IV. BIOCHEMICAL SYSTEMS 235 



tissues of an enzyme catalyzing the reduction of oxidized glutathione l)y 

 dehydrocoenzyme II. This enzyme, glutathione reductase, has l)een shown 

 to calalyzo the reaction 



GSSG + Co II ^H -^ 2GSn + Co II 



The reverse reaction could not be shown, even in the presence of excess of 

 GSH, indicating that the equilibrium of the reaction is in favor of the forma- 

 tion of reduced glutathione. The enzyme appears to be highly specific for 

 glutathione: it will not catalyze the reduction of cystine, homocystine, 

 a-glutamylcystine, or aspartathione. It is also specific for coenzyme II. 

 There is no reaction between dihydrocoenzyme I and oxidized glutathione. 

 The possibility of a hydrogen transfer system involving coenzyme II 

 glutathione and ascorbic acid is at once apparent. In pea seeds the transfer 

 of hydrogen from substrates such as isocitrate or malic to dehydroascorbic 

 and coenzj'-me II and GSSG has been shown to occur in accordance with 

 the following reactions:'" 



Co II + isocitrate ^g^^^.^^^.^^. > Co II H2 + oxalosuccinate 



dehydrogenase 



Oxalosuccinate — — — > a-Ketoglutarate + CO2 

 Co II H. + GSSG ,i,.t,thione -^ 2GSH + Co II 



reductase 



GSH + DHA , , , 7-. ^-^ AA + GSSG 



I ii... dehydroascorbic acid 

 reductase 



We do not know at present how important such a system is in the general 

 respiratory activity of plant or animal cells. As far as our present informa- 

 tion goes, the enzyme system, like that of cytochrome and the other ter- 

 minal oxidases, will be inhibited by cyanide. Direct evidence that there is 

 a connection between the state of oxidation of GSH and that of ascorbic 

 acid has recently been shown in potato tubers. When these are subjected 

 to atmospheres of pure oxygen there is, after a period in which no apparent 

 change occurs, first a fall in the GSH content of the tissue, followed by a 

 fall in the concentration of ascorbic acid and a rise in that of dehydroascor- 

 bic acid.'^^ 



b. Reduction of Dehydroascorbic Acid by Bacteria 



Several workers have reported that certain bacteria can reduce dehydro- 

 ascorbic acid to ascorbic acid.'''" Strains of Escherichia coli and of Aerobacter 



•'' J. Barker and L. W. Mapson, New Phytologist (In press.) 



'" I. C. Gunsalus and D. B. Hand, J. Biol. Chem. 141, 853 (1941); A. P. Stewart and 

 P. F. Sharp, Ind. Eng. Chem. Anal. Ed. 17, 373 (1945); L. W. Mapson and M. In- 

 gram, Biochem. J. 48, 551 (1951). 



