A further indication that the mechanism of enzyme activity is colloidal in character lies in the fact that, so far as is known, all reactions which are catalyzed by specific enzymes are reversible and the same enzyme will accelerate the velocity of the reaction in either direction, the direction in which the reaction goes being determined by the conditions surrounding the reacting material at the time. It was formerly supposed that enzymes catalyze only decomposition reactions and that the synthetic reactions of living tissues are produced by means of some other force or agency. This view supported the idea of a chemical union of the enzyme with the substrate which, when it breaks down, breaks the molecule of the substrate material into some simpler form, or forms. But it is now known that the reaction which is influenced by the enzyme will be catalyzed in either direction by the specific enzyme which "fits" the particular substrate material at every point of its molecular configuration, as the glove fits the hand. The contrast between this fitting of the enzyme to the entire configuration of the molecule, and the union at a single point or group which is characteristic of chemical linkages, is apparent. As examples of the synthetic action of the same enzyme which, under other conditions, accelerates the decomposition of the same material, there may be cited the demonstrated synthesis of isomaltose from glucose by maltase; the production of ethyl butyrate from alcohol and butyric acid; and the synthetic production of artificial fats, by the aid of the pancreatic lipase; and the apparent synthesis of a protein from the same amino-acids which may be obtained from it by hydrolysis under the influence of the same protease, but under different environmental conditions. ACTIVATORS AND INHIBITORS The activity of enzymes is strongly influenced by the presence in the solution of other bodies, usually, although not always, electrolytes. This is probably due, in most cases at least, to the action of the electrolyte upon the colloidal condition of the enzyme. All enzymes do not respond alike to the action of the same electrolyte, however. The activity of certain enzymes is enormously increased by the presence of a small amount of acid; while the action of another may be absolutely inhibited by the same acid in the same concentration. Thus, the activity of the amylase found in the endosperm of many seeds is instantly stopped by adding to the solution enough sulfuric acid to make it two-hundredth normal in strength; while the same concentration of acid actually accelerates the activity of some of the proteases.