108. Protein-Splitting Enzyms

Among these are at least two, and possibly several, which act on proteins, including trypsin (possibly not a single body), as the main one, and one that, like erepsin (see p. 105), splits peptones into simpler compounds and seems to supplement the action of trypsin. Trypsin acts in neutral or in alkaline solutions, a free mineral acid like hydrochloric completely stopping its operation. Organic acids, like lactic, do not seem to have this effect. In conjunction with other enzyms, it splits food proteins into simpler compounds, viz., monamino and diamino acids, tryptophane and other bodies, all of which may be regarded as the building stones of the original proteins. As we have seen, these simpler bodies are not the same in kind or proportions for all proteins.

109. Steapsin

The pancreatic secretion acts vigorously on fats, not only splitting them into fatty acids and glycerin, but, in conjunction with the bile; also effects their emulsification, this latter result being aided, doubtless, by the soaps which are formed from a union of the fatty acids and the alkaline bases (mostly sodium) in the bile. This is a true saponification. The cleavage of the fats is due to an enzym to which the name of steapsin is given, also called lipase.

110. Amylopsin

We have seen that starch is acted upon to a small extent by the saliva, and that this action is not prolonged in the stomach beyond the time when the stomach contents become fully acidified. Starch digestion is therefore carried on mainly in the intestines, chiefly, if not wholly, by a diastatic ferment in the pancreatic juice which has the power of hydrolyzing the starch mostly into maltose. This pancreatic diastase, called amylopsin by some authors, is not found in the digestive tract of infants until more than one month after birth. The presence of bile is very favorable to its action.

111. Intestinal Juices

Mention has been made of juices that are secreted by small glands distributed in the walls of the intestines. These appear to be quite important factors in digestion, as they supplement the action of the ferments of the pancreatic juice. It appears to be shown that an enzym erepsin is found in these juices, that is unable to act upon any of the native proteins except casein, but has the power of decomposing proteoses and peptones into simpler compounds, particularly the amino acids. These secretions seem to contain, also, a ferment that converts maltose into dextrose, and in infants and young animals they also contain a lactose- (milk sugar) splitting enzym. It is held that trypsin does nol exist as such in the pancreatic juice when poured into the small intestine, but that this enzym is formed from a mother substance (trypsinogen) in the pancreatic juice after it comes in contact with the intestinal juice, this result being accomplished through the action of a body, probably secreted from the intestinal walls and called by Pawlow enterokinase.