224 ASCORBIC ACID 



B. FUNCTION OF ASCORBIC ACID IN BIOCHEMICAL SYSTEMS 



The most obvious property of ascorbic acid is the ease with which it 

 may be oxidized and reversibly reduced, and the possibility has led many 

 investigators to attempt to show that this is its action in vivo. From the 

 fact that both the reduced and the oxidized forms are usually found in 

 animal and plant tissues and that its occurrence coincides quite generally 

 with tissues possessing high metabolic activity, it seems a priori probable 

 that it must possess properties which permit it to play an important role 

 in respiration. The demonstration by Szent-Gyorgyi (1931)," and since then 

 by many other workers, that accompanying ascorbic acid in plant tissues 

 there is an enzyme, ascorbic oxidase, capable of catalyzing a direct reaction 

 between ascorbic acid and molecular oxygen gave further impetus to the 

 idea that ascorbic acid may act as a respiratory catalyst, acting in a similar 

 capacity to that of cytochrome. In animal tissues it is true that no specific 

 oxidase catalyzing the direct oxidation of ascorbic acid has as yet been 

 isolated. Nevertheless, as will be shown later, other enzyme systems are 

 present in animal tissues which are capable indirectly of catalyzing the 

 oxidation of ascorbic acid. Neither do all plant tissues contain ascorbic 

 oxidase, yet all plants contain oxidase enzymes which can indirectly cata- 

 lyze the oxidation of ascorbic acid. 



1. Systems Catalyzing the Oxidation of Ascorbic Acid 



Ascorbic acid can be reversibly oxidized with extreme ease by a number 

 of reagents. Both copper and iron salts are known to catalyze its oxidation,*" 

 and the oxidative activity of many tissues may, in part, be attributed to 

 this cause, despite the fact that the activity of these metals is reduced by 

 other constituents such as glutathione, amino acids, or proteins.^' Other 

 substances, the hemochromagens,*^ have also been shown to be good cata- 

 lysts. Similarly, many quinones are capable of oxidizing it, and these sub- 

 stances may be formed from a variety of compounds present in V^oth animal 

 and plant tissues. 



In addition to these non-enzymic catalysts, others of an enz^'mic nature 

 are known to exist in both plant and animal tissues. There are at least five 

 oxidases present in the tissues of higher plants that could be responsible 

 for the entry of oxygen into the respiratory system. These are ascorbic 



79 A. Szent-Gyorgyi, J. Biol. Chem. 90, 385 (1931). 



*" H. von Euler, K. Myrbiick, and H. Larson, Hoppc-Seylcr^s Z. phi/siol. Chon. 245, 



217 (1933); C. M. Lyman, M. O. Schulize, and. C. G. King, J. Biol. Chon. 118, 



757 (1937); L. W. Mapson, Biochem. J. 39, 228 (1945). 

 " E. S. G. Barron, A. G. Barron, and F. KlempcMer, J. Biol. Chem. 116, 503 (1936). 

 82 E. S. G. Barron, R. H. Dc Meio, and F. Klemperer, J. Biol. Chem. 112, 625 (1936) ; 



R. Lemberg, B. Cortis-Jones, and M. Norrie, Biochem. J. 32, 149 (1938). 



