Page:EB1911 - Volume 19.djvu/959

 of enzymes, those which were living or associated with living cells, and those which were non-living. In 1897, however, E. Buchner and M. Hahn showed that from living cells (yeast) a ferment could be obtained which acted quite as well extracellularly as when it was bound up within the cell. Subsequent work has shown that other organisms act by the enzymes they contain, so that it is now recognized that there is no essential difference between the living or organized ferment and the non-living or unorganized ferment. All ferments probably act as catalysators or catalysts. Catalysis is the process by which reactions are either initiated or accelerated by the mere presence of certain substances which remain unchanged during the process; to these substances the name of catalysators has been given. As an example of such catalytic action the acceleration of the decomposition of hydrogen peroxide (H2O2) into water (H2O) and oxygen (O) by the action of a colloidal solution of platinum may be given. C. Oppenheimer defines an enzyme as a substance produced by living cells, which acts by catalysis. E. Fischer has shown that the action of ferments is specific, that is, the ferment only exerts its action on definite substances or substrates of definite structural arrangement. He has compared the relation of ferment to substrate to that of a key to its lock. Ferments which bring about the breakdown of proteins are without influence on fats and carbohydrates; those which decompose fats leave proteins and carbohydrates untouched, and so on.

The chemical composition of enzymes is unknown. It has been assumed that they are protein in nature, but this is mainly because it has been found that when they are extracted from tissues they are apparently in combination with proteins. In all probability the protein is there as an impurity owing to incomplete separation.

As regards the general properties of enzymes, most of them can be precipitated from their solutions by means of alcohol. They can also be carried down by fine precipitates of certain inorganic salts or by protein precipitation, e.g. when a precipitate of casein is produced by acidifying a casein solution with acetic acid. Most of the ferments are soluble in water or saline solutions, and in glycerin and water. The ferments are found to have an optimum temperature of action. This temperature in most cases ranges from 37° to 40° C. All true ferments are thermolabile, being destroyed at about 70° C. Ferments are hindered in their action to some extent by the general protoplasmic poisons, such as salicylic acid, chloroform, &c. The action of many of them is retarded when the products of their action are allowed to accumulate. Just as when a chemical reaction is set up its rate tends to decrease and finally comes to a standstill before the reaction is completed—an equilibrium being established—so the reactions set up by enzymes also tend to come to an equilibrium before the complete conversion of the original substance. In the case of certain enzymes at least this equilibrium may be reached from either side; thus the enzyme maltase may either bring about the breakdown of the sugar maltose to dextrose or cause a synthesis of dextrose to maltose.

A number of the body ferments have now been shown to exist in the tissues in an inactive form. This condition is known as the proferment or zymogen state, and before any action can be exerted it must be activated, usually by some specific substance, as in the case of the activation of trypsinogen by means of enterokinase. The following table gives a list of the principal ferments concerned in the digestion and metabolism of food-stuffs:—

Certain oxydases, catalases and de-amidizing enzymes are found in the tissues generally and play an important part in the various metabolic processes.

2. Digestion in the Mouth.—The first of the digestive secretions which food comes into contact with is the saliva. This is the mixed secretion from the various glands, salivary and other, the ducts of which open in the mouth. The saliva, which is for the most part produced by the three large salivary glands, the parotid, the sub-maxillary and the sub-lingual, is a colourless or a slightly turbid viscous fluid with a faintly alkaline reaction and of low specific gravity. It contains a very small proportion of solids, which vary somewhat in amount and character in the secretions of the different glands. Mucin and traces of other proteins are present. Small amounts of potassium sulphocyanide may nearly always be detected. The functions of the saliva are twofold. First, it has a mechanical action moistening the mouth and the food and thus aiding mastication and swallowing by securing the formation of a proper bolus of food; it also assists by binding the particles together, an action of special importance when the food is dry. Second, in man and in some of the lower animals the enzyme ptyalin exerts an action in digestion on part of the carbohydrates of the diet. The starches or polysaccharides are broken down, first of all to the simple dextrins and then to the still more simple disaccharide, maltose. The further breakdown of the maltose is carried out in the intestine by the action of a ferment maltase which does not exist at all or only in the merest traces in the buccal secretion. The action of ptyalin on starches is thus very similar to that of acids, except that it stops at the formation of maltose. Ptyalin acts best at a temperature of about 40° C. and in a neutral or faintly alkaline medium, its action being inhibited by the presence of even very dilute solutions of the mineral acids. If the acid be in sufficient amount the enzyme is destroyed. For this reason the action ceases in the stomach whenever the bolus is completely permeated by the gastric juice. As it takes time for the gastric juice thoroughly to permeate the food mass, which remains for a considerable period in the fundus of the stomach unmixed with the secretion, salivary digestion goes on for about half an hour after food is taken.

3. Gastric Digestion.—The passage of food from the mouth to the stomach will be dealt with later. The stomach has two digestive functions: (1) It acts as a store chamber permitting a full meal to be taken; (2) It acts as a digestive organ of importance in preparing the food for further attack in the intestinal canal. But the stomach cannot be regarded as an essential organ, since it has been removed in dogs and in man without apparent interference with nutrition and health.

Gastric digestion is brought about by the action of the gastric juice, a clear watery, colourless and strongly acid fluid with a specific gravity of about 1003. The amount of solids present is extremely small, about 0·3%. They consist of protein, nucleic acid, lecithin and inorganic salts, in addition to the more important constituents, the enzymes and hydrochloric acid.

The amount of hydrochloric acid present in the juice varies with the period of digestion. In man the maximum acid concentration is about 0·2%. The acid exists in the stomach in two forms as free hydrochloric acid and as combined hydrochloric acid. The amount of each depends on various factors: (1) the secretion itself; (2) the nature of the food; and (3) the rapidity with which the stomach empties itself, &c. For instance, after a protein-free meal the hydrochloric acid is for the most part free, whereas, when protein is present, it combines with it and, unless secreted in very large amount, most of the acid is in a fixed condition.

The hydrochloric acid is formed by the activities of certain gland cells in the middle region of the stomach, and the fact that it does not exist as such in the blood proves that it is formed within these cells. Further, it has been found that the gastric mucous membranes of starving dogs contain 0·74% of sodium and potassium chloride, much more than is present in any other organ or in the blood plasma. That the chlorine comes from the sodium chloride in the food has been shown by the fact that, when the tissues are deprived of this salt, and sodium bromide is given, hydrobromic acid may appear in the gastric secretion.