Digestion and Absorption of Ingested and Secreted Proteins Labeled with 75Se-Selenomethionine and 35S-Methionine in the Gastrointestinal Tract of the Rat

Abstract

By labeling exogenous and endogenous proteins with different radioactive amino acids an attempt was made to further establish their fate in the gastro-intestinal tract of the rat. Ovalbumin labeled with 75Se-selenomethionine and 35S-methionine was fed to rats and the rate of disappearance of both amino acids from the stomach and different segments of small intestine was studied. No differences could be observed in the rate of liberation during digestion and in the subsequent absorption of both amino acids. Similarly, a parallel rate of absorption of both methionines was observed from an intestinal segment in situ. Using ovalbumin labeled with 75Se-selenomethionine, the extent of digestion of dietary versus endogenous secreted proteins showed that exogenous proteins emptied from the stomach are rapidly digested and that the small fraction not digested accumulates in the distal jejunum and ileum, together with the majority of the endogenous protein and of the tryptic activity present in the small intestine. In the presence of soybean trypsin inhibitor added to the diet, a marked reduction in the digestion of the exogenous protein resulted, while the endogenous protein content of the intestine remained relatively constant. Labeled endogenous proteins, peptides and amino acids were isolated from the stomach and the small intestine if radioactive methionine was injected intravenously 2 hours before feeding. This indicates that some digestion of endogenous protein was also taking place; however no quantitative estimates could be obtained.