214 ASCORBIC ACID 



good precursors. Substances such as methyl glyoxal, pyruvate, glycerophos- 

 phate, glutamate, and pentose sugars were ineffective. Glycerol and disac- 

 charides, e.g., sucrose, were also found to act as precursors. ^^ 



Further studies with seedlings have indicated a close correlation between 

 the synthesis of hexose sugars during germination and that of ascorbic acid, 

 a correlation which holds when the seedlings are grown under conditions in 

 which the amount of ascorbic acid synthesized is altered by altering the 

 nutrient salts supplied. ^^' ^° Furthermore, it has been shown that the effect 

 of different nutrient salts is due to their effect on the pH of the cell sap. 

 Salts which decrease the pH decrease and salts which increase it increase 

 the synthesis of ascorbic acid. Similarly a shift of the pH of the cell sap to 

 more alkaline values increases, and a shift to more acid values decreases the 

 efficiency of the conversion of hexose sugars to ascorbic acid. 



The formation of ascorbic acid from hexose sugars has also been reported 

 to occur in animal tissues. Dextrose has been found to increase the ascorbic 

 acid content of slices of intestinal tissue, but not of liver, spleen, stomach, 

 or brain. "1 In these experiments, however, the vitamin was determined by 

 iodine titration, an unspecific method. The synthesis has also been observed 

 in spleen, liver, and cardiac muscle tissue in vitro from mannose. The mech- 

 anism of the transformation is claimed to be enzymatic in nature, the en- 

 zyme being a dehydrogenase.-'^ So far these results have not been confirmed 

 by other workers. 



Evidence along somewhat different lines has been produced by a study 

 of the increased excretion of ascorbic acid by the rat after administration 

 of certain drugs, notably the terpene-like cyclic ketones.-'- -* These drugs 

 appear to stimulate the synthesis of ascorbic acid and are themselves ex- 

 creted in combination with glycuronic acid. Ascorbic acid appears as such 

 in the urine but not in combination with any of the drugs. It is of interest 

 that there appears to be a correlation between the formation of glycuronic 

 acid needed for detoxication and the formation of ascorbic acid, and we 

 may speculate whether in the formation of glycuronic acid metabolic inter- 

 mediates are formed which also lead to an increased synthesis of the vi- 

 tamin. The increased synthesis of ascorbic acid appears to occur in the liver 

 and kidney, for liver and kidney slices from chloretone-treated animals 

 continue to synthesize ascorbic acid in greater quantity than the corres- 



18 S. N. Ray, Biochem. J. 28, 996 (1934). 



19 L. W. Mapson and E. M. Cruickshank, Biochem. J. 41, 197 (1947). 



20 L. W. Mapson, E. M. Cruickshank, and Y.-T. Chen, Biochem. J. 45, 2, 171 (1949). 



21 F. Widenbauer and K. Koschorrech, Biochem. Z. 291, 209 (1937). 



22 B. C. Guha and A. R. Ghosh, Nature 134, 739 (1934); 135, 871 (1935). 



23 H. E. Longenecker, R. R. Musulin, R. H. Tully, and C. G. King, J. Biol. Chem. 129, 

 445 (1939). 



24 J. Mosonyi, Hoppe-Seyler's Z. physiol. Chem. 230, 240 (1934). 



