Digestion and absorption of protein along ovine gastrointestinal tract

Digestion and absorption of protein were determined in ovine gastrointestinal tract with cerium-141 as an unabsorbed reference substance. Nitrogen flows changed little in rumen and reticulum, but in the proximal small intestine flows increased because of secretion of .9 g nitrogen per day per kg body weight. This secretion included trypsin, chymotrypsin, elastase, and carboxypeptidases A and B; maximal activity was in proximal segments of the small intestine and decreased with distance from the pylorus. Activity of chymotrypsin decreased more rapidly than that of trypsin. Amino acid flows reflected the influx of protein in the duodenum; absorption was approximately 55% in the terminal ileum. No major changes of proportions of individual amino acids were observed. Overall nitrogen absorption was 72.6% of which 6% was in the large intestine. The major soluble protein fraction in the gastrointestinal tract consisted of peptides with molecular weight 7,000 to 14,000 daltons. Soluble high molecular weight protein was observed only in rumen and duodenum. Low molecular weight peptides and amino acids accumulated only in the proximal small intestine. Solubilization of protein and breakdown of peptides of 7,000 to 14,000 molecular weight appear to be rate limiting for protein absorption in sheep.