THE CARBOHYDRATES AXD THEIR METABOLISM 245 



that have no frco aldehyde or ketone do not form osazones. Other than 

 the phenyl osazones, the disaccharides form no compounds that are char- 

 acteristic. 



In the determination of the configuration of the disaceharides, the 

 chief points to ho elucidated were (1) the nature of the component hexoses, 

 (2) whether the disaccharide was an a- or ^-glucoside, (3) the place of 

 union of the two monosaccharides. 



The nature of the component hexoses was determined by hydrolyzing 

 the disaccharide with acid and identifying the hexoses. The nature of the 

 iihicosidic union was established by the behavior of the disaccharide toward 

 maltose and emulsin. If the disaccharide is hydrolyzed by maltase, it is an 

 a-glucoside; if by emulsin, it is a p-glucoside. This point has also been de- 

 termined by studying the optical behavior of the hexoses as soon as formed 

 by the action of an enzyme, toward a drop of alkali. If the rotation is 

 increased, it indicates the presence of a (3-glucose; if the nrata rotation is 

 in the other direction, an a-glucose has been formed. 



Points of special interest of the individual disaccharides will now 

 bo presented. 



Sue rose.- Sucrose, known also as saccharose or cane sugar, is indus- 

 trially the most important of the disaccharides. It is very widely dis- 

 tributed in the plant world, where it serves chiefly as a reserve material. 



It crystallizes readily, is very soluble in water and very sweet. It 

 does not exhibit mutarotation in aqueous solution. It is dextrarotary and 

 has a specific rotation of + 66.5. When heated, it melts at 160 0., 

 and at 200 C. it darkens, forming caramel, in which process water is 

 given off. 



Chemically, sucrose behaves neither as an aldehyde nor as a ketone; 

 it does not form hydrazones or osazones, nor does it reduce Fehling's 

 solution. Sucrose is readily hydrolyzed by boiling with acids, one mole- 

 cule of glucose and one of fructose being formed. The same hydrolysis 

 may be brought about by an enzyme, invertase or sucrase, present in yeasts 

 and other fungi, as well as in many other plants and in the digestive tracts 

 of many animals. 



The products of hydrolysis of sucrose have a resultant levorotation, 

 since fructose is more levorotatory than glucose is dextrorotatory. This 

 process is therefore called inversion and the product invert sugar. Because 

 sucrose exhibits neither aldehyde nor ketone properties, it is believed that 

 the glucose and fructose molecules, that compose the sucrose raolecule, are 

 united in such a way that both aldehyde and ketone groups are destroyed. 

 The formula usually ascribed to sucrose, is Fischer's modification of the 

 Tollens formula, in which it is both a glucoside and a fructoside. 



Lactose. Lactose or milk sugar was first obtained about 1615 by 

 Fabricio Bartoletti. It is always found in the mammary secretion, but 

 has not been found in the vegetable kingdom. It is often found in the 



