THE PRODUCTION OF ENZYIVIES BY PLANTS 477 



Chemical Nature of Enzymes. — It is evident that in any ultimate analy- 

 sis enzymes must represent chemical compounds of definite molecular struc- 

 ture and configuration, and that these molecules, whether they exist in the 

 cells in true solution, in colloidal dispersion, or as adsorbed substances in a 

 colloidal complex, must contain certain chemical groupings which can ac- 

 complish their characteristic catalytic effects. It has long been suspected, 

 both because of their intimate relation to protoplasm, and their apparent 

 colloidal properties, that enzymes are either proteins or protein-like sub- 

 stances. At the present time evidence is rapidly accumulating in support of 

 the view that many enzymes, at least, are proteins. 



Sumner's work on urease (1926, 1928) was the first to point in this direc- 

 tion. Urease is the enzyme which catalyzes the hydrolysis of urea, as follows: 



T Jrp'i cp 



(NH2)2CO -f H2O > 1 NH3 + CO2 



Urea 



This enzyme is produced by many bacteria and fungi, and is also found in 

 a number of species of the higher plants. Sumner succeeded in isolating 

 from the seeds of the jack bean {Canavalia ensiformis) a crystalline protein 

 which he regards as the enzyme urease in pure form. This product was 

 crystallized out of acetone extracts of the ground seeds. His preparations 

 exhibit activities of from 730 to 1400 times that of the original material, and 

 appear to have the properties of a globulin. 



Recently a number of other enzymes have apparently been isolated in a 

 pure form. Among these are pepsin (Northrup, 1930), trypsin (Northrup 

 and Kunitz, 1932), amylase (Sherman, et al., 1931), and papain (Balls, 

 et al.j 1937). All of these enzymes in the isolated form appear to be crystal- 

 line proteins. 



It would be premature to conclude, however, that all enzymes are pro- 

 teins or protein-like compounds. In fact there is considerable evidence that 

 certain ones such as the oxidizing-reducing enzymes catalase and peroxidase 

 (Chap. XXX), or at least their active groups, are non-proteins. It is en- 

 tirely possible that some enzymes may be proteins while others are not. 



The Production of Enzymes by Plants. — It has already been pointed 

 out that plant enzymes are mostly of the endoenzyme type which means that 

 they generally operate within the cells in which they are produced. 



There are only two well-known examples of the formation of exoenzymes 

 by the higher plants. One of these is in the so-called insectivorous plants 

 such as certain pitcher plants and sundews. The leaves of these species 



