STUDIES IN THE RESPIRATORY AND CARBOHYDRATE METABOLISM OF PLANT TISSUES

Abstract

S ummary The changes of glucose‐6‐phosphate, fructose‐6‐phosphate, fructose diphosphate, dihydroxy‐acetonephosphate, 3‐phosphoglycerate, 2‐phosphoglycerate, phosphoenolpyruvate, adenosine triphosphate, adenosine diphosphate, adenosine 5′‐monophosphate and oxidized co‐enzyme I were determined in shelled green peas held in air and in nitrogen. In anoxia 3‐phosphoglycerate, 2‐phosphoglycerate, phosphoenolpyruvate, adenosine triphosphate and oxidized co‐enzyme I, each decreased markedly to new steady levels after ahout 15 minutes; except in one experiment, adenosine diphosphate increased markedly for about 15 minutes. Similarly, the development of the Pasteur effect in shelled green peas is rapid, i.e. less than 30 minutes (Wager, 1961). In contrast, glucose‐6‐phosphate decreased for some 24 hours of anoxia, while fructose diphosphate and dihydroxyacetonephosphate first increased, then decreased and finally might rise again. From our results, the metabolic fate of adenosine diphosphate appears to be of primary importance in controlling the rate of glycolysis. Thus, with transfer to anoxia, the most consistent ehange in the glycolytic intermediates is the rapid decrease in the contents of 3‐phosphoglycerate and of phosphoenolpyruvate. One explanation of these changes is that the pyruvate kinase reaction is accelerated, due possibly to a local increase of adenosine diphosphate. With a faster pyruvate kinase reaction, the additional adenosine triphosphate formed might hasten glycolysis and this faster rate might then be maintained. The fact that, in anoxia, smaller amounts of adenosine triphosphate support a higher rate of glycolysis than in air has suggested that in air this nucleotide is not readily available to the sugar‐phosphorylating enzymes. Similarly, the present results could be explained if the glycolytic enzymes were mainly confined to an organized structure showing a difference in accessibility for adenosine di‐ and triphosphates. On the other hand, our results may be related to a feed‐back mechanism not yet specified. How important such mechanisms might be is shown by the complex effeets of adenosine nucleotides, of inorganic phosphate and of other compounds on the activity of phospho‐fruetokinase in vitro . The changes m anoxia, stated above, of glueose‐6‐phospliate, of fructose diphosphate and of dihydroxyacetone phosphate are considered. A difference in the metabolic fate of adenosine triphosphate in aerobie and anaerobie conditions is described. The difference is readily explained in terms of a glyeolytic structure but could also be due to an acceleration of the hexokinase and phosphofructokinase reaetions in anaerohiosis either by activation or by a decrease in the inhibition of these enzymic stages. While the existence of a glycolytic structure has not been proved beyond doubt, evidence that such a strueture may exist is accumulating.