226 ASCORBIC ACID 



idases, such as hemin or hemoglobin, have an action similar to that of 

 peroxidase.^" 



h. Cyiochrome c-Cylochrome Oxidase System 



Cytochrome c will oxidize many mild reducing agents such as cystine, 

 p-phenylenediamine, hydroquinone, and other phenols. ^^ It is not surprising, 

 therefore, that it will oxidize ascorbic acid. If, in addition, cytochrome 

 oxidase is present, a cyclic oxidation system is established whereby ascorbic 

 acid may be oxidized to completion, the oxidation being carried to the 

 dehydroascorbic acid stage. ^^ Cytochrome oxidase, in the absence of cyto- 

 chrome c, will not oxidize ascorbic acid; hence the oxidation is an indirect 

 one.^^' ^^ The cytochrome system is widely distributed throughout animal 

 tissues, and evidence is fast accumulating of its equally wide distribution 

 and similar function in plant tissues. ^^ Cytochrome oxidase is inhibited by 

 HCN, II2S, and azide, and in the dark by carbon monoxide, ^^ the latter 

 inhibition being reversed in light which causes dissociation of the carbon 

 monoxide-enzyme complex. Such behavior is typical of an iron proteinate 

 and is, so far as is known, not true for any other heavy metal enzyme. 



c. Polyphenoloxidase 



The identification of polyphenoloxidase in potatoes as a copper protein- 

 ate,^"* together with evidence from succeeding studies on previously re- 

 ported oxidases, ^^' ^^ has indicated that copper associated with certain 

 specific proteins will account for the oxidase activity previously designated 

 as polyphenolase, potato oxidase, catechol oxidase, and tyrosinase. Poly- 

 phenolase converts 0- and p-polyhydroxyphenols to the corresponding qui- 

 none. These oxidation products will, in turn, oxidize ascorbic acid to de- 

 hydroascorbic acid, and the system becomes a cyclic one imtil all the 

 ascorbic acid is oxidized. The reaction may be expressed thus: 



O2 + phenolic compound Polyphenoloxidase_^ Quinone 



AA + quinone — > DHA + phenolic compound 



The most outstanding example of this type of reaction is the system in 



9° G. Bancroft and K. A. C. Elliott, Biochem. J. 28, 1911 (1934). 



" D. Keilin and E. F. Hartree, Proc. Roy. Soc. {London) B125, 171 (1938). 



92 E. Stotz, C. J. Harrer, M. O. Schultze, and C. G. King, J. Biol. Chem. 124, 745 

 (1938). 



93 R. Hill and K. Bhagvat, Nature 143, 726 (1939); K. Bhagvat and R. Hill, New 

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94 F. Kubowitz, Biochem. Z. 292, 221 (1937). 



96 F. R. Dalton and J. M. Nelson, J. Am. Chem. Soc. 60, 3085 (1938). 

 »« D. Keilin and T. Mann, Nature 143, 23 (1939). 



