Part 8

In this connection the question naturally arises how far we are to trust these results in their bearing on the natural process of digestion as it occurs in the living stomach. Obviously, the conditions are quite different in the two cases. In artificial digestions, we have especially the influence of an ever-increasing percentage of soluble products on the activity of the ferment, a condition of things generally considered as more or less inhibitory to enzyme action. We have attempted to measure the real value of this influence by experiments[141] conducted in parchment dialyzing tubes, in which the conditions are made favorable for the removal of at least some of the products of digestion as fast as they are formed. In these experiments, the dialyzer tubes containing the proteid and pepsin-acid were immersed in a large volume of 0.2 per cent. hydrochloric acid (about three litres), which was gradually changed from time to time, the whole mixture being kept at 40° C. during the entire period of the experiment. The extent of peptonization was then ascertained by analysis of both the contents of the dialyzer tubes and of the surrounding acid, the results being compared with those obtained from control experiments carried on in flasks. Without considering the results in detail, it may be mentioned that the slow and incomplete peptonization so characteristic of artificial gastric digestion is not materially modified by this closer approach to the natural process. The several digestions carried on in the dialyzer tubes were certainly accompanied by a fairly rapid withdrawal of the diffusible products of digestion, yet no noticeable increase in the amount of peptone formed was observed. The results certainly favor the view that the conversion of the primary products of gastric digestion into true peptone is a slow and gradual process, even under the most favorable circumstances, and that this lack of complete peptonization is not due to accumulation of the products of digestion, but is rather an inherent quality of pepsin-proteolysis under all circumstances.

[141] Chittenden and Amerman: A Comparison of Artificial and Natural Gastric Digestion, together with a Study of the Diffusibility of Proteoses and Peptones. Journal of Physiol., vol. xiv, p. 483.

In these dialyzer experiments it was observed that not only did peptones diffuse, but also the proteoses. In fact, it was found that six to eight per cent. of the proteoses formed passed through the parchment walls of the dialyzer tubes into the surrounding acid in the nine hours’ digestion. This led to a study of the diffusibility of proteoses in general, from which we were led to conclude that these bodies possess this power to a greater degree than had hitherto been supposed. As might be expected, it was also found that the attendant conditions modify materially the rate of diffusibility; the two factors especially prominent being temperature and the volume of the surrounding fluid. Thus, 1.9 grammes of protoalbumose dissolved in 200 c.c. of water and suspended in 4.5 litres of water heated to 38° C., diffused through the parchment tube to the-extent of 5.09 per cent., while at 10° C. diffusion amounted to only 2.57 per cent. Under somewhat similar conditions, pure peptone diffused to the extent of eleven per cent. in six hours at 38° C. Somewhat singular, however, was the result obtained with deuteroalbumose; this proteose showing a diffusibility considerably less than that of the proto-body. But as Kühne[142] has independently obtained essentially the same results, this apparent anomaly cannot depend upon any errors of work.

[142] Erfahrungen über Albumosen und Peptone. Zeitschr. f. Biol., Band 29, p. 20.

It is of course to be understood that diffusion experiments made with dead parchment membranes cannot necessarily be expected to throw much light upon the rate of absorption of these bodies through the living membranes of the stomach and intestine, where, as Waymouth Reid[143] has well said, we have to deal with an absorptive force dependent, no doubt, upon protoplasmic activity, and comparable, in part at least, to the excretive force of a gland-cell. Furthermore, in considering absorption as it occurs in the living stomach, we must necessarily give due weight to the selective power of the epithelial cells, a power which may be far more potent even than we suppose. Hence, without attempting at this point to draw any broad deductions from our experiments we may simply lay stress upon the facts themselves, viz., that the primary products of pepsin-proteolysis are diffusible, and, like true peptones, are capable of passing through animal and vegetable membranes, although to a less extent. We may further emphasize the fact that experiments of this character on diffusibility can, at the most, only indicate general tendencies, since every variation in the attendant conditions will exercise some influence upon the final result.

[143] Osmosis Experiments with Living and Dead Membranes. Journal of Physiol., vol. xi, p. 312.

With reference to the bearing digestive experiments made in dialyzer tubes have upon the natural process as carried on in the living stomach, we must necessarily grant that the conditions approximate only in the crudest way to those existent in the alimentary tract. At the same time, if complete peptonization is characteristic of pepsin-proteolysis in the stomach, and failure to obtain such results in an artificial digestion is due to lack of withdrawal of the diffusible products formed, then certainly the experiments carried on in dialyzer tubes, with abundant opportunity for diffusion, and with a large excess of free hydrochloric acid, should show some indications of increased peptone-formation. But none were obtained.

It is more than probable that the rate of absorption of diffusible products from the stomach has been overestimated. Lea,[144] for example, assumes that, “normally the products of digestion, whether proteid or carbohydrate, are never met with in either the stomach or intestine in other than the smallest amounts, frequently to be described as merely traces.” This certainly implies a far more rapid absorption of proteoses and peptones from the stomach than results seem to justify. Indeed, recent facts obtained by Brandl,[145] working under Tappeiner’s direction, tend to show that absorption from the stomach is, under some circumstances at least, comparatively slow. Brandl’s experiments were conducted on large and vigorous dogs with gastric fistulæ, the stomach being shut off from the intestine by the simple introduction of a small rubber balloon into the pylorus, which when dilated completely closed the orifice. By carefully conducted experiments, it was shown that pure peptone, entirely free from proteose, is absorbed from the empty stomach in proportion to the concentration of the peptone solution. Thus, 7.5 grammes of peptone dissolved in water in such proportion as to make a five per cent. solution, and allowed to remain in the stomach for two hours, lost by absorption only 0.28 gramme, equal to 2.68 per cent. of the peptone introduced. Under similar conditions, a ten per cent. aqueous solution of peptone lost only 4.5 per cent. by absorption. On the other hand, when peptone was introduced in larger quantity, viz., in a twenty per cent. solution, absorption amounted to thirteen per cent. in two hours.

[144] Journal of Physiol., vol. xi, p. 240.

[145] Ueber Resorption und Secretion im Magen, und deren Beeinflussung durch Arzneimittel. Zeitschr. f. Biol., Band 29, p. 277.

It is thus evident that pure peptones, even when taken into the stomach in fairly large amounts, and under conditions very favorable for rapid absorption, pass into the circulating blood very slowly. Obviously, however, one must not lose sight of the fact that when digestion is under way and the volume of blood consequently increased, there may be a corresponding rise in the rate of absorption. There is perhaps a hint of this conclusion in the influence of alcohol on the absorption of peptone as brought out by some of Brandl’s experiments. Thus, it was found that when alcohol was added in considerable quantity to a ten per cent. solution of peptone, the stomach-mucosa was greatly reddened, while in two hours the absorption of peptone amounted to 11.8 per cent. But in any event, these results certainly do not favor the view that the products of gastric digestion are absorbed as soon as they are formed. It is, no doubt, quite different in the intestine, but in the stomach, where pepsin-proteolysis occurs, we have, I think, no grounds for assuming that either peptones or proteoses are rapidly absorbed. Hence, it might perhaps be considered that the results of pepsin-proteolysis in the living stomach are much the same as those obtained in artificial digestion experiments.

Still, there are other differences between natural digestion and artificial proteolysis than those connected with the possible absorption of the more diffusible products of digestion. Thus, in the living stomach there is an ever-increasing secretion of hydrochloric acid, and perhaps also of pepsin, more or less proportional to the extent of proteolysis. On this point Brandl’s experiments again give us some light. Thus, it was found that the introduction of an aqueous solution of peptone into the empty stomach led to the secretion of an acid fluid containing on an average 0.24 per cent. HCl, while, under similar conditions, the introduction of sugar or potassium iodide was followed by the secretion of a fluid containing on an average only 0.13 per cent. HCl. Further, the absolute amount of acid found after the introduction of peptone was far greater than when sugar or iodide was introduced, since peptone led to an increase of at least fifty per cent. in the volume of fluid secreted. Hence, proteolysis in the living stomach may give rise to such an increased production and secretion of hydrochloric acid that formation of the terminal products of gastric digestion may be greatly accelerated. That such in fact is the case, I have no manner of doubt, but that it may result in the complete conversion of the so-called primary and secondary proteoses into peptone I very much question. In fact, such examinations as I have made of the stomach-contents after a suitable test-meal have always resulted in the finding of a relatively large amount of proteoses. To be sure, true peptone may be detected and in fairly large amounts, but whenever a quantitative determination of the relative proportion of the two has been made, the proteoses have always been in excess. I have already reported elsewhere the results of some experiments in this direction made on a healthy young man, where the stomach-contents were withdrawn at varying periods after the ingestion of weighed amounts of coagulated egg-albumin. Thus, in one experiment[146] the stomach was thoroughly rinsed with water, after which 138 grammes of finely divided coagulated-albumin, equal to 16 grammes of dry albumin, were ingested. Three-quarters of an hour thereafter, the stomach-contents were withdrawn by lavage and analyzed. As a result, 1.41 grammes of albumoses were separated and weighed, and 0.84 gramme of peptones, the relative proportion being expressed by sixty-two per cent. of albumoses and thirty-seven per cent. of peptones, calculated on the 2.25 grammes of soluble products recovered. This expresses the general character of the results obtained in experiments of this nature, and in my opinion adds emphasis to the statement already made, that complete peptonization is not a feature of pepsin-proteolysis, either in the artificial or in the natural process as it takes place in the living stomach.

[146] Journal of Physiology, vol. xiv., p. 501.

Gastric digestion is to be considered rather as a preliminary step in proteolysis, preparatory to the more profound changes characteristic of pancreatic digestion, in which the ferment trypsin is the important factor. We can thus see how, as in the case of Czerny’s dogs, an animal may be perfectly nourished without a stomach, digestive proteolysis being carried on solely by the pancreatic fluid. You will remember that two of the dogs operated on by Czerny and his pupils lived between four and five years after the operation, with the stomach completely removed, and yet during this period they were well nourished and ate all varieties of food with apparently a normal appetite.[147] Evidently, then, in some cases at least, digestive proteolysis can be carried on without this preliminary action of the gastric juice. Ogata[148] arrived at essentially the same conclusion by the establishment of a duodenal fistula, shutting off the stomach from the intestine by means of a small rubber ball which could be inflated with water. On then introducing coagulated egg-albumin and other forms of proteid matter into the duodenum, he found that digestion was at least sufficiently complete to satisfy all the demands of the system. The only unsatisfactory result was with collagenous foods, which plainly showed the need of a preliminary acid digestion. More recently still, Cawallo and Pachon,[149] working in Richet’s laboratory, have studied the digestibility of different kinds of proteid foods in a dog, upon which they had performed a gastrectomy; the entire fundus, as well as the pyloric portion, of the stomach having been removed. In an animal so operated upon, after recovery was complete, solid food, as meat, was completely digested when taken in small quantities at a time. Raw meat, however, was less completely utilized, the fæces showing portions of undigested fibres. Still, it was apparent that intestinal digestion alone was capable of accomplishing all that was necessary for the complete nourishment of the animal, when it had once become accustomed to the changed condition of its alimentary tract.

[147] Bunge’s Physiologische und Pathologische Chemie, p. 152.

[148] Ueber die Verdauung nach der Ausschaltung des Magens. Du Bois-Reymond’s Archiv f. Physiol, 1883, p. 89.

[149] Une observation de chien sans estomac. Comptes Rendus hebd. de la Société de Biologie, December 1, 1893.

These facts are cited not to belittle the importance of gastric digestion in the nutrition of the body, but rather to emphasize the probability that pepsin-proteolysis is simply a preliminary step in digestion; that its function is not in the direction of a complete peptonization of the proteid foods ingested, but that its action is especially directed to the production of soluble products, proteoses, which can be further digested in the small intestine, or perhaps directly absorbed after they have passed through the pylorus, or even from the stomach itself to a certain extent.

SOME PHYSIOLOGICAL PROPERTIES OF PROTEOSES AND PEPTONES.

It is very evident from what has been said that all forms of proteid matter, _i.e._, all the members of the three main groups spoken of in our classification of the proteids, excepting only nuclein, reticulin, and the keratins, are capable of undergoing proteolysis with pepsin-hydrochloric acid. Further, in every case the main products of the transformation are proteoses; viz., albumoses, caseoses, gelatoses, vitelloses, myosinoses, etc., according to the nature of the proteid undergoing proteolysis; true peptones being formed in less abundance. Corresponding to each of these groups are primary and secondary proteoses, all possessed of many points in common, both chemical and physiological, yet differing from each other in many minor respects. These are the important products of gastric digestion, of pepsin-proteolysis, and it may be well to consider for a moment some of the physiological properties of the proteoses and of peptones as well, in order that we may the better comprehend the general nature of these substances with reference to their possible action in the economy.

As far back as 1880, Schmidt-Mülheim[150] discovered that the injection of aqueous solutions of peptone into the blood-vessels of living dogs was attended by a series of remarkable phenomena. Thus, the animal passed at once into a condition of narcosis resembling that produced by chloroform, accompanied by a fall of general blood-pressure so great that the animal was liable to die, as from asphyxia. Further, there was evidence of some marked change in the condition of the blood, as indicated by loss of the power of spontaneous coagulation, while the peptone itself evidently underwent some alteration, or else was rapidly eliminated, since it could not be detected in the blood a short time after its introduction. These experiments, however, were not conducted with true peptone but with Witte’s “peptonum siccum,” which at that time, at least, was composed in great part of proteoses. The general character of these interesting results was confirmed by Fano,[151] who found that the injection of so-called peptone in the proportion of 0.3 gramme per kilo. of body-weight was sufficient to bring about complete narcosis, together with loss of coagulability on the part of the blood. Very suggestive, however, was the fact that Fano, on trying similar experiments with the peptone formed by pancreatic digestion, viz., with antipeptone, which presumably contained a far smaller proportion of proteoses, failed to obtain like results; the tryptone, so-called, being exceedingly irregular in its action, in many cases producing no effect whatever.

[150] Beiträge zur Kenntniss des Peptons und seiner physiologischen Bedeutung. Du Bois-Reymond’s Archiv f. Physiol., 1880, p. 33.

[151] Das Verhalten des Peptons und Tryptons gegen Blut und Lymphe. Ibid., 1881, p. 277.

The discovery at this date of the several albumoses, and their presence in large amounts in all so-called peptones, led to a study of their physiological action with special reference to the observations of Schmidt-Mülheim and Fano. Politzer,[152] working under Kühne’s guidance, was the first to experiment in this direction, and his results are full of interest as throwing light on the action of the individual albumoses. Thus proto, hetero, and deuteroalbumose are all active physiologically, giving rise when injected into the veins of dogs and cats to strong narcotic action, varying somewhat in intensity in different individuals. There is also produced a marked fall in blood-pressure, due apparently to vaso-motor paralysis, the action being manifested chiefly, if not wholly, on the splanchnic region. Thus, after an injection of one of these albumoses, the mesenteric vessels are always strongly congested, accompanied frequently by the appearance of a bloody serum in the peritoneal cavity. Narcotic action is manifested only so long as the blood-pressure remains sub-normal, and is due presumably to this marked accumulation of blood in the large abdominal veins, thus leading to anæmia of the brain. Albumoses and peptones injected into the jugular vein likewise produce fever, presumably through some action on the nervous system by which the equilibrium of tissue-metamorphosis is interfered with.[153]

[152] On the Physiological Action of Peptones and Albumoses. Journal of Physiol., vol. 7, p. 283.

[153] Ott and Collmar: Pyrexial Agents, Albumose, Peptone, and Neurin. Journal of Physiol., vol. 8, p. 218.

Further, Politzer found that all of the albumoses either delayed or prevented altogether the coagulation of the blood, in conformity with the observations of Schmidt-Mülheim and Fano. In all of these actions the primary albumoses appeared most effective, deuteroalbumose least so. Heteroalbumose, however, was constantly most active, especially in delaying the coagulation of the blood. With amphopeptone, there was far less narcosis and less diminution of blood-pressure, while the effect on the coagulability of the blood was more or less variable, frequently being entirely negative. Antipeptone, on the other hand, was found almost wholly wanting in any constant effects, although in one instance deep narcosis was produced. Thus, from Politzer’s experiments, it was made clear that the albumoses, when introduced directly into the blood-current, possess a far greater toxic action than either amphopeptone or antipeptone. Albumoses, in sufficiently large doses, were invariably fatal, while peptones never produced fatal results so long as the kidneys of the animal remained intact. The extreme solubility and diffusibility of peptones, coupled perhaps with their marked diuretic action, lead to rapid elimination through the kidneys, and their consequent removal from the system.

Many of these observations made with the albumoses I have repeated with several of the proteoses and peptones more recently studied, as protocaseose, protoelastose, the globuloses, and others. The results may be taken as practically confirmatory of the older observations, and I make mention of them in this general way simply to emphasize the fact that all of the proteoses, though perhaps showing individual peculiarities, are possessed of marked physiological properties, which plainly testify to their toxic nature, when introduced directly into the blood-current.

Young animals are particularly sensitive to the injection of proteoses into the blood, even when the introduction takes place very gradually.[154] Thus, a young, healthy dog of 2 kilos. body-weight, eight weeks old, died in one hour after the injection into the jugular vein of 1 gramme of protoalbumose in 20 c.c. of water, thus affording a good illustration of the extreme toxicity of this albumose when introduced directly into the blood.

[154] Neumeister: Ueber die Einführung der Albumosen und Peptone in den Organismus. Zeitschr. f. Biol., Band 24, p. 284.

Of greater interest, physiologically, are the changes the individual proteoses undergo after their injection into the blood. As already stated, peptone so injected may appear in the urine wholly unaltered. Thus, Neumeister[155] has made injections of both amphopeptone and antipeptone in the case of dogs, and was able to detect the peptone very quickly in the urine. I have made like experiments with other forms of peptone and obtained similar results; thus, a pure amphopeptone formed from casein by pepsin-proteolysis (2 grammes in 15 c.c. water) was injected into the jugular vein of a dog weighing 5 kilos. The urine collected during several hours after the injection was heated to boiling, and saturated while hot with ammonium sulphate. The filtrate, on being tested with cupric sulphate and potassium hydroxide, gave a fairly strong biuret reaction for peptone. Another similar experiment made with antipeptone, formed from the myosin of muscle-tissue, gave like results.

[155] Zeitschr. f. Biol., Band 24, p. 287.