Page:Popular Science Monthly Volume 72.djvu/394

 amino-acids were formed in ordinary intestinal digestion. But such data twenty-five years ago, and indeed up to very recent times, failed to attract much attention or were misinterpreted. Physiologists hastened to formulate a theory which would harmonize with existing views, and so arose the theory of "luxus consumption," in which it was held that when an excess of protein food was taken, far larger than the demands of the body called for, the organism was able to protect itself by virtue of this power possessed by trypsin of breaking down protein matter into simple decomposition products easily got rid of with less strain upon liver, kidneys and other organs and tissues.

Many of you doubtless remember the experiments of Schmidt-Mülheim and of Fano, who attempted to determine the amounts of proteoses and peptones present in the blood of dogs after a hearty meal of protein food; and how the negative results they obtained led finally to experiments on the injection of these substances directly into the blood, in which it was found that marked physiological action followed. In other words, proteoses and peptones are not normal constituents of the blood, even though there be a large amount of them in the intestine. They are plainly not absorbed as such, and this fact led to the theory—apparently supported by experiment—that proteoses and peptones in the very act of absorption, in their passage through the epithelial cells of the intestinal wall, are transformed into the proteins of the blood. This made a convenient way of explaining the facts, and one could well imagine that the system took this method of reinforcing the proteins of blood, lymph and tissue. Data, however, have been slowly accumulating which do not admit of such easy interpretation. Physiological chemists interested in enzyme action and equally interested in the chemical constitution of protein matter have been gradually collecting evidence of much significance. A row of diamino-acids, arginine, lysine and histidine, together with alanine, proline, cystine, tryptophane, etc., have been discovered as hydrolytic decomposition products of proteins, both by the action of pancreatic juice and by boiling dilute acids, in addition to the earlier known leucine, tyrosine, glycocoll, aspartic and glutaminic acids, etc. Further, it has been shown that pancreatic juice in artificial digestion experiments, if sufficient time be allowed, is able to bring about a complete breaking down of the protein molecule into these relatively simple amino-acids, so that the biuret reaction, for example, entirely disappears. For a time, this extremely significant fact was not accredited with much importance physiologically; it was interesting; it testified to the general lability of the protein molecule and it threw light on the nature of the building stones which make up the protein complex. Then came, as many of you know, the comparatively recent discovery by Cohnheim of the enzyme erepsin in the duodenal mucous membrane; an enzyme which