The most prominent characteristic of enzymes is that they bring about very great chemical changes without themselves being appreciably affected. This property is also shown by many inorganic substances which are spoken of as “catalytic agents” or “catalyzers” so that enzymes are sometimes called “organic catalyzers.” The function of catalytic agents seems to be to hasten the rate of a reaction which would occur spontaneously, though in a great many cases with extreme slowness. Just how enzymes act is not certain and probably will not be until their composition and constitution are known. Most probably they form a combination with the substance acted on (_the substrate_) as a result of which there is a rearrangement of the atoms in such a way that new compounds are formed, nearly always at least two, and the enzyme is at the same time set free. It is rather remarkable that chiefly optically active substances are split up by enzymes and where two modifications exist it is usually the dextro-rotatory one which is attacked. No single enzyme attacks both. This probably means that the structure of the enzyme corresponds to that of the substrate, “fits it as a key fits a lock,” as Emil Fischer says. The production of enzymes is by no means restricted to bacteria since all kinds of living cells that have been investigated have been shown to produce them and presumably _all_ living cells do. Hence the number of different kinds of enzymes and of substances acted upon is practically unlimited. Nevertheless they may be grouped into a comparatively few classes based on the general character of the change brought about by them. I. Class I is the so-called _“splitting” enzymes_ whose action is for the most part hydrolytic, that is, the substance takes up water and then splits into compounds that were apparently constituents of the original molecule. As examples may be mentioned _diastase_, the enzyme first discovered, which changes starch into a malt-sugar, hence is more commonly called _amylase_[16] (starch-splitting enzyme); _invertase_,[16] which splits cane-sugar into dextrose and levulose: C₁₂H₂₂O₁₁ + H₂O = C₆H₁₂O₆ + C₆H₁₂O₆. _Lipase_[16] or a fat-splitting enzyme, which decomposes fat into glycerin and fatty acid: C₃H₅(OC{n}H₂{n}₋₁O)₃ + 3H₂O = C₃H₅(OH)₃ + 3C{n}H₂{n}O₂. Fat Glycerin Fatty acid _Proteases_, which split up proteins into proteoses and peptones. Other classes of “splitting enzymes” break up the products of complex protein decomposition, such as proteoses, peptones and amino-acids. A variety of the “splitting enzymes” is the group of