28 * CARBOHYDRATES 



jj-Xylulose. This sugar, which has also been called L-xyloketose, is 

 a ketopentose excreted in cases of human pentosuria.^ From one to 

 several grams may be excreted daily by a patient. L-Xylulose is so 

 difficult to crystallize that it has been obtained only in the form of 

 sirups. Crystalline derivatives are known, however, which serve to 

 characterize and identify the sugar. It is an unusually strong reducing 

 agent, as is shown by its ability to reduce Benedict's solution even at 

 room temperature, whereas most other sugars give a positive result only 

 upon heating. 



Hexoses 



n-Glucose. This sugar is also called dextrose. From the biological 

 standpoint it is the most important carbohydrate in nature both because 

 of its wide distribution and because of its prominence in physiological 

 processes. It is the circulating carbohydrate of animals. Glucose is the 

 sugar into which all the available carbohydrates of food are converted 

 before oxidation in the body. 



In the free state it occurs in practically all fruits, being especially 

 abundant in grapes, figs, dates, and raisins. The blood contains about 

 0.08 per cent; in normal urine the amount may vary from traces to 

 about 0.2 per cent. In diabetic urine the sugar sometimes rises to 10 

 per cent. 



In the combined state it forms a part, or the whole composition, of 

 many other sugars such as sucrose, lactose, and maltose. Starch, glycogen, 

 and true cellulose yield glucose on complete hydrolysis. Certain sub- 

 stances known as glucosides yield on hydrolysis glucose together with 

 some nonsugar compound often of characteristic odor or taste. An 

 example of such a glucoside is amygdalin, the substance that gives the 

 almond its peculiar flavor. Mustard owes its strong taste and odor 

 to an oil produced from the glucoside, sinigrin. The following equation 

 illustrates the action of the enzyme myrosin upon sinigrin: 



C10H16NS2KO9 + H2O = CeHisOg + C3H5NCS + KHSO4 



Sinigrin Glucose Mustard Potassium 



oil acid sulfate 



Formation of Glucose in Nature. The plant is the factory in which 

 the world's food supply is manufactured. All animal life depends ulti- 

 mately upon the vegetable world for its sustenance. Even carnivorous 

 animals are indirectly supported by the plant; they prey on animals 

 that feed upon plants. Man being an omnivorous creature receives a 



^An abnormal condition characterized by the presence of a pentose sugar in the 

 urine. 



