168 PHYSIOLOGY 



and Isevo -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 Isevo -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 best explained by the assumptions (a) that the enzyme or a substance closely asso- 

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

 takes place between the enzyme and the ester undergoing hydrolysis. 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 substances, 

 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 combina- 

 tion ? 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 com- 

 position, such as molybdic acid or nitric oxide. In many cases the sub- 

 strate, 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 

 interaction between two substances in which the adsorption by the surfaces 

 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 be- 

 longs, therefore, to that special class of interactions, 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 proceeded 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 4 the same way it has been shown by Castle and Loewenhart that the 



