B. Twenty c.c. of the neutral albumen solution + 25 c.c. 0.2 per cent. HCl + 30 c.c. of the neutral pepsin solution. In this mixture, the proteid matter was obviously only half saturated with acid.
The two solutions were placed in a bath at 40° C., where they were allowed to remain for forty-four hours, a little thymol being added to guard against any possible putrefactive changes. At the end of this time the amount of undigested albumin was accurately determined. The 20 c.c. of original albumen solution contained 1.6338 grammes of dry coagulable albumin. At the end of the forty-four hours, A contained only 0.5430 gramme of unaltered albumin, or acid-albumin, while B contained 1.2225 grammes. That is to say, in the mixture A, where the acid existed wholly in the form of combined acid, but with the albumin completely saturated, 1.0908 grammes of the proteid were converted into soluble albumoses and peptones. In B, on the other hand, where the albumin was only half saturated with acid, 0.4113 gramme of the proteid was converted into soluble products. This difference in action is made more striking by the statement that where the proteid was only half saturated with acid, 25.1 per cent. of the albumin was digested; while with a complete saturation of the proteid, 66.7 per cent. of the albumin was digested.
To give emphasis to this matter, a second experiment may be quoted as follows: The proteid used was the same neutral solution of egg-albumen containing 0.8169 gramme of albumin per 10 c.c. Two mixtures were prepared as follows:
A. Ten c.c. of the neutral albumen solution + 21.7 c.c. 0.2 per cent. HCl, the amount needed to completely saturate the proteid, + 40 c.c. of a weak solution of pepsin, perfectly neutral.
B. Ten c.c. of the albumen solution + 10.9 c.c. 0.2 per cent. HCl + 40 c.c. of the pepsin solution, making a mixture half saturated with acid.
These two solutions were warmed at 40° C. for seventeen hours. The extent of digestive action was then determined, when it was found that in A only 0.1638 gramme of the proteid was undigested, while in B, 0.6088 gramme remained unaltered. In other words, where the proteid was completely saturated with acid, but with an utter lack of free acid, 79.9 per cent. of the albumin was converted into albumoses and peptone, while in the mixture half saturated with acid only 25.4 per cent. was digested.
These two experiments thus give striking proof that free acid is not absolutely essential for pepsin-proteolysis. Digestion is, to be sure, more rapid and complete when free hydrochloric acid is present, but proteolysis is still possible, and even vigorous, when there is a marked deficiency of free acid. Further, as we have seen, proteolysis may proceed to a certain extent even though the amount of acid available is not sufficient to combine with more than half the proteid matter present.
These facts at once raise the question whether the products of proteolysis may not have a stronger affinity for acid than the native proteids; an affinity so strong that they may be able to withdraw acid from the acid-albumin first formed. One of our conceptions regarding pepsin-proteolysis is that acid is necessary for every step in the proteolytic process. A primary albumose, for example, cannot be further changed by pepsin, unless there is acid present for it to combine with. This being true, it is clear, in view of the fact that even peptones may appear in a digestive mixture containing an amount of acid insufficient to combine even with the albumin present, that the products of proteolysis must withdraw acid from the acid-albumin first formed. In regard to the first point, my own experiments certainly tend to show that the products of gastric digestion do combine with larger amounts of hydrochloric acid than undigested proteids; and further, that of the several products of proteolysis, the secondary proteoses combine with a larger percentage of acid than the primary proteoses, while true peptones combine with still larger amounts. In other words, the simpler and more soluble the proteid, the larger the amount of acid it is capable of combining with; a statement which accords with results obtained by other workers[101] in this direction. Further, another factor of considerable importance in connection with the natural digestive process is that a dissolved proteid, such as protoalbumose for example, will combine more readily with free acid than an insoluble proteid; from which Gillespie[102] is led to infer that in pepsin-proteolysis where there is no free acid present, only acid-albumin, proteoses may be formed to a limited extent at the expense of some of the acid of the acid-albumin, a portion of the latter being perhaps reconverted into albumin. The ability of the proteoses, however, to withdraw acid from its combination with a native proteid is perhaps best indicated by Kossler’s[103] experiments, which show that a solution of acid-albumin containing only enough hydrochloric acid to hold the albumin dissolved, on being warmed at 40° C. for some hours with addition of a neutral solution of pepsin, may undergo partial conversion into albumose or peptone.
In spite of these facts, there is some evidence that while proteoses and peptones have the power of combining with more acid than a like weight of native proteid, the latter, leaving out all action of the pepsin, has a stronger affinity for the acid; in fact, the firmness or strength of the union appears to diminish as the products become simpler.[104] Hence, a peptone separated from a digestive mixture, will part with its combined acid somewhat more readily than acid-albumin for example, although on this point there is not complete unanimity of opinion.[105] In digestive proteolysis, however, where the pepsin is accompanied by a minimal amount of hydrochloric acid, insufficient perhaps to even half saturate the proteid present, the formation of proteoses and peptones must be accompanied by a withdrawal of acid from its combination with the native proteid.