The character and properties of the products formed in such digestion depend on the nature of the protein acted upon. As will be seen from the following table these proteins vary fairly widely in the proportion of amino acids which they contain.
| Caseinogen. | Gelatine. | Globine from Oxy· haemoglobin. |
Elastine. | |
| Glycoeoll | . . | 16·5 | . . | 25·75 |
| Alanine | 0·9 | 0·8 | 4·19 | 6·58 |
| Leucine | 10·5 | 2·1 | 29·04 | 21·38 |
| αProline | 3·1 | 5·2 | 2·34 | 1·74 |
| Phenyalanine | 3·2 | 0·4 | 4·24 | 3·89 |
| Glutamic acid | 10·7 | 0·88 | 1·73 | 0·76 |
| Aspartic acid | 1·2 | 0·56 | 4·43 | . . |
| Cystine | 0·065 | . . | 0·31 | . . |
| Serine | 0·23 | . . | 0·56 | . . |
| Oxyproline | 0·25 | 3·0 | 1·04 | . . |
| Tyrosine | 4·5 | . . | 1·33 | 0·34 |
| Lysine | 5·80 | 2·75 | 4·28 | . . |
| Histidine | 2·59 | 0·40 | 10·96 | . . |
| Arginine | 4·84 | 7·62 | 5·42 | 0·3 |
| Tryptophane | 1·5 | . . | Present | . . |
Whether any of the polypeptides found in digestion are further broken down in the course of normal pancreatic digestion is a moot point, but E. Fischer and E. Abderhalden have shown that many of the synthetic polypeptides prepared by them can be broken into their constituents by the action of trypsin. The previous peptic digestion seems to play some part in the extent to which tryptic digestion is carried out, as one of these observers has demonstrated that protein digested first with pepsin and then with trypsin gives a smaller yield of polypeptide and a larger yield of monamino acids than when digestion has been carried out with trypsin alone.
b. Diastase.—This ferment is found in the pancreatic juice apparently secreted in an active form, although some observers hold that it also is secreted in a zymogen form. It is practically identical in its action with the ptyalin of the saliva, converting starch into maltose. It deals with all the starchy food which has escaped conversion into the simple sugars by the ptyalin.
c. Lipase.—Most of this ferment, if not all, is apparently secreted in the form of a zymogen. There is evidence that the bile is the activating agent here, just as the enterokinase acts in the case of trypsin. Lipase can act in any medium acid. neutral, and alkaline, and both on emulsified and non-emulsified fats. It converts the fats by a process of hydrolysis into fatty acids and glycerin. Kastle and Loevenhart found that not only can this enzyme break up fats into their components, but that it also has the power to act in the reverse direction, and in this way bring about the union of fatty acids and glycerin so as to form fats, a process which occurs in the intestinal epithelial cells after absorption.
In addition to these three enzymes the pancreatic juice may contain traces of others, for example, a rennet-like ferment which curdles milk. This again, as in the case of the stomach rennet, is held by some to be only another phase of proteolytic action. Maltase is also said to be present in small amount, as is also lactase under certain conditions. In pancreatic, as in gastric digestion, the nature of the food is said to play a part in controlling the amount and the composition of the secretion with respect to its ferments. The action, if it does exist, is not very well defined.
b. Intestinal Digestion.—By this is meant the other digestive processes which go on in the intestine under the action of the secretion of Lieberkühn’s follicles—the succus entericus. This is a yellowish, often opalescent, strongly alkaline fluid. The alkalinity is due to the presence of sodium carbonate. It contains a small amount of protein, shed epithelial cells, &c. The secretion of some 170 c.c in 24 hours has been observed in a short loop of human intestine by H. S. Hamburger and E. Hekma, but it is almost impossible to get a measure of the actual amount of secretion from the whole gut. Most of the ferments are present in very small amount in the intestinal juice. They seem to be actually within the epithelial lining of the intestine, for extracts made from the intestinal mucous membrane are richer in ferments than the secretion.
Apparently the intestinal secretion contains no trace of a ferment acting on native protein, but a ferment—erepsin—is present in fair amount in the intestinal mucous membrane and in small amount in the secretion, which acts in an alkaline medium on proteoses, peptones, and on casein, converting them into crystalline products of the nature of amino acids.
Another ferment, arginase, has been isolated from the intestinal mucous membrane by A. Kossel and H. D. Dakin, which splits the diamino acid arginin into urea and ornithin. A lipase has also been detected which is very similar to pancreatic lipase; it, however, attacks only emulsified fats.
Several carbohydrate hydrolysing enzymes have been described in the small intestine. Invertin, the ferment which splits cane-sugar, is present in small amount in the secretion, more abundantly in the extract of mucous membrane. In all probability it deals with the saccharose after or in process of absorption. Maltase is also present in large amount, and here again in greater amount in the extract than in the secretion. The presence of lactase has been much discussed, and it seems probable that suckling animals do possess this enzyme. Some workers have stated that an intestinal diastase is to be found, but, if so, it is present in very small amount.
In the large intestine a small amount of erepsin has been discovered at the upper end. Any digestion which does take place is probably either bacterial in origin, or due to ferments which have originated in the lower end of the small intestine, and which have been carried down.
5. Bile.—This fluid, in all probability, has little direct action in ordinary digestion, although it contains substances which act indirectly. The bile salts act as solvents for fats and fatty acids, and as activators of pancreatic lipase. The salts also serve to keep cholestrin in solution. Bile is to be looked upon rather as the excretion, the result of the hepatic metabolism, than as a digestive juice. Various workers have shown that when the bile is prevented from entering the intestine owing to a fistula having been made, the animal or patient may continue to enjoy good health, thus proving that this fluid is not essential to any of the digestive processes which normally take place.
Bile as secreted has an orange-brown colour, but the colour varies according to the pigment present. It is more or less viscous (not so viscous as bile taken from the gall bladder) and has a specific gravity of about 1010. It has a slightly alkaline reaction, a bitter taste and a characteristic smell. The daily output is, for a normal individual, over 500 c.c On analysis it is found to have over 2% of solids, of which more than half are organic. It contains in addition to a nucleo-albumin, derived mainly from the bile passages and gall bladder, bile acids, bile pigments, cholesterin, lecithin, fats, &c. The most abundant solids are the salts of the bile acids, of which in man the most important is sodium glycocholate, sodium taurocholate being present in very small amount. The bile acids are formed in the liver cells, and when the duct is ligatured they tend to accumulate in the blood.
The pigments amount to only about 0·2%. In human bile the chief pigment is bilirubin, whilst in herbivora biliverdin is more abundant. They are derived from the haemoglobin of the blood, but the pigments are iron-free. They may be regarded as purely excretory products arising from the breakdown of the haemoglobin of effete blood corpuscles.
Cholesterin is a monatomic alcohol, and is probably a waste product. It occurs in the bile only in small amount, and there is some evidence that it is not secreted by the liver cells but is added to the bile from the bile passages. Fats and lecithin are both derived from the liver cells. Of the inorganic constituents phosphate of calcium is the most abundant.
The secretion of bile is practically continuous, but it seems to enter the duodenum intermittently. The taking of food increases the flow of bile, the amount of the increase depending to a certain extent on the nature of the food. A protein meal has been found to have the greatest effect and a carbohydrate one the least. The entry of the acid chyme into the duodenum is the stimulus which brings about the ejection of the bile. Pressure on the liver also seems to cause a flow (see section II.).
In connexion with bile secretion attention may be drawn here to a peculiar enterohepatic circulation which is stated to exist. The bile salts are partly absorbed from the intestine, to be carried again by the portal blood to the liver and to be again eliminated. By this circulation the entrance of various alkaloidal and ptomaine poisons into the general circulation may be prevented.
Faeces.—The bulk of the waste matter arising from the foods along with the secretions from the alimentary canal form the faeces. On an absorbable diet the faeces are almost purely intestinal in origin. As a channel of excretion of nitrogenous metabolic waste products they are not very important, although the work of C. Voit indicates that they do play a certain part. The nature of the excreted nitrogenous substances has not been fully examined. Of the inorganic constituents iron is probably for the most part excreted into the large intestine. It is, however, very difficult to come to any definite Conclusion as to what is unabsorbed material and what excreted.
II. The Mode of Formation of the Digestive Secretions
1. Salivary Glands.—The secretion from the various glands is generally evoked by nervous impulses, through the secretory
