II. The zymases. Their action seems to be to cause a “shifting on rearrangement of the carbon atoms” so that new compounds are formed which are not assumed to have been constituents of the original molecule. Most commonly there is a closer combination of the carbon and oxygen atoms, frequently even the formation of CO2 so that considerable energy is thus liberated. Examples are the zymase or alcoholase of yeast which converts sugar into alcohol and carbon dioxide; C6H12O6 = 2C2H6O + 2CO2: also urease, which causes the change of urea into ammonia and carbon dioxide. Another common zymase is the lactacidase in lactic acid fermentation.
III. Oxidizing enzymes also play an important part in many of the activities of higher plants and animals. Among the bacteria this action is illustrated by the formation of nitrites, nitrates and sulphates and the oxidation of alcohol to acetic acid as already described.
IV. Reducing enzymes occur in many of the dentrifying bacteria and in those which liberate H2S from sulphates. A very widely distributed reducing enzyme is “catalase” which decomposes hydrogen peroxide.
As previously stated, most of the physiological activities of bacteria are due to the enzymes that they produce. It is evident that for action to occur on substances which do not diffuse into the bacterial cell—starches, cellulose, complex proteins, gelatin—the enzymes must pass out of the bacterium and consequently may be found in the surrounding medium. Substances like sugars, peptones, alcohol, which are readily diffusible, may be acted on by enzymes retained within the cell body. In the former case the enzymes are spoken of as extra-cellular or “exo-enzymes,” and in the latter as intra-cellular or “endo-enzymes.” The endo-enzymes and doubtless also the exo-enzymes may after the death of the cell digest the contents to a greater or less extent and thus furnish substances that are not otherwise obtainable. This process of “self-digestion” is known technically as “autolysis.”
A distinction was formerly made between “organized” and “unorganized ferments.” The former term was applied to the minute living organisms, bacteria, yeasts, molds, etc., which bring about characteristic fermentative changes, while the latter term was restricted to enzymes as just described. Since investigation has shown that the changes ascribed to the “organized ferments” are really due to their enzymes, and that enzymes are probably formed by all living cells, the distinction is scarcely necessary at present.
PRODUCTION OF TOXINS.
The injurious effects of pathogenic bacteria are due in large part to the action of these substances, which in many respects bear a close relationship to enzymes. The chemical composition is unknown since no toxin has been prepared “pure” as yet. It was formerly thought that they were protein in character, but very pure toxins have been prepared which failed to show the characteristic protein reactions. It is well established that they are complex substances, of rather large molecule and are precipitated by many of the reagents which precipitate proteins. Toxins will be further discussed in [Chapter XXVII]. It will be sufficient at this point to enumerate their chief peculiarities in order to show their marked resemblance to enzymes.
1. Toxins are dead organic chemical substances.
2. They are always produced by living cells.
3. They are active poisons in very small quantities.[17]