564 B. A. RUBIN 



In the very most recent years, however, it has been shown that in plants 

 there is a special group of metalloflavine enzymes, some of which contain molyb- 

 denum (e.g. aldehyde or xanthine oxidase), or copper (butyryl-coenzymc A de- 

 hydrase). Cataiytically active iron derivatives of flavines have also been found. 

 Unlike the ordinary flavoprotein oxidases, all these enzymes are inhibited by 

 cyanide [4]. 



As well as the oxidation of sugars by glycolysis, the direct, so-called apotomic, 

 method of breakdown of carbohydrates must play an appreciable part in plants 

 [5]. The physiological significance of the presence in the cell of several enzymes 

 which catalyse one and the same chemical reaction is, it would seem, that the 

 individual representatives of the complex vary in their character according to 

 the environmental conditions. 



For example, it has been established that the influence of the partial pressure 

 of oxygen on the action of different plant oxidases is different. Cytochrome oxi- 

 dase has the greatest affinity for oxygen; its activity reaches the maximum when 

 the concentration of this gas is considerably less than that in the air. The com- 

 plete opposite is true of the flavine enzymes, the activity of which increases 

 markedly with an increase in the concentration of oxygen in the gaseous medium. 

 The copper-containing proteins (polyphenol oxidase and ascorbic acid oxidase) 

 occupy an intermediate position. 



It is interesting to note, in this connection, that in compact tissues (having a 

 parenchyma of many layers) cytochrome oxidase is considerably more active in 

 the deeper layers which are distinguished by their very low oxygen contents. 



There is a fairly definite distinction between the terminal oxidases of plants 

 in their relations to temperature. For instance, a high temperature coefficient, 

 often reaching 4-6, is characteristic of cytochrome oxidase. This is considerably 

 higher than the value of Oio for the flavoproteins of the same tissues in the same 

 phase of development of the plant. 



The significance of this peculiarity of the enzymes of the flavoprotein group 

 may be discussed with reference to plants of the Arum family. The respiration 

 of the spadix of these plants is completely resistant to the effects of cyanide and 

 other enzymic poisons, the intensity of the process increasing in direct propor- 

 tion to the concentration of oxygen (right up to loo'^'o). All this shows that the 

 respiration of the spadix is brought about at the expense of auto-oxidizable 

 flavoproteins. It is interesting that the formation of the spadix usually takes 

 place at the very beginning of spring, a period which is characterized by low 

 air temperatures. 



Polyphenol oxidase occupies a position similar to that of the flavine enzymes, 

 also having a characteristically low Qw. 



Another piece of evidence of the heterogeneity of the system of oxidases is 

 provided by the results of experiments in which it has been shown that a whole 

 number of enzymes, which have, until recently, been regarded as individual 

 compounds, are, in fact, systems having many components. Thus, in 1942, 

 when Theorell [6] first isolated a cr^'stalline preparation of peroxidase from the 

 horse radish, he showed, from its electrophoretic and other properties, that it 

 (peroxidase II) was substantially different from the non-crystalline fraction of 



