CARBOHYDRATES 71 



constituent of lactose or milk sugar and in certain substances in 

 brain and nerve tissue. It is prepared from milk sugar or from 

 various gums by hydrolysis. Galactose is somewhat less soluble 

 in water than glucose. The solution is dextrorotatory, the spe- 

 cific rotation being +81. Galactose is an aldose and gives the 

 usual reduction tests, and forms with phenylhydrazine an osa- 

 zone which melts at 192-195. Galactose ferments slowly but 

 completely with ordinary yeast. If heated with nitric acid it 

 forms mucic acid which is relatively insoluble and forms a fine 

 white precipitate. This test serves to distinguish galactose from 

 all sugars except lactose. It is of interest that the mammary 

 gland constructs it out of the glucose of the blood, and unites it 

 with glucose to form lactose or milk sugar. 



Amino Sugars. Closely allied to the monosaccharides are 

 the amino sugars, which differ from the simple sugars only in 

 having an amino group ( NH 2 ) instead of an OH attached 

 to the carbon atom next the aldehyde group. These compounds 

 have been obtained by the hydrolysis of complicated substances 

 occurring in the shells of lobsters and from the proteins mucin 

 and mucoid which are widely distributed in the animal world. 

 d-Glucosamine is an important member of this group. It is ob- 

 tained by boiling the chitin of lobster shells with hydrochloric 

 acid. It is readily soluble in water, the solution being alkaline. 

 Its hydrochloric acid salt shows solubilities similar to those of 

 the monosaccharides. The solution is dextrorotatory, having a 

 specific rotation of from + 70 to + 74 according to concen- 

 tration. It reduces the ordinary carbohydrate reagents and gives 

 an osazone identical with that formed from glucose. Gluco- 

 samine does not ferment, however. d-Glucosamine is an inter- 

 esting compound because in composition it stands midway be- 

 tween the carbohydrates and a group of substances called amino 

 acids, which are the simple units of which the proteins are com- 

 posed. 



d-Glucuronic Acid. This compound is obtained from glucose 

 by oxidation, and is found in the body in combination with 

 other substances. These compounds are called conjugated glu- 

 curonates. Glucuronic acid has the formula: 



