II TERMINAL H ELECTRON TRANSPORT SYSTEM 25 



Plant polyphenol oxidase has been employed as a terminal oxidase in experi- 

 mental studies with animal neoplasms (Kertesz and Albano, 1955). Thus, the 

 respiration of Ehrlich tumor cell homogenates in the presence of glucose is in- 

 creased by the addition of a terminal electron transport system consisting of DOPA 

 in catalytic amounts (soy) and plant polyphenol oxidase. Concommitantly, the 

 aerobic glycolysis of the tumor cells is markedly inhibited. 



8. Glutathione and ascorbic acid as respiratory catalysts 



Glutathione and ascorbic acid may also function as components of electron and 

 hydrogen transport systems in plants. Pyridine nucleotide linked-glutathione 

 reductase enzymes have been observed in various plant and animal tissues (Racker, 

 1955b; Conn and Vennesland, 1951; Rail and Lehninger, 1952). 



Many plant enzymes also catalyze the reduction of dehydroascorbic acid by 

 reduced glutathione. A variety of possible pathways exist for the aerobic oxidation 

 of ascorbic acid. For example, plants contain ascorbic acid oxidases in which 

 copper functions as the prosthetic group (Reactions 1-3). 



glutathione reductase 

 i) TPNH + H" + GSSG . 2GSH + TPN" 



dehydroascorbic reductase 



2) 2GSH + dehydroascorbic > GSSG + ascorbic 



Cu**-oxidase 



3) Ascorbic + '/2O2 »• dehydroascorbic + H2O 



Sum: TPNH J H* ^ 1/2O2 » TPN" + H2O 



The significance of the ascorbic acid-glutathione respiratory pathway in pea seedlings 

 was recently studied (Mapson and Moustafa, 1956). Dehydroascorbic reductase, gluta- 

 thione reductase and ascorbic oxidase were readily demonstrated in pea seeds which had 

 germinated for 1.5 or 7 days. An oxidase catalyzing the oxidation of ascorbic acid was 

 absent from mitochondria prepared from one day old seedlings but was present in those 

 particles prepared from seven days old seedlings. However, even in the latter case, it 

 accounted for but 10% of the total oxidase activity. It was inferred that approximately 

 25% of the respiration of pea seedling cotyledons pass over the glutathione-ascorbate 

 system. However, the suggestion also has been made that glutathione reductase functions 

 primarily in the maintenance of the reduced state of sulfhydryl containing protein enzymes, 

 rather than as a component of an electron transport chain. 



D. Localization of Enzymes within Cells 



Mention should be made of the distribution of energy yielding enzymes within the 

 cell. The enzymes concerned with glycolysis and the hexose monophosphate 

 shunt are to be found in the soluble cytoplasm of tissues. It is possible, however, 

 that these enzymes also occur in cell nuclei (Stern and Timonen, 1952). The nuclei 

 of wheat germ manifest vigorous glycolytic activity. GIucose-6-phosphate de- 

 hydrogenase activity was also found in the nuclei of thymus cells (Stern and 

 Mirsky, 1954). However, the enzymes concerned with hydrogen transport, cyto- 

 chrome c, cytochrome reductase, cytochrome oxidase, and succinoxidase, appear 

 to be absent from nuclei and the concentration of glutathione reductase is but a 

 fraction of that of the cytoplasm (Stern and Timonen, 1952). The present evidence 

 therefore suggests that the metabolic organization of the nucleus is anaerobic in 



Literature p. 124 



