io8 THE CARBOHYDRATES 



(b) Those giving rise on hydrolysis to two different hexoses 

 such as sucrose, turanose, lactose, and melibiose. 



The isomerism between the various members of the first 

 group may be due to a different mode of attachment of the two 

 hexoses, involving in some cases the reducing groups so that, as 

 in the case of trehalose, sucrose, and turanose, the resulting 

 disaccharide has no reducing properties. On the other hand, 

 two structurally identical sugars may differ in stereochemical 

 formula, i.e. in the spatial arrangement of the two constituent 

 sugars with the resultant production of two isomeric a- and 

 j8-disaccharides corresponding to the a- and j8-glucoses ; such 

 a relationship is found to exist between maltose and isomaltose, 

 the former of which is hydrolysed by maltase while the latter 

 is only attacked by emulsin. Similarly, the disaccharides 

 gentiobiose and cellobiose appear to belong to the jS-glucosides, 

 since they are not attacked by maltase but are acted upon by 

 emulsin. 



In addition to the above considerations, an exact knowledge 

 of the nature of the anhydride ring of the constituent mono- 

 saccharides is requisite for a complete understanding of the 

 constitution of given disaccharide ; this may be seen by the 

 alternative formulae given for glucose on page 83. 



Action of Enzymes on Disaccharides. 



{a) Hydrolytic Enzymes. — The hydrolysis of disaccharides 

 is effected by enzymes such as maltase and emulsin, which 

 act on more than one substrate, and in some cases the hydrolysis 

 can only be effected by a specific enzyme such as invertase 

 (sucrase), which acts only upon sucrose. 



Attempts to utilize enzymes for the synthesis of disac- 

 charides as well as for their hydrolysis were initiated by Croft- 

 Hill who, by acting upon a solution of glucose with a yeast 

 extract of maltase, was able to synthesize a disaccharide to 

 which he gave the name of revertose, but which was subse- 

 quently identified as isomaltose, the jS-glucosidic isomer of 

 maltose. Since then, largely as the result of the work of 

 Bourquelot * and his co-workers, gentiobiose, cellobiose, and 



* Bourquelot : " Ann. d. Chim.," 1920, (9), 13, 5. 



