20 PLANT ENZYMES [ch. 



The chemical nature of enzymes is at present unknown, because it 

 is difficult to purify them without destroying them, and hence to obtain 

 them of sufficient purity for chemical analysis. They were originally 

 thought to be proteins, but with the improvements in methods for puri- 

 fication, it has been found that the protein reactions disappear, although 

 the enzyme activity does not decrease. In solution they exist in the 

 colloidal condition. 



The questions as to their origin and their relation to the protoplasm 

 cannot yet be answered with any certainty. It is also impossible to say 

 whether the majority of chemical processes in the plant are catalyzed by 

 enzymes. 



A feature of enzyme action which is of considerable interest and 

 which has already been mentioned is the question as to whether enzymes 

 catalyze a reaction in both directions. Thus, in the case of hydrolytic 

 enzymes which constitute by far the greater number of known enzymes, 

 do they control the synthetic as well as the hydrolytic process ? There 

 is evidence that this is so, since, in many cases, the hydrolysis is not 

 complete. If the enzyme were a catalyst in one direction only, the 

 reaction would be complete. Further evidence is supplied by the fact 

 that, under suitable conditions, i.e. strong concentration of the substances 

 from which synthesis is to take place, certain syntheses have been carried 

 out in vitro. As an example may be quoted the synthesis of maltose 

 from a concentrated solution of glucose by maltase (Bayliss, 2). 



In the living cell it is supposed that the hydrolysis and synthesis 

 are balanced. On the " death " of the protoplasm, which may be caused 

 by mechanical injury, vapour of chloroform or toluol, etc. (Armstrong, 

 7, 8), the reactions catalyzed by enzymes cease to be balanced and pro- 

 ceed almost always in the direction of hydrolysis and the splitting up of 

 more complex into simpler substances. This phenomenon is obvious when 

 any of the products can be recognized by smell or colour, as, for instance, 

 the smell of benzaldehyde on injuring leaves of plants containing cyano- 

 genetic glucosides (see p. 161), or the production of coloured oxidation 

 products when some of the aromatic glucosides are decomposed (see 

 p. 124). 



If plant tissues are disintegrated, and the mass is kept at a tempera- 

 ture of about 38° C, the above-mentioned hydrolytic processes continue 

 to be catalyzed by the enzymes present until equilibrium is reached, 

 which will be near complete hydrolysis, especially if water has been added. 

 Such a process is termed " autolysis." 



The chief plant enzymes may be classified to a certain extent accord- 



