass and as undergoing the hydrolytic change. What 

 is the nature of this combination ? Ferments, which are all of a 

 colloid or semi-colloid character, cannot be dealt with in the same way 

 as the catalysts of definite chemical composition, such as molybdic 

 acid or nitric oxide. In many cases the substrate, e.g. starch or 

 protein, is also colloidal, and the combination therefore falls into the 

 class of combinations between colloids. In this we have an inter- 

 action between two substances in which the adsorption by the sur- 

 faces of the molecules of one or both substances plays an important 

 part, though this adsorption is itself determined or modified by the 

 chemical configuration of the molecules. The combination of ferments 

 with their substrates belongs, therefore, to that special class of inter- 

 actions, not entirely chemical and not entirely physical, but depending 

 for their existence on a co-operation of both chemical and physical 

 factors, which we have discussed earlier under the name of adsorption 

 compounds. 



FERMENTS AS SYNTHETIC AGENTS 



If maltase, obtained from yeast, or from the so-called takadiastase 

 (prepared from Aspergillus oryzce), be added to a solution of maltose, 

 the latter is hydrolysed to glucose. The process of hydrolysis 

 stops short of complete inversion at a point varying with the 

 concentration of the sugar solution. Thus in a 10 per cent, solution 

 of maltose, inversion proceeds until 98 per cent, of the maltose is 

 converted into dextrose, whereas in a 40 per cent, solution the change 

 stops short when 85 per cent, sugar has undergone inversion. Croft Hill 

 showed that if the maltase were added to a 40 per cent, solution of 

 dextrose, a change took place in the reverse direction, which pro- 

 ceeded until 85 per cent, of the glucose was left. The sugar formed, 

 which is a disaccharide, was regarded by Croft Hill as maltose. 

 According to Emmerling, however, it is the stereoisomeric sugar, iso- 

 maltose, which is formed ; and Croft Hill in his later papers spoke of the 

 sugar as revertose. 



In the same way it has been shown by Castle and Loewenhart 



