Century Dictionary and Cyclopedia
- n. The products of albuminous digestion. The first step in the process of proteolytic digestion depends upon the reaction of the digestive medium. If this is acid, acid albumin results; if it is alkaline, alkaline albuminate results. Coincidently the denaturization of the albuminous molecule is effected, whereby the individual physical characteristics of the different members of the group are destroyed. This is followed by the formation of albumoses, of which two classes are recognized, namely, primary and secondary forms. Conjointly the albumoses may be viewed as cleavage-products of the original albumins, and some of their characteristics no doubt are referable to the smaller size of the molecule. As a class they are much more soluble than the albumins, and some members of the group are not altogether indiffusible. Three primary products are recognized, namely, a heteroalbumose, a proto-albumose, and a gluco-albumose. The first mentioned is an antibody (in the sense of Kühne), the second a hemibody; the third is an exclusive carrier of the carbohydrate group. The secondary products, or deuteroalbumoses, result from the primary forms on further digestion, and are divided into three groups, according to their behavior toward ammonium sulphate. These are termed the deuterofractions A, B, and C. Of these A and B are composed of at least two albumoses each. One member of the A fraction is known as thio-albumose, from the large amount of sulphur which it contains. As regards the subsequent fate of the albumoses, it was formerly thought that peptic digestion led to an end-product, a peptone, in which the hemi- and anti-groups of the albuminous molecule were still united (Kühne's amphopeptone), while on tryptic digestion this complex was broken down, with the formation of hemipeptone and antipeptone. The hemipeptone was characterized by the supposed readiness with which it was further decomposed by trypsin into leucin, tyrosin, and other crystalline products; but as a matter of fact the substance itself has thus far had only a hypothetical existence. Kühne's antipeptone, on the other hand, could be isolated apparently as a chemical unity. It can be obtained from a mixture of digestive products, after removal of the soluble albumins and albuminates and after salting out the albumoses with ammonium sulphate in neutral, acid, and ammoniacal solution, by precipitation with absolute alcohol. The investigation of this antipeptone within recent years has shown that the substance is not a unity, however, but a mixture of different products, some of which still give the biuret reaction (peptones), while others are further removed in structure from the original albumins and no longer show the characteristic color-change of the ‘peptones’ (the peptoids). The various components of the antipeptone fraction have not all been isolated as yet, but it appears from Kutscher's researches in the case of fibrin antipeptone that this contains considerable amounts of arginin, lysin, and histidin. The antipeptone of Kühne, in the original meaning of the term, has thus no existence, and further researches have shown that an essential difference, as far as the end-products are concerned, does not exist between peptic and tryptic digestion, and that in either case proteolysis leads to the production of crystalline bodies, which partly belong to the mono-amido acids and partly to the diamido acids, while still others exist which are as yet but little known. Of the manner in which the cleavage of the albumoses to the end-products of proteolytic digestion occurs, and of the intermediary bodies which are formed during the process, our knowledge is still imperfect. But it appears from the researches of E. Fischer and his pupils that the cleavage may take its course over products in which amido-acid radicals are united with one another in gradually diminishing numbers. Such bodies are designated dipeptides, tripeptides, and polypeptides, according to the number of amido-groups which are present in combination. Bodies of this order have been synthetically produced, and have also been demonstrated among the products of peptic and tryptic digestion, such as Siegfried's kyrin, obtained from glutin, which apparently consists of one molecule of arginin, one of lysin, one of glutaminic acid, and two of glycocoll. While formerly only two proteolytic ferments were known to be concerned in the digestion of albumins, Cohnheim has recently shown that a third ferment of this order is found in the mucous membrane of the intestinal tract. This is known as erepsin. In its general behavior and the rapidity of its cleavage-action this ferment resembles trypsin, but it differs from this in its inability to attack the native albumins, while acid albumins and albumoses are readily broken down to end-products which no longer give the biuret reaction. All these data suggest that the older concept of the restitution of the albumins from relatively complex bodies in the intestinal mucous membrane must be abandoned and that the reconstruction of the molecule takes place from much simpler bodies. Where this occurs is as yet a matter of speculation, but there are reasons to believe that it may take place beyond the intestinal barrier, and possibly in the tissues of the body at large.
- n. The products of carbohydrate digestion. Carbohdrate digestion is effected by the inverting ferments of the saliva, the pancreatic juice, and the enteric juice. The effect is quite analogous to what occurs in the case of the albumins, that is, there is a cleavage of the more complex bodies to relatively simple substances. The number of these end-products, owing to the smaller size of the molecule of the original material, is, however, much smaller, and all belong to one group, the monosaccharides, of which dextrose (glucose) and levulose are familiar examples. These are derived from corresponding disaccharides, which can be compared to the dipeptides, and in which two monosaccharine molecules still exist in combination. The most notable representatives of this order are maltose, which on further digestion yields two molecules of dextrose; lactose, which breaks up into dextrose and galactose; and cane-sugar, which similarly gives rise to dextrose and levulose. The disaccharides, in turn, result from the complex polysaccharides, with the intermediary formation of bodies which are comparable to the albumoses. Starch, which is the most important foodstuff belonging to this order, is thus first transformed into erythrodextrine, this into achroödextrine, and this in turn into the disaccharides maltose and isomaltose. Absorption of polysaccharides and disaccharides as such does not take place in the gastro-intestinal canal.
- n. The products of fat-digestion. The digestion of fats is effected by the lipase or steapsin of the pancreatic juice and the enteric juice. This is quite analogous to the digestion of albumins and carbohydrates, and also leads to the liberation of the essential radicals of the original food-material. The end-products are glycerin and the fatty acids which enter into the construction of the fat-molecule, which latter, accordingly, vary with the nature of the fat ingested, the most important being palmitic acid, stearic acid, and oleic acid. In this form the fats are absorbed and then reconstructed from the component radicals.
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