IV. mOCHEMICAL SYSTEMS 233 



sulfhydryl coinpoiiiuls. lie showed the presence in potato juice of an enzyme 

 A\hich catalj'zed the reduction of DHA by cysteine. The catalyst responsible 

 was found to be a heat-lai)ile substance and to be precipitated by protein 

 reagents. This work was extended notably by Hopkins and his collabo- 

 rators/-' who demonstrated the presence of an enzyme, dehydroascorbic 

 reductase, which catalyzed the reduction of dehydroascorbic acid by GSH 

 in accordance with the following reaction: 



2GSH + DHA ^ GSSG + AA 



The enzyme was prepared from cauliflower and separated from ascorbic 

 oxidase. It had the properties of an enzyme in that it was thermolabile, 

 was nondiffusible, and was precipitated by (NH4)2S04, and its activity was 

 affected by pH in a manner characteristic of enzj^mes. The enzyme cata- 

 lyzed the reduction of DHA by cysteine, thiolactic acid, or GSH, although 

 the latter compound was twice as effective as cysteine and four times as 

 effective as thiolactic acid in equimolar concentration. The distribution of 

 the enzyme was investigated and was found to be present in twenty-two 

 of the thirty species examined, the most active sources being cauliflower 

 and l)road beans. So far no enzyme of this type has been found in animal 

 tissues. 



Other workers'^" were unable to repeat the observations of Hopkins and 

 his collaborators for reasons which are obscure. However, a re-examination 

 of the earlier work produced confirmatory evidence for the existence of this 

 enzyme. The presence of a dehydroascorbic reductase in leguminous seeds 

 was also found by Volinar and Sansom, although their work suggested that 

 it was not glutathione but another system that was oxidized by dehydro- 

 ascorbic acid. The present writer (unpublished experiments) has been able 

 to substantiate the work of Hopkins and his coworkers in showing the 

 presence of an enzyme catalyzing the reaction between GSH and DHA in 

 cress seedlings, potatoes, and peas. 



The possibility that the dehydroascorbic acid-glutathione system is one 

 which may form a hydrogen transfer system similar to that of the cyto- 

 chrome system was suggested by Crook (1941).^-' The system was visual- 

 ized as acting as follows: 



H j— ^^ > — S — S T-r-:i vT — ^ I^HA ZTTT.—^ Atmospheric O2 



plant dehydroascorbic ascorbic ^ 



sutetrates reductase oxidase 



128 F. Pfankuch, Naturwissenschnflen 22, 821 (1934). 



'29 F. G. Hopkins and K. J. IMoiKnn, Biochem. J. 30, 1446 (1936); E. M. Crook and 



F. G. Hopkins, Biochem. J. 32, 1.3.56 (1938); E. M. Crook, Biochem. J. 35, 226 (1941); 



E. M. Crook and E. J. Morgan, Biochem. J. 38, 10 (1943) ; E. F. Kohman and N. H. 



Sanborn, Ind. Eng. Chem. 29, 189, 1195 (1937). 

 '30 A. Fujita and I. Numata, Biochem,. Z. 299, 249 (1938); Z. I. Kertesy, Biochem. J. 



32, 621 (1938). 



