IV. BIOCHEMICAL SYSTEMS 229 



witli oxidation antl ivtluctioii of ascorbic acid in the cell are poised at a level 

 wliicii is suflicitMit to maintain most of the ascorbic acid in the rcdnced 

 form. This balance is normally maintained throughout the life of the cell. 

 It may, however, be disturbed (1) by mechanical damage to the cell,"' 

 (2) by the action of narcotics, and (3) by the action of certain enzymic 

 poisons. "^^ Under such conditions, there is a rapid conversion of ascorbic 

 acid to dehydroascorbic acid. The exact reason for these effects is not 

 known. On the one hand it may be explained by the hypothesis that dam- 

 age to the cell allows the oxidative enzyme systems to come into more 

 elTective contact with their substrates, resulting in a greatly increased rate 

 of oxidation; a second explanation is that the enzymic systems concerned 

 with the reduction of ascorbic acid are damaged to a greater extent than 

 the oxidase systems. Whichever explanation is correct, it seems certain that 

 in tissues reducing systems are present which maintain ascorbic acid in the 

 reduced form. Thus both animal and plant tissues are capable of forming 

 ascorbic acid from dehydroascorbic acid. Dehydroascorbic acid has been 

 shown to possess antiscorbutic properties'^'' and to form ascorbic acid on 

 entry into plant cells. ^*' "^ 



Although the enzymic oxidation of ascorbic acid is relatively easy to 

 demonstrate, the demonstration of enzymes or enzymic systems responsible 

 for the reduction of dehydroascorbic acid presents a more difficult problem. 

 The failure to discover any appropriate reducing system has in the past been 

 the chief obstacle to accepting Szent-Gyorgyi's suggestion that in some 

 species ascorbic acid might play the role of a respiratory catalyst. 



Evidence of a reducing system in barley has, however, been found by 

 James and his collaborators.''" Having demonstrated the existence of an 

 ascorbic oxidase in barley, these workers found that, out of several organic 

 acids tried, three acids, namely glycollic, lactic, and tartaric, increased the 

 oxygen consumption of barley saps in the presence of ascorbic acid. The 

 increased oxygen consumption due to the presence of lactate was greater 

 than with the other two acids. In the latter case, pyruvate was isolated as 

 the oxidative product. The increased oxygen consumption due to these 

 acids was not accompanied by any increased loss of ascorbic acid, and it 

 appeared, therefore, that this effect was due to the oxidation of the organic 

 acid, and not to that of ascorbic acid. They further demonstrated that 

 barley sap increased the reduction of dyes such as methylene blue or 2,6- 



"5 A. Szent-Gyorgyi, /. Biol. Chevi. 90, 385 (1931); W. Stone, Biochem. J. 31, 508 



(1937). 

 "^ K. I. Strakitskil and B. A. Rubin, Biokhimiya 1, 642 (1936). 

 "s W. O. James and J. M. Cragg, New Phytologist 42, 28 (1943); VV. O. James, C. R. C. 



Heard, and G. M. James, ibid. 43, 62 (1944). 



