CARBOHYDRATES 



27 



such as cherry gum, mesquite gum, and gum arable. Such gums are 

 frequently found associated with pectin in phmt materials. Arabinose 

 has also been obtained from peas and beans. In tlie laboratory it is 

 generally made from sugar beet pulp or mesquite gum. The latter re- 

 sembles gum arabic, being produced by a shrub which grows abundantly in 

 Arizona and other states of the Southwest. Yields of arabinose amounting 

 to 20 per cent can be readily obtained from the gum. Arabinose, like 

 xjdose, finds its chief use in bacteriological laboratories. 



D-Ribose. Although from the standpoint of obtaining it in quantity, 

 D-ribose is an exceedingly rare and expensive sugar (about $400 per 

 pound); yet from the standpoint of its occurrence and functions in 

 living organisms, it is one of the most common and important of the 

 carbohydrates. It is present in all living cells as a component of ribose- 

 nucleic acids (see Chap. 6) and also as a part of several coenzymes 

 (p. 273) . Furthermore, two of the key substances involved in the process 

 of muscle contraction, adenosine diphosphate (ADP) and adenosine tn- 

 phosphate (ATP), are D-ribose derivatives. 



The pure sugar may be obtained by hydrolysis of yeast nucleic acid, 

 or prepared synthetically from D-arabinose, The formula is indicated 

 by the diagram on p. 23. 



2-Desoxy-D-ribose. This sugar has been found only as a component 

 of desoxyribonucleic acids, which are present in the nuclei of all living 

 cells, specifically in the chromosomes of the nucleus (see Chap. 6) . 

 Therefore, it is of great interest as one of the chemical substances in- 

 volved in the transmission of hereditary characteristics from one genera- 

 tion to the next. 



As its name implies, the substance has the formula of an aldopentose, 

 except that the oxygen on carbon 2 is missing, and has the configuration 

 of D-ribose: 



CHO CHjOH 



1 I 



CH: CO 



I I 



HCOH HC— OH 



I I 



HCOH HO-CH 



I I 



CH2OH CH2OH 



2-Desoxy- D-ribose L-Xylulose 



It is much more reactive and less stable than the ordinary aldoses or 

 ketoses and is particularly distinguished by giving a positive aldehyde 

 test with the Schiff reagent. This property of desoxyribose is the basis 

 for the Feulgen and diphcnylamine tests for desoxyribosenucleic acids. 



