230 ASCORBIC ACID 



dichlorophenolindophenol under anaerobic conditions when a-hydroxy acids 

 were added. They concluded, therefore, that a mechanism for the reduction 

 of dehydroascorbic acid exists in barley involving the participation of a 

 dehydrogenase (or group of dehydrogenases) of a-hydroxy acids. They 

 postulated that the following reactions may occur: 



R • CHOH • COOH + DHA — R • CO • COOH + AA (1) 



AA + O2 -> DHA + H2O (2) 



In further experiments it was shown that, with barley saps to which 

 hexose diphosphate and ascorbic acid was added, an increased oxygen up- 

 take occurred in excess of that caused by the addition of ascorbic acid 

 alone; the oxygen uptake was still further increased by the addition of 

 coenzyme I (diphosphopyridine nucleotide). The hexose diphosphate was 

 broken down to phosphoglyceric acid, and it was shown that the reaction 

 was stimulated by the addition of ascorbic acid. The course of hydrogen 

 transport in these experiments is therefore believed to be triose phosphate 

 -^ coenzyme I — > ascorbic acid — ^ O2. How far coenzyme I was active in 

 the former experiments was not and has not since been determined. The 

 evidence in these studies is highly suggestive of a reducing mechanism, 

 probably involving coenzyme I, for the reduction of dehydroascorbic acid. 

 It is, however, only suggestive and not conclusive, for the authors did not 

 demonstrate either with their lactate dehydrogenase or hexose diphosphate 

 systems the direct reduction of dehydroascorbic acid to ascorbic acid. 



Results of a similar character have been found in pea seeds and pea 

 seedlings. ^^^ These tissues contain an active formic dehydrogenase which will 

 reduce in the presence of coenzyme I dyes such as Nile blue; the reduced 

 dye is in turn oxidized on the addition of dehydroascorbic acid. The oxygen 

 consumption of such tissue is increased when formate is added and still 

 further increased on the addition of ascorbic acid. Such evidence has been 

 taken to indicate that hydrogen is transported via the coenzyme I-dehj'dro- 

 ascorbic acid-ascorbic acid system. As in the work of James et al., no direct 

 demonstration of the reduction of dehydroascorbic acid in the presence of 

 tissue extract, formate, or coenzyme I was made. 



Further support for the hypothesis that any system producing dihydro- 

 coenzyme I in cells may produce ascorbic acid from dehydroascorbic acid 

 comes from recent work on wheat seedlings.^* Cell-free extracts of wheat 

 seedlings were found to contain a malic dehydrogenase-reducing coenzyme 

 I as well as peroxidase and ascorbic oxidase systems. When to such extracts 

 malic acid, coenzyme I, ascorbic acid, and a fixative for the oxalacetate 

 formed in the reaction were added, the system absorbed oxygen in excess 

 of that required for the complete oxidation of ascorbic acid. A similar 



"« D. C. Davison, Proc. Linnean Soc. N.S. Wales 74, 37 (1949). 



