80 DISACCHARIDES, POLYSACCHARIDES AND GLUCOSIDES 



No enzyme has yet been isolated which is capable of bringing about 

 this transformation. 



It is for this reason and possibly also for others connected with the 

 metabolism of the intestinal bacteria, that maltose or cane-sugar can- 

 not be regarded as satisfactory substitutes for milk-sugar in the diet 

 of young infants. On the other hand, the assimilation-limit or quantity 

 which may be ingested at once without leading to alimentary glycosuria, 

 is lower for lactose than for the other sugars, so that if large quantities 

 of sugar have to be given to make up the requisite calorific value of the 

 diet, as in the case of fat-intolerant infants, maltose may be used as an 

 accessory to lactose in the food. Cane-sugar being the disaccharide 

 which is most foreign to animal tissues, and also the sweetest in taste, 

 is much less suitable than milk-sugar or maltose for the dietary of 

 young infants. 



The galactose which milk-sugar yields on hydrolysis is of low rotatory 

 power, and its rotation increases on adding ammonia. Hence lactose 

 is glucose-/3-galactoside, since it can be shown by forming the osazone 

 of the sugar and hydrolyzing, when the phenylhydrazine remains 

 attached to the sugar with the free (potential) aldehyde group, that 

 it is the glucose radical which contains the potential aldehyde-group, 

 the aldehyde-group of the galactose offering the point of union for the 

 glucose molecule. The potential aldehyde-group of the glucose radical 

 can exist either in the a- or the /3-form. The a-lactose (rotatory power 

 = -f- 86) is therefore a-glucose-jft-galactoside, while ,8-lactose (rotatory 

 power = + 35) is /3-glucose = 0-galactoside. No derivative of a- 

 galactose is certainly known to occur in nature. If, however, kephir 

 lactase be allowed to act upon a concentrated mixture of equal parts of 

 glucose and galactose, two isomeric lactoses are produced, both 

 exhibiting mutarotation, and both yielding d-glucose and d-galactose 

 on hydrolysis. One of these is ordinary lactose, the other has been 

 termed Isolactose and is possibly a mixture of a- and 0-glucose-a:- 

 galactosides. 



Each of the disaccharides which contains a potentially active 

 aldehyde-group can, therefore, exist in four different forms. Thus for 

 maltose we have: 



a glucose-a-glucoside \ a- 



glucose-a-glucoside / ft- maltose 



a glucose-/3-glucoside \ a- . ., 

 ft glucose-0-glucoside / ft- ^omaltose 



and for lactose we have: 



a glucose-a-galactoside \ a- . , 



ft glucose-a-galactoside / ft- lsolactose 



a glucose-/3-galactoside \ a- . 

 ft glucose-0-galactoside / ft- lact< 



