SOLUBLE FERMENTS OR ENZYMES. 499 



in many plant cells. The produets of the activity of cytase have as yet 

 been imperfectly studied, and it is possible that several different 

 enzymes, corresponding to the different kinds of cellulose, are at 

 present included under this name. Cytase may be useful in various 

 ways to the organisms secreting it. Some parasitic fungi are able to 

 attack and penetrate the cell-walls of the host-plant by virtue of the 

 dissolving power of the cytase secreted in the tip of the growing hypha3. 

 Enzymes acting upon sugars. Another group of enzymes is con- 

 cerned with the conversion of the higher sugars or polysaccharides 

 into the lower. The classic instance is the so-called 'inversion' of 

 cane-sugar or sucrose into equal parts of grape-sugar and fruit-sugar, 

 according to the equation : 



sucrose or dextrose or levulose or 



cane-sugar. ' grape-sugar, fruit-sugar. 



A solution of cane-sugar turns a ray of polarized light to the right, 

 but the mixture of dextrose and levulose, owing to the superior Isevo- 

 rotatory power of levulose, turns the ray to the left, whence the term 

 'inversion' as applied to this process. The enzyme that is able to pro- 

 duce the inversion of cane-sugar was discovered by Berthelot in 1860. 

 Sucrase, or invertase, is found in many yeasts and other fungi, in 

 pollen-grains, in the beet-root, and to a slight extent in some animal 

 secretions. Cane-sugar is not directly assimilable by the animal body, 

 and if injected into a vein is excreted almost unchanged. The inver- 

 sion which occurs in the intestine when the cane-sugar is taken into the 

 body by way of the alimentary tract seems to be a necessary prelimi- 

 nary to the utilization of this sugar as a food substance. 



The same is true to a certain extent of maltose, which is a sugar 

 of the same percentage composition as sucrose, but with a different ar- 

 rangement of the atoms within the molecule. Maltose is split up by 

 the action of the enzyme maltase into two molecules of dextrose. Mal- 

 tose, like sucrose, is only with difficulty assimilable, and its conversion 

 into dextrose constitutes an important phase of carbohydrate nutrition. 

 Maltase, like sucrase, is a widely distributed enzyme and is found in 

 many animal and plant tissues. 



The action of maltase upon maltose presents a significant example 

 of what chemists call a 'balanced' action. When a certain proportion 

 of maltose has been converted into dextrose, the action of the maltase 

 ceases, and if now an excess of dextrose be added the action of the 

 enz}Tne is reversed, and a certain proportion of the dextrose is con- 

 verted back into maltose until a new equilibrium be reached. This re- 

 versibility of action has been thought to indicate that the action of 

 maltase falls in line with other chemical reactions and is not essentially 

 different from that evinced by many well-studied inorganic substances. 



