CHEMISTRY OF DIGESTION The essential step which prepares the ordinary food for utiliza tion in the body, for the change into living matter, is digestion, a process which the food undergoes under the influence of the ferments or enzymes present in the gastrointestinal tract. By this process it is broken down into simpler substances, which can be utilized by the body tissues for conversion into protoplasm and as the supply of energy.
The substances which bring about this change are known as ferments, enzymes or zymins. Formerly it was believed that there were two distinct classes 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 be tween the living or organized ferment and the non-living or unorganized ferment. All ferments probably act as catalysts. Catalysis (q.v.) is the process by which reactions are either initiated or accelerated by the mere presence of certain substances which remain unchanged during the process. It has been shown that the action of ferments is specific ; i.e., the ferment only exerts its action on definite substances or substrates of definite structural arrangement. The relation of ferment to substrate has been compared 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 pro teins and carbohydrates untouched, and so on.
The chemical composition of enzymes is unknown, but it would seem certain that all enzymes have not the same chemical structure. One of their most important physical properties is their colloidal nature, shown by the fact that they cannot, or only with extreme difficulty, pass through a parchment membrane.
Most of the ferments are soluble in water or saline solutions, and in glycerol and water. The ferments are found to have an optimum temperature of action. This temperature in most cases ranges from 37° to 4o° C. All true ferments are thermolabile, being destroyed at about 70° C. The action of many of them is retarded when the products of their action are allowed to accumu late. 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.
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 prof erment 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 entero kinase. The table on page 649 gives a list of the principal ferments concerned in the digestion and metabolism of food-stuffs.
Certain oxydases, catalases and de-aminizing enzymes are found in the tissues generally and play an important part in the various metabolic processes.
The first of the digestive secretions which food encounters is the saliva. It is produced by the three large salivary glands, the parotid, the sub-maxillary and the sub-lingual, is a colourless or slightly turbid viscous fluid with a faintly alkaline reaction and of low specific gravity, and exercises a twofold function. First, it has a mechanical action moistening the mouth and the food and thus aiding mastica tion 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 dex trins and then to the still more simple disaccharide, maltose. 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 per meated 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.