Enzymes are found in all parts of the animal organism and those which are active in the digestion of food, the metabolism of digested material, the coagulation of blood, etc., have been extensively studied. A discussion of these animal enzymes would be out of place in such a text as this, however, and the following list includes only enzymes which are known to occur in plant tissues. These well-known enzymes will serve as examples of the several general types which have thus far been isolated and studied.
The above list includes only the more common and best-known plant enzymes. It seems reasonable to suppose that for every individual type of organic compound which may occur in general plant groups, or even in single species, there is a corresponding enzyme available to affect its physiological alterations. Indeed, new preparations of active enzymes from special types of plants and new evidences of the existence of enzymes in various plant organisms are continuously being reported.
A few of the most common specific representatives of individual groups of enzymes may be briefly described, as follows:
Amylase (or diastase, as it was first named and is still commonly called) is probably the most widely distributed enzyme of plants. It is found in practically all bacteria and fungi; in practically all seeds (it has been found in active form in seeds which were known to be over fifty years old); in all roots and tubers; and in practically all leaves, where it is located in the stroma of the chloroplasts.
It appears to exist in two modifications, known, respectively, as (a) translocation diastase and (b) diastase of secretion. The first form is found in the cells of ungerminated seeds, in leaves, shoots, etc. It remains in the cells where reserve starch is stored and aids in the transformation of starch into soluble materials for translocation from cell to cell. It is active at a lower temperature than the second form, its optimum temperature being 45° to 50°. The second form is secreted by the scutellum, and perhaps by the aleurone cells, of germinating seeds, being produced by special glandular tissue. It aids in the hydrolysis of the starch for the use of the growing embryo. Its optimum temperature is 50° to 55°.
The activity of amylase is accelerated by the presence of small quantities of neutral salts, especially by sodium chloride and disodium phosphate. It acts best in neutral solutions, its activity being inhibited, although the enzyme itself is not destroyed, by the presence of more than minute traces of free mineral acid or alkali.
Sucrase (or invertase) is present in almost all species of yeasts, where it serves to convert unfermentable sucrose into glucose and fructose, which are readily fermentable. Invertase is also present in moulds and other microorganisms; and in the buds, leaves, flowers, and rootlets of those higher order plants which store their carbohydrate reserves in the form of sucrose. It appears that sucrose, while easily soluble, is not readily translocated, or utilized, by plants until after it has been hydrolyzed into its constituent hexoses.
The optimum temperature for invertase is 50° to 54°; it is killed if heated, in the moist condition, to 70°. Its activity is increased by the presence of small amounts of free acids; but is inhibited by free alkalies.