IV. HIOCHEMICAL SYSTEMS 227 



which catechol acts as carrier, large amounts of ascorbic acid being rapidly 

 oxidized in the presence of quite small amounts of catechol." Polyphenol- 

 oxidase is inhibited by cyanide, sulfide, and carbon monoxide,'^ the inhibi- 

 tion due to the latter not l)eing reversed bj' light. The wide distribution 

 of this type of enzyme in plants and fungi suggests that it may be an im- 

 portant terminal oxidase, and in potatoes it has been claimed that, as such, 

 its activity may account for two-thirds of the total respiratory activity. ^^ 

 However, this viewpoint has recently been contested.^"" Many plants which 

 do not possess ascorbic oxidase contain a high concentration of polyphenol- 

 oxidase in their tissues. 



d. Laccase 



This enzyme resembles polyphenoloxidase, since it catalyzes the oxida- 

 tion of 0- and p-phenolic derivatives to o- and p-quinones, but it differs in 

 that it will not oxidize tyrosine or p-cresol.^^ Crude preparations of the 

 enz>Tne oxidize ascorbic acid, but this activity decreases as the enzyme is 

 purified; it can be restored by the addition of substances like phenylene- 

 diamine but not by catechol. The oxidation of ascorbic acid is therefore 

 indirect. The enzyme is a copper proteinate containing 0.24 % copper. An 

 enzyme resembling laccase has been obtained from animal sources; it wall 

 oxidize catechol and hydroquinone, V)ut not tyrosine or p-cresol;^"^ its action 

 on ascorbic acid has not been reported. 



e. Ascorbic Oxidase 



An enzyme catalyzing the oxidation of ascorbic acid by atmospheric 

 oxygen was first described by Szent-Gyorgyi in 1931.^^ • '°- Since these ex- 

 periments, man}'' workers have prepared the enzyme from a variety of plant 

 tissues. Almost all authors agree that the enzyme is absent from mammalian 

 tissues. The enzj^me has been purified and has been obtained in crystalline 

 form from the fruit of Cucurhita}^^ It is a blue or greenish-blue copper pro- 

 teinate containing approximately 0.25% copper. It is the only enzyme 

 known which catalyzes a direct oxidation of ascorbic acid by molecular 

 oxygen to dehydroascorbic acid. Unlike the corresponding oxidation cata- 

 lyzed by 0X1++,'"^ no H2O2 is found during the reaction. ^"^ Artificial copper 



«'D. Keilin and T. Mann, Proc. Roy. Soc. (London) B125, 187 (1938). 



»8 D. Keilin, Proc. Roy. Soc. (London) B104, 206 (1929). 



9» J. G. Boswell and G. C. Whiting, Ann. Bat. 2, 847 (1938). 

 '»« H. Levy and A. Schade, Arch. Biochem. 19, 273 (1948); A. Schade and H. Levy 



ibid. 20, 211 (1949). 

 •oi J. F. Cadden and L. V. Dill, J. Biol. Chetn. 143, 105 (1942). 

 »°2 A. Szent-Gyorgyi, Science 72, 125 (1930). 



"" T. Tadokoro and H. Takasugi, J. Chem. Soc. Japan 60, 188 (1939). 

 »" L. W. Mapson, Biochem. J. 39, 228 (1945). 



