238 ASCORBIC ACID 



extent as an equivalent amount of L-ascorbic acid.'^^ This indicates that 

 the chemical behavior of L-ascorbic acid in these reactions is not one of its 

 essential roles in metabolism. The suggestion has been made that these 

 substances accelerate the oxidation of the aromatic keto acids by virtue of 

 their effect on the oxidative reduction potential of the system. 



4. Action of Ascorbic Acid on Enzymes 



Ascorbic acid has been found to have an effect on a number of enzymes. 

 In some cases the effect observed has been one of activation, e.g., arginase,^^" 

 papain, ''^^ /3-amylase'^'- from liver and pancreas, liver esterase, catalase,^'^ 

 and cathepsin;^^" in others inhibitory effects have been described, e.g., 

 urease^^^ and jS-amylase of plants. ^^^ Several workers have attempted to 

 demonstrate the effect of ascorbic acid by comparing the enzymic activity 

 of extracts from tissues of normal and scorbutic animals. The blood ester- 

 ase of scorbutic guinea pigs was found to be lower than in normal animals'^^ 

 and to be increased with the administration of ascorbic acid.^^^ Similar 

 results were observed in deficient guinea pigs for other enzymes, notably 

 liver esterase, succinic dehydrogenase, and cytochrome oxidase.^"' ^** The 

 lowered enzymic activity of tissue extracts from deficient animals does not 

 necessarily indicate, however, that the lowered activity observed is directl}'' 

 due to an absence of the vitamin ; it may well be caused by inanition accom- 

 panying the deficient state. 



In the case of liver esterase the claim was made that the enzyme consists 

 of a protein (apoenzyme) combined with ascorbic acid (coenzyme), when 

 it was found that liver esterase loses its activity on dialysis with dilute 

 HCP^® but can be reactivated by the addition of ascorbic acid. These results 

 were not confirmed by other workers. ^*^ 



Many enzymes have been shown to depend for their activity on the 

 integrity of an — SH group in the molecule, and the activating effect of 

 ascorbic acid has been suggested as being due to the protection of such 

 — SH groups from oxidation. However, it has been shown that the activity 

 of one such enzyme, urease, is inhilnted by ascorbic acid although this 



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