334 ASCORBIC ACID 



land^"^^ have presented evidence suggesting that "ascorbic acid plays its 

 part in tyrosine metabolism by acting as a coenzyme in the oxidation of 

 the deaminated amino acid." They postulated that "the enzyme would be 

 regarded as removing hydrogen from the tyrosine keto acid by means of 

 the dehydroascorbic acid form of the vitamin, with subsequent transfer to 

 oxygen and regeneration of dehydroascorbic acid." Such a regeneration of 

 the dehydroascorbic acid would aid in maintaining a balance between the 

 two forms of the vitamin in the catalytically induced oxidation-reduction 

 cycle described by Barron et al.^^ in which hydrogen peroxide is presumably 

 formed. The oxidative action of hydrogen peroxide in association with 

 ascorbic acid in influencing polymerization of mucopolysaccharides and in 

 promoting oxidation of unsaturated fatty acids has been discussed pre- 

 viously. Cavallini^^^ reported results which suggest that the physiological 

 role of ascorbic acid may be to form a continuous supply of hydrogen 

 peroxide. He found that oxidation of ascorbic acid with CuCL in a Warburg 

 apparatus resulted in oxidation of added pyruvate and the amount of 

 pyruvate oxidized was proportional to the rate of oxidation of ascorbic acid 

 and was independent of the catalyst employed. The oxidizing action on 

 pyruvate was considered to be due to the production of hydrogen peroxide 

 during the oxidation of ascorbic acid. He postulated that in the organism 

 ascorbic acid is present in a state of dynamic balance with respect to oxida- 

 tion and reduction, creating thereby a constant flow of hydrogen peroxide 

 proportional to the amount of the vitamin and to the agents which reduce 

 it, i.e., cytochrome c and sulfhydryl compounds. Cavallini considered it 

 equally possible that coupled oxidation reactions can take place under 

 defense conditions on the part of the disturbing substances or by a closely 

 connecting mechanism due to which there is a direct transposition of ele- 

 ments in the molecular complex rather than a two-phase reaction compris- 

 ing first a hydrogen peroxide formation and then an action of the latter on 

 the substrate. 



More information is needed before a full interpretation can be made of 

 the role of ascorbic acid in cellular respiration. At present no definite ex- 

 planation can be made of the increased respiratory rate which has been 

 observed^^'' • ^"^ to occur during the onset of scurvy, but it seems possible, 

 as previously stated, that with a deficiency or lack of the vitamin the 

 normal oxidation-reduction cycle is upset, resulting in imbalances probably 

 involving the action of hormones, enzymes,'*'^^ and other vitamins. 



7. Excretion and Retention 



The kidneys have a rather definite threshold for excretion of ascorbic 

 acid. A rapid rise in excretion of the vitamin occurs in man when the plasma 



"s R. R. Sealock and R. L. Goodland, Science 114, 645 (1951). 



