The Chemistry of Digestion. Enzymes and Catalysers.— The chemistry of digestion involves a knowledge of the enzymes. The sub stance upon which the enzymes act is called the substrate ; thus the substrate for pepsin is proteid and the substrate for the diastatic ferment amylopsin is starch. The ferment attaches itself to the substrate and makes a new co “und with it, which is capable of standing a .• er temperature than the f alone. Proteo ytic ferments, like trypsin of the pancreatic juice makes, distribute themselves on several different kinds of proteids, if such are present at the same time; but then the protein of the more difficult digestibility acts as an inhibiting sub stance to the ferment. An example of this is the manner of action of trypsin in a mixture of casein, fibrin and egg albumen. This inhibitive effect gave rise to the idea of in such solutions. The action of enzymes is very similar to that of the catalyzers of inorganic chemistry, but they do not act exactly as the catalyzers do.
According to Ostwald catalyzers simply ac celerate a process that is already started and going on. However, it is well known that carbohydrates, proteins and peptones can be kept for years without any change if kept free of bacteria. Hence, ferments cannot be claimed to simply accelerate a process already begun.
The beginning of ferment action is extremely rapid. Sugar can be detected in starch almost immediately when saliva touches it. There is a glycolytic ferment in muscles which forms CO, explosively from glucose, provided pancreatic extract has been added. The rapid beginning is a constant sign if the enzymes are allowed to work under the correct conditions, but in experi mental conditions this action ceases quickly; also, no matter how much glucose is present, the glycolytic action and CO, formation ceases in 2-10 hours. This is explained by the following possibilities: (1) Ferments may be used up. (2) They may be destroyed because they are labile. (3) They may be inhibited by products.
The characteristic manner of action of enzymes is quick onset and quick cessation. They may act by their presence, they may not be consumed and they do not appear as part of the end products.
For some enzymes it has been demonstrated that their action is reversible — that is, they may take place in opposite directions. One of the first demonstrations of this double effect was made by Kastle and Loevenhart in experiment ing with an enzyme of the animal body known as Lipase. A necessary step in the digestion and absorption of fats is their splitting up into fatty acids and glycerine. These two chemists found
that when they made use of a simple ester chemically analogous to the fats, namely, ethyl butyrate, that it was not only split up by Lipase into ethyl alcohol and butyric acid, but that the two last-named substanCes could be resynthe sized into ethyl butyric and water. In this case this reversibility can be chemically expressed by the following formula; CaTITCOOCiHs+H20 CJITCOOH+C.H.OH Ethyl-Butyrate Butyric acid-Ethyl-alcohol The Functions of the Gastric Juice — The Pawlow Experimental Stomach.— By means of a very ingenious surgical operation the Russianphysiologist Pawlow prepared a second ary artificial stomach from the walls of the en tire organ in the dog. The aim of the operation was to separate the stomach into a smaller and larger stomach in such a manner that the two portions did not communicate, but were sepa rated by a partition made of the mucous mem branes of the stomach. The larger portion of the stomach is so sewed up that it remains in continuity with the general digestive tract. The smaller stomach is made to open on the surface of the abdomen by a fistula. It was found by experiment that the juice secreted by the smaller stomach has the same composition as that produced by the large stomach. It had, moreover, the advantage of being free from admixture with food because no food could get into the lesser or artificial stomach.
The gastric juice thus obtained has a specific gravity of 1003-1005. It consists of 99 per cent water and nearly 1 per cent solids. The juice also contains free HC1 acid in the proportion of 0.2 per cent. It is this HC1 acid that explains the inversion of cane-sugar into equal molecules of glucose and fructose in the stomach, for• it has been erroneously thought that the gastric juice contains a ferment to which the name Invertase is given, and to which this action was assigned. Similarly a variety of sugar which occurs in dahlia tubers and known as Inulin is converted by the HCl acid of the gastric juice into fruc tose.
Gastric juice has three functions in addition to that already mentioned. It converts proteids into proteoses by virtue of the HCl acid and pepsin it contains; secondly, it coagulates milk, and, thirdly, it acts on neutral fats but only such as are in the form of fine emulsion, such as yolk of eggs and milk.
The stages of peptic digestion of protein may be represented in tabular form thus— Protein.