PLANT METABOLISM 



405 



animal respiration where cytochrome oxidase is the only oxidase assum- 

 ing an important role. 



Peroxidases are widely distributed in plants, but their function is not 

 well defined. They use H0O2 for the oxidation of a variety of phenols 

 and aromatic amines. The peroxidase from horse radish has been 

 crystallized and shown to be a hematin compound. Catalase, which con- 

 verts hydrogen peroxide to water and Oo, and has peroxidatic activity, 

 likewise is a hematin enzyme widely distributed in plants. Hemoglobin, 

 cytochrome c, peroxidase, and catalase each has hematin as its prosthetic 

 group. 



Many organic acids are relatively abundant in plants, and they serve 

 as substrates for their respective dehydrogenases. Malic, citric, and 

 oxalic acids are quantitatively the most important plant acids. Among 

 these, malic acid is a particularly active metabolite; citric acid is inter- 

 mediate in activity; and oxalic acid is rather inert. Glycolic acid and 

 lactic acid are rapidly oxidized in plants by glycolic and lactic acid 

 dehydrogenases. 



The formation, interconversion, and oxidation of organic acids in plants 

 are competing processes which give rise to spectacular changes in the 

 organic acid levels from day to day. For example, the succulent plants 

 accumulate high concentrations of organic acids at night and convert 

 them to starch during the day (isocitric acid, a rare acid, may constitute 

 18 per cent of the dry weight of the leaves of the succulent plant, Bryo- 

 phyllum calycinum) . In the dark, a large share of the malic acid of 

 tobacco leaves is converted to citric acid. 



Glycolysis in plants and animals yields pyruvic acid, which may be 

 reduced to lactic acid or oxidized by aerobic processes. The oxidation 

 of pyruvate via the Kreb's tricarboxylic acid cycle has been described for 

 animal tissue in Chap. 13. Although information is much less complete 

 for plants, there is good evidence that oxidation can occur by the same 

 pathway outlined for animal tissue. Early work depended upon evidence 

 that in the presence of inhibitors, such as malonate, intermediates in 

 the tricarboxylic acid cycle accumulated. Now it has been possible to 

 show that preparations of washed mitochondria ^ from mung bean and 

 lupine seedlings can oxidize all the intermediates of the tricarboxylic 

 acid cycle and can use some of the energy so obtained for converting 

 ADP to ATP. 



^ Mitochondria are small, discrete particles existing within cells which can be 

 separated from the rest of the cell contents by breaking the cell walls mechanically, 

 suspending the mixture in a suitable liquid medium (e.g., 0.4.1/ sucrose), and cen- 

 trifuging. The mitochondria settle out only after a certain centrifugal force has been 

 reached. Larger particles settle first and are discarded, and smaller particles remain 

 in suspension. Washing is accomplished by resuspending in fresh liquid and recen- 

 trifuging. 



