IV. BIOCHEMICAL SYSTEMS 225 



oxidase, polyphenoloxidase, eytochrome oxidase, laccase, and peroxidase. 

 All these enzymes are capable of catalyzing the direct or indirect oxidation 

 of ascorbic acid to dchydroascorhic acid. Thus, ascorbic acid is oxidized by 

 the catechol-catechol oxidase system, ])y the cytochrome c-cytochrome oxi- 

 dase system, by the fla\'one-peroxidase system, and by laccase and ascorbic 

 oxidase. All these enzymes are proteins containing copper or iron as an 

 integral part of their active prosthetic groups. 



The mode and pathway of the oxidation of ascorbic acid occurs in two 

 stages. The primary reversible oxidation product is clehydroascorbic acid, 

 which is the lactone of 2,3-diketogulonic acid: the second stage of oxidation 

 involves a cleavage of diketogulonic acid with the production of oxalic and 

 L-threonic acids.^' The conversion of dehj^droascorl)ic acid into 2,3-diketo- 

 gulonic acid is dependent on pH ;^^ in neutral or alkaline solution this reac- 

 tion is ^'ery rapid. The enzymic systems present in plant or animal tissues 

 carry the oxidation only as far as dehj^droascorbic acid. Little is known of 

 any enzyme system affecting conversion of dehydroascorbic acid to 2,3- 

 diketogulonic acid, although indication of the presence in wheat tissue of a 

 thermolabile inhibitor of this reaction has been reported. ^'^ Theranostable 

 substances such as borate^^ or cyanide^^ have, however, been found to ac- 

 celerate the opening of the lactone ring of dehydroascorbic acid. 



a. Peroxidase 



The first of the enzymic mechanisms catalyzing the indirect oxidation of 

 ascorbic acid to be described was the peroxide-peroxidase system.^* Purified 

 peroxidase preparations with H2O2 failed to catalyze the reaction and did so 

 only after the addition of small amounts of plant juices.^* The substances 

 present in the plant juice which were responsible for restoring the activity 

 were found to be phenolic compounds capable of quinone formation. Hence 

 the reactions involved may be generally expressed as: 



Phenolic comi)Ouiul + H.O2 — peroxidase — ^ Quinone 



Quinone + AA — > DIIA + phenolic compound 



All ciuinone-forming phenols are capable of acting as carriers in the reac- 

 tion, though the most efficient were the naturally occurring o-dihydroxy- 

 benzopyrene pigments (flavones, flavonones, and flavonols).^'-' Pseudoperox- 



" R. W. Herbert, E. L. Hirst, G. V. Percival, R. Reynolds, and F. Smith, J. Chem. 



Soc. 52, 1270 (1933). 

 " J. R. Penney and S. Zilva, Biochem. J. 37, 39 (1943). 

 « E. R. Waygood, Can. J. Research 28, 7 (1950). 

 " F. E. Huelin, Australian J. Set. Research 2, 346 (1949). 

 «^ L. W. Mapson and iM. Ingram, Biochem. J. 48, 551 (1951). 

 8« A. Szent-Gyorgyi, Biochem. J. 22, 1387 (1928). 

 "S. Huzak, Hoppe-Seyler's Z. physiol. Chem. 243, 239 (1937). 



