228 ASCORBIC ACID 



proteinates have been prepared which exhibit many of the properties of 

 ascorbic oxidase. It would appear premature, however, to consider the 

 natural enzyme simply as a complex of copper with a non-specific protein. 

 Ascorbic oxidase oxidizes, besides L-ascorbic acid, certain analogs of ascor- 

 bic acid; D-araboascorbic acid, L-glucoascorbic acid, and L-galactoascorbic 

 acid are equally oxidized by the enzyme. ^"^ Compounds related to ascorbic 

 acid in which the oxygen bridge is on the right side of the carbon chain 

 are directly oxidized by the enzyme at a much higher rate than are their 

 enantiomorphs. Of these latter the six-membered series (n-ascorbic acid 

 and D-araboascorbic acid) are oxidized more rapidly than the seven-mem- 

 bered series (n-glucoascorbic acid and D-galactoascorbic acid) . Later work^"^ 

 has shown that other dienols (e.g., reductone and reductic acid) may be 

 oxidized by the enzyme although at a much slower rate than the L-ascorbic 

 acid series. The main specificity of the enzyme seems centered round the 

 dienol grouping. The most noteworthy feature in the kinetics of ascorbic 

 oxidase is the characteristic linear oxidation rates obtained,"^ which show 

 no falling off in velocity until the substrate is completely oxidized. This 

 implies a very low Michaelis constant, which has been determined to be of 

 the order of 2 X 10~^ M.^"^ The enzyme is extremely sensitive to cyanide; 

 concentrations of 10~' M will inhibit completely, and definite inhibitions 

 may be observed with 10~^ M."** 



2. Enzymic Systems Associated with the Reduction of 

 Dehydroascorbic Acid 



That ascorbic acid and dehydroascorbic acid are interconvertible in plant 

 and animal tissue seems certain. In some fruits, notably apples, the propor- 

 tion of dehydroascorbic acid to that of ascorbic acid is high, but as the 

 fruit approaches maturity the concentration of dehydroascorbic acid de- 

 creases while that of ascorbic acid increases.^^^ In germinating seeds it has 

 also been found that the proportion of dehydroascorbic acid falls as the 

 ascorbic acid increases. i^- 



In most fresh plant or animal tissues, the concentration of dehydro- 

 ascorbic acid is very small, compared to the concentration of ascorbic acid. 

 It is, therefore, reasonable to assume that the enzymic systems associated 



"6 H. G. Steinman and C. R. Dawson, /. Am. Chem. Soc. 64, 1212 (1942). 

 "6 8. W. Johnson and S. S. Zilva, Biochem. J. 31, 1366 (1937). 

 10^ G. A. Snow and S. S. Zilva Biochem. J. 32, 1926 (1938). 



108 F. G. Hopkins and J. Morgan, Biochem. J. 30, 1446 (1936). 



109 E. M. Crook, Ph.D. Dissertation, Cambridge, 1941. 

 "(»M. Srinivasan, Biochem. J. 30, 2077 (1936). 



111 S. S. Zilva, F. Kidd, and C. West, New Fhytologisi 37, 345 (1938). 



112 L. W. Mapson, Unpublished data. 



