DISACCHARIDES 79 



which is found in certain plant-tissues and which has no action upon 

 maltose, hydrolyzes this sugar with the production of .two molecules 

 of glucose. This glucose, unlike the glucose which is produced in the 

 hydrolysis of maltose, is of low initial rotatory power. On adding a 

 drop of ammonia to its solution the rotatory power increases. Hence, 

 this sugar, which is called Isomaltose, is a derivative of 0-glucose. It is 

 a mixture of a-glucose-/3-glucoside, and /3-glucose-/3-glucoside. 



Milk-sugar, also called Lactose, has not been encountered in the 

 vegetable kingdom. It does not occur preformed in any item of our 

 diet excepting in milk, nor does it appear likely that one of its con- 

 stituent hexoses, galactose, is commonly obtainable from any other 

 dietary source than milk. Of course it might be obtained from brain- 

 tissue, but this cannot be regarded as a customary item of our dietary. 

 Lactose yields, on hydrolysis, one molecule of d-glucose and one of 

 d-galactose. Lactose exhibits mutarotation, reduces Fehling's solution, 

 and forms a phenylosazone. Lactose is not hydrolyzed by maltase, 

 invertase, diastase or emulsin, but it is hydrolyzed by a specific fer- 

 ment designated Lactase, and found in the gastric mucous membrane 

 and in a few yeasts such as Kephir yeast. This yeast is employed by 

 the Arabs to make a sparkling alcoholic beverage, "Kephir," from 

 the milk of mares. 



Milk-sugar is found in varying amounts in the milk of all mammals. 

 During pregnancy it is often found in small quantities in the urine; 

 and after weaning it also tends to escape for a few days through the 

 kidneys. Extirpation of the mammary glands in milch-goats and 

 cows gives rise to a notable increase in the amount of sugar in the 

 blood (Glucohemia) and also to the appearance of glucose in the urine 

 (Glycosuria) . It thus appears probable that in the mammary glands 

 milk-sugar is formed from glucose alone, and not from glucose and 

 galactose in the diet. On comparing the formulae of glucose and 

 galactose: 



HCOH HCOH 



- ;: 



HCOH\ \ HCOH 



> < 



HOCH / \ HOCH 



CH CH 



HCOH HCOH 



CH 2 OH CH 2 OH 



d-glucose. d-galactose. 



it will be seen that the transformation of galactose into glucose involves 

 the rupture of the oxide-ring and its closure again on the opposite side. 



