in many plant cells. The products 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 hyphæ.
Enzymes acting upon sugars. Another group of enzymes is concerned 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:
| C12H22O11 H2O C6H12O6 C6H12O6 | |||
| 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 lævo-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 produce 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 inversion 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 preliminary 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 arrangement of the atoms within the molecule. Maltose is split up by the action of the enzyme maltase into two molecules of dextrose. Maltose, 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 converted back into maltose until a new equilibrium be reached. This reversibility 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.
