78 



THE CARBOHYDRATE ECONOMY OF CACTI. 



In the aldose monosaccharides the first carbon atom or the carbonyl 

 group, CH : O, is the most reactive and is largely responsible for the 

 great reactivity of these sugars. In the disaccharides and polysaccharides 

 found in these plants this active carbonyl group is so united with other 

 groups that it no longer forms the point of attack in chemical reaction. 

 These sugars are therefore first affected on the opposite end of the chain of 

 carbon atoms, at the CH 2 OH group. 1 Such a reaction results in a 

 primary formation of glucuronic acid, CHi: O(CHOH) 4 OOOH. 



This substance has been found as a product of glucose metabolism in 

 mammals. When substances such as chloral or camphor, which unite with 

 the carbonyl group of glucose, are fed, glucuronic acid, conjugated with 

 these substances, is found in the urine. It has been established now that 

 glucuronic acid is also present in the cacti. The discovery of this substance 

 in plants is especially significant, because it permits the formulation of a 

 rational theory of the formation of pentoses in plants. 



A very general property of acids of this character is the splitting-off of 

 C0 2 from the carboxyl group when solutions thereof are exposed to the 

 sunlight. An example of this action has already been furnished in the 

 case of malic acid, which loses two molecules of carbon dioxid to form ethyl 

 alcohol. 3 



In this manner glucuronic acid would form 1-xylose : 



OH H OH OH OH H OH 



COH- 



COOH 



COH 



CH 2 OH 



H OH H H H OH H +C0 3 



Solkowski and Neuberg * have reported the breaking-down of glucuronic 

 acid into 1-xylose and carbon dioxid by means of bacteria. 



Further support of this interpretation of the formation of pentoses is 



obtained from a consideration of the structural relations of the various 



sugars concerned. If the pentoses were derived from a direct oxidation of 



the hexoses, d-glucose would yield d-arabinose and d-galactose give d-lyxose. 



OH H OH OH H OH OH 



1 FISCHEB, E., and O. PILOTY. Reduction der Zuckersaeure. Ber. d. deut. chem. Ges., 



24, 524, 1891, 



a SPOEHK, H. A. Biochem. Zeitschr., 57, 101, 1913. 

 * SOLKOWSKI, E., and C. NEUBERG. Die Verwandlung von d-glucuronsaeure in 



1-xylose. Chem. Zentr., 1902, 1, 1098. 



