The Digestion of Proteins in Vivo

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Abstract
Dogs were fed protein-containing and non-protein test meals and sacrificed approximately one and one-half hours later. The contents of the gastrointestinal tract were analyzed for total nitrogen, 70% ethanol-soluble nitrogen, and 15 “free” amino acids. In the small intestine all 15 amino acids were present in approximately constant proportions whether protein or nonprotein test meals were fed. Lysine, for example, which is absent from zein, one of the test-meal proteins, was present in chyme derived from zein in approximately the same proportion as in chyme derived from egg albumin. The mixture present during digestion appears to contain amino acids derived not only from the hydrolysis of the food proteins but also from autodigestion of the hydrolytic enzymes themselves and other endogenous proteins. It is concluded, therefore, that the qualitative amino acid composition of intestinal contents is not greatly altered by changing from a non-protein to a protein-containing test meal.